The Ball Bar (Dumbbell) Thing

The Ball Bar (Dumbbell) Thing (Part # BB-TH-1) is a simple device that was developed primarily as a Monday morning check for evaluating the "Performance of Coordinate Measuring Machines" in accordance with ANSI B89.4.1-1997 specification. It answers the need for a very fast, dependable method of checking the interim performance of these complex machines.

The importance of an interim checking device was brought home by an Air Force study, indicating that the majority of C.M.M.s are not functioning properly at the time of their annual recalibration.

The Ball Bar (Dumbbell) Thing consists of a very rigid aluminum base ring that holds two high precision Cantilever Ball Bars (see Technical Data Sheet C.M.M.-16, Page 1.). Each Ball Bar (Dumbbell) consists of two ultra precise spheres of exactly the same diameter securely fastened to opposite ends of a rather long rigid bar.

One Ball Bar (Dumbbell) is held horizontal and the second is inclined at an angle. They are indexed at 90 degrees to each other.

The base ring rides against the top surface of the C.M.M. table on three hardened stainless steel feet that are lapped flat and coplanar.

There are eight 3/4 inch (1.9cm) diameter precision spheres spaced every 45 degrees around the periphery of the base ring. In use, alternate pairs of these spheres are located in a Kinematic Coupling consisting of a magnetic Vee-Block formed by two hardened stainless steel cylinders and the hard flat face of a rare earth magnet.

Although a length calibration of the Ball Bar (Dumbbell)s is not essential, it does give an added level of certainty to the evaluation. A certification of the exact dimension of the sphere center to center can be provided, with traceability to N.I.S.T.

To resist any deflection caused by the contact force of the C.M.M. measuring probe, the support spheres of these Ball Bar (Dumbbell)s are held kinematically on robust aluminum rails. One of the support spheres sits in a precision three ball kinematic cup at one end of the aluminum rail while the other is supported in a cradle formed by two precision cylinders at the other end. There is a powerful rare earth magnet in the center of each of the kinematic couplings that hold, the Ball Bar (Dumbbell) rigidly in position.

The robust aluminum rails are rigid supporting devices only and the high rate of their thermal expansion is not involved in the measurement of the Ball Bar (Dumbbell)s.

There is no air pressure required for this device. There are no air lines, air cylinders or air valves involved.

In use, the aluminum base ring is simply rotated by hand to locate sequentially in each one of the eight positions. The heat of hand contact is thoroughly isolated from the Ball Bar (Dumbbell)s.

The end result of the horizontal and the 45 degree Ball Bar (Dumbbell)s being indexed to the eight positions is the generation of a three dimensional Ball Plate with thirty two physical coordinates.

The Kinematic Vee-Block and the hardened magnetic plane that are used to locate the device are rigidly attached to an aluminum bar (Part # BB-TH-KB) that is solidly clamped to the top surface of the measuring machine table. This aluminum bar is built so that is can be bolted down in the same location on the C.M.M. table every time the "Ball Bar (Dumbbell) Thing" is used. The table hole patterns of several different C.M.M. can be machined into a single aluminum bar or additional bars (Part # BB-TH-KC) are available separately.

On Direct Computer Controlled C.M.M.s the locations of both Ball Bar (Dumbbell)s, in all eight positions, can be programmed into the computer memory one time so that all future measurements will be carried out automatically.

The proper use of Ball Bar (Dumbbell)s for the "Performance Evaluation of Coordinate Measuring Machines" is outlined in the ANSI B89.4.1-1997 specification. This is available from the American National Standards Institute. 25 West 43rd Street, 4 floor, New York, NY 10036, USA
Tel: 1.212.642.4900, Fax: 1.212.398.0023

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