A jig for coaxially welding together a pair of generally coaxial tubular pipe ends meeting at a joint has a pair of support bodies in the pipe ends flanking the joint and each having an outer surface tapering axially inward toward the joint. Respective arrays of wedges are axially displaceable on the surfaces so that as the arrays are displaced apart the respective wedges move radially outward. Respective springs engaging the arrays urge the respective jaws radially together and urge the respective arrays axially toward each other. An abutment between the arrays has annular end faces engageable axially outward with the respective wedges. Thus when the bodies are drawn axially toward each other and the respective wedges are forced radially outward so that, when the arrays are flanking a joint and displaced radially outward, they coaxially center and lock together the pipe ends.