Rotary table for a coordinate measuring machine and method of determining the axis of table rotation

Claim 1

In combination, a coordinate measuring machine (10) having a probe tip (18) movable along three orthogonal axes (X,Y,Z) within a measuring envelop, distance transducers (24) generating electrical signals corresponding to said movement to enable measurements to be made by moving said probe tip (18) between points on a workpiece (W), a workpiece support table (12) having a top plate (36) supported on a table base (40), said table top plate (36) providing a supporting surface (20) for a workpiece (W) located within said measuring envelop to enable access by said probe tip (18), bearing means (116, 128, 136) supporting said table top plate for rotation about an axis (a) and drive means (38) for rotating said table top plate (36) about said axis characterized by: a retractable datum ball assembly (140) including a locating feature (144), means (142) fixing said datum ball assembly (140) relative to said table base (40) adjacent said table top plate (36), means (146) for selectively extending and retracting said locating feature (144) between an up position above said table surface (20) and within said measuring envelop and a down position below said table surface (20) and outside said measuring envelop, said locating surface (144) positioned to be accessible by said probe tip (18) when in said up position; and locating means (178, 182) acting on said datum ball assembly to locate said locating feature in space within said measuring envelop above said table in a predetermined, precisely repeatable relationship with said table axis of rotation whenever said locating feature is extended to said up position.

Claim 12

A method of determining the axis of rotation of a table top plate (36) rotatably supported on a table base (40), said table top plate (36) having an upper surface (20) for supporting a workpiece (W) for measurement thereof within a measuring envelop defined above said table upper surface by a coordinate measuring machine (10) comprising the steps of fixing a datum feature (144) with respect to said base (40) by a bracket (142); determining the location of said feature relative to said axis of rotation (a); extending said datum feature above the level of said surface (20) when said axis of rotation (a) must be determined to an extended position precisely located in the space above said table within said measuring envelop, and measuring the location of said datum feature (144) at said extended position within said measuring envelop, to determine the location of said axis of rotation (a); and retracting said datum feature (144) out of said measuring envelop when measurements are being conducted on said workpiece (W).

Claim 13

A method of determining the axis of rotation of a table top plate (36) rotatably supported on a table base (40), said table top plate (36) having an upper surface (20) for supporting a workpiece (W) for measurement thereof within a measuring envelop defined above said table upper surface by a coordinate measuring machine (10) comprising the steps of fixing a datum feature (144) with respect to said base (40) by a bracket (142); determining the location of said feature relative to said axis of rotation (a); extending said datum feature above the level of said surface (20) when said axis of rotation (a) must be determined to an extended position precisely located in the space above said table within said measuring envelop, and measuring the location of said datum feature (144) at said extended position within said measuring envelop, to determine the location of said axis of rotation (a); and retracting said datum feature (144) out of said measuring envelop when measurements are being conducted on said workpiece (W).