Established in 1952

Page: 1860 Technical Data Sheet: CMM-10, Page 1

Adjustable Ball Bars for CMM Evaluation

Bal-tec adjustable ball bar ( dumbbell ) kit

The concept of the Ball Bar is almost too simple. Two very round, dimensionally matched spheres are securely attached to opposite ends of a rather long rigid bar. The center to center distance between these two spheres will remain constant regardless of what X-Y-Z position the bar may be held in. In practice, the center to center distance between the two spheres is measured many times while the Ball Bar is relocated in a variety of positions throughout the Coordinate Measuring Machines dimensional envelope. Any variation in these measurements indicates an error in the CMM System.

The Adjustable Ball Bar Kit is made as versatile as possible. A complete eight piece kit will allow for assembly of Ball Bars with sphere center to center dimensions from 100mm (3.937 inch ) to 1650mm (64.96 inch), in 50mm (1.969 inch) increments.

The precision spheres incorporated in the adjustable Ball Bar are made of ultra fine grain, high chrome, high carbon, stainless steel. They are hardened to 58 Rc for wear-resistance and ultra cold-cycled for long term dimensional stability. The quality of the spheres meet all of the requirements for Performance Evaluation of Coordinate Measuring Machines ANSI/ACME B89.4.1-1997. They are five times as accurate as high quality bearing balls, They are precision lapped spherical within 5 millionths of an inch (127nm) with a maximum surface roughness of 0.5 micro inches (12.7nm) Ra.

These precise spheres are not just glued on to the mounting posts. A steel pin is inserted into a deep, small diameter, hole in the sphere and in the post as the high strength glue is applied during assembly. The result is an exceptionally strong bond between the sphere and the post. The dimension from the flat end of the post to the sphere center is 50mm (1.969 inch).

There is a 3/16 inch ( 0.1875", 4.7525 mm ) diameter clearance hole drilled through the post and through the cylinder of each extension bar. We supply high tensile stainless steel pins that are used as wrenches to assemble and remove the various components. The posts and the extension ends are provided with an attractive black oxide finish which resists corrosion.

The 50mm (1.969 inch) extension is machined from a solid steel bar. The extensions from 100mm (3.937 inch) to 800mm (31.496 inch) are an assembly. Two rugged steel plugs that are tapped to accept a 5/16 inch (7.9mm) threaded stud are joined to a 3/4 inch (19.05mm) heavy walled steel tube. This provides a very stiff light weight assembly. This steel tubing is electroless nickel plated to resist corrosion.

From a broad perspective, there are two ways to support a Ball Bar for CMM evaluation. The first and by far the simplest is the free standing Ball Bar. The center of the Ball Bar is clamped to the vertical post of a gage stand and the position of the spheres are measured a large number of times as the gage stand is moved to various locations within the measuring envelope of the CMM. This method is simple and inexpensive, but its accuracy is somewhat limited.

The force applied by the CMM measuring probe bends the shaft of the Ball Bar slightly; but the long unsupported lever formed by the Ball Bar twists and bends the vertical post of the support stand a considerable amount. Instead of causing a simple cosine error in the position of the spheres as might be expected, the Ball Bar is very unevenly deflected. The maximum deflection occurs perpendicular to the axis of the bar and is almost zero at the very ends. This condition is interpreted by the computer's spherical fit algorithm as a Ball Bar with smaller diameter spheres that are positioned further apart. This condition can be minimized by using a very robust vertical stand such as the "FS" series (see Technical Data Sheet CMM 2) or it can be all but eliminated by supporting the Ball Bar at its ends. The very rigid support is accomplished by using the truly cylindrical diameter of the optional accessory, ABB-C-PB-100. This cylinder is made of the same high quality, very fine grain, hardened stainless steel as the spheres. The central area of this cylinder is precision lapped straight and round within 5 micro inches (127nm). This excellent cylinder makes intimate contact with the two balls of a Kinematic coupling (Part Number 2 B-KM- Technical Data Sheet CMM3), so that not a single micro inch of backlash or hysteresis remains.

An extra feature of this coupling is a safety tether which consists of a loop of plastic covered steel cable that prevents a crash from sending the Ball Bar bouncing across the CMM table.

The Ultra Precise Test Sphere at the opposite end of the Ball Bar is supported directly in the nest formed by the three .750" (19.05mm) hardened stainless steel balls of the Kinematic Platform (Part Number 3B-KM, see Technical Data Sheet CMM-4). This platform can be solidly clamped to the CMM table through either a 3/8 inch or 10mm diameter socket head screw.

We strongly recommend that three of the post mounted precision spheres (Part Number ABB-PB100) be purchased at the time the set is acquired. The adjustable Ball bar requires assembly in the field where there is a high risk of damaging one of the spheres. Although we carry a good inventory of replacement spheres in stock, several days could elapse in their transportation to your site.

Each component in the Adjustable Ball Bar line is priced separately so that special combinations may be purchased to satisfy specific needs. When the main need is for evaluating very large gantry machines, several of the 800mm (31.496 inch) sections may be desirable. If the set is transported frequently, consider having the set broken down into several shorter bars.

Available Components for Adjustable Ball Bar Kit

In addition to the standard components listed, a wide variety of custom made items can be economically produced. Some of the more common of these include extra long extensions made of larger diameter tubing for greater rigidity, spheres of different diameters or materials, kits made of special materials to meet specific thermal expansion characteristics, and magnetic sockets.