U.S. patent application number 12/288578 was filed with the patent office on 2009-05-07 for safety system for power equipment.
Invention is credited to Stephen F. Gass.
Application Number | 20090114070 12/288578 |
Document ID | / |
Family ID | 36568753 |
Filed Date | 2009-05-07 |
United States Patent
Application |
20090114070 |
Kind Code |
A1 |
Gass; Stephen F. |
May 7, 2009 |
Safety system for power equipment
Abstract
Safety systems for power equipment are disclosed. The safety
systems include a detection system adapted to detect a dangerous
condition between a person and a working portion of a machine, and
a reaction system associated with the detection system to cause a
predetermined action to take place upon detection of the dangerous
condition. The reaction system may be adapted to perform some
action upon detection of the dangerous condition with sufficient
speed so that the person is cut no deeper than 1/8th of an inch
when contacting or approaching the blade at any point above the
table at a rate of one foot per second.
Inventors: |
Gass; Stephen F.;
(Wilsonville, OR) |
Correspondence
Address: |
SD3, LLC
9564 S.W. Tualatin Road
Tualatin
OR
97062
US
|
Family ID: |
36568753 |
Appl. No.: |
12/288578 |
Filed: |
October 21, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11447449 |
Jun 5, 2006 |
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12288578 |
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09676190 |
Sep 29, 2000 |
7055417 |
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11447449 |
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60157340 |
Oct 1, 1999 |
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60182866 |
Feb 16, 2000 |
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Current U.S.
Class: |
83/76.8 ;
83/477.2 |
Current CPC
Class: |
H01H 3/022 20130101;
F16P 3/12 20130101; Y10T 83/175 20150401; H01H 27/00 20130101; Y10S
83/01 20130101; Y10T 83/7788 20150401; B23D 59/001 20130101; H01H
9/161 20130101; Y10T 83/081 20150401; Y10T 83/773 20150401; Y10T
83/089 20150401; Y10T 83/178 20150401; B27B 5/38 20130101; B27G
19/02 20130101; Y10T 83/7726 20150401 |
Class at
Publication: |
83/76.8 ;
83/477.2 |
International
Class: |
B26D 5/20 20060101
B26D005/20; B27B 27/04 20060101 B27B027/04 |
Claims
1. A method of operating a table saw, where the table saw includes
a table, a circular blade adapted to spin and to extend at least
partially above the table to cut a work piece on the table, a
detection system capable of detecting contact or dangerous
proximity between a person and the blade when the blade is
spinning, and a reaction system adapted to perform some action upon
detection of such contact or dangerous proximity with sufficient
speed so that the person is cut no deeper than 1/8.sup.th of an
inch when contacting or approaching the blade at any point above
the table at a rate of one foot per second, the method comprising:
spinning the blade, detecting contact or dangerous proximity
between the person and the blade, and if contact or dangerous
proximity between the person and the blade is detected, then
triggering the reaction system to perform an action with sufficient
speed so that the person is cut no deeper than 1/8.sup.th of an
inch when contacting or approaching the blade at any point above
the table at a rate of one foot per second.
2. The table saw of claim 1, where the action performed by the
reaction system is to stop the blade.
3. The table saw of claim 1, where the action performed by the
reaction system is to retract the blade.
4. The table saw of claim 1, where the action performed by the
reaction system is to move the blade away from the person.
5. The table saw of claim 1, where the action performed by the
reaction system is to stop and retract the blade
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/447,449, filed Jun. 5, 2006, which is a
continuation of U.S. patent application Ser. No. 09/676,190, filed
Sep. 29, 2000, issuing as U.S. Pat. No. 7,055,417 on Jun. 6, 2006,
which application claims the benefit of and priority from U.S.
Provisional Patent Application Ser. No. 60/157,340, filed Oct. 1,
1999 and U.S. Provisional Patent Application Ser. No. 60/182,866,
filed Feb. 16, 2000. All of the above applications are hereby
incorporated by reference in their entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to safety systems and more
particularly to a high-speed safety stop for use on power
equipment.
BACKGROUND OF THE INVENTION
[0003] Beginning with the industrial revolution and continuing to
the present, mechanized equipment has allowed workers to produce
goods with greater speed and less effort than possible with
manually-powered tools. Unfortunately, the power and high operating
speeds of mechanized equipment creates a risk for those operating
such machinery. Each year thousands of people are maimed or killed
by accidents involving power equipment.
[0004] As might be expected, many systems have been developed to
minimize the risk of injury when using power equipment. Probably
the most common safety feature is a guard that physically blocks an
operator from making contact with dangerous components of
machinery, such as belts, shafts or blades. In many cases, guards
are effective to reduce the risk of injury, however, there are many
instances where the nature of the operations to be performed
precludes using a guard that completely blocks access to hazardous
machine parts.
[0005] Various systems have been proposed to prevent accidental
injury where guards cannot effectively be employed. For instance,
U.S. Pat. Nos. 941,726, 2,978,084, 3,011,610, 3,047,116, 4,195,722
and 4,321,841, the disclosures of which are incorporated herein by
reference, all disclose safety systems for use with power presses.
These systems utilize cables attached to the wrists of the operator
that either pull back a user's hands from the work zone upon
operation or prevent operation until the user's hands are outside
the danger zone. U.S. Pat. Nos. 3,953,770, 4,075,961, 4,470,046,
4,532,501 and 5,212,621, the disclosures of which are incorporated
herein by reference, disclose radio-frequency safety systems which
utilize radio-frequency signals to detect the presence of a user's
hand in a dangerous area of the machine and thereupon prevent or
interrupt operation of the machine.
[0006] U.S. Pat. Nos. 4,959,909, 5,025,175, 5,122,091, 5,198,702,
5,201,684, 5,272,946, and 5,510,685 disclose safety systems for use
with meat-skinning equipment, and are incorporated herein by
reference. These systems interrupt or reverse power to the motor,
or disengage a clutch, upon contact with a user's hand by any
dangerous portion of the machine. Typically, contact between the
user and the machine is detected by monitoring for electrical
contact between a fine wire mesh in a glove worn by the user and
some metal component in the dangerous area of the machine. Although
such systems are suitable for use with meat skinning machines, they
are relatively slow to stop the motion of the cutting element
because they rely on the operation of solenoids or must overcome
the inertia of the motor. However, because these systems operate at
relatively low speeds, the blade does not need to be stopped
rapidly to prevent serious injury to the user.
[0007] U.S. Pat. Nos. 3,785,230 and 4,026,177, the disclosures of
which are herein incorporated by reference, disclose a safety
system for use on circular saws to stop the blade when a user's
hand approaches the blade. The system uses the blade as an antenna
in an electromagnetic proximity detector to detect the approach of
a user's hand prior to actual contact with the blade. Upon
detection of a user's hand, the system engages a brake using a
standard solenoid. Unfortunately, such a system is prone to false
triggers and is relatively slow acting because of the solenoid.
U.S. Pat. No. 4,117,752, which is herein incorporated by reference,
discloses a similar braking system for use with a band saw, where
the brake is triggered by actual contact between the user's hand
and the blade. However, the system described for detecting blade
contact does not appear to be functional to accurately and reliably
detect contact. Furthermore, the system relies on standard
electromagnetic brakes operating off of line voltage to stop the
blade and pulleys of the band saw. It is believed that such brakes
would take 50 ms-1s to stop the blade. Therefore, the system is too
slow to stop the blade quickly enough to avoid serious injury.
[0008] None of these existing systems have operated with sufficient
speed and/or reliability to prevent serious injury with many types
of commonly used power tools. Although proximity-type sensors can
be used with some equipment to increase the time available to stop
the moving pieces, in many cases the user's hands must be brought
into relatively close proximity to the cutting element in the
normal course of operation. For example, many types of woodworking
equipment require that the user's hands pass relatively close to
the cutting tools. As a result, existing proximity-type sensors,
which are relatively imprecise, have not proven effective with this
type of equipment. Even where proximity sensors are practical,
existing brake systems have not operated quickly enough to prevent
serious injury in many cases.
[0009] In equipment where proximity-type detection have not proven
effective, the cutting tool must stop very quickly in the event of
user contact to avoid serious injury. By way of example, a user may
feed a piece of wood through a table saw at a rate of approximately
one foot per second. Assuming an average reaction time of
approximately one-tenth of a second, the hand may have moved well
over an inch before the user even detects the contact. This
distance is more than sufficient to result in the loss of several
digits, severing of vital vessels and tendons, or even complete
severing of a hand. If a brake is triggered immediately upon
contact with the saw's blade, the blade must be stopped within
approximately one-hundredth of a second to limit the depth of
injury to one-eighth of an inch. Standard solenoids or other
electromagnetic devices are generally not designed to act in this
time scale, particularly where significant force must be generated.
For instance, in the case of solenoids or electromagnetic brakes
that operate on 60 hz electrical power, it is possible that the
power line will be at a phase that has low voltage at the time the
brake is triggered and several milliseconds may elapse before the
voltage reaches a sufficient level even to begin physical
displacement of the brake, much less achieve a complete stoppage of
the blade or cutting tool.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic view of a machine with a fast-acting
safety stop according to the present invention.
[0011] FIG. 2 is a schematic view of a contact detection system and
a brake system of a fast-acting safety stop according to the
present invention.
[0012] FIG. 3 is a schematic circuit diagram of an exemplary first
electrical system, an exemplary second electrical system, and an
exemplary firing system according to the present invention.
[0013] FIG. 4 is a sectional view of a saw blade mounted on an
arbor according to the present invention.
[0014] FIG. 5 is a sectional view of an alternative charge plate
configuration according to the present invention.
[0015] FIG. 6 is a schematic circuit diagram of an alternative
first electrical system, an alternative second electrical system,
and an alternative firing system according to the present
invention.
[0016] FIG. 7 is a side elevation showing an alternative
arrangement of a fusible member according to the present
invention.
[0017] FIG. 8 is a front elevation of the alternative arrangement
of FIG. 7.
[0018] FIG. 9 is a flowchart illustrating an exemplary control
logic power-on sequence according to the present invention.
[0019] FIG. 10 is a detailed sectional view of an alternative brake
system using an explosive charge according to the present
invention.
[0020] FIG. 11 is a schematic view of an alternative pawl
configuration according to the present invention.
[0021] FIG. 12 is a schematic view of an alternative pawl
configuration according to the present invention.
[0022] FIG. 13 is a schematic view of an alternative pawl
configuration according to the present invention.
[0023] FIG. 14 is a schematic view of an alternative pawl
configuration according to the present invention.
[0024] FIG. 15 is a detail view of an exemplary pawl configuration
to automatically detect correct blade-to-pawl spacing according to
the present invention.
[0025] FIG. 16 is a side elevation of an alternative pawl
configuration according to the present invention.
[0026] FIG. 17 is a top plan of the alternative pawl configuration
of FIG. 16.
[0027] FIG. 18 is a side elevation of an exemplary implementation
of a safety stop according to the present invention in the context
of a table saw.
[0028] FIG. 19 is a detail view of a cartridge-type safety stop
according to the present invention.
[0029] FIG. 20 is a side elevation of an exemplary implementation
of a safety stop according to the present invention in the context
of a miter saw.
[0030] FIG. 21 is a top cross-section view taken along the line
21-21 in FIG. 20.
[0031] FIG. 22 is an alternative implementation of the safety stop
of FIG. 20 in which the miter saw blade is adapted to swing upward
when stopped by the pawl.
[0032] FIG. 23 is a side elevation of an exemplary implementation
of a safety stop according to the present invention in the context
of a radial arm saw.
[0033] FIG. 24 is a side elevation of an exemplary implementation
of a safety stop according to the present invention in the context
of a circular saw.
[0034] FIG. 25 is a side elevation of an exemplary implementation
of a safety stop according to the present invention in the context
of a band saw.
[0035] FIG. 26 is a section view taken along the line 26-26 in FIG.
25 and showing an alternative charge plate arrangement according to
the present invention.
[0036] FIG. 27 is a schematic view of an alternative brake system
according to the present invention.
[0037] FIG. 28 is a side elevation of an exemplary implementation
of a safety stop according to the present invention in the context
of a jointer.
DETAILED DESCRIPTION AND BEST MODE OF THE INVENTION
[0038] A machine according to the present invention is shown
schematically in FIG. 1 and indicated generally at 10. Machine 10
includes a motor assembly 12 adapted to drive a cutting tool 14.
Motor assembly 12 includes one or more motors, at least one of
which is adapted to drive cutting tool 14. For example, machine 10
may include one or more motors adapted to drive tool 14 as well as
one or more motors adapted to feed work pieces, such as wood, into
contact with the cutting tool. Cutting tool 14 typically includes
one or more blades or other suitable cutting implements that are
adapted to cut or remove portions from the work pieces. The
particular form of cutting tool 14 will tend to vary depending upon
the various embodiments of machine 10. For example, in table saws,
chop and miter saws, circular saws and radial arm saws, cutting
tool 14 will typically include a circular rotating blade having a
plurality of teeth disposed along the perimetrical edge of the
blade. For a jointer or planer, the cutting tool includes a
plurality of radially spaced-apart blades. For a band saw, the
cutting tool includes an elongate, circuitous tooth-edged band.
[0039] A controller is disposed between motor assembly 12 and a
power source 22, and typically will include one or more switches
used to start and stop the operation of the machine. As shown, the
controller takes the form of an electromagnetic contactor 16 that
includes start and stop switches 18 and 20. An example of a
suitable start switch 20 is a momentary contact switch that
establishes a circuit to close the contactor upon operation. When
the contactor is closed, power is delivered to the motor. The
contactor includes an internal switch that maintains the circuit
once the contactor is closed. As example of a suitable stop switch
22 is a normally-closed momentary switch that upon operation
interrupts the circuit holding the contactor closed, thereby
stopping the motor. It will be appreciated that the above
description is intended only to provide an illustrative example of
such a contactor, and any other suitable actuator for starting and
stopping the operation of the machine, such as solid-state devices,
may be used.
