U.S. patent application number 10/773045 was filed with the patent office on 2005-01-13 for stir-friction hot working control system.
Invention is credited to Adams, Glynn Paul, Loftus, Zachary Sean Samuel, McCormac, Joseph Nathan, Venable, Richard Allen.
Application Number | 20050006441 10/773045 |
Document ID | / |
Family ID | 22944529 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050006441 |
Kind Code |
A1 |
Adams, Glynn Paul ; et
al. |
January 13, 2005 |
Stir-friction hot working control system
Abstract
A stir-friction hot-working or welding arrangement uses a pin
tool having a ligament 22 and a shoulder 224. The force required
for incremental penetration increases markedly when the shoulder is
reached. A control system for maintaining a set penetration depth
includes a load cell for measuring force or pressure applied to the
pin tool. The control system compares a reference signal
representing the desired force with the actual force from the load
cell, to produce an error signal which controls the penetration
force, thereby tending to maintain a desired penetration depth. In
a particular embodiment, the reference signal ramps up from a low
or zero value at turn-on, to reduce forces applied upon initial
penetration. In another embodiment, position signals are used to
control a modulator or multiplier, which changes the error signal
applied at certain positions of penetration, or at certain
velocities of penetration.
Inventors: |
Adams, Glynn Paul; (Slidell,
LA) ; Loftus, Zachary Sean Samuel; (New Orleans,
LA) ; McCormac, Joseph Nathan; (Huntsville, AL)
; Venable, Richard Allen; (Grant, AL) |
Correspondence
Address: |
MARSH, FISCHMANN & BREYFOGLE LLP
3151 SOUTH VAUGHN WAY
SUITE 411
AURORA
CO
80014
US
|
Family ID: |
22944529 |
Appl. No.: |
10/773045 |
Filed: |
February 5, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10773045 |
Feb 5, 2004 |
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10193943 |
Jul 12, 2002 |
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10193943 |
Jul 12, 2002 |
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09249680 |
Feb 12, 1999 |
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6421578 |
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Current U.S.
Class: |
228/114.5 |
Current CPC
Class: |
B23K 20/123
20130101 |
Class at
Publication: |
228/114.5 |
International
Class: |
B23K 020/12; B23K
031/02 |
Claims
1-8. (Canceled)
9. A stir-friction welder, comprising: a frame; a lead screw
rotatably interconnected with said frame; a first drive
interconnected with said lead screw; a first sensor located between
said frame and said lead screw; a controller interconnected with
said first sensor and said first drive; a head mounted on said lead
screw; and a pin tool interconnected with said head, wherein
rotation of said lead screw by said first drive, responsive to an
input by said first sensor to said controller, changes a position
of said head along said lead screw, and thereby a plunge depth of
said pin tool relative to a workpiece.
10. A stir-friction welder, as claimed in claim 9, wherein: said
first sensor is a load cell.
11. A stir-friction welder, as claimed in claim 9, further
comprising: a bearing holder and a bearing mounted in said bearing
holder, wherein said lead screw is rotatably supported by said
bearing, wherein said first sensor interfaces with said bearing
holder.
12. A stir-friction welder, as claimed in claim 9, wherein: said
head comprises a second drive interconnected with pin tool, wherein
said second drive rotates said pin tool relative to the
workpiece.
13. A stir-friction welder, comprising: a frame; a lead screw
rotatably interconnected with said frame; a first drive
interconnected with said lead screw; a first sensor that is
stationary; a controller interconnected with said first sensor and
said first drive; a head mounted on said lead screw; and a pin tool
interconnected with said head, wherein rotation of said lead screw
by said first drive, responsive to an input by said first sensor to
said controller, changes a position of said head along said lead
screw, and thereby a plunge depth of said pin tool relative to a
workpiece.
14. A stir-friction welder, as claimed in claim 13, wherein: said
first sensor is located between said frame and said lead screw.
15. A stir-friction welder, as claimed in claim 13, wherein: said
first sensor is a load cell.
16. A stir-friction welder, as claimed in claim 13, further
comprising: a bearing holder and a bearing mounted in said bearing
holder, wherein said lead screw is rotatably supported by said
bearing, wherein said first sensor interfaces with said bearing
holder.
17. A stir-friction welder, as claimed in claim 13, wherein: said
head comprises a second drive interconnected with pin tool, wherein
said second drive rotates said pin tool relative to the workpiece.
