Abstract: Bevelled end edges (37, 38) of gasket lengths (21, 36) are fused into miter joints (39) at corners (18) by welding apparatus having jig and clamp sets (64) on plates (49, 50) that are mounted on rails (59, 60) and separated by cam followers (81, 82) which interact against the bias of a spring (87) with a cam (88) which is rotated by a handler (93) on a shaft (73) to set the spacing of the sets (64). End edges (37, 38) are aligned int he spacing gap (54) against surfaces (185, 186) of a sissy bar (180) which is lifted up when the cam (88) is turned to its position of longest spacing. The bar (180) is then lowered and a heater block (29) is moved and locked into an advanced position within the gap (54) pulling a handle (115) of a rod (116), by spring finger (122) which shifts onto a lockwheel (125). The cam (88) is turned to an intermediate spacing position to melt the edges (37, 38) against the block (79), and then moved to a shortest spacing position to fuse the melted edges together.