Expansible mandrel

An expansion mandrel for forming an internal annular groove in a tubular element of a synthetic plastic material comprising first and second groups of plate segments being positionable alternately in side-by-side relation in the expanded position, said first plate segments being larger than said second plate segments, means on said mandrel for mounting said first group of plate segments for movement inwardly and outwardly in a radial plane of the mandrel, an operating rod axially reciprocable along the longitudinal axis of the mandrel, a conical portion on said rod engageable with said first plate segments to cam said first segments radially outwardly into the expanded position upon axial movement of said rod in a first direction, a plurality of wedge-shaped elements on said rod subsequent to said conical portion with respect to said first direction of movement and having inclined surfaces thereon with dove-tail guides therein slidably engaging said second segments, said second plate segments being movable axially and then radially, and stop means on said mandrel for stopping the axial movement of said second segments in the said radial plane of said first segments so that axial movement of said rod in said first direction will cam said second segments radially outwardly into the expanded position in alternate side-by-side relation with said first segments.