A keyed dop chuck is a lapidary holding device in which the dop stick locks into the chuck in only one rotational position by means of a key, slot, or pin arrangement. The mechanism allows the cutter to remove the stone from the faceting machine, return it later, and resume cutting in exactly the same orientation. It is a small refinement of the basic dop chuck but a consequential one for any work requiring multiple sessions or tooling changes.

Function

Faceting demands precise indexing in three axes: rotation around the dop, tilt against the lap, and angle of the dop stick within the chuck. The keyed chuck addresses the third by ensuring the dop is replaced in only one orientation. Without the key, a small rotation between sessions would shift the index of every facet by a corresponding amount, requiring re-establishment of the reference point.

Variants

Common implementations include a flat machined onto the dop stick that engages a corresponding flat in the chuck, a pin-and-hole arrangement, or a hexagonal cross-section. Modern faceting machines from manufacturers such as Facetron, Ultra Tec, and OMF use proprietary keyed systems that are not interchangeable across brands. Cutters working with multiple machines therefore standardise on one system or accept the rework involved in transferring stones.

Use in practice

The keyed chuck is essential for faceters who polish a stone in stages or who must change cutting laps mid-stone. It is less critical for cabochon work, where indexing is freehand. Properly used, it eliminates a substantial source of error that would otherwise require time-consuming realignment.