Established in 1952

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Three Dimensional Calibration Device
3D Ball Plate

The three-dimensional, 3D, ball plate is one of the oldest calibration devices for the evaluation of Coordinate Measuring Machines. It dates back to the beginning of the commercial use of C.M.M.'s, where it was used with hard probes (conical cups) to evaluate the machines' overall performance. There are three mating pairs of precision ground and lapped cylinders that are rigidly mounted in three precision-machined trenches in the bottom surface of the upper platform. These three pairs of cylinders form three Kinematic Vee Blocks. This upper platform containing the nine spherical masters can be removed and replaced any number of times, within less than one microradian. Building a practical three dimensional ( 3-D, 3D ) calibration artifact for modern Coordinate Measuring Machines is a tricky proposition. To be of any practical value, the artifact must be fairly large. It also must be rugged enough to be mechanically stable. This means that it must be able to withstand at least moderate handling and shipping abuse, but still be able to perform its task. It must remain dimensionally stable over long periods of time. The materials used must not twist or turn. They must not grow or shrink with time. This job is accomplished by erecting a series of nine very rigid vertical posts, of different heights, from the surface of a stiff, flat pallet. The standard post heights are 12 inches (305 mm), 6 inches (152 mm) and 2 inches (51 mm).

Base for Three Dimensional Ball Plate

On the top of each of the nine vertical posts is rigidly attached a very precise master sphere.

For use on machines with touch trigger, or other mechanical probes, a one-inch (25.4 mm) diameter sphere is normally used. These spheres are made of a very high chrome, high carbon, Martensitic stainless steel. This ultra fine grain material is hardened and thermal cycled to produce a minimum hardness of 58 HRC (Hardness on the Rockwell "C" scale). This special thermal cycle promotes long term dimensional stability.

All nine of the balls are precision lapped spherical and exactly same the diameter within five micro-inches (one hundred twenty five nanometers) tolerance.

When the C.M.M. probe used is a non-contact laser scanner of other optical device, the balls used are usually a satin finished white ceramic material. The reduced surface quality of these satin finished balls limits the sphericity and common diameter to twenty-five micro-inches (0.63 micrometers). Balls made of many other materials, and diameters, can be supplied on special order. Satin finished steel and titanium are two of the more common.

The rather large dimensions of the 3 D Ball plate, that are 12 inches (305 mm) by 16 inches (406 mm), requires an exceptional mounting method to avoid mechanical distortion. This is made more important, as any bending of the plate will cause serious first order errors in the spacing of the balls on the tops of the rather long posts.

In order to gain perfect repeatability of all the elastic deflections, the platform of the 3 D Ball plate is Kinematically mounted.

The base for this kinematic mount consists of a triangular aluminum casting that is rigidly attached to the CMM table. Three high quality, hard Martensitic stainless steel, spheres, are rigidly mounted at the corners of this triangle.

The Kinematicaly mounted upper platform is equipped with two sets of isolated handles to facilitate placing it in position on the triangular platform, without any personal contact with the metrology elements.

Even though the 3-D Ball Plate is rather large and very rugged, it weighs only 30.5 pounds (13.84 kilograms). This is well below the 44-pound O.S.H.A. limit, to be handled by a single technician.

The device can be custom built in other dimensional configurations. It can be configured with taller or shorter posts. It can also be built on a larger or smaller pallet. It can have a totally different aspect ratio, so that it is long and narrow.

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