1.  Field of the Invention
The present invention is directed to a holder for a resistive welding electrode.  More specifically, the present invention is directed to a holder having a compressible biasing member that provides a counter force. 
2.  Description of the Related Arts
Modern metal fabrication facilities utilized resistive welding equipment.  A resistive welder contacts opposite surfaces of a work piece with welding electrodes.  An electric current applied to the electrodes causes the work piece to weld.  Hundreds of resistive welders are used by an automotive assembly facilities to join the various body components.  The welders require a measured application of force between the electrodes and the work piece.  If the applied force is too high, the work piece or electrodes may be damaged.  If the applied force is too low, the resulting weld may be weakened. 
Most resistive welding equipment is constructed of opposed pneumatically or hydraulically operated cylinders.  Electrodes are attached to each cylinder and contact the work piece from opposite sides.  The cylinders are generally positioned horizontally, whereby both the upper and lower cylinders are moved into contact with the work piece.  Moveable cylinders are used to provide an opening for the work piece and to apply a known force to the electrodes. 
The welding operation used to produce automotive bodies requires thousands of welds.  Each welder is operated many times per day.  Each use of the welder causes a small amount of wear to the electrode.  As the electrode wears, the cylinder must travel a greater distance to apply the desired force.  Hence, it has been necessary to use opposed cylinders that trap the work piece.  If one electrode were mounted to a stationary surface, it would lose contact with the work piece as the electrode wore, or it would cause an unwanted deflection in the work piece as the work piece is forced into contact with the worn electrode. 
When the work piece to be welded is a vehicle body, it is desirable that the welding operation not impart any distortion or damage to the joined metal members.  Consequently, it is desirable to attach the welding electrodes to holders that apply force to both sides of the work piece. 
As previously described, most current automotive assembly operations utilize opposed moveable cylinders operated by air or oil.  The cylinders require numerous electrical and fluid connections.  These connections occupy floor space in the vicinity of the vehicle body and prevent access to the vehicle by people and equipment.  Because the electrodes require periodic replacement, it is desirable to maintain the area near the welder accessible to people and equipment.  Additionally, when the cylinders are mounted in a vertical orientation, the welding operation causes sparks and hot metal debris to fall on the lower cylinder and connections.  These debris can be trapped by the exposed cylinders and may damage the connections. 
It is desirable to replace at least one of these electrode holders with a self-contained holder that does not require any external connection for motion.  The holder would be accessible to people and equipment and would not be as susceptible to damage by falling welding debris. 
It is known to include a biasing member in combination with a moveable cylinder as shown in U.S. Pat.  No.  3,731,046.  The electrode holder illustrated in this patent uses elastic bushings that provide a very small follow-up force during the welding process while also providing electrical isolation between the electrode and the moveable cylinder.  This device still requires a moveable cylinder to move the electrode holder into position because the follow-up force described in this patent occurs when the metal work piece becomes molten.  The patent describes a follow-up movement occurring in 1/20 of a second during which the metal work piece is molten.  The bushing is placed radially between the piston and cylinder wall.  The bushings are bonded to both the piston and cylinder wall.  The bonding operation makes assembly and repair of the electrode holder very difficult.  The present invention is directed to a simpler design that eliminates the need for the moveable cylinder by providing greater electrode travel.  The present design may be easily assembled and repaired because it does not use a bushing attached to either the cylinder wall or piston. 
One design that utilizes a single moveable cylinder and a force equalizing means is illustrated in U.S. Pat.  No.  4,549,457.  A moveable cylinder is mounted in a pivoting frame.  The frame pivots the tool into position on opposite sides of a work piece.  A biasing means attached between the frame and the cylinder apply an equalized force to opposite sides of the work piece.  This design is not used as an electrode holder, but rather, as a forming anvil to mechanically join metal pieces.  The equalizing means requires that the cylinder and frame be connected and moveable so as to bias the cylinder with respect to the work piece.  The equalizing means is a complex spring and cam mechanism that balances the force applied by the cylinder.  The electrode holder of the present invention does not require the bulky frame that serves to retain both the cylinder and the anvil (electrodes as used in the present invention) nor the complex equalizing means described. 
These deficiencies and problems as well as others are overcome by the present invention.