U.S. patent number 3,905,161 [Application Number 05/471,559] was granted by the patent office on 1975-09-16 for grinding machine with feed rate changing apparatus.
This patent grant is currently assigned to Toyoda Koki Kabushiki Kaisha. Invention is credited to Hayashi Kodama, Tamaki Tomita.
United States Patent |
3,905,161 |
Tomita , et al. |
September 16, 1975 |
Grinding machine with feed rate changing apparatus
Abstract
A grinding machine includes means for changing the feed rate of
a grinding wheel relative to a workpiece when the grinding wheel
approaches the workpiece. The grinding wheel is insulatedly mounted
on a wheel slide which is moved by a feed device toward and away
from the workpiece. An electrode device is mounted adjacent to the
grinding wheel and is connected to an electric power source for
generating a difference of potential between the grinding wheel and
the workpiece. A detecting device detects the change of the
difference of potential caused by the approach of the grinding
wheel to the workpiece to generate a control signal. A control
device controls the feed device so as to change the feed rate of
the wheel slide in accordance with the control signal.
Inventors: |
Tomita; Tamaki (Okazaki,
JA), Kodama; Hayashi (Kariya, JA) |
Assignee: |
Toyoda Koki Kabushiki Kaisha
(JA)
|
Family
ID: |
13075978 |
Appl.
No.: |
05/471,559 |
Filed: |
May 20, 1974 |
Foreign Application Priority Data
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May 24, 1973 [JA] |
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48-58151 |
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Current U.S.
Class: |
451/11; 451/26;
451/450; 204/400 |
Current CPC
Class: |
B24B
47/20 (20130101) |
Current International
Class: |
B24B
47/00 (20060101); B24B 47/20 (20060101); B24B
049/08 () |
Field of
Search: |
;51/165R,165.77,165.87,165.88,165.92,267 ;204/129.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1,371,307 |
|
Jul 1963 |
|
FR |
|
782,432 |
|
Sep 1957 |
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GB |
|
Primary Examiner: Whitehead; Harold D.
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A grinding machine comprising:
a bed;
a wheel slide slidably mounted on said bed;
a wheel shaft rotatably mounted on said wheel slide;
a grinding wheel attached to said wheel shaft with electrically
non-conductive relationship therebetween;
a drive motor for rotating said grinding wheel;
a work table slidably mounted on said bed for supporting a
workpiece;
feed means for moving said wheel slide toward and away from said
workpiece;
an electric power source;
electrode means mounted adjacent to said grinding wheel and
connected to said electric power source for generating a difference
of potential by creating an electric condenser between said
grinding wheel and said workpiece;
coolant means for connecting said electrode means and said grinding
wheel so as to charge said grinding wheel by creating a coolant
layer thereon;
detecting means for detecting the change of the difference of
potential caused by engagement between said coolant layer on said
grinding wheel and said workpiece to generate a control signal;
and
control means for controlling said feed means so as to change the
feed rate of said wheel slide in accordance with said control
signal.
2. A grinding machine according to claim 1, further comprising a
safety means connected to said control means and said drive motor
for preventing said grinding wheel from being damaged by colliding
with said workpiece when said electrode means or said detecting
means malfunction.
3. A grinding machine according to claim 2, wherein said safety
means comprises:
a current transformer connected to said drive motor for
transforming a high input current applied to said drive motor to a
low alternating current;
a rectifier for rectifying said low alternating current to generate
a direct current, said direct current increasing when an excessive
load is applied to said grinding wheel;
a setting means for setting a safety reference current; and
a comparator circuit connected to said rectifier and said setting
means for comparing said direct current with said safety reference
current to generate a safety signal when said direct current
attains said safety reference current, said control means being
operable to cause said feed means to retract said wheel slide in
response to said safety signal.
4. A grinding machine according to claim 1, wherein said electrode
means has a coolant nozzle of said coolant means for wetting said
grinding wheel with a conductive coolant.
