Established in 1952

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Technical Data Sheet: CMM-1, Page 1		per ANSI / ASME B89.4.1-1997
Ball Bar (Dumbbell)s with shipping case		Accuracy The Ball Bar (Dumbbell) is the most accurate long gage device ever developed. The Ball Bar (Dumbbell) has no moving parts, it generates no heat, uses no magnetic fields and has absolutely no friction, stiction or hysteresis. Cost Ball Bar (Dumbbell)s cost only a small fraction of their more complex but far less accurate counterparts, such as long gage blocks.

The Concept

The concept of the Ball Bar (Dumbbell) is almost too simple. Two very round dimensionally-matched spheres are securely attached to opposite ends of a long rigid bar. The distance between these spheres remains constant regardless of the attitude of the bar. In the Free Standing Ball Bar (Dumbbell) system, the Ball Bar (Dumbbell) itself is clamped at its center to a rigid vertical post and the position of each of its two spheres is measured every time the position of the Ball Bar (Dumbbell) is changed.

The extremely fine-grained stainless steel spheres used are 1.00 inch (25.4 mm) diameter. They are hardened to 58 HRC for long wear and ultra-cold cycled for long-term dimensional stability. They are held spherical within 5 microinches (127 nm) and both balls are exactly the same size within 5 microinches (127 nm). This meets the quality requirements for “ Performance Evaluation of Coordinate Measuring Machines” ANSI / ASME-B89.4.1-1997. They are five times more accurate than high quality bearing balls.

Total System Evaluation

In use, measurements of the distance between the two spheres are made with the Ball Bar (Dumbbell) positioned at various X-Y-Z locations throughout the C.M.M.'s volumetric capacity. From these data, the overall accuracy of the C.M.M. system may be determined. The C.M.M. system consists of the geometric or metrology frame, the test probe, the scale system, the computer and its software—plus the environment within which the system operates, including the temperature, vibration and the utilities. Because all of these elements are factored in by the Ball Bar (Dumbbell), it is a truly functional test of the entire C.M.M. system.

Using this approach, the test probe actually contacts the spheres. The system actually senses the probe trigger point, it records the scale position at that instant and then the computer calculates the true size and position of the spheres. Any errors of the C.M.M. system will cause erroneous readings or variations in the distance between the two spheres in the various positions.

Multiple Ball Bar (Dumbbell)s

For ideal evaluation—and to help isolate errors—several lengths of Ball Bar (Dumbbell)s are desirable. The longest Ball Bar (Dumbbell) for a given calibration is usually 80% of the longest axis of the C.M.M. Because of sag, Standard Ball Bar (Dumbbell)s are seldom made over 39.37 inches (1000 mm) long. Giant Invar Ball Bar (Dumbbell)s of composite design are the best choice for longer Ball Bar (Dumbbell)s.

The cause of significant errors can be separated by systematically orienting the Ball Bar (Dumbbell) or Bars in particular patterns such as in-line with one axis or another to isolate a specific cause.

Design

The precision stainless steel spheres are attached to the ends of the bar by gluing. A steel pin is inserted into a hole in the sphere and in the bar as the high strength glue is applied during assembly. The result is an exceptionally strong bond between the sphere and the bar.

To provide a stiff, lightweight assembly, the bar itself is made of 3/4 inch (19.05 mm) diameter heavy walled steel tubing.

This steel tubing is electroless nickel-plated to avoid corrosion.

Temperature

Ball Bar (Dumbbell)s are much longer than most familiar gages and are therefore far more susceptible to temperature influence. Our Standard Ball Bar (Dumbbell)s have the same coefficient of thermal expansion as most steel and iron (6.3 microinches per inch per degree Fahrenheit, 11.4 micrometers per meter per degree Celsius).

The most frequent source of rapid thermal change is caused by the machine's operator handling the Ball Bar (Dumbbell). Even when wearing well insulated gloves, the Ball Bar (Dumbbell) should be handled as little as possible. The Ball Bar (Dumbbell) should be allowed to soak in the C.M.M. environment for 30 minutes for ordinary quality machines and two hours before taking any serious measurements.