Description
FIELD OF THE INVENTION
[0001] This invention relates to control arrangements for
stir-friction welders, and more particularly to automatic
positioning systems for stir-friction welders.
BACKGROUND OF THE INVENTION
[0002] FIG. 1 is a simplified illustration of a prior-art
stir-friction welding arrangement 10. In FIG. 1, an anvil 12
supports a flat workpiece 14, illustrated in phantom to make other
portions of the arrangement more obvious. A friction-stir pin tool
and spindle arrangement designated generally as 16 includes a
spindle 18 which rotates about its axis 8, carrying with it a pin
tool or ligament holding arrangement 20. The pin tool itself
includes an elongated pin or ligament 22, which also rotates in
consonance with the rotating spindle 18. As the spindle 18, holder
20, and pin tool 22 rotate, a relative motion is introduced between
the anvil 12 (carrying the workpiece 14) and the
spindle-and-pin-tool arrangement 16. The motion is represented by
an arrow 24, which suggests movement of the spindle-and-pin-tool to
the right in FIG. 1, assuming that the anvil 12 and workpiece 14
remain fixed. As a consequence of the friction resulting from the
rotation of the pin tool 22 within the workpiece 14, that portion
of the workpiece near the pin tool 22 is heated and becomes
plastic. The relative motion represented by arrow 24 indicates that
the portion 14a of workpiece 14, which lies to the right of pin
tool 22 in FIG. 1, has not yet been welded or hot worked, while
that portion 14b of workpiece 14, which lies to the left of the pin
tool 22, has already been hot-worked, as suggested by the dashed
hatching. Those skilled in the stir-friction hot working arts know
that the described motion results in a line weld or hot working of
the workpiece.
[0003] In order to make appropriate welds, the pin tool or ligament
22 of FIG. 1 must be maintained at a depth which provides full hot
working of the desired region of the workpiece. The depth of plunge
of the pin tool 22 is the depth to which the tip 28 of the pin tool
22 extends below the upper surface 14us, which is the surface from
which the pin tool is introduced into the workpiece. The depth of
plunge cannot be such that the tip 28 of pin tool 22 extends beyond
the second or lower surface 14 of the workpiece, because this might
weld the workpiece 14 to the anvil 12, or might damage the
workpiece or pin tool. In U.S. patent application Ser. No.
09/006,915, the position of the pin tool is maintained at the
appropriate level by a set 26 of rollers, only one of which, namely
roller 26a, is illustrated. Roller 26a is affixed by a shaft 26 to
the spindle or tool holder, and rotates therewith, bearing on the
upper surface 14us of the workpiece 14. The position of the tip 28
of the pin tool 22 in this prior-art arrangement extends below (or
beyond) the lower rolling surface of the rollers by the desired
penetration of the workpiece. The penetration of the workpiece may
be termed "axial" penetration, because the penetration is in a
direction coincident with, or at least parallel to, the axis of
rotation 8 of the spindle 18 and the pin tool 22.
[0004] The roller arrangement 26 for controlling the depth of
penetration of the pin tool during stir-friction welding or hot
working is effective, but the roller apparatus makes it
inconvenient to change the penetration depth from one workpiece to
another, and it is not possible to make small adjustments in the
depth of penetration during a weld or hot-working procedure.
[0005] Another prior-art method which can be used to control the
depth of penetration of a stir-friction pin tool into a workpiece
is by the use of distance measuring devices or sensors (not
illustrated) which measure the distance between the upper surface
of the workpiece, corresponding to surface 14us in FIG. 1, and a
reference point on the spindle holding structure. Sensors which
might be used in such a prior-art arrangement include laser systems
and linear variable differential transformers (LVDTs). The sensor
signal can be compared with a reference value representing the
desired depth of penetration, and the spindle can be moved up or
down, in the absence of rollers such as 26a, in the direction of
double-headed arrow 26 of FIG. 1, in response to deviation of the
measured position from the calculated position. This technique
provides easy change of depth of penetration, by simply adjusting
the signal representing the desired penetration, but is subject to
error due to the large number of dimensions which must be added and
subtracted in order to arrive at the calculated value, and because
of axial position variation or changes due to slack in the spindle
bearings, and similar tolerances.
[0006] Improved control arrangements are desired for stir friction
welding.
SUMMARY OF THE INVENTION
[0007] A method according to the invention for stir-friction
welding a planar workpiece uses a rotating pin tool which includes
a pin or ligament. The pin or ligament defines a diameter at
locations closer to the tip of the pin tool than a particular
location along its length, and also includes or defines a shoulder
at the location. The shoulder has a larger diameter than the pin.