5. A grinding machine according to claim 4, wherein said detecting
means comprises:
an amplifier for amplifying the difference of potential to generate
an output voltage;
a first setting means for setting a first reference voltage equal
to the output voltage appearing at an output terminal of said
amplifier when a predetermined gap is formed between an outer
peripheral edge of said grinding wheel and said workpiece when said
feed means moves said wheel slide toward said workpiece at a rapid
feed rate; and
a first comparator circuit connected to said amplifier and said
first setting means for comparing said output voltage with said
first reference voltage to generate said control signal when the
output of said amplifier voltage attains said first reference
voltage, said control means causing said feed means to change the
feed rate of said wheel slide from a rapid feed rate to a slow feed
rate in accordance with said control signal.
6. A grinding machine according to claim 5, wherein said detecting
means further comprises:
a second setting means for setting a second reference voltage equal
to the output voltage appearing at an output terminal of said
amplifier when said grinding wheel is in a desired wet state;
and
a second comparator circuit connected to the output terminal of
said amplifier and said second setting device for comparing the
output voltage of said amplifier with said second reference voltage
to generate a confirmation signal, said control means being
permitted in accordance with said confirmation signal to cause said
feed means to move said wheel slide toward said workpiece.
7. A grinding machine according to claim 5, wherein said electric
power source is a direct current source.
8. A grinding machine according to claim 5, wherein said electric
power source is an oscillator and further comprises a rectifier
connected to and between said amplifier and said first
comparator.
9. A grinding machine according to claim 8, wherein said electrode
means comprises:
a pair of electrodes mounted adjacent to a periphery of said
grinding wheel, one of said electrodes being connected to said
oscillator and the other of said electrodes being connected to said
amplifier;
an atomizer means for supplying an atomized coolant, and
a coolant nozzle mounted adjacent to the pereiphery of said
grinding wheel for wetting the same with said atomized coolant.
10. A grinding wheel according to claim 8, wherein said detecting
means further comprises:
a second setting means for setting a second reference voltage equal
to the output voltage appearing at an output terminal of said
amplifier when said grinding wheel is in a desired wet state;
and
a second comparator circuit connected to the output terminal of
said amplifier and said second setting device for comparing the
output voltage of said amplifier with said second reference voltage
to generate a confirmation signal, said control means being
permitted in accordance with said confirmation signal to cause said
feed means to move said wheel slide toward said workpiece.
11. A grinding machine according to claim 9, wherein said electrode
means comprises:
an electrode mounted adjacent to a periphery of said grinding
wheel, said electrode being connected to said amplifier; and
a coolant nozzle mounted below said grinding wheel for supplying a
conductive coolant, said coolant nozzle being connected to said
oscillator.
12. A grinding machine according to claim 11, wherein said
detecting means further comprises:
a second setting means for setting a second reference voltage equal
to the output voltage appearing at an output terminal of said
amplifier when said grinding wheel is in a desired wet state;
and
a second comparator circuit to the output terminal of said
amplifier and said second setting device for comparing the output
voltage of said amplifier with said second reference voltage to
generate a confirmation signal, said control means being permitted
in accordance with said confirmation signal to cause said feed
means to move said wheel slide toward said workpiece.
13. A grinding machine according to claim 1, wherein said coolant
means is operable to supply coolant to said grinding wheel in
response to said control signal.
14. A grinding machine according to claim 1, wherein said coolant
means comprises:
a first means for creating a desired coolant layer on said grinding
wheel so as to connect said electrode means and said workpiece;
and
a second means for supplying a large amount of coolant to said
grinding wheel and the workpiece in response to said control
signal.
15. A grinding machine according to claim 14, which further
comprises a variable throttle valve for controlling an amount of
coolant supplied to said first means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to grinding apparatus, and more
particularly to a grinding machine provided with an apparatus for
changing the feed rate of a grinding wheel relative to a workpiece
when the grinding wheel approaches or contacts the workpiece.
2. Description of the Prior Art
For the purpose of detecting the contact of a grinding wheel with a
workpiece, conventional feed rate-changing devices utilize a change
of input current of a grinding wheel-driving motor caused by the
contact of the grinding wheel with the workpiece, a change of
moment of the rotating grinding wheel, or a vibration or a noise
caused by the contact of the grinding wheel with the workpiece. The
feed rate changing devices using such detecting methods can not,
however, change the feed rate of the grinding wheel just before or
at the moment.
Another method that detects the contact of the grinding wheel with
the workpiece utilizes a conductive circuit formed by the contact
between a grinding machine and a conductive grinding wheel.