[0040] Machine 10 includes a safety stop 30 that stops the cutting
tool abruptly upon contact between the cutting tool and the user's
body. As used herein the phrases "virtually instantaneously,"
"immediately," "instantly," "rapidly," "abruptly," and "suddenly"
mean sufficiently quickly to prevent serious injury to a user, such
as amputation of one or more of the user's fingers, in the normal
course of machine operation (i.e., the user is using the machine as
intended, with normal feed rates, etc.). Stopping times of less
than 10 milliseconds from initial contact between the cutting tool
and the user are preferred, with stopping times of less than 5
milliseconds, or preferably less than 2 milliseconds being more
desirable and potentially attainable with the safety stop of the
present invention. Alternatively, the stopping times may be longer
where such would be sufficient to prevent serious injury under the
circumstances of normal use. For example, stopping times of 20, 50
or 100 milliseconds may be sufficient.
[0041] As will be described in more detail below, safety stop 30
includes two major subsystems, a contact detection system 32 and a
brake system 34. The contact detection system monitors the cutting
tool for contact with a user. Upon detection of such contact,
system 32 triggers the brake system. In response to actuation by
detection system 32, the brake system stops the cutting tool (or
takes other action to eliminate the danger of injury) abruptly.
[0042] In addition to actuating brake system 34, contact detection
system 32 preferably also interrupts the power to the motor
assembly, or at least the portion of motor assembly 12 adapted to
drive the cutting tool. For example, when machine 10 includes a
magnetic contactor, such as described in the above example, the
detection system may be adapted to interrupt the circuit holding
the magnetic contactor closed so that power to the motor is
interrupted. It should be understood that this step is optional, in
that interrupting power to the machine's motor assembly is not
necessary or sufficient to prevent serious injury to the user when
the user touches the machine's cutting tool. Therefore, the
principal benefit of this step is to reduce the likelihood of
damaging the motor assembly or drive system while the brake system
is preventing rotation or other movement of the cutting tool.
[0043] It will be appreciated that detection system 32 may employ
any one or more of a wide variety of methods for detecting contact
or proximity between the cutting tool and a user's body. In view of
the relatively high response speed of electronic signals and
circuits, one suitable method includes using electrical circuitry
to detect an electronic connection between a user and the cutting
tool. It is well known among those of skill in the art that the
capacitance of the human body is approximately 300 picofarads. As a
result, when a user contacts cutting tool 14, the capacitance of
the user's body is electrically coupled to the inherent capacitance
of the cutting tool, thereby creating an effective capacitance that
is larger than the inherent capacitance of the cutting tool alone.
Thus, detection system 32 may be electrically coupled to measure
the capacitance of the cutting tool, so that any substantial change
in the measured capacitance would indicate contact between the
user's body and the cutting tool. The various prior art safety
systems described above also provide various ways to detect contact
or proximity that can be used to trigger the brake system.
[0044] A specific example of a capacitive touch-sensing detection
system 32 is schematically illustrated in FIGS. 2 and 3. Contact
detection system 32 includes a first electrical system that is
configured to generate an input signal and that is capacitively
coupled, via the cutting tool, to a second electrical system
configured to detect the input signal. In such an arrangement, any
change in the capacitance of the cutting tool changes the input
signal reaching the second electrical system. Thus, contact between
a user's body and the cutting tool causes the second electrical
system to detect a change in the input signal.
[0045] In the illustrated example, detection system 32 includes a
first electrical system 38 configured to generate an input signal.
First electrical system 38 is connected to a charge plate 62 that
is mounted close to, but spaced-apart from, a cutting tool 14,
which in FIG. 2 is shown in the form of a saw blade 64. Plate 62 is
capacitively coupled to the saw blade by virtue of its size and
placement parallel to and spaced-apart from the saw blade. A second
charge plate 66 is also mounted close to the saw blade to establish
a second capacitive coupling. It is within the scope of the present
invention that the number, size and placement of charge plates may
vary.
[0046] The effect of the arrangement shown in FIG. 2 is to form two
capacitors in series through the blade, creating a capacitive shunt
at the junction between the capacitors. Thus, the input signal is
transmitted from charge plate 62 through the blade and onto charge
plate 66. As illustrated, exemplary contact detection system 32
also includes a second electrical system 88 connected to charge
plate 66, and configured to detect changes in the input signal
received at charge plate 66.
[0047] When a user touches the saw blade, the capacitance of the
user's body creates a capacitive load on the blade. As a result,
the size of the capacitive shunt between the charge plates and the
blade is increased, thereby reducing the charge that reaches plate
66. Thus, the magnitude of the input signal passed through the
blade to plate 66 decreases when a user touches the blade. As will
be discussed in more detail below, second electrical system 88 is
configured to respond to this change in the input signal with an
output signal to brake system 34.
[0048] In some cases, there may be a significant amount of
resistance at the contact point of the user's dry skin and the
blade. This resistance may reduce the capacitive coupling of the
user's body to the blade. However, when the teeth on the blade
penetrate the outer layer of the user's skin, the moisture inherent
in the internal tissue of skin will tend to decrease the resistance
of the skin/blade contact, thereby establishing a solid electrical
connection. Moreover, as will be described below, the sensitivity
of second electrical system 88 can be adjusted as desired to
recognize even slight changes in the input signal.
[0049] While one exemplary system and method for detecting contact
between the user's body and the blade is described herein, many
other systems and methods are available and within the scope of the
invention. For example, the detection system may sense the
resistance of the human body upon contact between the user's body
and the blade to detect contact. As another example, a radio signal
may be broadcast near the blade so that the user's body acts as an
antenna to change the signal received through the blade when
contact is made between the user's body and the blade. Similarly, a
signal could be applied to the user's body by virtue of standing on
a signal transfer mat or wearing a transmitter and the detection
system could monitor for reception of the signal upon contact with
or proximity to the cutting tools.
[0050] As a further example, a proximity detector may be used as
described above. Since brake system 34 is capable of stopping the
blade within a few milliseconds, the proximity detector may be set
to trigger only upon extremely close positioning of the user's body
relative to the blade. This would allow the user to manipulate the
work piece close to the blade without triggering the brake system.
As another example, the wire-mesh glove detection system described
above in connection with the meat cutting equipment may be used.
Thus, it will be appreciated that while an exemplary embodiment has
been described which uses a capacitive contact detection system,
any suitable method of detecting contact or close proximity of the
user's body to the blade may be used.
[0051] FIG. 3 illustrates one suitable configuration of first
electrical system 38 and second electrical system 88. However, it
will be appreciated by those of skill in the electrical arts that
the exemplary configuration of detection system 32 illustrated in
FIG. 3 is just one of many configurations which may be used. Thus,
it will be understood that any suitable embodiment or configuration
could be used within the scope of the invention.
[0052] As shown in FIG. 3, first electrical system 38 includes an
oscillator circuit that generates a wave input signal, such as a
square wave signal, at a frequency of approximately 200 khz and
voltage amplitude of 12 volts. Alternatively, first electrical
system 38 may be configured to generate a signal of a different
frequency and/or a different amplitude and/or different waveform.
The oscillator is formed by a pair of inverters 40, 42 from a
CD4040 configured as a bistable oscillator. The output of inverter
40 is connected to a 100 pF capacitor 44, which is connected
through a 100 k.OMEGA. resistor 46 to the input of inverter 42. A
10 k.OMEGA. resistor 48 is connected between the output of inverter
42 to the junction between capacitor 44 and resistor 48. The output
of inverter 42 is connected to the input of inverter 40. A 10
k.OMEGA. resistor 50 connects the output of inverter 40 to the
input of another inverter 52, which serves as an output buffer to
drive the input wave signal onto the blade. A 2 k.OMEGA. series
resistor 54 functions to reduce any ringing in the input signal by
damping the high frequency components of the signal.
[0053] It will be appreciated that the particular form of the
oscillator signal may vary and there are many suitable waveforms
and frequencies that may be utilized. The waveform may be chosen to
maximize the signal-to-noise ratio, for example, by selecting a
frequency at which the human body has the lowest resistance or
highest capacitance relative to the workpiece being cut. In
addition, there are many different oscillator circuits that are
well known in the art and which would also be suitable for
generating the input signal.
[0054] The input signal generated by the oscillator is fed through
a shielded cable 60 onto charge plate 62. Shielded cable 60
functions to insulate the input signal from any electrical noise
present in the operating environment, insuring that a "clean" input
signal is transmitted onto charge plate 62. Alternatively, other
methods may be used to prevent noise in the input signal. As a
further alternative, second electrical system 88 may include a
filter to remove any noise in the input signal or other electrical
noise detected by charge plate 66.
[0055] Generally speaking, the spacing of the charge plates from
the blade is not critical. However, it may be desirable to separate
the plates from the blade by a distance selected to reduce the
effect of deflections in the blade on the capacitance between the
blade and the plates. For instance, if the blade is displaced 1/32
of an inch toward one of the plates by loads created during cutting
operations, the capacitance to that plate is increased. Since the
capacitance is proportional to the area of the plate divided by the
spacing, a relatively large spacing reduces the relative effect of
a given blade displacement. Similarly, placing the plates
relatively close to the center of the blade is preferable because
the blade undergoes minimal lateral displacement nearer the arbor
upon which it is mounted. Distances in the range of approximately
1/32 inch and approximately 1/2 inch have proven effective,
although values outside this range could be used under appropriate
circumstances.
[0056] In an alternative embodiment, at least one of the charge
plates may include one or more insulating spacers 68 mounted on the
side of the charge plate adjacent the blade, such as shown in FIG.
2. Spacers 68 act as physical barriers to prevent the blade from
deflecting too close to the charge plate. This may be especially
useful when the distances between the charge plates and the blade
are relatively small. The spacers may be constructed of any
suitable electrically insulating material, including ceramic,
glass, etc. In the exemplary embodiment depicted in FIG. 2, spacers
68 cover only a small portion of the area between the charge plates
and the blade. As a result, the spacers have relatively little
effect on the capacitance between the blade and the plate.
Alternatively, the spacers may cover a substantially larger
portion, or even all of the space between the charge plates and the
blade. In this latter case, the spacer will function, at least
partially, as the dielectric between the conductive surfaces of the
charge plates and the blade. Thus, the capacitance between the
blade and the charge plates will vary depending on the dielectric
constant of the spacer. In addition to one or more spacers 68
mounted between the charge plates and the blade, opposing spacers
(not shown) may be mounted on the side of the blade opposite the
charge plates to prevent the blade from deflecting too far from the
charge plates. The spacers may be designed to slide on the surface
of the blade so that the plates move with any deflections of the
blade. An advantage of this arrangement is the close spacing that
can be established and maintained, thereby reducing the size of the
plates.
[0057] It will be appreciated that the size of charge plates 62 and
66 may also vary. Typical plate areas are between 1 and 10 square
inches, although many different sizes may be used, including sizes
outside of this typical range. An example of a suitable plate
material is copper-plated printed circuit board, which is
relatively rigid and thin. Other examples include any relatively
electrically conductive material such as gold, aluminum, copper,
steel, etc. Where there are large grounded metal structures near
the blade, a larger driving charge plate 62 can be used to
partially shield the blade from capacitive coupling to the grounded
structure. Although the larger plate also will have increased
capacitive coupling to the grounded structure, this does not
interfere with the operation of the system since first electrical
system 38 is capable of driving much larger capacitance loads than
are created under these circumstances.
[0058] As described above, the input signal is coupled from charge
plate 62 to charge plate 66 via blade 64. As shown in FIG. 3, the
signal received on charge plate 66 is then fed via a shielded cable
90 to second electrical system 88. The second electrical system is
configured to detect a change in the signal due to contact between
the user's body and the blade. It will be appreciated that second
electrical system 88 may be implemented in any of a wide variety of
designs and configurations. In the exemplary embodiment depicted in
FIG. 3, second electrical system 88 compares the amplitude of the
input signal received at charge plate 66 to a determined reference
voltage. In the event that the input signal received at charge
plate 68 falls below the reference voltage for a determined time,
the second electrical system produces an output signal to brake
system 34. The brake system is configured to receive the output
signal and immediately act to stop the blade.
[0059] The particular components of second electrical system 88 may
vary depending on a variety of factors including the application,
the desired sensitivity, availability of components, type of
electrical power available, etc. In the exemplary embodiment, a
shielded cable 90 is connected between charge plate 66 and a
voltage divider 91. Voltage divider 91 is formed by two 1M.OMEGA.
resistors 92, 94 connected in series between the supply voltage
(typically about 12 volts) and ground. The voltage divider
functions to bias the output signal from charge plate 66 to an
average level of half of the supply voltage. The biased signal is
fed to the positive input of an op-amp 96. Op-amp 96 may be any one
of many suitable op-amps that are well known in the art. An example
of such an op-amp is a TL082 op-amp. The negative input of the
op-amp is fed by a reference voltage source 97. In the exemplary
embodiment, the reference voltage source is formed by a 10 k.OMEGA.
potentiometer 98 coupled in series between two 10 k.OMEGA.
resistors 100, 102, which are connected to ground and the supply
voltage, respectively. A 0.47 .mu.F capacitor 104 stabilizes the
output of the reference voltage.
[0060] As will be understood by those of skill in the art, op-amp
96 functions as a comparator of the input signal and the reference
voltage. Typically, the voltage reference is adjusted so that its
value is slightly less than the maximum input signal voltage from
charge plate 66. As a result, the output of the op-amp is low when
the signal voltage from the charge plate is less than the reference
voltage and high when the signal voltage from the charge plate is
greater than the reference voltage. Where the input signal is a
periodic signal such as the square wave generated by first
electrical system 38, the output of op-amp 96 will be a similar
periodic signal. However, when a user contacts the blade, the
maximum input signal voltage decreases below the reference voltage
and the op-amp output no longer goes high.
