Stiebel, U.S. Patent No. 4,419,558 (December 6, 1983) and A. Stiebel, C. Ulmer, D. Kodrack, B. Holmes, "Monitoring and Control of Spot Weld Operations," ME Technical Paper Series. No. 860579 (1986), Issue No. 148-7191 describe monitoring and controlling electrical resistance spot- welding by measuring displacements of the electrodes during welding. After the squeezing force is applied by the electrodes to the work pieces and the supply of welding current is initiated, the metal at the work site first expands thermally as it heats (expansion) and then flows plastically as it softens and fuses (indentation). The electrodes are displaced by the expansion and indentation of the metal at the weld site as well as by the expansion and contraction of the electrodes. Thus, measurements of the displacement of the electrodes during formation of the weld contain information indicative of the state of the metal at the weld site.

It has long been known that moderate indentation almost always ensures a good weld. The ability to measure the onset of indentation makes it possible, therefore, to shut off the welding current upon detection of indentation with a high level of assurance that a good weld has been formed. The Stiebel patent and the Stiebel et al technical paper referred to above are incorporated by the foregoing reference to them into the present specification. In the method and apparatus of the Stiebel patent (and the Stiebel et al. technical article) consistent measurements of displacement are assured by interposing a mechanical compression spring between the piston of an air cylinder (or its equivalent) that moves the movable electrode into engagement with the work piece and the movable electrode. A load cell associated with the spring detects the changes in the load imposed on the spring as the movable electrode is displaced upon expansion and indentation of the metal of the work pieces at the weld site during

formation of the weld. Compressing the spring during expansion provides changes in the resulting forces in the spring and thus on the load cells that are directly proportional to the displacement of the movable electrode. Without the spring, for example with a hydraulic or pneumatic cylinder directly working on the movable electrode, the piston is theoretically free to displace with the movable electrode in direct correspondence with the electrode movements, this providing no change in load and no opportunity to detect electrode displacements by detection of load changes.

Summary Of The Invention It has been discovered that greatly improved welding characteristics may be had by fixing the electrodes in position during the application of welding current. Disclosed is a welding apparatus having a frame supporting a pair of arms having welding electrodes. At least one of the pair of arms is connected to a pneumatic cylinder operable for moving an electrode against a work piece, the pneumatic cylinder includes a brake for preventing an outward displacement of electrodes and pneumatic cylinder during thermal expansion of the weld nugget. The brake includes a plurality of wedges which are disposed in an axial bore within the piston rod and movable outwardly to frictionally engage the inner surface of the axial bore of the piston rod. Also disclosed is a method of welding, including the steps of moving a pair of electrodes into abutment with opposing sides of a work piece, fixing the electrodes against outward displacement, and applying a welding current to form a weld and releasing the brake after formation of the weld nugget.

The method and apparatus of the invention provides a weld with greatly improved welding characteristics. By mechanically locking the electrodes in position during the application of welding current, there is a reduction in sparking and resulting spark erosion of the weld nugget. Additionally, the fixing of the electrodes against outward displacement results in increased pressure being applied to the weld nugget as a result of the expansion of the

nugget during the application of the weld current. This increased pressure results in an improved weld.

Additionally, the apparatus and method are advantageously used in conjunction with piezoelectric strain gauges for contiolling weld current, as disclosed in co-pending patent application Serial No. 694,937.

Description Of The Drawing The present invention will be more fully understood by reference to the detailed description of the preferred embodiments of the present invention when read in conjunction with the accompanying drawing, in which like reference characters refer to like parts throughout the views, and in which:

Fig. 1 is a side view of a welding apparatus in accordance with the invention shown in a retracted position;

Fig. 2 is a partial side view of the welding apparatus in accordance with the invention shown with an electrode arm in an extended position for welding;

Fig. 3 is a sectional view of a pneumatic cylinder with the piston in a retracted position;

Fig 4 is a sectional view of a portion of the pneumatic cylinder with the piston in an extended position and a brake in an unlocked position; Fig. 5 is a cross-sectional view of the pneumatic cylinder and brake taken along lines 5-5 of Fig. 4; and

Fig. 6 is a sectional view of the pneumatic cylinder with the brake in a locked position.

Detailed Description of a Preferred Embodiment Shown in Fig. 1 is an embodiment of a welding apparatus 10 embodying the invention of a type for use in the welding of a work piece 12, particularly suitable for welding car and truck bodies. The welding apparatus 10 has a rigid frame 14 including flanges 16 extending along one side of the frame. Each flange 16 has a bore 18 for accepting a fastener to secure the

frame to a robotic arm (not shown) which is used to position the welding apparatus.

Mounted to a top surface 20 of the frame 14 is a pressure equalizing device for use in balancing the pressure exerted on the work piece by the welding apparatus. The pressure equalizing device includes a biasing member

22, such as a spring. The biasing member is connected by a threaded rod 24 to a slide bar 26. The slide bar 26 is reciprocally movable between a pair of rails 28 along the top surface 20 of the frame 14. The pressure equalizing device as is discussed below is operable to balance the pressure on the work piece when an inner arm 32 is extended to move an electrode 54 against the work piece as shown in Fig. 2. As shown in Fig. 1, the inner arm

32 and an outer arm 34 are fixedly mounted to the slide bar 26. The outer arm has a mounting bracket 36 having an elongated portion 32 mounted to the slide bar 26. Extending from a free end of the mounting bracket 36 is an L-shaped member 38 supporting a conventional electrode holder 40 and welding electrode 42. Electrical current is delivered to the electrode 42 by conductive material disposed within the electrode holder 40 and L-shaped member 38. Electrical current is carried from a supply of electrical power carried at the frame by an extension bar 43 and flexible wire 44. The flexible wire 44 permits positioning of the outer arm 34 by the pressure equalizing device. The electrode holder is removable from the L-shaped member by rotating threaded locking members 46 to release the electrode holder.

