An expandable mandrel for holding a part has a shank with inclined radial slots slidably engaged by keys. The keys are coordinated by a collar which engages an extension on each key. A platen affixes to the shank of the mandrel to provide a stop that will index the position of a part mounted onto the mandrel. The platen has radial key passages to allow longitudinal movement of the keys in the slots. The platen preferably has resilient indexing nubs to engage the part. Preferably, the position of the platen on the mandrel can be adjusted by a set screw engaging one of several indexing holes or engaging a longitudinal guide track. For machining in a wet environment, the slots can be provided with longitudinal rails and the keys provided with transverse channels to remove liquid from between the keys and the slots.