[0061] The output of op-amp 96 is coupled to a charging circuit
106. Charging circuit 106 includes a 240 pF capacitor 108 that is
connected between the output of op-amp 96 and ground. A 100
k.OMEGA. discharge resistor 112 is connected in parallel to
capacitor 108. When the output of op-amp 96 is high, capacitor 108
is charged. Conversely, when the output of op-amp 96 is low, the
charge from capacitor 108 discharges through resistor 112 with a
time constant of approximately 24 .mu.s. Thus, the voltage on
capacitor 108 will discharge to less than half the supply voltage
in approximately 25-50 .mu.s unless the capacitor is recharged by
pulses from the op-amp. A diode 110 prevents the capacitor from
discharging into op-amp 96. Diode 110 may be any one of many
suitable diodes which are well known in the art, such as a 1N914
diode. It will be appreciated that the time required for capacitor
108 to discharge may be adjusted by selecting a different value
capacitor or a different value resistor 112.
[0062] As described above, charging circuit 106 will be recharged
repeatedly and the voltage across capacitor 108 will remain high so
long as the detected signal is received substantially unattenuated
from its reference voltage at op-amp 96. The voltage from capacitor
108 is applied to the negative input of an op-amp 114. Op-amp 114
may be any one of many suitable op-amps which are well known in the
art, such as a TL082 op-amp. The positive input of op-amp 114 is
tied to a reference voltage, which is approximately equal to
one-half of the supply voltage. In the exemplary embodiment
depicted in FIG. 3, the reference voltage is provided by reference
voltage source 97.
[0063] So long as charging circuit 106 is recharged, the output of
op-amp 114 will be low. However, if the output of op-amp 96 does
not go high for a period of 25-50 .mu.s, the voltage across
capacitor 108 will decay to less than the reference voltage, and
op-amp 114 will output a high signal indicating contact between the
user's body and the blade. As will be described in more detail
below, the output signal from op-amp 114 is coupled to actuate
brake system 34 and stop the blade. The time between contact and
braking can be adjusted by selecting the time constant of capacitor
108 and resistor 112.
[0064] It should be noted that, depending on the size,
configuration and number of teeth on the blade and the position of
contact with the operator, the electrical contact between the
operator and blade might be intermittent. As a result, it is
desirable that the system detect contact in a period less than or
equal to the time a single tooth would be in contact with a user's
finger or other body portion. For example, assuming a 10-inch blade
rotating at 3600 rpm and a contact distance of about one-quarter of
an inch (the approximate width of a fingertip), a point on the
surface of the blade, such as the point of a tooth, will be in
contact with the user for approximately 100 .mu.s. After this
period of contact, there will normally be an interval of no contact
until the next tooth reaches the finger. The length of the contact
and non-contact periods will depend on such factors as the number
of teeth on the blade and the speed of rotation of the blade.
[0065] It is preferable, though not necessary, to detect the
contact with the first tooth because the interval to the second
tooth may be substantial with blades that have relatively few
teeth. Furthermore, any delay in detection increases the depth of
cut that the operator will suffer. Thus, in the exemplary
embodiment, the charging circuit is configured to decay within
approximately 25-50 .mu.s to ensure that second electrical system
88 responds to even momentary contact between the user's body and
the blade. Further, the oscillator is configured to create a 200
khz signal with pulses approximately every 5 .mu.s. As a result,
several pulses of the input signal occur during each period of
contact, thereby increasing the reliability of contact detection.
Alternatively, the oscillator and charging circuit may be
configured to cause the detection system to respond more quickly or
more slowly. Generally, it is desirable to maximize the reliability
of the contact detection, while minimizing the likelihood of
erroneous detections.
[0066] It will be appreciated by those of skill in the art that
blade 64 should be insulated from ground to allow the input signal
to be capacitively coupled from charge plate 62 to charge plate 66.
In the exemplary embodiment depicted in FIG. 4, blade 64 is
electrically isolated from arbor 70 on which it rides, thus
insulating the blade from ground and the remaining structure of the
machine. There are a variety of suitable arrangements for providing
electrical insulation between the blade and the arbor, which may
vary depending on the particular configuration of machine 10. For
example, in the case of a 5/8-inch arbor shaft 70, blade 64 can be
formed with a one-inch diameter hole into which a 3/16-inch thick
cylindrical plastic bushing 72 is fitted, such as shown in FIG. 4.
Insulating washers 74, 76 are disposed on either side of the blade
to isolate the blade from the arbor flange 78 and arbor washer 80.
The insulating washers should be thick enough that only negligible
capacitance is created between the blade and the grounded arbor
flange and washer. A typical thickness is approximately 1/8-inch,
although 1/32-inch or less may be suitable depending on other
factors. In addition, it is possible to construct some or all of
the arbor components from non-conductive materials, such as
ceramic, to reduce or eliminate the need for electrical isolation
from the arbor.
[0067] An arbor nut 82 holds the entire blade assembly on arbor 70.
Friction established by tightening the arbor nut allows torque from
the arbor to be transmitted to the saw blade. It is preferable,
although not essential, that the blade be able to slip slightly on
the arbor in the event of a sudden stop by the brake to reduce the
mass that must be stopped and decrease the chance of damage to the
blade, arbor, and/or other components in the drive system of the
saw. Furthermore, it may be desirable to construct the bushing from
a material that is soft enough to deform when the blade is stopped
suddenly. For example, depending on the type of braking system
used, a substantial radial impact load may be transmitted to the
arbor when the brake is actuated. A deformable bushing can be used
to absorb some of this impact and reduce the chance of damage to
the arbor. In addition, proper positioning of the brake in
combination with a deformable bushing may be employed to cause the
blade to move away from the user upon activation of the brake, as
will be discussed in further detail below.
[0068] In an alternative embodiment, the arbor and/or part of its
supporting framework is electrically isolated from ground instead
of isolating the blade from the arbor. One benefit of this
embodiment is that if the blade is electrically connected to the
arbor, then the arbor itself can be used to capacitively couple the
input signal from charge plate 62 to charge plate 66. While the
particular implementation of this alternative embodiment will vary
with the configuration of the cutting tool, one exemplary
implementation is depicted in FIG. 5. As shown, blade 64 is mounted
directly onto arbor 70. As in FIG. 4, the blade is secured to the
arbor by arbor flange 78, arbor washer 80 and arbor nut 82.
[0069] The arbor is supported for rotational movement by a pair of
bearings 79 spaced along the elongate axis of the arbor. However,
bearings 79 do not contact the arbor directly. Instead,
electrically insulating sleeves 84 are disposed between the arbor
and the bearings. Bearings 79 are mounted in a movable arbor block
81. The arbor block allows the blade to be raised and lowered, as
well as to be inclined for angled cuts. A motor (not shown) drives
the arbor through a belt 83 that loops over a pulley 86 on the end
of the arbor opposite the blade. The belt typically is
non-conducting and thus does not electrically couple the arbor to
ground.
[0070] Sleeves 84 may be constructed of any suitable material that
is relatively durable and non-conductive, including plastic,
ceramic, etc. The sleeves may be configured to fit over a
constant-diameter arbor as shown, or the arbor may be notched to
receive the sleeves so that the outer diameter of the sleeves are
flush with the outer diameter of the arbor. Furthermore, it will be
appreciated that there are many other arrangements for electrically
insulating the arbor. As just a few examples, sleeves 84 may be
disposed between bearings 79 and arbor block 81, or at least
portions of the bearings may be constructed of non-conductive
materials. Alternatively, larger portions of the arbor assembly may
be isolated from the rest of the saw.
[0071] In any event, charging plates 62 and 66 are disposed
alongside, but slightly spaced from, the arbor. The charging plates
typically are shaped and arranged relative to the arbor to ensure
adequate capacitive coupling. For example, the charging plates may
be trough-shaped to conform to the cylindrical shape of the arbor,
as illustrated in FIG. 5. Alternatively, the plates may be
ring-shaped to completely surround axially-spaced portions of the
arbor. The charging plates typically are supported on arbor block
81, such as by mounts 85 extending from the frame. This arrangement
ensures that the charging plates will move in tandem with the arbor
when the position or angle of the blade is adjusted. The mounts
usually will be configured to electrically insulate the charging
plates from the frame. The charge plates can be positioned very
close to the arbor because it does not deflect during use like the
blade, thereby allowing smaller charge plates to the utilized.
[0072] While a few exemplary arrangements for capacitively coupling
the charging plates to the arbor have been described, it will be
understood that there are many suitable arrangements and that the
invention is not limited to any particular one. For example, one or
both of the charging plates may be positioned on the other side of
the pulley (as illustrated in FIG. 5) where there is insufficient
room between the bearings, or between the bearings and the belt.
Also, a direct rather than capacitive electrical connection to the
blade can be maintained to detect the capacitive load of the user
upon contact.
[0073] Turning attention now to brake system 34, there are many
possible methods of stopping blade 64 once detection system 32
signals a contact between the user's body and the blade. In one
embodiment, brake system 34 includes one or more pawls configured
to move into contact with the blade and bring the blade to an
immediate stop. The pawls typically are positioned in relatively
close proximity to the blade to reduce the amount of time required
to move the pawl into contact with the blade. A driving mechanism
is responsive to the output signal of detection system 32 to move
the pawl into contact with the blade. The pawls may engage any one
or more portions of the blade, including the teeth, the sides, etc.
Alternatively, the pawls may engage the arbor provided the blade is
rigidly attached to the arbor to prevent rotation of the blade
during a sudden stop of the arbor. Some arbor assemblies include an
arbor flange 78 having a pin 77 that extends parallel to arbor
shaft 70 to engage a hole in the blade spaced from the center hole,
such as shown in dashed lines in FIG. 4. If present, pin 77 may be
a shear pin to allow the blade to be stopped virtually
instantaneously without also stopping the arbor.
[0074] One exemplary embodiment of brake system 34 is illustrated
schematically in FIG. 2. It will be appreciated that the
arrangement of the various elements of the brake system will vary
with the configuration of the particular cutting tool. As shown, a
single brake pawl 140 is pivotally mounted to the saw frame over a
shoulder bolt 142 that is screwed into, or otherwise secured to,
the frame of the saw adjacent the blade. The pawl is mounted to
engage the teeth at the periphery of the blade. The end of the pawl
adjacent the blade may be beveled or otherwise shaped to ensure
that the pawl engages the teeth completely and immediately upon
contact. In addition, the shape of the pawl may be designed such
that once contact with the teeth occurs, the motion of the blade
pivots the pawl further and drives the pawl more tightly against
the teeth.
[0075] In the exemplary embodiment, the driving mechanism includes
a spring 130 adapted to press the free end of the pawl against the
blade. Typically, the spring will be configured to exert continuous
force on the pawl in the direction of the blade once the brake
system is actuated. This will ensure that the pawl does not bounce
backward from contact with the blade. It will be appreciated that
many spring configurations may be used, including compression
springs, tension springs, torsion springs, etc. In the exemplary
embodiment, the spring is formed of a section of 1/8-inch piano
wire with a coil 132 turned in the center and mounted over a bolt
(not shown) to a portion of the saw frame. However, other sizes and
types of wire would be suitable as well. The wire nose gear used in
radio-controlled airplanes is another example of a suitable spring.
In any event, the spring is biased adjacent the pawl to force the
pawl against the blade. Typically, the spring will supply 1-500
pounds of force to the pawl, with values between 15-100 being more
preferred. Greater forces provide faster actuation, but make the
system to release the pawl more complicated.
[0076] In the depicted embodiment, one end of the spring is biased
against another bolt 134 mounted to the saw frame. The end of
spring 130 opposite bolt 134 contacts the free end of pawl 140.
Coil 132 and bolt 134 are positioned to hold the spring in flexion
against the pawl even when the pawl is in contact with the blade. A
restraining member 122 holds the spring in slightly further flexion
to allow the pawl to pivot slightly away from the blade during
normal use. When detection system 32 produces an output signal
indicating contact between the user's body and the blade, the
restraining member releases the spring, which drives the pawl into
engagement with the blade. In the event that brake system 34 is
activated and the spring is released, the system can be reset by
pulling the spring away from the pawl and reconnecting and/or
replacing the restraining member.
[0077] Restraining member 122 can be configured in any of a variety
of ways. Typically, the restraining member is configured to release
the spring virtually instantaneously upon receipt of an output
signal from the detection system. In the exemplary embodiment,
restraining member 122 takes the form of a fusible member or wire.
Fusible member 122 is connected to a firing system 115 (described
below) that melts the wire in response to an output signal by the
detection circuit. Once the wire melts, the spring is released to
drive the pawl into the blade. To reset the exemplary brake system,
a new fusible member is installed to hold the spring away from the
blade. The pawl may also be replaced.
[0078] Fusible member 122 may be any of a variety of materials that
are well known in the art. For example, one suitable fusible member
is a 0.010-inch nichrome wire or a steel strand. Generally, the
fusible member should have a high tensile strength so that the
strength is maximized relative to the heat that is required to melt
the member. In the exemplary embodiment, the wire is formed with a
loop at each end. The overall length is generally less than about
an inch, with a break region of about 1/16- to 1/2-inch between the
loops. A short break region is beneficial to focus the power
delivered to a small region when the brake is tripped. It may be
desirable to form the fusible member from a larger wire with a
reduced waist section of small diameter to achieve a higher current
density at in the waist section for more focused heating.
[0079] One end loop of the fusible member is connected to the end
of brake bias spring 130 that is opposite bolt 134. The other end
of the loop is mounted to an electrically isolated contact stud
136. When current is applied to the contact stud, it flows through
the fusible member to spring 130. The conductivity of the spring
and bolt electrically connects the fuse to the saw frame, which is
grounded. Thus, when current is supplied to contact stud 136, it
flows through the fusible member to the grounded saw frame. When
sufficient current is supplied, the fusible member will melt and
release the spring to drive the pawl against the blade. It will be
appreciated that the fusible member can be arranged in many
alternative ways within the scope of the invention. As one example,
one loop of the wire can be attached to contact stud 136 and the
opposite loop attached to a grounded stud (not shown). If the
middle of the wire is placed over the end of the spring adjacent
the pawl, the spring will be released when the wire is melted. In
this arrangement, the current to melt the fusible member travels
only from the contact stud, through the fusible member and into the
grounded stud.
