Part No.: 2B-KM

A Ball Bar (Dumbbell) consists of two very round, exactly matched spheres which are securely attached to opposite ends of a rather long rigid bar. The exact distance between the two spheres remain constant while the Ball Bar (Dumbbell) is measured in various positions throughout the X-Y-Z envelope of a Coordinate Measuring Machine. Using this inexpensive tool, an entire CMM system can be quickly and accurately evaluated.

In order to improve the performance of the Ball Bar (Dumbbell), a two ball kinematic coupling was developed. This device provides a rigid support near the very end of the Ball Bar (Dumbbell). The cylinder of the Ball Bar (Dumbbell) rests in a cradle formed by two precision balls. This support eliminates the large bending and twisting errors that normally occur when only the center of the Ball Bar (Dumbbell) is supported. These errors are caused by a multiplication of the contact force of the measuring probe deflecting long unsupported sections of the Ball Bar (Dumbbell).

This Ball Bar (Dumbbell) holding device consists of two very precise, fine grain stainless steel balls that are hardened to 58 Rockwell C for wear resistance. The two balls are coupled together and rigidly connected to a support.

In addition to the obvious cradle formed by the two balls when the coupling is at right angles to the cylindrical bar, the support stand can be rotated more than 30 degrees in either direction, and it will still render the same micro inch accuracy.

There are three different, alternative, support structures to choose from.

The first alternative supporting structure (Part Number 2B-KM) is a simple cylindrical shaft. The two balls are held on the end of this robust cylinder by a socket head cap screw. This screw passes through concentric holes drilled in the balls. This cylinder in turn is held in the head of one of the Quick Grip Vertical Stands (see Technical Data Sheet CMM-6, Part Numbers QG-14, QG-24 or QG-36).

The second alternative is similar to the first, but the two balls are coupled to a powerful magnet by the socket head cap screw. This magnetic base supports the two ball coupling and the coupling supports the Ball Bar (Dumbbell). This inexpensive magnetic coupling, when used with any ordinary shop quality angle plate forms a very rigid deflection-resistant support for the upper end of the Ball Bar (Dumbbell). (Part Number 2B-KM-M).

The third alternative version of this product (Part Number 2B-KM-H) is made to fit into a height gage or transfer gage stand. Although this type of stand is not as rugged as would be desired, it does form a convenient system that is far superior to supporting the Ball Bar (Dumbbell) by its center. The standard dimensions of the clamping shank is 0.25 inch (6.35mm) wide by 0.5 inch (12.7mm) high. Oversize blanks are kept in stock so that they can be quickly machined to provide custom sized shanks.

The couplings are provided with a safety tether consisting of a loop of plastic covered steel cable that prevents a crash from sending the Ball Bar (Dumbbell) bouncing across the CMM table.

The sphere is supported in the nest of a three ball kinematic platform (see Technical Data Sheet CMM-4, Part Number 3B-KM ) for improved rigidity.

At first glance, it may not be obvious, but these two simple devices have eliminated five of the six degrees of freedom for the Ball Bar (Dumbbell) while giving it very rigid support but without sacrificing one microinch of distortion due to over-constraint from rigid clamping.

For more information on Ball Bar (Dumbbell) evaluation see ANSI/ASME-B89.4.1- 1997 for Performance Evaluation of Coordinate Measuring Machines.