However, this method suffers from the drawback that a
non-conductive grinding wheel can not be utilized.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a
grinding machine provided with new and improved apparatus for
changing the feed rate of a grinding wheel just before the grinding
wheel contacts the workpiece.
It is another object of the present invention to provide a grinding
machine provided with new and improved apparatus wherein a
difference of potential is generated between a grinding wheel and a
workpiece and the change of the difference of potential caused by
the approach of the grinding wheel to the workpiece is detected to
change the feed rate of the grinding wheel.
Other objects will appear hereinafter.
According to the present invention these and other objects are
achieved by a grinding machine which comprises a bed, a wheel slide
slidaby mounted on the bed, a wheel shaft rotatably mounted on the
wheel slide, a grinding wheel attached to the wheel shaft, a drive
motor for rotating the grinding wheel, a work table slidably
mounted on the bed for supporting a workpiece and a feed device for
moving the wheel slide toward and away from the workpiece. An
electrode device is mounted adjacent to the grinding wheel, which
is in non-conductive relationship with the wheel shaft, and is
connected to an electric power source for generating a difference
of potential between the grinding wheel and the workpiece. A
detecting device detects the change of the difference of potential
caused in the approach of the grinding wheel to the workpiece to
generate a control signal. A control device controls the feed
device so as to change the feed rate of the wheel slide in
accordance with the control signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects of the present invention will be
more fully appreciated as the same becomes better understood from
the following detailed description of preferred embodiments when
considered in connection with the accompanying drawings, in
which:
FIG. 1 is a schematic view showing a first embodiment according to
the present invention;
FIG. 2 is a relay circuit;
FIG. 3 is a schematic view showing a second embodiment;
FIG. 4 is a cross sectional view, taken along the line IV--IV of
FIG. 3;
FIG. 5 is a cross sectional view showing another embodiment of a
nozzle;
FIG. 6 is a schematic view showing a third embodiment; and
FIG. 7 is a schematic view showing a fourth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like reference numerals and
signs refer to identical or corresponding parts throughout the
several views, a grinding machine in accordance with the present
invention is described with reference to FIGS. 1, 3, 6 and 7 which
show four different embodiments of the present invention.
A work table 2 is slidably mounted on a bed 1 which fixedly mounts
a head stock 3 and a tail stock (not shown) for rotatably
supporting a workpiece W therebetween. A wheel slide 4 is slidably
mounted on the bed 1 so as to be moved toward and away from the
supported workpiece W. The above-mentioned grinding machine parts
are made of conductive material. A grinding wheel G is fixed to a
wheel shaft 5, made of conductive material, and is in
non-conductive relationship with the wheel shaft 5. The wheel shaft
5 is rotatably mounted on the wheel slide 4 and is drivingly
connected to a drive motor 7 through a belt 6. In case a
nonconductive grinding wheel, such as, for example a vitrified
grinding wheel is mounted on the wheel shaft 5, it is not necessary
to interpose an insulated material between the grinding wheel G and
the wheel shaft 5. Accordingly, in both cases, an electric
condenser is created between the grinding wheel G and the workpiece
W.
A feed cylinder 11 is mounted on the bed 1 for slidably receiving a
piston. A piston rod 13 is connected to the lower portion of the
wheel slide 4. Fluid under pressure is applied to and is discharged
from the cylinder 11 through a magnetic change-over valve 14 for
changing the advance and the retraction of the wheel slide 4.
Another change-over valve 15 is used for changing the feed rate of
the wheel slide 4. A check valve 16 is connected to and between the
cylinder 11 and the change-over valve 14 in parallel with the
change-over valve 15 for causing the wheel slide 4 to retract
rapidly.
A coolant nozzle 8 is secured on the top of a grinding wheel guard
12 for discharging a conductive coolant to the grinding wheel G and
the workpiece W during a grinding operation. A pump 17 is connected
to the coolant nozzle 8 by means of a conduit 9 for supplying the
conductive coolant from a coolant reservior 18 to the coolant
nozzle 8. A magnetic chang-over valve 10 is connected to and
between the coolant nozzle 8 and the pump 17 for opening and
closing the conduit 9.
The first embodiment is described in detail below with reference to
FIG. 1.