[0080] It will be appreciated that the size of the fusible member
will depend, at least partially on the force required to restrain
the spring. In general, greater spring forces are desirable to
increase the speed and force with which the pawl contacts the
blade. Where more pressure is required, a larger diameter fusible
member may be needed, thereby requiring a larger amount of current
to melt the fusible member. In the exemplary embodiment depicted in
FIG. 2, the spring typically applies between 5 and 15 pounds of
force against the fusible member and subsequently against the brake
pawl, when released. It should be understood that a wide range of
forces and mechanisms can be used to shift the pawl, up to hundreds
of pounds or more. Under some circumstances, it may also be
desirable to use a two-stage release system, such as used in many
traps, to reduce the force that the fusible member is required to
exert. See FIG. 28, below. This may allow use of a smaller fusible
wire that can be melted more quickly and or with a smaller current
surge.
[0081] As mentioned above, fusible member 122 is melted by a firing
system 115 (shown in FIG. 3) that produces a sudden current surge
to the fusible member in response to an output signal from the
contact detection system. For the exemplary fusible member
described above, approximately 20-100 Amps are required to ensure
complete and rapid melting. As will be appreciated by those of
skill in the art, there are many circuits suitable for supplying
this current surge. The exemplary embodiment of firing system 115
shown in FIG. 3 includes one or more charge storage devices that
are discharged through fusible member 122 in response to an output
signal from second electrical system 88. The use of charge storage
devices obviates the need for a large current supply to melt the
fusible member. It will be appreciated, however, that a current
supply may be used instead of charge storage devices.
Alternatively, other devices may be used to supply the necessary
current, including a silicon-controlled rectifier or triac
connected to supply line 22.
[0082] The firing system of the exemplary embodiment includes a
pair of relatively high-current transistors 118 coupled to pass the
current stored in the storage device to fusible member 122.
Transistors 118 are switched on by the output signal from second
electrical circuit 88. As illustrated in FIG. 3, the output of
op-amp 114 is connected by a 10 k resistor 116 to ground and to the
gates of transistors 118. Any suitable transistors may be used,
such as IRFZ40 MOSFET transistors, which are well known in the art.
The transistors are connected in parallel between charge storage
devices 120 and fusible member 122. In the exemplary embodiment,
charge storage devices 120 are in the form of a 75,000 .mu.F
capacitor bank. A 100-ohm resistor 124 connected to a 24-volt
supply voltage establishes and maintains the charge on the
capacitor bank. When the output of op-amp 114 goes high,
transistors 118 allow the charge stored in the capacitor bank to
pass through the fusible member. The sudden release of the charge
stored in the capacitor bank heats the fusible member to its
melting point in approximately 1-5 ms. Alternatively, one or more
of the transistors may be replaced by other switching devices such
as SCR's. One advantage of using stored charge to fuse the fusible
member is that the firing system does not rely on the capacity of
line power or the phase of the line voltage.
[0083] As described above, the contact between a user's body and
the teeth of blade 64 might be intermittent depending on the size
and arrangement of the teeth. Although second electrical system 88
typically is configured to detect contact periods as short as 25-50
.mu.s, once the first tooth of the blade passes by the user's body,
the contact signal received by the second electrical circuit may
return to normal until the next tooth contacts the user's body. As
a result, while the output signal at op-amp 114 will go high as a
result of the first contact, the output signal may return low once
the first contact ends. If the output signal does not remain high
long enough to fully discharge the charge storage devices, the
fusible member may not melt. Therefore, second electrical system 88
may include a charging circuit 117 on the output of op-amp 114,
similar to charging circuit 106. Once op-amp 114 produces a high
output signal, charging circuit 117 functions to ensure that the
output signal remains high long enough to sufficiently discharge
the charge storage devices to melt the fusible member. In the
exemplary embodiment, charging circuit 117 includes a 0.47 .mu.F
capacitor 119 connected between the output of op-amp 114 and
ground. When the output of op-amp 114 goes high, capacitor 119
charges to the output signal level. If the output of op-amp 114
returns low, the voltage across capacitor 119 discharges through 10
k resistor 116 with a time constant of approximately 4.7 ms. A
diode, such as an 1N914 diode, prevents capacitor 119 from
discharging through op-amp 114.
[0084] The above-described system is capable of detecting contact
within approximately 50 .mu.s and releasing the brake in
approximately one to approximately three milliseconds. The brake
then contacts the blade in approximately one to approximately three
milliseconds. The blade will normally come to rest within not more
than 2-10 ms of brake engagement. As a result, injury to the
operator is minimized in the event of accidental contact with the
cutting tool. With appropriate selection of components, it may be
possible to stop the blade may within 1 ms, or less. Alternatively,
the brake system may be configured to stop the blade in 5, 10, 15,
20 or 50 milliseconds depending on various parameters such as
spring force, pawl size and shape, blade type, blade speed,
etc.
[0085] While exemplary embodiments of first electrical system 38,
second electrical system 88, and firing system 115 have been
described above with specific components having specific values and
arranged in a specific configuration, it will be appreciated that
these systems may be constructed with many different
configurations, components, and values as necessary or desired for
a particular application. The above configurations, components, and
values are presented only to describe one particular embodiment
that has proven effective, and should be viewed as illustrating,
rather than limiting, the invention.
[0086] FIG. 6 shows alternative embodiments of first electrical
system 38, second electrical system 88, and firing system 115 which
may form part of safety stop 30. Alternative first electrical
system 38 is configured to generate a square wave signal using only
a single comparator 400 such as an LM393 comparator. A 1M resistor
402 is connected between the high input terminal of comparator 400
and ground. Another 1M resistor 404 is connected between the high
input terminal of comparator 400 and a low voltage supply V. A 1M
resistor 406 is connected between the high input terminal of the
comparator and the output of the comparator. A 100 pF capacitor 408
is connected between the low input terminal of the comparator and
ground. A 27 k resistor 410 is connected between the low input
terminal of the comparator and the output of the comparator. A 3.3
k resistor 412 is connected between the low voltage supply V and
the output of the comparator. The alternative oscillator circuit
illustrated in FIG. 6 produces a square wave having a frequency of
approximately 500 khz. A 1 k resistor 414 is connected between the
output of the comparator and shielded cable 60 to reduce ringing.
It will be appreciated that the values of one or more elements of
alternative first electrical system 38 may be varied to produce a
signal having a different frequency, waveform, etc.
[0087] As in the exemplary embodiment described above, the signal
generated by alternative first electrical system 38 is fed through
shielded cable 60 to charge plate 62. The signal is capacitively
coupled to charge plate 66 via blade 64. Alternative second
electrical system 88 receives the signal from charge plate 66 via
shielded cable 90 and compares the signal to a reference voltage.
If the signal falls below the reference voltage for approximately
25 .mu.s, an output signal is generated indicating contact between
the blade and the user's body.
[0088] Alternative second electrical system 88 includes a voltage
divider 91, which is formed of 22 k resistors 416 and 418. The
voltage divider biases the signal received via cable 90 to half the
low voltage supply V. The lower resistance of resistors 416, 418
relative to resistors 92, 94 serves to reduce 60 hz noise because
low-frequency signals are attenuated. The biased signal is fed to
the low input terminal of a second comparator 420, such as an LM393
comparator. The positive terminal of comparator 420 is connected to
reference voltage source 422. In the depicted embodiment, the
reference voltage source is formed by a 10 k.OMEGA. potentiometer
424 coupled in series between two 100 k.OMEGA. resistors 426, 428
connected to the low voltage supply V and ground, respectively. A
0.1 .mu.F capacitor 430 stabilizes the output of the reference
voltage. As before, the reference voltage is used to adjust the
trigger point.
[0089] The output of second comparator 420 is connected to the base
terminal of an npn bipolar junction transistor 431, such as a
2N3904 transistor. The base terminal of transistor 431 is also
connected to low voltage supply V through a 100 k resistor 432, and
to ground through a 220 pF capacitor 434. Potentiometer 432 is
adjusted so that the voltage at the positive terminal of comparator
420 is slightly lower than the high peak of the signal received at
the negative terminal of the second comparator when there is no
contact between the blade and the user's body. Thus, each high
cycle of the signal causes the second comparator output to go low,
discharging capacitor 434. So long as there is no contact between
the blade and the user's body, the output of the second comparator
continues to go low, preventing capacitor 434 from charging up
through resistor 432 and switching transistor 431 on. However, when
the user's body contacts the blade, the signal received at the
negative terminal of the second comparator remains below the
reference voltage at the positive terminal and the output of the
second comparator remains high. As a result, capacitor 434 is able
to charge up through resistor 432 and switch transistor 431 on.
[0090] The collector terminal of transistor 431 is connected to low
voltage supply V, while the emitter terminal is connected to
680.OMEGA. resistor 436. When transistor 431 is switched on, it
supplies an output signal through resistor 436 of approximately 40
mA, which is fed to alternative firing system 115. The alternative
firing circuit includes fusible member 122 connected between a high
voltage supply HV and an SCR 438, such as an NTE 5552 SCR. The gate
terminal of the SCR is connected to resistor 436. Thus, when
transistor 431 is switched on, the approximately 40 mA current
through resistor 436 turns on SCR 438, allowing the high voltage
supply HV to discharge to ground through fusible member 122. Once
the SCR is switched on, it will continue to conduct as long as the
current through fusible member 122 remains above the holding
current of approximately 40 mA, even if the current to the gate
terminal is removed. Thus, the SCR will conduct current through the
fusible member until the fusible member is melted or the high
voltage source is removed.
[0091] FIG. 6 also illustrates an exemplary electrical supply
system 440 configured to provide both low voltage supply V and high
voltage supply HV from standard 120 VAC line voltage. Electrical
supply system 440 is connected to provide low voltage supply V and
high voltage supply HV to alternative first electrical system 38,
alternative second electrical system 88, and alternative firing
system 115. The line voltage is connected through a 100.OMEGA.
resistor 442 and a diode 444, such as a 1N4002 diode, to a 1000
.mu.F charge storage capacitor 446. The diode passes only the
positive portion of the line voltage, thereby charging capacitor
446 to approximately 160V relative to ground. The positive terminal
of capacitor 446 serves as the high voltage supply HV connected to
fusible link 122. When SCR 438 is switched on upon detection of
contact between the blade and the user's body, the charge stored in
capacitor 446 is discharged through the fusible link until it
melts. It will be appreciated that the size of capacitor 446 may be
varied as required to supply the necessary current to melt fusible
member 122. It should be noted that the use of a HV capacitor leads
to a much higher current surge, and therefore a faster melting of
the fusible member than is the case with a low voltage system.
[0092] The positive terminal of capacitor 446 also provides a
transformer-less source of voltage for low voltage supply V, which
includes a 12 k resistor 448 connected between the positive
terminal of capacitor 446 and a reverse 40V Zener diode 450. Diode
450 functions to maintain a relatively constant 40V potential at
the junction between the diode and resistor 448. It can be seen
that the current through the 12 k resistor will be about 10 mA.
Most of this current is used by the low voltage circuit, which has
a relatively constant current demand of about 8 mA. Note that while
resistor 448 and diode 450 discharge some current from capacitor
446, the line voltage supply continuously recharges the capacitor
to maintain the HV supply. A 0.1 .mu.F capacitor 452 is connected
in parallel with diode 450 to buffer the 40V potential of the
diode, which is then connected to the input terminal of an
adjustable voltage regulator 454, such as an LM317 voltage
regulator. The ratio of a 1 k resistor 456 connected between the
output terminal and adjustment terminal, and a 22 k resistor 458
connected between the adjustment terminal and ground, set the
output voltage of regulator 454 to approximately 30 VDC. A 50 .mu.F
capacitor 460 is connected to the output terminal of regulator 454
to buffer sufficient charge to ensure that low voltage supply V can
provide the brief 40 mA pulse necessary to switch on SCR 438. The
described low voltage source is advantageous because of its low
cost and low complexity.
[0093] It should be noted that when high voltage supply HV is
discharged through fusible member 122, the input voltage to voltage
regulator 454 may temporarily drop below 30V, thereby causing a
corresponding drop in the low voltage supply V. However, since the
brake system has already been triggered, it is no longer necessary
for the detection system to continue to function as described and
any drop in low voltage supply V will not impair the functioning of
safety stop 30.
[0094] It will be appreciated by those of skill in the electrical
arts that the alternative embodiments of first electrical system
38, second electrical system 88, firing system 115, and electrical
supply system 440 may be implemented on a single substrate and/or
in a single package. Additionally, the particular values for the
various electrical circuit elements described above may be varied
depending on the application.
[0095] FIG. 7 shows an alternative arrangement of brake system 34
configured to focus the current supplied by firing system 115 to a
very small area of fusible member 122. Alternative brake system 115
includes a pawl 140 positioned to engage blade 64 under the urging
of brake spring 130, which is compressed between the pawl and a
spring block 470. The pawl is held slightly spaced from the blade
against the urging of brake spring 130 by fusible member 122. The
fusible member is attached to the pawl, such as by looping it
around a hook 472 formed on the pawl. Alternatively, the fusible
member may be attached to the pawl by a screw or other known
mechanisms.
[0096] The fusible member also loops around a high voltage contact
stud 474 and a discharge contact stud 476. The high voltage contact
stud is connected to high voltage supply HV of electrical supply
440, while the discharge contact stud is connected to SCR 438 of
alternative firing system 115. When SCR 438 is switched on, the
charge supplied by the high voltage supply HV is discharged
primarily through a portion 478 of fusible member 122 between studs
474 and 476. This serves to focus the discharge current to a
relatively small portion of the fusible member, ensuring complete
and rapid melting of the fusible member with less stored charge. In
general, it is desirable to make section 478 as short as possible
to minimize the length of fuse that needs to be heated. As shown in
FIGS. 7-8, high voltage stud 474 and discharge stud 476 may be
configured as opposite sides of a single structure with an
insulating layer 479 disposed therebetween. Typical spacing between
the studs would be approximately 1/64-inch to 1/8-inch. Once the
fusible member has melted, the brake spring presses the pawl into
engagement with the blade as described above. While the alternative
arrangement of brake system 34 has been described in combination
with alternative firing system 115, it will be appreciated that
alternative brake system 34 may also be used in combination with
the embodiment of firing system 115 depicted in FIG. 3. In such
case, high voltage contact stud 474 would be connected to the
emitters of transistors 118, while discharge contact stud 476 would
be connected to ground.
