Abstract: A mandrel for forming an annular groove in the bell of a plastic pipe which comprises a plurality of identically formed segments. The segments are radially urged identically from an initial position which is inwardly of the bell to an expanded position wherein the segments cooperate and interact to form an expanded ring of diameter greater than the pipe bell to thereby impress a circular groove therein. An actuating shaft rotatively reciprocates in unison a pair of cam plates, which cam plates are characterized by a plurality of arcuately curved guide slots formed therein. The segments include follower pairs which respectively ride within the guide slots to urge the segments from their initial positions to their expanded positions and back upon rotative reciprocation of the cam plates.