A conductive nozzle 20 is fixed to the grinding wheel guard 12 so
as to be electrically insulated from the same for discharging the
conductive coolant into the periphery of the grinding wheel G to
thereby wet the same. The conductive nozzle 20 which serves also as
an electrode to provide electric power is connected to the conduit
9 through a relatively long nonconductive pipe 21 for preventing
leakage of current to the conductivity of coolant. Conduit 9 is
connected to the pump 17. A variable throttle 22 is interposed
between the conductive nozzle 20 and the conduit 9 for adjusting
the quantity of the coolant applied to the grinding wheel G so as
not to wet the vicinity of the wheel shaft 5 and so as to apply an
appropriate coolant layer to the periphery of the grinding wheel
G.
A direct current source 25 is used as an electric power device in
this embodiment. The direct current source 25 is connected at its
positive output terminal to the conductive nozzle 20 through a
variable resistance R and is grounded to the head stock 3 at its
negative output terminal. An amplifier 27 is connected at its
positive input terminal to the variable resistance R and the
conductive nozzle 20 and is grounded at its negative input terminal
for amplifying a difference of potential appearing between the heat
stock 3 and the nozzle 20 to generate an output voltage.
A first setting device 30 sets a first reference voltage equal to
the output voltage appearing at the output terminal of the
amplifier 27 when a predetermined gap is formed between the outer
peripheral edge of the grinding wheel G and the workpiece W during
the rapid advance of the wheel slide 4. A first comparator circuit
28 is connected to the output terminal of the amplifier 27 and the
first setting device 30 for comparing the output voltage of the
amplifier 27 with the first reference voltage to thereby generate a
control signal when the output voltage falls to the first reference
voltage. The output terminal of the first comparator circuit 28 is
connected to a relay circuit 40, described hereinafter, which
changes the magnetic change-over valves 10 and 15 to their ports I
in accordance with the control signal. As mentioned above, a feed
rate-changing device is constituted by the first setting device 30,
the first comparator circuit 28, the amplifier 27, the relay
circuit 40 and the magnetic change-over valve 15.
A second setting device 31 sets a second reference voltage equal to
the output voltage appearing at the output terminal of the
amplifier 27 when the periphery of the grinding wheel G is in the
desired wet state. A second comparator circuit 29 is connected to
the output terminal of the amplifier 27 and the second setting
device 31 for comparing the output voltage of the amplifier 27 with
the second reference voltage to thereby generate a confirmation
signal when the output voltage attains the second reference
voltage. The output terminal of the second comparator circuit 29 is
connected to the relay circuit 40 to change the magnetic
change-over valve 14 to its port II in accordance with the
confirmation signal.
A safety device, used also in the other embodiments, for preventing
the grinding wheel G from colliding with the workpiece W at the
rapid feed rate when the feed rate-changing device malfunctions is
described hereunder.
A current transformer 35 is connected to the power input terminal
of the drive motor 7 for transforming a high input alternating
current applied to the power input terminal to a predetermined low
current adequate for the components of the safety device. When an
excessive load is applied to the grinding wheel G, an excessive
current appears at the power input terminal of the drive motor 7
and is detected by the safety device. A rectifier 36 is connected
to the current transformer 35 for converting the alternating
current to a direct current. A setting device 38 sets a safety
reference current for preventing a collision of the grinding wheel
G with the workpiece W. A comparator circuit 37 is connected to the
setting device 38 and the rectifier 36 for comparing the direct
current transmitted from the rectifier 36 with the safety reference
current to thereby generate a safety signal when the direct current
becomes larger than the safety reference current. The comparator
circuit 37 is connected at its output terminal to the relay circuit
40 to change the magnetic change-over valve 14 to its port I in
accordance with the safety signal.
The relay circuit 40 is described hereunder with reference to FIG.
2 wherein a sign A designates a normally open contact closed in
accordance with the confirmation signal, a sign B designates a
normally open contact closed in accordance with the control signal,
a sign C designates a normally open contact closed in accordance
with the safety signal and a sign D designates a normally open
contact closed in accordance with a retraction command signal,
described hereinafter. Normally open contacts Cr1 to Cr4 are
adapted to be closed in accordance with energizations of relays CR1
to CR4, respectively. Normally closed contacts Cr2x and Cr5x are
adapted to be opened in accordance with energizations of relays CR2
and CR5, respectively. A sign PB1 designates an emergency push
button.