[0097] Since contact detection system 32 and brake system 34 are
configured to stop cutting tool 14 upon contact with a user's body,
it may also be desirable to stop motor assembly 12 to prevent
damage to the motor as it tries to drive the stopped cutting tool.
However, since machine 10 typically is designed with the
expectation that the cutting tool may stop due to binding, etc., it
will usually be sufficient to turn off the motor assembly within a
few seconds. This can be accomplished simply by cutting power to
the motor. Thus, in one alternative embodiment, the brake
activation signal from second electrical system 88 is also used to
activate a normally closed switch relay (not shown) in the coil
circuit of contactor 16. Similar to pressing stop switch 20, when
the normally closed switch relay is activated, power is disengaged
from the contactor coil and thereby the motor. It will be
appreciated that there are many other suitable ways of stopping
motor assembly 12 which are within the scope of the invention. As
one example, power to the motor assembly may be controlled directly
by safety stop 30 (e.g., through solid state on/off switches,
etc.). Also, it is possible to simply allow existing overload
circuitry to trip in to turn off the stalled motor.
[0098] Since the detection circuitry described above relies on
certain electrical properties of the human body, the use of safety
stop 30 while cutting some materials, such as foil-coated
insulation, may cause the detection circuitry to falsely register
contact with a user. Therefore, it may be desirable to provide a
disablement control that prevents the brake from operating for a
particular cutting operation. A suitable disablement control may
include a mechanical switch between fusible member 122 and firing
system 115. Alternatively, the switch may be a single-use switch
configured to reset itself after each use. As a further
alternative, safety stop 30 may include sensors adjacent the
workpiece to detect the presence of foil, etc., and disable the
brake system. This latter alternative relieves the user of having
to remember to disable and re-enable the brake system.
[0099] As an alternative to disabling safety stop 30, the contact
detection system may be configured to distinguish between metal and
a user's body. For example, when a user's body comes into contact
with the blade, the input signal received by second electrical
system 88 is attenuated but not grounded. However, if a metal work
piece (which was in contact with the grounded machine frame)
contacted the blade, the input signal coupled onto the blade would
be discharged to ground so that second electrical system 88 would
receive no signal at all. Thus, second electrical system 88 may be
configured to output an activation signal to firing system 155 only
if an attenuated signal is received that has a peak below the
previously described threshold and above a second threshold.
[0100] It should be understood that in a commercial embodiment of
the present invention, much of the electronic functionality can be
incorporated in a custom application specific integrated circuit
(ASIC). In addition, at least a portion or all of the electronics
may be powered by battery rather than by the power supply to the
machine. However, the charge necessary to melt fusible member 122
will preferably be supplied by the line current to conserve the
battery and because of the higher voltage available.
[0101] It also may be desirable to provide a logic control system
configured to conduct various self-test safety checks, etc., when
the machine is switched on or off and during use, to ensure that
the safety stop is operating properly and to prevent inadvertent
triggering of the brake system. Such a logic control system may be
implemented in any of a variety of ways using any desired test
sequence. A flowchart illustrating an exemplary logic sequence is
shown in FIG. 9. The exemplary sequence begins when the user
initially supplies power to the system, indicated at 600. The logic
system first checks to determine whether the spacing between the
blade and charge plates is correct, as indicated at 602. The
blade-to-charge plate spacing may be measured by any suitable
mechanism such as described in more detail below. If the spacing is
outside acceptable limits, the system responds with an error
signal, indicated at 604. The error signal may be an audible and/or
visible signal, etc. Preferably, the logic system remains in the
error state and prevents further operation of the machine until the
correct blade-to-charge plate spacing is detected.
[0102] If the blade-to-charge plate spacing is acceptable, the
logic system determines whether the proper (i.e., unattenuated)
input signal from the first electrical system is being detected by
the second electrical system, as indicated at 606. This step
ensures that the brake will not be triggered accidentally upon
start-up due to a fault in the first and/or second electrical
systems, a grounded blade, incorrectly placed charge plates, etc.
If the proper input signal is not detected, the system responds
with an error signal 604, which can take the form of a beeper or
flashing light or similar indicator. It will be appreciated that
either the same or a different error signal may be produced for
each fault condition.
[0103] If the proper input signal is detected, the system proceeds
to determine whether a fusible member is present, as indicated at
step 608. The presence of a fusible member may be determined by any
suitable means including measuring the conductivity between contact
stud 474 and discharge stud 476. If no fusible member is present,
the system returns an error signal 604.
[0104] The system then checks the charge stored in the charge
storage device, as indicated at 612. This step ensures that
sufficient charge is present to melt the fusible member if contact
is detected. If sufficient charge is not detected, the logic system
responds with an error signal if sufficient charge is not detected
within a determined time period.
[0105] In the exemplary sequence, after the predetermined checks
are completed, the logic system allows power to be sent to motor
assembly 12, as indicated at 614. It will be appreciated that the
electrical sequence described above typically is completed within
less than a second if no faults are detected. Alternatively,
additional steps may be taken before the motor assembly is
powered.
[0106] In addition to an initial power-up sequence, a logic system
may be configured to perform any of a variety of checks during
operation. For example, the rotation of the blade may be monitored
by known mechanisms and the firing system may be disabled when the
blade is not moving. This would allow the user to touch the blade
when it is stopped to prevent engaging the pawl. Some embodiments
may be configured to ensure the safety stop will continue to
function even after power to the machine is turned off if the blade
continues to rotate. As another example, power to the motor
assembly may be shut off if an error occurs other than contact
detection such as incorrect blade-to-charge plate spacing,
insufficient charge on the charge storage devices, etc. It will be
appreciated that a logic control system may be implemented to
provide any of a variety of safety and/or operational functions
desired.
[0107] In addition to the spring and fusible member systems
described above, other systems can also be used to shift the pawl
or pawls into contact with the blade. For example, as shown in FIG.
10, a relatively small explosive charge 150, in the form of a squib
or detonator, can be used to drive the pawl against the blade. An
example of a suitable explosive charge is an M-100 detonator
available, for example, from Stresau Laboratory, Inc., of Spooner,
Wis.
[0108] An exemplary embodiment of an explosive charge actuated
system is depicted in FIG. 10. Although any suitable explosive
charge system may be used, the exemplary embodiment preferably uses
a self-contained charge or squib 150 to increase safety and focus
the force of the explosion along the direction of movement of the
pawl. A trigger line 164 extends from the charge to cause
detonation.
[0109] Explosive charge 150 can be used to move pawl 140 by
inserting the charge between the pawl and a stationary block 166
adjacent the charge. When the charge detonates, the pawl is pushed
away from the block. A compression spring 130 is placed between the
block and pawl to ensure the pawl does not bounce back from the
blade when the charge is detonated. Prior to detonation, the pawl
is held away from the blade friction-fit of the charge in both the
block and pawl. However, the force created upon detonation of the
charge is more than sufficient to overcome this friction fit.
Alternatively, the pawl may be held away from the blade by other
mechanisms such as a frangible member, gravity, a spring between
the pawl and block (not shown), etc. It will be appreciated that
the position of the charge relative to pivot bolt 142 will
determine the distance that the opposite end of the pawl travels.
The closer the charge is positioned to bolt 142, the further the
opposite end of the pawl will travel. However, the amount of force
needed to move the pawl will increase as the charge is moved closer
to bolt 142 due to the smaller moment arm. Thus, the pawl may move
more slowly for a particular amount of explosive force.
[0110] Furthermore, it will be appreciated that there are many
other ways to drive the pawl into contact with the cutting tool in
addition to the spring and explosive charge embodiments described
above. For example, a DC solenoid can be over-driven with a current
surge to create a rapid displacement. Alternatively, a pressurized
air or gas cylinder can be used to supply the pressure in place of
the brake spring or charge. As another alternative, the pawl may be
supported on an electromagnet activated by the firing system to
either repel the pawl against the blade, or release a spring-loaded
pawl toward the blade.
[0111] Focusing now more closely on the pawl, it will be understood
that the pawl may be constructed from one or more of a variety of
materials. Examples of suitable materials include plastics, such as
polycarbonate, rubber and wood, or even soft metals, such as lead
or aluminum. It is generally desirable, though not required, that
the pawl be sufficiently strong that the blade does not simply cut
through it, but also soft enough to absorb some of the impact of
the blade coming to a sudden stop. After the brake has been
triggered, it is normally necessary to replace the fuse and perhaps
the pawl. Depending on the speed with which the blade is stopped,
the blade may also be damaged, or at least require removal of any
portions of the pawl engaged thereto. Indeed, in an alternative
embodiment of the brake system, the pawl is constructed of a
sufficiently strong and hard material to break the teeth off of the
blade rather than stopping the blade.
[0112] Pawl 140 may also be configured to have an elongate contact
surface that engages a large portion of the blade. Examples of this
alternative configuration are illustrated schematically in FIGS.
11-13. As can be seen, pawl 140 is mounted on a pair of pivot arms
141. The pivot arms can have the same or different lengths, and can
be mounted to pivot anchors (not shown) positioned outside or
inside the perimeter of the blade. One advantage of a pawl with an
elongate contact surface is that the force exerted by the pawl is
distributed across a larger portion of the blade, thereby allowing
the blade to be stopped more quickly. The longer contact surface
can also be used to reduce the chance of damage to the blade
because the braking force is spread over more teeth.
[0113] Although the exemplary embodiments are described above in
the context of a single brake pawl that engages the teeth of a
blade, the brake system may incorporate two pawls that engage two
or more locations on the perimeter of the blade to decrease the
stopping time or spread the stopping forces. It is also possible to
utilize pawls that contact opposed points on the sides of the
blade. FIG. 14 illustrates an exemplary embodiment using two pawls
to contact opposite sides of the blade. As shown, pawls 140 are
pivotally mounted on either side of blade 64. Each pawl includes a
contact head 170 adjacent the blade, and a lever arm 172 opposite
the contact head. The pawls are pivotally mounted on pins 174 that
pass through pivot holes in the pawl between the contact head and
the lever arm. Thus, when the lever arms of each pawl are pivoted
upward (as shown in FIG. 14), the contact heads close together. The
pawls are mounted relative to the blade so that the contact heads
pivot toward the blade in the direction of blade travel. Once the
pawls contact and grip the blade, they continue to pivot inward
pulled by the downward motion (as shown in FIG. 14) of the blade.
As a result, the blade is pinched more and more tightly between the
contact surfaces of the pawls until the pawls can close no further,
at which point the blade is stopped between the pawls.
[0114] To ensure that both pawls close together on the blade, a
linkage 176 is provided which is attached, at either end, to lever
arms 172. The central portion of linkage 176 extends beyond the
blade to rest on the end of brake spring 130. As discussed above,
the brake spring is held in flexion by fusible member 122,
connected to contact stud 136. The spacing of the pawl contact
heads from the blade is controlled by adjusting the position of the
spring. When the fusible member is melted, the spring drives the
linkage upward. If one pawl contacts the blade first, the upward
motion of the linkage is transferred to the other pawl until it
makes contact with the blade as well.
[0115] It will be appreciated that the dual-pawl system described
above may be implemented with many variations within the scope of
the invention. For example, the linkage may be driven upward by any
of the other actuating means described above, including an
explosive charge, solenoid, compressed gas, etc. As another
example, one or more pawls may be positioned to contact only one
side of the blade. Additionally, the linkage may be omitted, and
each pawl actuated by a separate spring, explosive charge,
solenoid, etc.
[0116] To increase the gripping action of the pawls on the blade,
the contact surface of the pawls may be coated with a relatively
high-friction material such as rubber. Alternatively, the pawls may
be constructed of a harder material than the blade and have a
ridged surface to "bite" into the blade, as illustrated.
Alternatively, or additionally, the blade may be configured with
grip structure such as coatings of high-friction material, grooves,
notches, holes, protuberances, etc., to further increase the
gripping action of the pawls.
[0117] As mentioned above, it will usually be desirable to locate
the pawl relatively close to the blade to reduce the amount of time
necessary to move the pawl into contact with the blade. While the
optimum pawl-blade spacing may vary depending on the configuration
of the particular cutting tool, the detection system, and/or the
brake system, it has been found that a space of approximately
1/32-inch to 1/4-inch between the pawl and blade provides suitable
results in the spring-actuated embodiment described above. However,
as is well known in the art, many cutting tools such as saw blades
do not have precisely uniform dimensions. For example, for circular
saw blades having a nominal diameters of 10-inches and nominal
thicknesses of 0.125-inch, actual blades from various manufacturers
or for different applications may have diameters that range between
9.5-inches and 10.5-inches and thicknesses that range between
0.075-inch and 0.15-inch Therefore, to ensure uniform braking
speed, it may be necessary to adjust the position of the pawl
whenever a blade is replaced.
[0118] Optionally, brake system 34 may include automatic and/or
self-adjusting mechanism for ensuring that the spacing between the
pawl and the cutting tool is within an acceptable range. It will be
appreciated that there are a variety of mechanisms for
automatically determining the pawl spacing, including electrical,
mechanical, optical, etc. As one example, FIG. 15 illustrates a
pawl 180 having a capacitive system for detecting correct pawl
spacing. Similar to pawl 140 shown in FIG. 2, pawl 180 may include
a portion 182 that is beveled or otherwise shaped to quickly and
completely engage the teeth of a cutting tool. In addition, pawl
180 includes a pair of generally parallel, spaced-apart arms 184
which extend beyond portion 182. Arms 184 are disposed to extend on
either side of the blade, without touching the blade, when the pawl
is in place adjacent the blade. Each arm includes a capacitor plate
186 disposed on the inside surface of the arm adjacent the blade.
Conductive leads 188 run from each capacitor plate 186 to suitable
blade detector circuitry (not shown).
[0119] Capacitor plates 186 are positioned on arms 184 such that,
when the pawl spacing is within a desired range, the blade extends
between the two capacitor plates. It will be appreciated that the
capacitance across plates 186 will vary depending on whether the
blade is positioned between the plates. The blade detector
circuitry is configured to drive an electrical signal through
conductive leads 188 and detect changes in the capacitance across
the plates. Suitable circuitry that may be used with pawl 180 is
well known to those of skill in the art, and may include systems
similar to first and second electrical systems 38 and 88 described
above. The capacitor plates can optionally be shaped to detect when
the pawl is too close to the blade as well as not close enough.
Alternatively, two pairs of capacitor plates may be positioned on
the pawl: one pair to detect if the pawl is too close to the blade,
and the other pair to detect if the pawl is too far from the blade.
In any event, the detector circuitry is configured to produce an
error signal and/or disable the machine if the correct pawl spacing
is not detected.
[0120] While one exemplary automatic pawl spacing detection system
has been described above, it will be appreciated that there are
many possible variations within the scope of the invention. For
example, both capacitor plates may be positioned on the same side
of the blade rather than on opposite sides. The capacitor plates
and/or blade detection circuitry may be separate from the pawl. In
the latter case, for example, the capacitor plates and detection
circuitry may be mounted on a separate electronics board associated
with the pawl. Alternatively, the capacitor plates may be replaced
with one or more light-emitting diodes and detectors such that,
when the pawl is properly positioned, the blade obstructs the
optical path between the diodes and detectors. Other methods of
detecting the proximity of the blade to the pawl are also possible.
As a further option, capacitors 186, 188 may function as charge
plates 62, 66 instead of, or in addition to, pawl-spacing
detectors. In addition, a detection plate may be mounted on face
182 of the pawl. This plate can be used to detect the drive input
signal use for contact detection. The amplitude of the signal
detected at the plate will be inversely proportional to the space
between the plate and the teeth of the blade. If this signal does
not have an amplitude over a given threshold, the system would
interpret this as indicating that the pawl face is not close enough
to the blade.
[0121] Since the height and/or angle of the blade for many cutting
machines are adjustable, it may be desirable to mount the pawl to a
portion of the machine frame that moves with the blade. This
arrangement will ensure that the pawl is maintained in a
predetermined position relative to the blade. Similarly, the
charging plates are preferably mounted to move with either the
blade or arbor to maintain a predetermined and constant spacing
thereto. One exemplary embodiment where the charging plates are
mounted to move with the arbor has been described above.
[0122] Alternatively, a stationary pawl may be configured to engage
the blade in any orientation. One exemplary implementation of such
a stationary pawl is depicted in FIGS. 16 and 17. In this exemplary
embodiment, brake system 34 includes an elongate pawl 200. The pawl
is sized and shaped to extend along the outer perimeter of blade 64
as it is adjusted vertically. Similarly the width of pawl 200 is
sized to extend the breadth of the incline of blade 64. As shown in
FIGS. 16 and 17, pawl 200 is mounted generally parallel with the
vertical of travel of the blade, and generally normal to the axis
of incline of the blade. As a result, the spacing between the blade
and contact surface 202 remains constant regardless of the position
or orientation of the blade.
[0123] The upper end of pawl 200 is pivotally attached to upper
pivot arms 204 by pivot pins 206 that pass through one end of arms
204 into the sides of the pawl. The other ends of pivot arms 204
are pivotally attached to one or more mounts (not shown), by pivot
pins 210. The lower end of pawl 200 is pivotally attached to lower
pivot arms 203 by pivot pins 205 that pass through one end of arm
203 into the sides of the pawl. The lower pivot arms are pivotally
attached to mounts (not shown) by pivot pins 209. Springs 212 are
attached to the lower pivot arms on the side of pivot pins 209
opposite pivot pins 205. Thus, pawl 200 is configured to pivot
toward or away from blade 64.
[0124] Pivot arms 204 and 209 are sized and arranged such that pawl
200 cannot pivot up past the blade without striking the edge of the
blade. When the pawl strikes the blade while the blade is rotating,
the movement of the blade causes the pawl to continue pivoting
upward until the pawl is firmly wedged between the blade and pivot
arms, thereby stopping the blade. The contact surface of the pawl
may be textured, coated, etc., to enhance the gripping action
between the pawl and the blade.
[0125] Pawl 200 is biased upward to pivot toward the blade by
springs 212, which are anchored, for example, to the saw frame.
Thus, when the pawl is free to pivot, springs 212 drive the pawl
quickly toward the blade. Similar to the exemplary embodiment
described above, fusible member 122 is connected to the pawl to
hold it away from the blade. The fusible member is sized to hold
the pawl spaced slightly away from the edge of the blade. However,
when a sufficient current is passed through the fusible member the
fusible member will melt, causing the pawl to pivot toward the
blade under the bias of springs 212.
[0126] It will be appreciated that many variations to the exemplary
embodiment depicted in FIGS. 16 and 17 are possible within the
scope of the invention. For example, the pawl may be configured to
pivot toward the blade solely due to gravity. Alternatively,
springs 212 may be compression springs which normally hold the pawl
away from the blade until it is pivoted upward under the force of
another spring, an explosive charge, a solenoid, gas pressure, etc.
Further, the pawl may be mounted on the other side of the blade to
pivot downward into the blade under the force of a spring, an
explosive charge, a solenoid, gas pressure, etc.
[0127] As described above, the invention provides a reliable
system, referred to herein as a safety stop 30, for stopping the
cutting tool of a machine upon contact with a user's body. While
several exemplary embodiments of safety stop 30 have been depicted
and described, it will be appreciated that the specific
implementation of the safety stop may vary depending on the
particular cutting tool it is installed on. For the purpose of
providing additional illustrations of the flexibility and
versatility of safety stop 30, exemplary implementations of the
safety stop in the context of several common machines will be
described briefly.
[0128] FIG. 18 shows safety stop 30 in the context of a typical
table saw. Saw blade 64 is mounted to rotate on arbor 70. The arbor
extends outward (as viewed in FIG. 18) from a swing arm 220, which
pivots about an axle 222 to raise and lower the blade. A worm gear
224 engages an arcuate rack on the swing arm to pivot the swing arm
about the axle. Safety stop 30 includes a bracket 228 that attaches
to swing arm 220, for example, by one or more bolts 230 extending
through the bracket. Disposed on mounting bracket 228 are charging
plates 62 and 66. The charging plates are positioned parallel to,
and slightly spaced from, blade 64 to create the capacitive shunt
between the plates. The mounting bracket may be constructed of an
electrically insulating material or include electrical insulation
between the bracket and the charging plates.
[0129] Mounting bracket 228 extends from the end of swing arm 220
beyond the edge of blade 64. A pawl 140 is pivotally mounted on a
bolt 142 extending from the bracket. The free end of the pawl is
biased toward the edge of the blade by a compression spring 236.
The spring is held in compression between the pawl and a spring
block 238, which extends from the bracket. A fusible member 122 is
anchored to a pair of contact studs 240. The fusible member is
coupled to the pawl and holds it away from the edge of the blade
against the spring bias.
[0130] An electronics unit 242 contains a contact detector such as
first electrical system 38 and second electrical system 88
described above. Shielded cables 60 and 90 extend from the
electronics unit to charging plates 62 and 66, respectively.
Electronics unit 242 also includes a current generator, such as
firing system 115 described above, which is connected to contact
studs 240. A power cable 244 extends from electronics unit 242 to a
suitable power source (not shown). When the contact detector
detects contact between the user's body and the blade, the firing
circuit melts the fusible member, thereby releasing the pawl, which
engages and abruptly stops the blade.
[0131] It should be noted that by placing the pawl and the charging
plates on bracket 228 which is attached to the swing arm, the pawl
and charging plates move with the blade when it is adjusted. This
eliminates the need to reposition the pawl and/or the charging
plates whenever the blade is moved. Furthermore, the embodiment of
safety stop 30 depicted in FIG. 18 is suitable for easy
installation or retrofit of existing table saws which do not
currently have a safety stop. The only requirement to retrofit an
existing saw is to tap one or more holes into the end of the swing
arm to receive bolts 230. The exact positioning of bracket 228 can
be adjusted as necessary, for example to extend downward, to fit
within the particular saw housing.
[0132] While one particular implementation of safety stop 30 has
been described in the context of a table saw, it will be understood
that any of the implementations and variations described above may
also be used in the table saw system. In addition, while safety
stop 30 is depicted in the context of one particular table saw
configuration, it will be understood that the safety stop may be
implemented in any table saw configuration using any of the various
embodiments and implementations within the scope of the invention.
For example, as shown in FIG. 18, the swing arm is configured to
pivot about an axle adjacent the front of many table saws. As a
result, when the pawl engages the blade and the angular momentum of
the blade is transferred to the swing arm, the swing arm may tend
to rise upward depending on its weight and the amount of play in
worm gear 224. If the swing arm rises upward the blade will also
rise, perhaps increasing the injury to the user. This transfer of
angular momentum is unlikely to be a problem where the pawl is
mounted to the frame of the saw. However, where the pawl is mounted
to the swing arm, it may be desirable to position the pivot point
of the swing arm at the rear of the table saw. This configuration
can be visualized in FIG. 18 by reversing the direction of blade
rotation and moving the brake onto the swing arm. The front of the
saw will be on the left as viewed in FIG. 18. In this
configuration, the angular momentum of the blade, when transferred
to the swing arm by the pawl, would tend to push the blade downward
rather than upward. Additionally, the worm gear may be held in
place by a spring, permitting the worm gear shift to allow the
swing arm to deflect downwardly when the blade is stopped.
[0133] In addition, where a plastic bushing is placed between the
blade and the arbor as described above, the substantial force
created by stopping the blade almost instantly may cause the
bushing to deform. Typically, the edge of the mounting hole of the
blade will bite into the bushing as the blade attempts to rotate
about the pawl. Therefore, if the pawl is mounted at the back of
the blade (as shown in FIG. 18), then the blade will tend to move
downward into the bushing and away from the user when the pawl
engages the blade. It is because of this effect that it is
generally preferable to mount the pawl on the swing arm if the
pivot is moved to the back of the saw as described above.
[0134] As discussed above, in some implementations of safety stop
30, it may be necessary to replace portions of the safety stop
(e.g., pawl 140, fusible member 122, etc.) after the safety stop
has been triggered to stop the cutting tool. Therefore, in another
alternative embodiment of the invention, a portion or all of safety
stop 30 is contained in a cartridge or module that can easily be
replaced. One example of a cartridge safety stop is shown in FIG.
19, in the context of the table saw implementation described above
and depicted in FIG. 18. Cartridge 246 typically includes a frame
or housing 248 attachable to a support surface, such as mounting
bracket 228, adjacent blade 64. Cartridge 246 may be attachable to
the support surface by any suitable mechanism including bolts 250.
Alternatively, or additionally, the cartridge may be configured to
fit within a socket or other suitable receiver on the mounting
bracket. Preferably, the cartridge has a non-symmetrical shape so
that it can only be attached to the support surface in the correct
orientation. In the exemplary embodiment shown in FIG. 19, the
mounting bracket includes a raised shoulder 252 corresponding to a
bevel 254 in housing 248 to prevent installation of the cartridge
unless the bevel is aligned with the shoulder. It will be
appreciated that there are many other ways of ensuring proper
orientation of the cartridge within the scope of the invention.
[0135] Cartridge 246 also includes an electrical connector 256
configured to operably engage plug 258, attached to cable 260. The
cable includes conductors for supplying electrical power to the
electronic units, as well as shielded cables 60 and 90 and other
input signal lines. The cable may also conduct output signals from
the electronics unit, such as a cutoff signal to stop motor
assembly 12. Although plug 258 and cable 260 are shown as being
freely movable, it will be appreciated that plug 258 may be rigidly
mounted to the support surface. Further, plug may be rigidly
positioned to ensure that the cartridge is properly aligned and
oriented when the connector is engaged with the plug. As a further
safety measure, which will be described in more detail below, the
electronics unit may be configured to provide an enable signal to
the saw power supply, thus preventing use of the saw unless the
cartridge was properly installed.
[0136] Housing 228 includes an aperture 262 through which the pawl
can protrude when released. Alternatively, the pawl can be
constructed to be flush with the housing until deployed. In any
event, the pawl is pivotally coupled to the housing by bolt 142 to
allow the pawl to move toward and engage the blade once contact
between the user's body and the blade is detected. Bolt 142 may be
mounted to housing 228, or it may be mounted to the support surface
to extend through apertures in both the housing and the pawl. It
may be useful to cover the pawl and opening with a foil 229, etc.,
to prevent contamination or inadvertent extension of the pawl.
[0137] Typically, cartridge 246 is configured to allow the
pawl-to-blade spacing to be adjusted as necessary. For example, the
position of the cartridge relative to the blade may be adjustable
such as by pivoting or sliding the cartridge relative to one or
more of the mounting bolts. In which case, pawl-to-blade spacing
may be determined indirectly by measuring the pawl-to-cartridge
spacing if desired. Alternatively, the cartridge may be stationary
and the pawl may be adjustable within the cartridge. As a further
alternative, both the cartridge and pawl are adjustable. Similarly,
the position of the mounting bracket may be adjustable relative to
the blade.
[0138] In the exemplary embodiment, cartridge 246 contains most of
safety stop 30 including the pawl, spring, fusible member, contact
stud, ground stud, and electronics unit. Additionally, charge
plates 62 and 64 may also be contained in cartridge 246. Placing
most of safety stop 30 in the cartridge allows manufacturers to
develop improved electronics, additional functions, etc., without
requiring major retrofits to the machine. Alternatively, only a
portion of safety stop 30 may be placed in the cartridge such as
the pawl, spring and fusible member. An advantage of this
alternative is that it would reduce the cost of the cartridge. As a
further alternative, safety stop 30 may comprise a plurality of
cartridges. For example, one cartridge may contain the pawl,
spring, fusible member and contact/ground studs, while another
cartridge may contain the electronics unit.
