Established in 1952

Page: 2020

Introduction to the Ball Bar (Dumbbell)

	The Ball Bar (Dumbbell) is ultra precise and ultra simple, with the power to evaluate the performance of the entire Coordinate Measuring Machine System in just a matter of minutes. The first step is to understand the Ball Bar (Dumbbell) concept.
Figure 1., The Ball Bar (Dumbbell) ( shown with heavy duty stand and clamp, priced separately)

This tool consists of two ultra precise spheres of exactly the same diameters that are firmly attached to the opposite ends of a rather long rigid bar. The accuracy of the Ball Bar (Dumbbell) is only limited by the roundness of the two spheres and their common size. The ANSI-B89.4-1-1997 specification for "The Performance Evaluation of Coordinate Measuring Machines" requires that the quality of the Standard Ball Bar (Dumbbell) Spheres be round and common size within five millionths of an inch (.11 micrometers). The spheres for the Ultra Precise Series Ball Bar (Dumbbell) are held to two and one half microinches (.06 micrometers). For all practical purposes, these highest quality Ball Bars (Dumbbells) can be considered perfect artifacts (See Figure # 1.).

For proper evaluation, the Ball Bar (Dumbbell) must be held in a fixed position on the C. M. M. table by an extremely rigid Heavy Duty Stand (See Figure # 1.).

The function of the Ball Bar (Dumbbell) is so unique that it requires an open mind to really appreciate it. The center to center distance between the two spheres is absolutely fixed.

A significant characteristic of the Ball Bar (Dumbbell) is that it has no cosine or alignment errors at all as contrasted with gage blocks. This is because the length of the line that defines the distance between the centers of the two spheres is represented by the distance between two infinitely small center points in space.

When this tool is placed in various locations through out the envelope of the measuring machine, the sphere center to center distance should measure exactly the same. Any variation in the measurement of this distance indicates an error somewhere in the C. M. M. system.

This concept is referred to as a Volumetric check. The truly hard thing to accept is the fact that you are not even required to know the true center to center distance between the two spheres to effectively use the device. A knowledge of the exact sphere center to center gives the Ball Bar (Dumbbell) additional capabilities especially in evaluation of the scales.

Understanding the Implications of Various Deviations in the Ball Bar ( Dumbbell ) Length

Strangely, the first step in the Ball Bar (Dumbbell) process does not involve the Ball Bar (Dumbbell) at all, but uses the probe characterization sphere instead.

This first step is very important. The triggering characteristics of the probe head and the elastic deflections through out the machine caused by probe contact force must be compensated for (See Figure # 3.). Form errors in the contact tip must also be corrected for before any meaningful information can be derived from Ball Bar (Dumbbell) checks.

Before initiating the computerized probe characterization sequence, take a good look at the data derived from measuring the almost perfect characterization sphere. In most cases, the machine will report substantial deviations in both size and roundness of the master sphere. During the probe characterization sequence, the computer software will automatically compensate for roundness errors of the contact tip, elastic deflections throughout the machine and for lobbing in the triggering mechanism of the measuring probe.		Probe Characterization Sphere
		The real magic in Ball Bar (Dumbbell) evaluation comes from criss crossing the Ball Bar (Dumbbell). By measuring an angled Ball Bar (Dumbbell) placed right an angles to any axis of the C. M. M., and then simply rotating it's position 180 degrees and measuring it again, the squareness of that axis can be critically evaluated. By moving the same angled Ball Bar (Dumbbell) to the other side or to the other end of the table and measuring it again, rotational errors around each axis can be determined.
Ball Bar (Dumbbell) with probe

*See the specific technical data sheet for the current prices.

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