The method includes the steps of rotating the tool, and applying
force to the pin tool with the pin tool plunged into one side of
the workpiece, and with the shoulder essentially coincident with
the surface of the one side of the workpiece, so that the rotating
pin creates a friction-stirred region. According to an aspect of a
method according to the invention, the workpiece and the rotating
tool are moved laterally (in a direction orthogonal to the axis of
rotation) relative to each other, so that the friction-stirred
region progresses along the workpiece. During the moving step, a
signal is generated which is representative of the force applied to
the pin tool. A reference signal is generated which is
representative of that force which is sufficient to maintain the
shoulder against the one surface of the workpiece. The signal
representative of the force applied to the pin tool is compared
with the reference signal, for generating an error signal
representative of the difference between the force applied to the
pin tool and the reference signal. The error signal is used to
control the step of applying force in a manner tending to maintain
the shoulder in contact with, or essentially coplanar with, the one
surface of the workpiece, as a result of which, or whereby, the pin
maintains substantially constant plunge depth.
[0008] In a particularly advantageous mode of practicing a method
according to the invention, the step of applying force includes the
steps of coupling a lead screw to the pin tool and to a fixed
reference point, so that rotation of the lead screw applies
pressure or force to the pin tool. The shaft of a force motor is
coupled to the lead screw, for rotating the lead screw in response
to rotation of the force motor, as a result of which, or whereby,
the force applied to the pin tool is responsive to the rotational
position of the shaft of the force motor. The shaft of the force
motor is rotated in response to at least the magnitude of the error
signal, and preferably in opposite rotational directions in
response to the variation of the error signal relative to a
particular value of the error signal, which is preferably a zero
value. In a most preferred embodiment of the invention, the maximum
force which can be applied to the pin tool is limited.
[0009] The method according to the invention may further include
initial steps which cause the pin tool to plunge into the
workpiece. These steps include positioning the tip of the pin tool
adjacent the one surface of the workpiece, and generating a signal
representing the plunge of the pin tool relative to the one surface
of the workpiece. Other steps include generating a monotonically
changing signal which represents a profile of the desired depth of
plunge as a function of time, generating a difference signal
representing the difference between the actual plunge of the pin
tool and the desired depth of plunge, rotating the pin tool, and
controlling the force in response to the difference signal in such
a manner that the force increases when the actual plunge is less
than the desired plunge, and decreases when the actual plunge is
more than the desired plunge. The step of moving the workpiece and
the rotating tool laterally relative to each other begins when the
actual plunge equals the desired plunge.
BRIEF DESCRIPTION OF THE DRAWING
[0010] FIG. 1 is a simplified diagram illustrating a prior-art
stir-friction technique using rollers to properly locate the
rotating pin tool within the workpiece;
[0011] FIG. 2 is a simplified diagram similar to FIG. 1,
illustrating a pin tool in accordance with the invention, which
includes a shoulder designed to ride on the upper surface of the
workpiece;
[0012] FIG. 3 is a perspective or isometric view of a tool with a
shoulder, separately from the spindle;
[0013] FIG. 4 is a simplified diagram illustrating how axial
pressure may be applied to the pin tool according to one aspect of
the invention;
[0014] FIG. 5a is a side elevation view of a pin tool according to
an aspect of the invention, and FIG. 5b is a plot of force or
pressure for incremental penetration at a depth of penetration
corresponding to the position along the tool of FIG. 5a;
[0015] FIG. 6 is a simplified diagram of a feedback control loop
according to an aspect of the invention, suitable for use in the
arrangement of FIG. 4;
[0016] FIG. 7 is a plot of reference signal versus time for use in
the arrangement of FIG. 6 according to an aspect of the
invention;
[0017] FIG. 8 is a simplified diagram similar to FIG. 4, including
a distance measuring device;
[0018] FIG. 9 is a simplified diagram of a feedback control system
using the distance measurement to aid in generating the control
signal; and
[0019] FIG. 10 is a dimensioned illustration of one embodiment of
the pin tool in accordance with an aspect of the invention.
DESCRIPTION OF THE INVENTION
[0020] FIG. 2 is a simplified diagram, similar to FIG. 1, showing a
pin tool 222 which includes a ligament 22 having a diameter d at
locations away from the rounded tip, and also including a shoulder
portion 224 having a larger diameter D than the diameter d of the
ligament portion 22. As illustrated in FIG. 2, the shoulder portion
224 rides on the upper surface 14us of the workpiece 14 during
operation, and the length of the ligament 22 in relation to the
intended depth of hot-working or welding is selected to
approximately equal the pin-tip-to-shoulder distance. Thus, the
distance between the pin tip 28 and shoulder 224 in FIG. 2 is about
equal to the depth of hot-working or welding.
[0021] FIG. 3 illustrates the pin tool 222 of FIG. 2 separate from
the spindle 18. As illustrated in FIG. 3, the pin tool 222 includes
a shank 310 in addition to the shoulder portion 224 and pin or
ligament 22. The grooves cut into the pin support in FIG. 3 are for
______.
[0022] FIG. 4 is a simplified diagram illustrating one method for
applying force to the pin tool in accordance with an aspect of the
invention. In FIG. 4, a fixed frame or housing 410 defines an
aperture 412 for accommodating and supporting a first bearing 414,
and also defines a second mounting region 416, to which a load cell
418 is mounted. Load cell 418 is a device which transduces force or
pressure into electrical signals, which are available on a signal
conductor set 420. Load cell 418 supports a bearing holder 422,
which holds a bearing 424. A threaded shaft or lead screw 430
extends between bearings 414 and 424, and is rotatable about its
axis 408. A first motor 450 is connected to a gear or worm 451
which engages lead screw 430, for, when the motor is energized,
driving lead screw 430 to rotate around its axis 408. A mounting or
head 440 is mounted in a movable fashion, and is mechanically
coupled to the threads of lead screw 430 by a pair of threaded
bosses or travelling nuts 440.sub.1 and 440.sub.2. Head 440 is
controllably driven in the directions of arrow 46 by rotation of
lead screw 430. Thus, rotation of motor 450 in a first direction
results in linear motion of head 440 in a first direction, and
rotation of motor 450 in a second direction results in linear
motion of head 440 in a second direction. Motor 450 is energized
with electrical power transmitted over conductor set 452. A
computer or processor (PROC) 460 produces appropriate control
signals, which are amplified by a power amplifier illustrated as
454 before application by way of conductors 452 to motor 450.
Processor 460 receives load-cell signals over conductor set 420,
and processes the signals as described in more detail below, to
control the position of head 440 and its pin tool 222.
[0023] It should be noted that pressure divided by area equals
force, so they are not identical measures. However, with a
constant-area system, force and pressure are proportional, and they
are often used interchangeably.
[0024] Head 440 of FIG. 4 supports shaft or spindle 18 on a pair of
bearings 441a and 441b. Spindle 18 is affixed to a circumferential
gear 442. A spur gear 444 driven by a second or spindle motor 446
engages circumferential gear 442, and drives the spindle with a
rotary motion in response to energization of motor. During
operation of the apparatus of FIG. 4, spindle motor 446 is driven
with a constant velocity, while lead-screw motor 450 is driven in
response to signals from load cell 418.
[0025] As illustrated in FIG. 4, the workpiece 14 has its upper
surface adjacent to shoulder 224 of the pin tool 222. Workpiece 14
is mounted on anvil 12, and anvil 12 has a rack gear 472 on its
underside. Rack gear 472 is driven by a gear 474 which is, in turn,
driven by a third or workpiece motion motor 470.
[0026] FIG. 5a illustrates details of the shoulder 224, and FIG. 5b
illustrates as a plot 500 the relative force tending to resist the
plunging of the rotating tool 222 into a workpiece, plotted against
the position or plunge depth of the pin tool 22, As can be seen in
FIG. 5a, the shoulder 224 is not absolutely flat, but is slightly
cone-shaped or tapered, in such a manner that the outer edges of
the shoulder 524r, as measured from the axis 28 of rotation, are
slightly behind (more distant from) the inner or leading portion
524, as measured from the tip 28. The plot 500 of FIG. 5b can be
interpreted by noting that at the moment at which the tip 28 of pin
tool 22 comes into contact with the workpiece, corresponding to
position 510 of FIGS. 5a and 5b, the force is at or near zero. As
the pin tool is plunged through the material of the workpiece so
that the tapered tip portion 28t of the pin tool enters the
material of the workpiece, the force represented by plot 500
increases, and eventually reaches a level A. The level A is reached
as the plunge depth is such as to bring the upper surface of the
workpiece to the position indicated as 512. Further plunging of the
tool 222 into the workpiece, represented by those positions between
position 512 and 514 of FIG. S, cause a slight increase in the
force necessary to produce further penetration, because a portion
of the cylindrical portion of the pin tool lies within the
workpiece, and its sides are in contact with workpiece, and tend to
resist axial motion. At some level of penetration, illustrated as
level 516, the upper surface 14u of the workpiece begins to contact
the leading edge 524 of shoulder 224, and the force required for
further penetration increases sharply. The corner is designated
501, and the corresponding force is designated B. The maximum force
C illustrated in plot 500 represents a condition in which the upper
surface 14u of the workpiece 14 is even with outer portion 524r of
the shoulder 224 at its maximum radius from the axis 28.