[0139] Optionally, the cartridge may be provided in different sizes
or configurations to accommodate different blade sizes. For
example, a longer version of the cartridge shown in FIG. 19 may be
used for a smaller diameter blade. Furthermore, different
cartridges may be provided for different applications that use
different types of blades (e.g., cross-cutting, ripping, plywood,
etc.). For example, a first cartridge having a first type pawl may
be provided for a first type blade, while a second cartridge having
a second, different pawl may be provided for a second, different
blade. Alternatively, the electronics of one cartridge may be
different from those of another cartridge to allow for different
applications (e.g., cutting plastic rather than wood).
Additionally, it is within the scope of the invention to use plural
cartridges simultaneously to ensure the safety stop responds
optimally for each material.
[0140] Turning now to FIGS. 20 and 21, an exemplary safety stop 30
is shown in the context of a miter saw 270 (also commonly referred
to as a chop saw). It will be understood that miter saw 270 may be
any type of miter saw including a simple miter saw, compound miter
saw, sliding compound miter saw, etc. Typically, miter saw 270
includes a base or stand 272 adapted to hold the workpiece to be
cut. A swing arm 274 is pivotally coupled to base 272 to allow the
arm to pivot downward toward the base. Attached to arm 274 is a
housing 276 adapted to at least partially enclose a circular blade
64. A motor assembly 12 is coupled to the housing, and includes a
rotating arbor 70 on which the blade is mounted. Motor assembly 12
includes a handle 278 with a trigger 280 operable to run the saw.
Blade 64 rotates downward toward swing arm 274. An optional blade
guard (not shown) may extend from the bottom of housing 276 to
cover any portion of the blade exposed from the housing.
[0141] Any of the various configurations and arrangements of safety
stop 30 described above may be implemented in miter saw 270. In the
exemplary embodiment depicted in FIGS. 20 and 21, safety stop 30 is
a cartridge-type system. With the exception of charging plates 62
and 64, both brake system 34 and detection system 32 are contained
within cartridge 246. The cartridge is configured to be mounted on
the front inside surface of housing 276 by one or more bolts 250.
The housing may include a movable panel or door 277 to allow access
to the cartridge. A pawl 140 is mounted in the cartridge and is
positionable in front of the blade. Charge plates 62 and 66 are
attached to the inside wall of housing 276 by one or more mounts
282. The mounts are attached to the housing by any suitable
mechanism such as bolts 284, and are configured to position the
charge plates parallel to, and closely adjacent, blade 64. As shown
in FIG. 21, the spacing between the charge plates and the blade is
preferably much less than the spacing between the charge plates and
the housing to minimize any parasitic capacitance between the
charge plates and the housing. Alternatively, the housing may be
constructed from an electrically non-conductive material.
[0142] Cables 60 and 90 connect the charge plates to the cartridge.
Electrical power for safety stop 30 is provided by a cable (not
shown) extending from motor assembly 12. In addition to engaging
the pawl with the blade, the electronics unit within cartridge 246
is also configured to interrupt the power to motor assembly 12 when
contact between the user's body and the blade is detected.
[0143] As discussed above in connection with table saws, a circular
blade spinning at several thousand revolutions per minute possesses
a substantial amount of angular momentum. Thus, when the pawl
engages a circular blade such as is found on miter saw 270 and
stops the blade within a few milliseconds, the angular momentum
must be transferred to the brake. Because the swing arm of the
miter saw is free to pivot in the direction of blade rotation, the
angular momentum of the blade may be transferred to the swing arm
when the blade is suddenly stopped, causing the swing arm to swing
downward. This sudden and forceful downward movement of the swing
arm may cause injury to the user if a portion of the user's body is
beneath the blade. Therefore, an alternative embodiment of the
miter saw implementation of safety stop 30 also includes means for
preventing the swing arm from moving downward when the blade is
stopped. In addition, the pawl typically is mounted at the front of
the miter saw to urge the blade to climb upward away from the user
(i.e., deforming the plastic bushing) when engaged by the pawl.
[0144] It will be appreciated that there are many suitable means
for preventing sudden downward movement of the swing arm. For
example, the pivotal connection between the swing arm and the base
of the miter saw may be electrically lockable, for example using an
electromagnetic leaf spring, to prevent the arm from pivoting. The
signal to lock the connection may be provided by the detection
system. Alternatively, a shock absorber may be connected between
the swing arm and the base to limit the speed with which the swing
arm can pivot relative to the base. This arrangement also serves to
limit how far the blade moves between the time contact between the
blade and user is detected, and the time the blade is stopped by
the pawl. While there are many other ways of connecting the swing
arm to the base to prevent sudden movement of the arm toward the
base, most such arrangements transfer the angular momentum to the
swing arm/base assembly. Depending on the weight and balance of the
saw, the angular momentum may be sufficient to cause the entire saw
to overturn. Therefore, it may be desirable to secure the base to a
stable surface with clamps, bolts, etc.
[0145] Alternatively, the miter saw can be configured to absorb any
angular momentum without allowing the swing arm to move downward.
For example, the exemplary embodiment depicted in FIGS. 20 and 21
is configured with a pivotal motor assembly to allow the blade to
move upward into the housing upon engagement with the pawl. Motor
assembly 12 is connected to housing 276 via pivot bolt 286,
allowing the motor assembly to pivot about bolt 286 in the
direction of blade rotation. A spring 292 is compressed between the
housing and an anchor 294 to bias the motor assembly against the
direction of blade rotation. The motor assembly may include a lip
296, which slides against a flange 298 on the housing to hold the
end of the motor assembly opposite the pivot bolt against the
housing.
[0146] When the saw is in use, spring 292 holds the motor assembly
in a normal position rotated fully counter to the direction of
blade rotation. However, once the pawl is released to engage the
blade, the motor assembly and blade to pivot upward against the
bias of the spring. In this embodiment, the pawl is positioned at
the front of the blade so that the pivot bolt 286 is between the
pawl and the arbor. This arrangement encourages the blade to move
upward into the housing when stopped. The spring is selected to be
sufficiently strong to hold the motor assembly down when cutting
through a workpiece, but sufficiently compressible to allow the
blade and motor assembly to move upward when the blade is
stopped.
[0147] While one exemplary implementation of safety stop 30 in the
context of a miter saw has been described, the invention should not
be seen as limited to any particular implementation as the
configuration and arrangement of safety stop 30 obviously may vary
among miter saws and applications. For example, the pivoting motor
assembly configuration may also be combined with one or more of the
other systems described above which prevent the swing arm from
pivoting suddenly toward the base. Further, it will be appreciated
that the blade and motor assembly may be configured in any of a
variety of ways to at least partially absorb the angular momentum
of the blade.
[0148] FIG. 22 shows an alternative configuration of miter saw 270
adapted to absorb the angular momentum of the blade. In this
configuration, the miter saw includes two swing arms 275 and 274.
One end 300 of each swing arm 275, 274 is connected to base 272,
and the opposite end 302 of each swing arm is connected to housing
276, blade 64, and/or the motor assembly (not shown). The position
of the swing arms relative to each other may vary depending on the
swing arm motion desired. In FIG. 22, swing arm 275 is connected to
base 272 somewhat below and forward of swing arm 274. Typically,
the motor assembly is rigidly attached to end 302 of swing arm 275,
while housing 276 is connected to rotate about end 302 of swing arm
275. End 302 of swing arm 274 is connected only to the housing.
This arrangement replicates the motion of the motor assembly and
trigger found on many conventional miter saws. Alternatively, the
motor assembly may be connected to rotate about end 302 of swing
arm 275 along with the housing.
[0149] The configuration shown in FIG. 22 causes the housing and/or
motor assembly to rotate as the swing arms pivot. Significantly,
when the swing arms move upward, the housing and/or motor assembly
rotate in the same direction in which the blade rotates during
cutting. As a result, when the pawl engages the blade and transfers
the angular momentum of the blade to the housing and/or motor
assembly, the housing and/or motor assembly tend to rotate in the
same direction as the blade. This causes the swing arms to pivot
upward, drawing the blade away from the workpiece and the user's
body. Thus, as described above, the miter saw configuration
illustrated in FIG. 22 is adapted to absorb the angular momentum of
the blade and translate that angular momentum into an upward force
on the swing arm.
[0150] The configuration shown in FIG. 22 and described above
illustrates a further alternative embodiment of safety stop 30.
Specifically, the safety stop may be configured to move the blade
of the cutting tool rapidly away from the user when contact with
the user's body is detected in addition to, or instead of, stopping
the blade. This alternative embodiment may be implemented in the
context of any of the cutting tools described herein. For example,
swing arm 220 of the table saw depicted in FIG. 18 may be
configured to be disengaged from worm gear 224 when contact is
detected, and to pivot downward to pull the blade beneath the upper
surface of the saw. A spring (not shown) may be coupled to the
swing arm to increase the speed with which it drops downward. It
will be appreciated that similar implementations may be configured
in the context of all the saws described herein. In the case of the
miter saw, the electromagnetic leaf brake discussed above can be
used to stop the movement of the arm upon contact with a user. In
addition, the release system can be used to release a spring to
push the arm upward upon contact of the blade and user. With such
systems, it may be unnecessary to abruptly stop the blade to avoid
injury.
[0151] FIG. 23 illustrates an exemplary implementation of safety
stop 30 in the context of a radial arm saw 310. Typically, radial
arm saw 310 includes a horizontal base 312, a vertical support
column 314 extending upward from base 312, and a guide arm 316
which extends from column 314 vertically spaced above base 312. A
carriage 318 is slidably coupled to the underside of guide arm 316.
The bottom end of carriage 318 is connected to the saw housing 320
and motor assembly 322, allowing blade 64 to be pulled across the
base to cut workpieces (not shown) supported on the base.
[0152] Any of the various configurations and arrangements of safety
stop 30 described above may be implemented in saw 310. For example,
safety stop 30 may be implemented in a cartridge 246 mountable to
housing 320 adjacent the blade. Charge plates 62 and 66 (not shown,
may also be mounted within the housing or other suitable locations
as described above. Pawl 140 is configured to engage and stop blade
64 upon contact between the blade and the user's body. Although the
pawl is shown mounted adjacent the rear of the blade, it may
alternatively be mounted adjacent the front of the blade or any
other desired location. It should be noted that if the blade is
isolated from the arbor by a deformable bushing as described above,
then the blade will tend to move upward into the housing if the
pawl engages the front of the blade.
[0153] Carriage 318 typically is mounted to guide arm 316 to
prevent pivoting of the bracket relative to the arm. Therefore, the
transfer of the angular momentum of the blade to the housing and
guide arm usually does not cause the arm to move toward the user's
body upon braking. However, since the user may be pulling the saw
toward his or her body when contact is detected, the saw may
continue to move toward the user even after safety stop 30 has
stopped the blade. This continued movement may cause the stopped
blade to be driven over a portion of the user's body (e.g., the
user's hand), causing further injury. Thus, it may be desirable to
provide an additional brake system to stop movement of the carriage
and saw along the guide arm once contact is detected between the
blade and the user's body. This brake system may also be used as an
alternative to stopping the blade, particularly where the system is
configured to draw the carriage backward away from the user upon
activation.
[0154] It will be appreciated that there are a wide variety of ways
to brake the sliding movement of bracket 318 along arm 316. FIG. 23
illustrates just two examples of the many suitable bracket brake
configurations within the scope of the invention. One of the
illustrated bracket brake configurations includes a pivoting pawl
assembly 324. Assembly 324 includes a pawl 328 pivotally coupled to
guide bracket 318. An actuator 330 mounted on bracket 318 is
operatively coupled to safety stop 30 and configured to engage pawl
328. During normal operation, actuator 330 maintains the pawl
spaced-apart from guide arm 316. However, once contact between the
blade and the user's body is detected, safety stop 30 sends an
actuation signal to actuator 330. The signal sent to actuator 330
may be the same signal that activates firing system 115, or it may
be a different signal. In any event, upon receipt of the actuation
signal, the actuator drives against pawl 328, causing it to pivot
into the guide arm, preventing further movement of the guide
bracket forward along the guide arm. The pawl may be constructed or
coated with a high friction material such as rubber, and/or may be
configured with teeth, etc., to increase its braking action.
[0155] The other exemplary braking configuration illustrated in
FIG. 23 includes a lockable spool assembly 332. Assembly 332 may be
used in place of, or in addition to, wedge assembly 324. In any
event, the lockable spool assembly includes a spring-loaded spool
334 mounted on support column 314. One end of a tether or cable 336
is attached to guide bracket 318, while the other end is wound
around spool 334. As the user pulls the saw across the base, the
spool unwinds, allowing the tether to extend. The spring-loading of
the spool ensures that the spool maintains a slight tension on the
tether and retracts the tether around the spool when the user
pushes the saw back toward the support column. Assembly 332 also
includes a spool brake, such as pawl 338, operatively coupled to
safety stop 30. Thus, when contact between the blade and the user's
body is detected, an actuation signal is sent from safety stop 30
to the spool brake, causing the spool to lock. Once the spool
locks, the tether prevents further movement of the saw away from
support column 314. In an alternative implementation of spool
assembly 332 not shown in FIG. 23, the lockable spool may be
contained in, or placed adjacent to, cartridge 246, in which case
the tether would run from the spool backward to support column
314.
[0156] It will be appreciated that there are many alternative
methods, devices, and configurations for stopping the travel of the
guide bracket and the saw along the guide arm Any one or more of
these alternatives may be used in place of, or in addition to, the
braking configurations illustrated in FIG. 23 and described
above.
[0157] FIG. 24 illustrates safety stop 30 implemented in the
context of a hand-held circular saw 340. Typically, circular saw
340 includes a housing 342 that contains a motor assembly (not
shown), a guide plate 344, and a retractable blade guard 346. Blade
64 is coupled to the motor assembly by arbor 70. Safety stop 30 may
be implemented on saw 340 according to any of the embodiments and
configurations described above. In the exemplary implementation
depicted in FIG. 24, the safety stop is illustrated as a
cartridge-based system. Cartridge 246 includes a pawl 140, and is
attachable to housing 342 so that the pawl may engage the blade.
Charge plates (not shown) may be mounted to an inner surface of the
housing adjacent the blade or any other location suitable to
capacitively couple the input signal across the blade. The
cartridge and pawl are shown as mounted adjacent the front of the
blade to avoid interference with blade guard 346. Alternatively,
the pawl and cartridge may be mounted adjacent any other portion of
the blade.
[0158] While safety stop 30 has been described above in the context
of table saws, miter saws, radial arm saws, and circular saws, it
will be appreciated that similar implementations of safety stop 30
will also be suitable for virtually any type of machine using a
rotating blade with a perimetrical cutting edge.
[0159] As a further example of the versatility and
multi-configurability of the invention, FIG. 25 illustrates another
embodiment of safety stop 30 in the context of a band saw 350.
Typically, band saw 350 includes a main housing 352 enclosing a
pair of vertically spaced-apart wheels 354. The perimeter of each
wheel 354 is coated or covered in a high-friction material such as
rubber, etc. A relatively thin, continuous loop blade 64 tightly
encircles both wheels. A workpiece is cut by passing it toward
blade 64 in a cutting zone 355 between wheels 354. An upper-blade
guide assembly 356 and a lower blade-guide assembly 358 maintain
the revolving blade in a stable path within cutting zone 355. The
workpiece is passed toward the blade on a table 360, which forms
the bottom of the cutting zone.
[0160] The blade should be electrically insulated from the main
housing, which usually is grounded. Thus, blade guide assemblies
356 and 358, which may include ball-bearing guides and/or friction
pads, etc., are constructed to electrically insulate the blade from
the main housing. In addition, the high-friction coating on wheels
350 electrically insulates the blade from wheels 354.
Alternatively, the wheels may be constructed of electrically
non-conductive material.
[0161] Charge plates 62 and 66 may be arranged in a variety of ways
depending on the application and the space constraints within the
main housing. Two possible arrangements are illustrated in FIG. 25.
In the first arrangement, charge plates 62 and 66 are disposed
closely adjacent the blade as it rides along one of the wheels 354.
The charge plates may be formed in an arc to match the perimeter of
the wheel and maintain a constant spacing with the blade. This
arrangement has the advantage of easily maintaining a constant
blade-to-charge plate spacing since the blade is held in a constant
path against the perimeter of the wheel. The charge plates may be
connected to the main housing via a non-conductive mount to
maintain electrical insulation from the housing.
[0162] Another of the many possible arrangements for the charge
plates includes a charge plate block 362 which is configured to
extend along the blade as it travels between wheels 354. As can
best be seen in the detail view of FIG. 26, the charge plate block
includes charge plates 62 and 66. In the depicted implementation,
the charge plate block has a substantially C-shaped cross-section
sized to fit around the sides and back edge (i.e., non-toothed
edge) of the blade. The charge plate block is mounted on main
housing 352 and resiliently biased, such as by one or more springs
364, toward the moving blade. Since blade 64 may tend to move or
deflect slightly in its path, springs 364 ensure that the charge
plate block is able to move along with blade. Charge plate block
362 typically is made of a durable, electrically non-conductive
material such as ceramic, plastic, etc. Charging plates 62 and 66
are disposed on or within the charge plate block. Although the
charging plates are illustrated as being disposed on opposite sides
of blade 64, the charging plates may alternatively be on the same
side of the blade. The self-aligning configuration of the charge
plate block ensures that the blade-to-charging plate spacing is
substantially constant despite the motion of the blade.
[0163] Safety stop 30 may include a brake system 34 having one or
more pawls (not shown) configured to engage and stop the blade upon
detection of contact between the blade and the user's body. The
pawl(s) may be arranged to engage the teeth of the blade or the
sides of the blade as described above. Due to the high friction,
high tension fit of the blade over the wheels, it may be necessary
to stop one or both wheels as well as the blade. Thus, the pawl(s)
may alternatively be arranged to engage one or both wheels 354.
However, due to the relatively high mass of typical bandsaw wheels
354, the angular momentum of the spinning wheels may be
substantial, requiring a large amount of force to stop the blade
and wheels virtually instantaneously. Therefore, a further
alternative embodiment of brake system 34 is illustrated in FIG.
27. As described below, alternative brake system 34 is configured
to sever blade 64 upon detection of contact between the user and
the blade. By severing the blade, the tension fit of the blade
around wheels 354 is released, allowing the blade to be stopped
without stopping the wheels.
[0164] As can best be seen in the detail view of FIG. 27,
alternative brake system 34 includes an explosive cable- or
bolt-cutting device 366 positionable adjacent blade 64 to sever the
blade upon receipt of an activation signal. Suitable cable cutting
devices 366 are available from a variety of sources, including
Cartridge Actuated Devices, Inc., of Fairfield, N.J. The size and
configuration of device 366 may vary depending on such factors as
the size and width of blade 64, the blade material, blade speed,
etc. Typically, cutting device 366 will be positioned closely
adjacent the underside of table 360 to block the continued downward
movement of the blade after it is severed. An electronics unit 242
similar to those described above is operatively coupled to device
366 to transmit an activation signal to the device once contact
between the user's body and the blade is detected by the
electronics unit. Device 366 then severs the blade virtually
instantaneously, thereby releasing the tension fit of the blade
around wheels 354. Once severed, the blade substantially stops
moving even though wheels 354 continue to turn. As described above,
the safety stop may optionally be configured to shut off the motor
to band saw 350 as well as to sever the blade. Additionally, one or
more pawls (not show) may be configured to engage and stop the
blade at the same time device 366 severs the blade, thereby
ensuring the blade does not continue to move after being
severed.
[0165] In addition to cutting machines using toothed blades,
several common machines such as jointers, planers, etc., use
generally cylindrical cutting heads having one or more elongate
blades mounted on the outer surface of the cutting head. In
operation, the cutting head is rotated about its cylindrical axis.
When a workpiece is passed across the cutting head, the blades make
wide cuts into the adjacent surface of the workpiece. As with the
cutting machines described above, machines using cylindrical
cutting heads may also cause severe injury if the blades come into
contact with the user's body during operation. Therefore, FIG. 28
shows exemplary configurations of the safety stop for use on a
jointer 500 to prevent severe injury to a user. For clarity, many
of the components of safety stop 30 are not shown in FIG. 28 since
they are similar to the safety stop components described above in
the context of other cutting machines.
[0166] Jointer 500 includes a generally cylindrical cutterhead 502
mounted to rotate on an arbor 504. The arbor typically is mounted
in one or more bearing assemblies (not shown) and rotationally
driven by a motor assembly (not shown), which is coupled to the
arbor either directly or by a belt-and-pulley system. The
cutterhead is mounted in a main frame assembly 506 to extend upward
in the space between infeed table 508 and outfeed table 510. A
workpiece is cut by sliding it along infeed table 508, past the
cutterhead and onto outfeed table 510. Typically, the vertical
positions of the infeed and outfeed tables are independently
adjustable to control the depth of cut into a workpiece.
[0167] The cutterhead is usually constructed of metal, such as
steel, and typically includes three knife blades 512 mounted to
extend above the surface of the cutterhead. It will be appreciated
that fewer or more knife blades may be used and that the utility of
safety stop 30 is not limited by the number of blades on cutterhead
502. One or more electrically non-conductive bushings 514 are
placed between the cutterhead and arbor to insulate the cutterhead
and blades from frame 506. Charge plates 62 and 66 may be placed
adjacent the cutterhead to couple the signal generated by first
electrical system 38 across the cutterhead. In FIG. 28, the charge
plates (shown in dashed lines) are mounted adjacent one flat end of
the cutterhead. Alternatively, the arbor may be insulated from the
frame and the charge plates may be positioned around the arbor as
described above in connection with FIG. 5.
[0168] Due to the relatively few blades, first contact between the
user's body and the cutterhead may be on one of the blades or on
the surface of the cutterhead itself. However, the blades and
cutterhead are electrically coupled so that any contact with the
user's body is detected regardless of whether or not it occurs on
the blades. Once contact is detected, a brake system is actuated to
quickly stop the rotation of cutterhead 502.
[0169] The brake system may include a pivotal pawl disposed to
swing into and engage one of the blades as described above. The
pawl may be constructed to be generally as wide as the cylindrical
length of the cutterhead to engage the entire length of the blade
rather than a relatively small portion of the blade. Typically, the
pawl will not stop rotation of the cutterhead until a blade strikes
the pawl because the surface of the cutterhead is relatively
smooth. The pawl may be shaped to extend into the slots in the
cutterhead that house the blades, thereby ensuring the pawl engages
the cutterhead itself, in addition to the blade.
[0170] Instead of the pivotal pawl system just described, FIG. 28
shows an alternative pawl configuration which includes a plate 520
supported on sliding blocks 522 beneath outfeed table 510. When
plate 520 is slid into contact with cutterhead 502, the plate
engages the next passing blade, stopping rotation of the
cutterhead. Plate 520 may be constructed of any suitable material
capable of stopping the cutterhead including metal, plastic, etc.
The plate may be slid against the cutterhead by a spring 130 held
by a fusible member 122. The fusible member is looped around a pair
of contact studs 524 connected to a firing system (not shown) so
that when contact with the user's body is detected, the fusible
member is melted and spring 130 is released. It should be noted
that the spring is released through a compound mechanism to allow
for a stronger spring without increasing the size of the fusible
member. While plate 520 is shown positioned on the upper, back side
of the cutterhead, the plate may alternatively be positioned at any
point adjacent the cutterhead and beneath the infeed and outfeed
tables. For instance, the plate could be placed under the infeed
table as shown in dashed lines. In addition, plural plates may be
used and positioned at various locations such as at the front and
back of the cutterhead.
[0171] For a typical cutterhead having three blades and rotating at
approximately 5000 rpm, the cutterhead makes a complete rotation in
approximately 12 ms, and contact between a first blade and the next
will be approximately 4 ms. Therefore, the cutterhead is preferably
stopped in less than 8 ms and more preferably in less than 4 ms
from first contact with the user's body. In the former case, no
more than two blades would contact the user, while in the latter
case, no more than one blade would contact the user. However,
longer stopping times may also be acceptable. It will be
appreciated that to optimize the performance of the safety stop,
the position of the pawl around the cutterhead may be
adjusted--based on the time between contact detection and pawl
engagement--to ensure that the pawl engages the cutterhead just in
front of a passing blade rather than just behind one. Otherwise,
the cutterhead would continue to rotate until the next blade
strikes the pawl. Alternatively, the cutterhead may be constructed
with one or more slots, such as shown in dashed lines at 529, along
its length adapted to receive the pawl. Then, if the pawl engaged
the cutterhead just behind a passing blade, the cutterhead would
only rotate to the next passing slot rather than the next
blade.
[0172] In addition to the pawl-based brake system described above,
FIG. 28 also shows an alternative system configured to cover the
blades to prevent them from causing injury to the user.
Specifically, the alternative system includes a substantially
flexible sheet material 530 such as plastic, rubber, metal foil,
metal sheet, etc. Material 530 includes a hook 532 adapted to
engage any of the blades 512. When hook 532 is pushed against
cutterhead 502, the next passing blade catches the hook, causing
material 530 to wrap around the cutterhead as it rotates. Thus, the
blades are covered by material 530, which protect the user from
serious injury. The hook may be moved into contact with the
cutterhead by being mounted to the front of plate 520 or other
actuator assembly. Typically, the outer surface of hook 532 is
rounded or beveled to prevent injury to the user when the hook is
pulled around the cutterhead. Material 530 may be stored on a spool
or may hang free beneath the hook until wrapped around the
cutterhead. Preferably, material 530 is long enough to completely
cover all blades. Alternatively, the end of material 530 opposite
the hook may be anchored to stop the cutterhead before it makes a
full rotation. Additionally or alternatively, the jointer motor
assembly may be shut off to stop rotation of the cutterhead. A
similar system can be implement for circular blade saws to cover
the sharp teeth.
[0173] Similarly to jointers, planers also use cylindrical
cutterheads having one or more knife blades along their surface.
Typically, the cutterhead is mounted above a bed having one or more
feed rollers, and a workpiece is passed along the rollers beneath
the cutterhead. It will be appreciated that safety stop 30 may be
adapted for use with planers using configurations similar to those
described above in connection with jointers. Accordingly, an
implementation of safety stop 30 in the context of planers will not
be separately described. In addition, it will be appreciated that
safety stop 30 may similarly be adapted for use with other machines
using rotating cutting heads, including shapers, routers, etc.
[0174] While safety stop 30 has been described above as configured
to detect contact between the blade and the user's body, it will be
appreciated that safety stop 30 may be additionally, or
alternatively, configured to detect other dangerous conditions. For
example, safety stop 30 may be configured to detect potential
kickback by monitoring the blade speed or deflection. If a rapid
decrease in blade speed (while under power) or a substantial
deflection of the blade is detected, the brake system would be
triggered to stop the blade. Blade speed may be measured by any
suitable device such as a tachometer, etc. Blade deflection may be
detected using micro-switches positioned adjacent the blade,
electrical detectors (e.g., optical, resistive, capacitive,
magnetic, etc.) adjacent the blade, or the like. Other dangerous
conditions may also be monitored. Indeed, it may be desirable for
safety stop 30 to trigger upon the occurrence of any of multiple
conditions.
[0175] It should be understood that the detection and brake systems
of the present invention and components thereof are independently
useful. For instance, the present brake system could be utilized
with a glove-based contact system as found in meat skinning
machines and described in the background, or any other suitable
contact detection system. Similarly, many different brake systems
can be used with a contact detection system constructed according
to the present invention. Further, the fusible member and firing
system may be used in any system requiring the rapid release of a
biased structural member.
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