The operation of the abovementioned first embodiment is described
hereunder.
FIG. 1 shows a state before a grinding operation in which the wheel
slide 4 and the other accompanying parts are in their initial
positions or states. The drive motor 7 is operated to rotate the
grinding wheel G and the pump 17 is driven to supply the coolant to
the nozzle 20 through the variable throttle 22. Since the periphery
of the grinding wheel G is brought into the wet state, the direct
current source 25 is electrically connected to the periphery of the
grinding wheel through the nozzle 20 and the coolant. Because of
the application of the coolant having a large resistivity and the
relatively long nonconductive pipe 21, an electric conduit formed
by the pipe 21 and the coolant is given a large electrical
resistance, so that the coolant nozzle 20 is effectively insulated
from the coolant reservior 18. Also, the grinding wheel G is
electrically isolated from the wheel shaft 5 as described
hereinbefore. Thus, since a leakage current from the direct current
source 25 is effectively suppressed, a voltage closely
approximating the output voltage of the direct current source 25
appears at the nozzle 20.
The voltage at the nozzle 20 is amplified by the amplifier 27 and
is in turn applied to comparator circuits 28 and 29 to be compared
with the first and the second reference voltages. When the grinding
wheel G is in a sufficiently wet state and is not electrically
shorted with the wheel shaft 5, the output voltage of the amplifier
27 is greater than the second reference voltage. Conversely, when
the periphery of the grinding wheel G is electrically shorted with
the wheel shaft 5, the difference of potential between the coolant
nozzle 20 and the head stock 3 decreases and the output voltage of
the amplifier 27 decreases below the second reference voltage with
the result that the confirmation signal is not transmitted from the
second comparator circuit 29.
When the start push buttom PB2 is operated after the confirmation
signal is generated, the relay CR1 is energized to thereby close
its self-holding contact Cr1. When the contact B is closed by the
confirmation signal, the relay CR3 is energized to thereby energize
the solenoid of the magnetic change-over valve 14 and change the
magnetic change-over valve 14 to its port II. Then, fluid under
pressure is applied to the right chamber of the feed cylinder 11 to
thereby move the wheel slide 4 toward the workpiece at the rapid
feed rate. The relay CR4 for the rapid feed rate is energized by
the contact Cr3 which is closed by the energization of the relay
CR3. When the coolant film formed on the periphery of the grinding
wheel G is brought into contact engagement with the workpiece W by
the rapid advance of the wheel slide 4, the conductive nozzle 20 is
electrically shorted with the workpiece W or the head stock 3
through the coolant film. Accordingly, the difference of potential
applied to the amplifier 27 rapidly decreases and the output
voltage of the amplifier 27 decreases below the first reference
voltage causing a control signal to be transmitted from the first
comparator circuit 28. The contact A is closed in accordance with
the control signal to thereby energize the relay CR2 for a slow or
grinding feed rate. The magnetic change-over valve 10 is changed to
its port I by the energized solenoid thereof to thereby supply
considerably coolant for the grinding operation through the coolant
nozzle 8. At the same time that the magnetic change-over valve 15
is changed to its port I by the energized solenoid thereof, the
wheel slide 4 is advanced at the slow or grinding feed rate and the
relay CR4 for the rapid feed rate is deenergized by the opening of
the normally closed contact Cr2x.
As mentioned above, the feed rate of the wheel slide 4 is changed
to the slow feed rate just before the grinding wheel G is brought
into direct contact with the workpiece W which is ground by the
grinding wheel G at the changed slow feed rate. When the workpiece
W is ground and finished to a predetermined dimension and shape,
the retraction command signal is transmitted from an appropriate
device (not shown), such as, e.g. a sizing device, to thereby close
the contact D. The relay CR5 for commanding the retraction of the
wheel slide 4 is energized to thereby deenergize relays CR1, CR2
through their normally closed contacts Cr5x and in turn to
deenergize the relay CR3 through the opening of the contact Cr1.
The above-described magnetic changeover valves, 10, 15 and 14, the
solenoids of which are deenergized, are changed to their initial
ports to thereby stop the supply of coolant and to thereby move the
wheel slide 4 away from the workpiece W. Thus, all elements return
to their initial positions as shown in FIG. 1.