[0027] It should be understood that in the plot 500 of FIG. 5b, the
actual pressure exerted on the underlying material of the workpiece
may differ from the force illustrated, because in the range between
penetrations 510 and 512, and between 514 and 516, the area of
contact is increasing at a rate greater than the rate of increase
of the force. Thus, the forces and the pressures may not track each
other exactly in these plots.
[0028] Considering the characteristics of plot 500 of FIG. 5b, it
will be understood that the plotted forces tending to resist
further penetration of the rotating pin tool increase in a
monotonic manner with increasing penetration, although with
different slopes for different penetrations. FIG. 6 is a simplified
diagram of a feedback control arrangement, which may be implemented
by processor 460 of FIG. 4, for control of lead-screw motor 450 in
such a manner as to tend to maintain constant pressure or force on
the pin tool 222. In FIG. 6, processor 460 includes a source of
signal 610 which represents the desired pressure or force which is
to be applied to the pin tool. The signal representing the desired
pressure or force is applied to the noninverting (+) input port of
a summing circuit 612. Summing circuit also receives at an
inverting (-) input port a signal representing the actual force
applied to the load cell 418. The force applied to the load cell
418 is the reaction force arising from the force applied by the
lead screw 430 to the head 440 to drive the head, and the pin tool
with it, toward the workpiece 14. Summing circuit 612 acts as an
error signal generator, and subtracts the actual-pressure or
actual-force signal from the desired-pressure or desired-force
signal, to thereby produce an error signal. The error signal is
applied to control the lead-screw motor 450 in a bidirectional
manner. The motor is connected by way of mechanical elements,
illustrated as a block 614, to the load cell 418. The connections
as described in conjunction with FIG. 6, together with the
mechanical arrangements discussed in conjunction with FIG. 4, make
it clear to those skilled in the art that the system as so far
described constitutes a degenerative feedback loop which tends to
maintain the desired pressure or force on the pin tool. The
pressure or force applied to the pin tool by the feedback
arrangement can readily be changed by simply changing the reference
signal produced by block 610.
[0029] The feedback arrangement as described in conjunction with
FIGS. 4 and 6 tends to maintain the pressure or force on the
spindle 18 and the pin tool 22 at a constant, selected target
level. According to an aspect of the invention, the target pressure
or force is selected to be that pressure or force represented by B
of FIG. 5b, which corresponds to the corner 501, where there is a
marked change from a relatively low rate of increase of force or
pressure to a relatively high rate of increase as a function of
penetration depth. This selection makes it easy for moderate
pressures or forces (those lying between A and B of FIG. 5b) to
cause penetration to depth 510, but any error in the feedback which
gives a value of error signal greater than that required to reach
depth 510 results in a relatively small additional penetration.
[0030] The arrangement of FIGS. 4 and 6 may tend to apply forces
larger than desired to the pin tool at the beginning of
penetration, before the pin tool has reached its full operating
depth. According to an aspect of the invention, the reference
signal source 610 of FIG. 6 produces a ramp voltage at initial
start-up, which ramps from zero to the desired value, and then
remains at the desired maximum value. FIG. 7 is a plot 710 of
reference signal versus time according to this aspect of the
invention. In plot 710, the amplitude of the error signal ramps
from zero value at zero time to the desired steady-state value at a
time t1, and remains at the steady-state value thereafter (or until
a new plunge is undertaken).
[0031] FIG. 8 is a simplified illustration of a stir friction
hot-working or welding arrangement similar to that of FIG. 4, but
including a measuring device or sensor in the form of a linear
variable differential transformer (LVDT) 710, which produces
signals on a signal path 712 for application to the controller 460.
FIG. 9 is a simplified block diagram of the control system of the
arrangement of FIG. 8. Elements of FIG. 9 corresponding to those of
FIG. 6 are designated by like reference numerals. In FIG. 9, motion
of the pin tool caused by movement of motor 450 is coupled to LVDT
710, which produces a signal representing pin tool position
relative to the surface. The position signals are applied to an
encoder 912, which may be as simple as a differentiator, for
determining the rate of change of position. The rate of change of
position signal is applied to a modulator or multiplier 910, which
couples more or less of the pressure error signal from error
detector 612 to the motor, depending upon the rate of change
signal. The purpose of this arrangement is to prevent application
of excessive force to the pin tool during initial penetration,
which might damage the system.
[0032] Other embodiments of the invention will be apparent to those
skilled in the art. For example, while a general-purpose computer
processor has been described as controlling the feedback, a
dedicated processor could also be used, and might even be
advantageous, by virtue of simplicity and reliability. While the
term "motor" has been used to describe a transducer from
electricity to mechanical energy, other devices than a conventional
rotary motor may be used, as for example piezoelectric or magnetic
devices, or linear motors. While the feedback loops have shown
simple feedback schemes, but many types of feedback control may be
used, including proportional-integral-derivative (PID) control.
[0033] Thus, a method according to an aspect of the invention for
stir-friction hot-working or welding a planar workpiece (14) uses a
rotating pin tool (22) which includes a pin or ligament defining a
diameter at locations closer to the tip of the pin tool (22) than
at a particular location along its length, and also including or
defining a shoulder (224) at the location. The shoulder (224) has a
larger diameter (D) than the pin diameter (d). The method includes
the steps of rotating the tool, and applying pressure or force to
the pin tool (22) with the pin tool (22) plunged into one side
(14us) of the workpiece (14), and with at least a portion (524) of
the shoulder (224) essentially coincident with the surface (14us)
of the one side of the workpiece (14), so that the rotating pin
creates a friction-stirred region. According to an aspect of a
method according to the invention, the workpiece (14) and the
rotating tool (22) are moved laterally (in a direction orthogonal
to the axis of rotation) relative to each other (by motor 470, rack
472 and gear 474), so that the friction-stirred region (14b)
progresses along the workpiece (14). During the moving step, a
signal is generated, which is representative of the pressure or
force applied to the pin tool (22). A reference signal is generated
(block 610) which is representative of that force (B) which is
sufficient to maintain the shoulder (224) against the one surface
(14us) of the workpiece (14). The signal representative of the
force applied to the pin tool (22) is compared (in error signal
generator 612) with the reference signal, for generating an error
signal representative of the difference between the force applied
to the pin tool (22) and the force represented by the reference
signal. The error signal is used to control the step of applying
force in a manner tending to maintain the shoulder (224) in contact
with, or essentially coplanar with, the one surface of the
workpiece (14), as a result of which, or whereby, the pin maintains
substantially constant plunge depth.
[0034] In a particularly advantageous mode of practicing a method
according to the invention, the step of applying force includes the
steps of coupling a lead screw (430) to the pin tool (22) and to a
fixed reference point (410), so that rotation of the lead screw
(430) applies pressure or force to the pin tool (22). The shaft of
a force or lead-screw motor (450) is coupled (by gear 451) to the
lead screw (430), for rotating the lead screw (430) in response to
rotation of the force or lead-screw motor (450), as a result of
which, or whereby, the pressure or force applied to the pin tool
(22) is responsive to the rotational position of the force motor
(450) shaft (450s). The shaft (450s) of the force motor (450) is
rotated in response to at least the magnitude of the error signal
(generated by error signal generator 612), and preferably in
opposite rotational directions in response to the variation of the
error signal relative to a particular value of the error signal,
which is preferably a zero value. In a most preferred embodiment of
the invention, the maximum pressure or force which can be applied
to the pin tool (22) is limited.
[0035] The method according to the invention may further include
initial steps which cause the pin tool (22) to plunge into the
workpiece (14). These steps include positioning the tip of the pin
tool (22) adjacent the one surface of the workpiece (14), and
generating a signal representing the plunge of the pin tool (22)
relative to the one surface of the workpiece (14). Other steps
include generating a monotonically changing signal which represents
a profile of the desired depth of plunge as a function of time,
generating a difference signal representing the difference between
the actual plunge of the pin tool (22) and the desired depth of
plunge, rotating the pin tool (22), and controlling the force in
response to the difference signal in such a manner that the force
increases when the actual plunge is less than the desired plunge,
and decreases when the actual plunge is more than the desired
plunge. The step of moving the workpiece (14) and the rotating tool
laterally relative to each other begins when the actual plunge
equals the desired plunge.
* * * * *