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

• US 4,958,438 A
• Filed: 03/30/1989
• Issued: 09/25/1990
• Est. Priority Date: 03/30/1989
• Status: Expired due to Fees

• Current Assignee: Warner & Swasey Company Sheffield Measurement DIV
• Original Assignee: Warner & Swasey Company
• Inventors: Hemmelgarn, Thomas L.
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US07/332,077
• Time in Patent Office: 544 Days
• Field of Search: 33/1 M, 33/503, 33/504, 33/559, 33/561, 33/569, 33/572, 33/556, 33/558, 74/208, 74/813 R, 74/813 C
• US Class Current: 33/503
• CPC Class Codes: G01B 5/0004 Supports G01B5/025 takes precedence

Abstract

A rotary workpiece support table (12) for a horizontal arm coordinate measuring machine (10) having a retractable datum ball (144) for determining the axis of rotation of a table top plate (36) on a table base (40). A frictional drive mechaism (38) has a drive wheel (72) urged into engagement with a perimeter surface (74) and rotates the table top plate (36), with a counterforce roller (98) neutralizing the side loading on an air spindle (116). The table base (40) is positioned on a machine base (30) and spacer (32) by a rotational connection (46) and a diametrically opposite block and cam follower connection (48) to avoid the effects of differential thermal expansion between the table base (40) and the machine base (30) and spacer (32).

Claims

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.

2. The coordinate measuring machine according to claim 1 wherein said means (146) for extending and retracting includes a vertically mounted power cylinder means having a piston (158) and a cylinder (146), said piston (158) mounted in a piston bore (160) in said cylinder (146) with a piston rod (166) attached to said piston (158), said piston rod (166) having a ball (144) attached comprising said feature (144).

3. The coordinate measuring machine according to claim 2 wherein said locating means (178, 182) acts between said piston (158) and said cylinder cap (148) with said piston (158) in said up position, to precisely locate said feature (144) in space above said cylinder (146).

4. The coordinate measuring machine according to claim 1 further including temperature sensor means (186) generating electrical signals corresponding to ambient temperatures, and means (26) to compensate for changes therein causing a change in correspondence between said datum feature (144) and the location of said axis of rotation (a) of said table top plate (36).

5. In combination, a coordinate measuring machine (10) having a probe tip (18) movable along three orthogonal axes (X,Y,Z), distance transducers (24) generating electrical signals corresponding to said movement along each axis to enable measurements to be made by moving a 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 to enable access by said probe tip (18), bearing means (116, 128, 136) supporting said table top plate (36) for rotation about an axis (a) and drive means (38) for rotating said table top plate (36), characterized by said bearing means (116, 128, 136) comprising an air spindle (116) received in said table tip plate (36) rotatably guiding said table top plate (36) and air supply means (128) for introducing an air flow over said air spindle (116), and wherein said rotary drive means (38) includes a rotary drive member (72) rotatable about an axis of rotation parallel to said axis of rotation of said table and continuously forced laterally against a cylindrical perimeter surface (74) of said table top plate (36);

and, a counterforce roller (98) rotatable about an axis parallel to the axis of rotation of said drive member continuously urged into engagement with said perimeter surface (74) at a diametrically opposite location from said rotary drive member (72).

6. The coordinate measuring machine according to claim 5 wherein said rotary drive member comprises a friction wheel (72) frictionally engaged with said perimeter surface (74).

7. The coordinate measuring machine according to claim 6 further including spring means (90, 92) for applying an adjustable spring force urging said drive member (72) into engagement with said perimeter surface (74) and spring means (100, 102) applying adjustable spring force urging said roller into engagement with said perimeter surface (74), said spring means (90, 92) also allows for slight radial movements to accommodate out of roundness in the drive surfaces.

8. In combination, a coordinate measuring machine (10) having a probe tip (10) movable along three orthogonal axes (X,Y,Z) distance transducers (24) generate electrical signals corresponding to said movement to enable measurements to be made by moving a probe tip (18) between points on a workpiece (W), a workpiece support table (12) having a ductile iron top plate (36) supported on a ductile iron table base (40), said table top plate (36) providing a supporting surface (20) for a workpiece (W) located 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) characterized by a machine base (30) including a granite upper portion (32) and means for attaching said table base (40) to said granite upper portion (32) of said machine base (30), comprising a rotary connection (46) and block and cam follower connection (48) composed of elongated block (56) and straddling rollers (57) each mounted to either said table base (40) or said machine base granite upper portion (32) respectively at diametrically opposite locations on said machine base granite upper portion (32), whereby differential thermal growth between said table base (40) and said machine base granite upper portion (32) are accommodated.

9. 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).

10. The method according to claim 9 further including the steps of sensing the ambient temperature of said datum feature (144) and bracket (142), and compensating for changes in relationship between the location thereof and said axis of rotation (a) caused by changes in ambient temperatures.