Since the input current of the drive motor 7 increases because of
the grinding resistance due to the cutting engagement of grinding
wheel G with the workpiece W, the output current value of the
rectifier 36 increases beyond the safety reference current so that
the contact C is closed. Accordingly, when the aforementioned feed
rate changing device malfunctions, for example, if the coolant is
not supplied from the coolant nozzle 20 or the rapid feed rate of
the wheel slide 4 is not changed to the slow feed rate when the
grinding wheel G is engaged with the workpiece W, the relay CR5 is
energized to rapidly move the wheel slide 4 away from the workpiece
W because the contact C is closed in addition to the still closed
contact Cr4.
The experimental results of the abovementioned first embodiment are
described below.
When the peripheral speed of the grinding wheel G is set to 30
meters/second and the output voltage of the direct current source
25 is changed between 30 and 35 volts,
Vo = 10- 12 volts
Vc = 7-9 volts
wherein Vo designates the voltage appearing at the input terminal
of the amplifier 27 in the disengagement state of the grinding
wheel G with the workpiece W and Vc designates the voltage
appearing at the input terminal of the amplifier 27 when the
coolant film on the grinding wheel G contacts the workpiece W.
The aforementioned feed rate-changing device is not affected in its
operation even when the coolant discharged from the conductive
nozzle 20 electrically connects the lower portion of the grinding
wheel G with the grinding wheel guard 12 or the bed I by its
continuous fall because the falling coolant has a high
resistance.
An alternating current source such as an oscillator may be applied
in substitution of the direct current source 25. The conductive
nozzle 20 and the coolant nozzle 8 may be combined into a single
nozzle which is adapted to supply a different quantity of coolant
during the operating time of the feed rate-changing device and
during the grinding operation time. The coolant nozzle 8 may be
fixed to the lower portion of the grinding wheel guard 12 so as to
discharge coolant upwardly into the grinding point.
The second embodiment of the present invention is described
hereunder with reference to FIG. 3 in which certain above-mentioned
parts are omitted in the description of the second embodiment.
An electrode 250 is fixed to the grinding wheel guard 12 so as to
be electrically insulated from the same through an insulated member
19 which opposes the side of the grinding wheel G with a small
clearance as best shown in FIG. 4. The electrode 250 may be
disposed opposite to the periphery of the grinding wheel G,
although the electrode 250 must allow the decrease of the diameter
of the grinding wheel G so that the clearance between the electrode
250 and the periphery of the grinding wheel G is kept constant.
Furthermore, electrodes 250 may be fixed on both sides of the
grinding wheel G.
The electrode 250 is connected to one of the output terminals of an
oscillator 251 through a variable resistance R, the other output
terminal being grounded on the head stock 3 or the bed 1. The
amplifier 27 is connected at one terminal to the electrode 250 and
the variable resistance R for detecting and amplifying the
difference of potential between the grinding wheel and the
workpiece. The other terminal of amplifier 27 is grounded to the
bed 1. The output voltage of amplifier 27 is applied to a rectifier
252 which converts an alternating current to a direct current. The
output of rectifier 252 is connected to the inputs of the same
comparator circuits 28 and 29 as is shown in the first embodiment.
Descriptions for first and second setting devices 30 and 31 and the
relay circuit 40 are omitted because the same devices and circuit
are described in the first embodiment.
The nozzle 20 is connectd to an atomizer 253 to receive a
conductive atomized coolant to adequately wet only the peripheral
portion of the grinding wheel G. The atomizer 253 comprises a
filter 254 for cleaning compressed air supplied from an air pump
(not shown), a regulator 255 for maintaining the compressed air
constant and a mist making device 256 for atomizing coolant
supplied by the compressed air.
The operation of the second embodiment as described hereunder.
The compressed air is applied to the atomizer 253 to make the
atomized coolant, which is discharged from the nozzle 20 into the
periphery of the rotating grinding wheel G to thereby wet the
peripheral portion. Then, since the vicinity of wheel shaft 5 is
not wetted by the operation of centrifugal force of the grinding
wheel, an insulated state is maintained between the grinding wheel
G and the wheel shaft 5. The peripheral portion of the grinding
wheel G is rendered conductive by being wetted by the applied
coolant. As a result, a condenser is formed by the grinding wheel G
and the electrode 250 between which an alternating current can be
conducted.
The grinding wheel G is, accordingly, grounded by the coolant film
formed thereon in its approach to the workpiece, an electric
conduit being formed by the oscillator 251, the variable resistance
R, the electrode 250, the coolant film, the workpiece W and the bed
1. Then, the difference of potential applied to the amplifier 27
decreases so that coolant is supplied from the coolant nozzle 8.
The feed rate of the wheel slide 4 is changed to the slow feed rate
by the operation of the comparator circuit 28 and the relay circuit
40 as aforementioned with respect to the first embodiment. The
second comparator circuit 29, also, performs the same operation as
is described in the first embodiment.
Although the second embodiment is constituted in such a manner that
the coolant nozzle 20 delivers the atomized coolant to the grinding
wheel G, an adequate amount of coolant, not atomized, may be
delivered to the grinding wheel G as well. The nozzle 20 may be
omitted if the peripheral portion of the grinding wheel G can be
maintained adequately wet by the coolant supplied in the previous
grinding operation for a short time from the accomplishment of a
previous grinding operation to the start of the next grinding
operation. In the experiment, it was found that the peripheral
portion of grinding wheel G was still adequately wet for 2 minutes
after the cessation of the coolant supply. The atomized coolant may
be discharged toward the peripheral side surface of the grinding
wheel or toward both the peripheral and the side thereof in
accordance with the peripheral speed of the grinding wheel G as
shown in FIG. 5.
For the purpose of maintaining the peripheral portion of the
grinding wheel G wet so as to cause the second embodiment to
operate normally, the atomized coolant may be discharged (1) to the
outer peripheral edge or the side of the grinding wheel G at a
peripheral speed of 30 meters/second, (2) to both sides of the
grinding wheel G at a peripheral speed of 45 meters/second and (3)
to both sides and the outer peripheral edge at a peripheral speed
of 60 meters/second.
The above-mentioned modifications with respect to the second
embodiment can be applied to the third embodiment described
hereunder.
FIG. 6 shows the third embodiment, which is different from the
second embodiment only in the detection of the difference of
potential between the grinding wheel G and the workpiece W.
Therefore, only this difference is described in detail
hereunder.
A second electrode 360 is secured to the grinding wheel guard 12 so
as to be electrically isolated from the same through an isolated
member 19 which opposes the peripheral and side portion of grinding
wheel G with a small clearance. Second electrode 360 is insulated
from the first electrode 250, mentioned hereinbefore, which is
connected only to the oscillator 251 through the variable
resistance R. The second electrode 360 may be secured so as to be
opposed to the outer periphery or opposed to both sides of the
grinding wheel G as well as the first electrode 250. Furthermore,
first and second electrodes 250 and 360 may be positioned at upper
and lower positions, respectively, so as to put the grinding point
therebetween. Both electrodes 250 and 360 may be disposed on the
same side, for example, on the upper or the lower side, with
respect to the grinding point.
When the peripheral portion of grinding wheel G is wet and thereby
conductive, a condenser is formed by the first electrode 250, the
second electrode 360 and the wet peripheral portion of the grinding
wheel G. Therefore, an alternating current which is transmitted
from the oscillator 251 flows between the first electrode 250 and
the second electrode 360 through the wet peripheral portion of the
grinding wheel G. The difference of potential caused between the
second electrode 360 and the bed 1 is applied to the amplifier 27
to be amplified.
When the coolant film encircling the grinding wheel G contacts with
or approaches the workpiece W causing an electrical short circuit
to the bed 1, the difference of potential between the second
electrode 360 and the bed 1 is markedly decreased. In accordance
with the change of the difference of potential, the coolant is
supplied from the coolant nozzle 8 and the rapid feed rate of the
wheel slide 4 is changed to the slow or grinding feed rate.
The experimental results of the above-mentioned third embodiment
are described below.
When the peripheral speed of the grinding wheel G is at 30
meters/second and 45 meters/second and the output voltage of the
oscillator 251 is changed between 30 and 35 volts,
Vo = 30-50 milli-volts
Vc = 0-10 milli-volts
wherein Vo designates the voltage appearing at the input terminal
of amplifier 27 in the disengagement state of the grinding wheel G
with the workpiece W and Vc designates the voltage appearing at the
input terminal of the amplifier 27 when the coolant film on the
grinding wheel G approaches and contacts the workpiece W.
The fourth embodiment is described hereunder with reference to FIG.
7. This embodiment is constituted, as mentioneed below, so as to
prevent the aforementioned confirmation signal from being
transmitted because of the electrical connection of the grinding
wheel G with the workpiece W which is effected by considerable
conductive coolant scattered about the grinding wheel G,
notwithstanding that the grinding wheel G is relatively distant
from the workpiece.
The nozzle 20 is mounted on the lower portion of the grinding wheel
guard 12 below the grinding wheel G so as to be electrically
insulated from the grinding wheel guard 12. The conductive coolant
discharged from the nozzle 20 is carried by the rotation of
grinding wheel G to the grinding position through the back of the
grinding wheel G. Thus, the conductive coolant applied to the
workpiece W at the grinding position is reduced to a suitable
amount. Furthermore, a baffle plate 400 is attached to the lower
portion of a front cover 12a which is mounted on the grinding wheel
guard 12 for covering the upper front of the grinding wheel G for
preventing the coolant from scattering to the front of the grinding
wheel G. The baffle plate 400 is effectively used when the
peripheral speed of the grinding wheel G is below 30 meters/seconds
but is not necessary when the peripheral speed of grinding wheel G
is over 30 meters/second because almost all the coolant is
displaced toward the back of grinding wheel G. The baffle plate 400
is operable to intercept an air film rotating with the grinding
wheel G for ensuring a positive supply of coolant to the grinding
point of the grinding wheel G.
The nozzle 20 is insulatedly fixed to the grinding wheel guard 12
and is connected to the reservoir 18 through the pump 17. One
output terminal of oscillator 251 is connected to the nozzle 20 by
variable resistance R, the other output terminal being
grounded.
An electrode 450 is mounted on the grinding wheel guard 12 so as to
be opposed to the side of the grinding wheel G and separated
therefrom by a small clearance. The electrode 450 provides a
condenser having a grinding wheel G as an opposite electrode as
well as the electrode 250 of the second embodiment, shown in FIG.
3. The amplifier 27 is connected to and between the electrode 450
and the bed 1 for amplifying the difference of the potential
detected therebetween. The amplifier 27 is connected to the
rectifier 252 which is connected to first and second comparator
circuits 28 and 29 which are connected to first and second setting
devices 30 and 31, respectively. First and second comparator
circuits 28 and 29 are connected to relay circuit 40.
When the peripheral portion of the grinding wheel G is wet from the
coolant discharged from the nozzle 20, the alternating output
voltage of the oscillator 251 appears at the electrode 450 through
the condenser which is constituted by nozzle 20, the coolant, the
grinding wheel G and the electrode 450. This voltage is applied to
the amplifier 27. The output voltage of the electrode 450 is
markedly reduced when the coolant film encircling the grinding
wheel G contacts or approaches the workpiece W. Then, the rapid
feed rate of wheel slide 4 is changed to the slow or grinding feed
rate.
The experimental results of the above-mentioned fourth embodiment
are described below.
When the peripheral speed of the grinding wheel G is set to 30
meters/second, 45 meters/second and 60 meters/second and the output
voltage of the oscillator 251 is changed between 30 and 35
volts,
Vo = 200-400 milli-volts
Vc = 80-150 mille-volts
wherein Vo designates the voltage appearing at the input terminal
of the amplifier 27 during the disengagement state of grinding
wheel G with workpiece W and Vc designates the voltage appearing at
the input terminal of the amplifier 27 when the coolant film
encircling the grinding wheel G contacts and approaches workpiece
W.
In the aformentioned embodiments, the insulated member may be
provided for covering the wheel shaft 5 to prevent the insulation
between the wheel shaft 5 or the wheel slide 4 and the grinding
wheel G from being broken by coolant wetting the wheel shaft 5.
Furthermore, in the third and fourth embodiments, the oscillator
251 and the amplifier 27 may be reversely connected to the two
elements. In the fourth embodiment, another nozzle as well as
nozzle 20 may be disposed in substitution of the electrode 450.
Although certain specific embodiments of the invention have been
shown and described, it is obvious that many modifications and
variations thereof are possible in light of the above teachings. It
is understood, therefore, that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
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