Friction welding machine having an expanding mandrel backstop assembly

In a friction welding machine for welding a first hollow, internally lobed workpiece and a second workpiece together, the friction welding machine including workhead means for rotating and laterally displacing the second workpiece, and a backstop assembly opposing said workhead means for maintaining the first workpiece in a stationary position, the backstop assembly comprising:

• stop ring means for buttressing one end of the first workpiece,
• mandrel means projecting from said stop ring having an outer configuration which complements the lobed interior of the hollow first workpiece and having a length measured from said stop ring which is slightly less than the length of said first workpiece,
• said mandrel means having an intermediate portion sandwiched between rigidly connected body and end portions,
• said intermediate portion mounted for limited rotation out of alignment with said body and end portions, and
• means for rotating said intermediate mandrel portion to effect internal gripping of the first workpiece whereby when the first workpiece becomes heated during a friction welding operation, any outward bowing or workpiece distortion attributable to said mandrel means will occur proximate said intermediate mandrel portion and will not become fixed by the weld.