Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
1999-02-12
2002-07-16
Picard, Leo P. (Department: 2125)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C700S147000, C228S112100
Reexamination Certificate
active
06421578
ABSTRACT:
FIELD OF THE INVENTION
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
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
14
a
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
14
b
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.
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
14
us
, 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
141
s
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. 09/036,915, now U.S. Pat. No. 5,971,247, the position of the pin tool is maintained at the appropriate level by a set
26
of rollers, only one of which, namely roller
26
a
, is illustrated. Roller
26
a
is affixed by a shaft
26
s
to the spindle or tool holder, and rotates therewith, bearing on the upper surface
14
us
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
.
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.
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
14
us
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
26
a
, in the direction of double-headed arrow
27
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.
Improved control arrangements are desired for stir friction welding.
SUMMARY OF THE INVENTION
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 at a particular location along its length. The pin or ligament 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.
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.
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 diff
Adams Glynn Paul
Loftus Zachary Sean Samuel
McCormac Joseph Nathan
Venable Richard Allen
Duane Morris LLP
Frank Elliot
Lockheed Martin Corporation
Picard Leo P.
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