As best shown in Fig. 1, the inner arm 32 includes a pneumatic cylinder 48 having a piston rod 50 supporting an electrode holder 52 and electrode 54. The pneumatic cylinder 48 is mounted to the mounting bracket 36 of the outer arm in a conventional manner, such as bolts (not shown).

The pneumatic cylinder 48 is mounted to the mounting bracket 36 of the outer arm so that after advancement of the piston rod against the work piece, additional pressure on the piston rod overcomes the biasing force of the

biasing member 22, resulting in movement of the slide bar and outer arm towards the biasing member. Sufficient pressure is applied to the pneumatic cylinder to draw the electrode 42 into contact with the work piece.

As best shown in Fig. 3, the pneumatic cylinder 48 includes a cylinder barrel 56 having an end piece 58 at one end and a cap 62 at the opposite end.

The piston rod 50 extends through a bore 60 in the end piece 58 and a bore 64 in a center port member 66 to a main piston 68. An auxiliary piston 70 is mounted to the piston rod to move within the barrel between the center port member 66 and end piece 58. The main piston 68 and auxiliary piston 70 each have annular grooves containing O-rings 71 to provide a seal with the interior surface of the barrel 56.

A primary port member 72 is mounted between the center port member and cap. The primary port member 72 has a forward stroke port P, and the center port member has a forward stroke port P ₂ . The forward stroke ports (Pi and P ₂ ) are connected to a source of pressurized fluid, such as compressed air

(not shown). When compressed air is introduced into the cylinder through Pj and P ₂ . The main piston 68 and auxiliary piston 70 are forced to move the piston rod and electrode 54 in a direction shown by Arrow A in Fig. 4 forward towards the work piece 12. The center port member 66 has a port P ₃ for introduction of pressurized air into the barrel to act on the main piston 68 to return the piston rod 50 from the work piece in a direction shown by Arrow B of Fig. 6.

As shown in Figs. 3, 4 and 6, the piston rod 50 has an axial bore 74 extending from the end piston 68 to the center piston 70. Disposed within the bore 74 is a brake rod 76 supporting a cylindrical spacer 78 and brake wedges

80 for mechanically locking the piston rod in position. One end of the spacer 78 is fixedly mounted within a bore of the primary port member 68. An axial throughbore extends through the spacer 78 and primary port member 68 and brake wedges 80 for slidingly accepting the brake rod 76.

A frusto-conical mandrel 82 is attached at one end of the brake rod adjacent the brake wedges 80. A brake piston 84 is attached to the opposite end of the brake rod 76. The brake piston 84 is disposed in the cylinder barrel between the primary port member 68 and cap 62 and has an O-ring 86 disposed in an annular groove, as best shown in Figs. 5 and 6. The brake piston 84 is displaceable away from the primary port member 68 by compressed air received through port P ₄ . Movement of the brake piston 84 away from the primary port member 68 results in the movement of the piston rod and brake mandrel 82 in the direction shown by Arrow B inwardly against the brake wedges 80. As the brake mandrel moves inwardly, the brake wedges 80 are forced radially outward by the brake cap against an interior cylindrical surface 88 of the bore of the piston rod, as shown in Fig. 5. Each brake wedge is formed of a suitable rigid heat resistant material, such as asbestos, which is used in automotive brake linings. The brake wedges have a cylindrical outer surface having a circumference slightly smaller than the interior surface 88 of its base.

The brake wedges are released by introducing pressurized air through port P ₅ to act against the brake piston 84 to move the brake rod 76 and brake mandrel 82 in a direction away from the brake wedges shown by Arrow A of Fig. 4.

The method of operation of the improved welding apparatus according to the invention includes first positioning the electrodes 42 and 54 on either side of the work piece 14, as best shown in Fig. 1. As shown in Fig. 2 and Fig. 4, pressurized air is introduced into ports P, and P ₂ to extend the piston rod 50 and electrode 54 against the work piece. The pressure of the electrode on the work piece forces the outer arm to slide inwardly, drawing electrode 42 against the biasing member to abut the work piece as shown in Fig. 2. The pressure of the electrodes on either side of the work piece is equalized by movement of the slide bar against the biasing force of the spring. When sufficient pressure, for instance, 800 lbs/sq. in., is reached on both sides of

the work piece, the electrodes are fixed against outward movement. The brake wedges are set by introducing pressurized air through P ₄ , thereby forcing the brake wedges apart against the inner surface of the piston rod bore to mechanically lock the piston rod in position. The welding current is then applied to form a weld nugget. After the nugget has been formed, the current is terminated and the brake is released. Because an important aspect of the invention is preventing outward displacement of the electrodes during the application of weld current, it is within the contemplation of the invention to adjust the pressure setting the brake at a level which will permit the cylinder to move the electrodes together after expansion of the weld nugget. Thus, the electrodes are free to move inwardly firmly contacting the weld during contraction of the nugget as it cools.

After the weld has cooled, the electrodes are moved apart and the work piece removed to complete the cycle. It has been found that mechanically locking the piston rod reduces spark erosion and the increased pressure which results from the expansion of the weld nugget during the application of the weld current results in a much improved weld over previous methods.

It should be clear that variations of the invention may be made without departing from the scope and spirit of the invention. For instance, it is not necessary to have a double piston arrangement as disclosed. Likewise, many variations in the manner in which the electrodes are fixed against outward expansion during the application of the welding current are within the scope of the invention. I claim: