Established in 1952		Page: 3070
		CMM-23

Portable Arm Coordinate Measuring Machines Calibration

For reliable evaluation of the Portable Arm Coordinate Measuring Machines, rather large calibration artifacts are required. Although a number of different length artifacts have been supplied the industry is trying to standardize on 2.3 meters ( 90.55"). These large calibration devices must remain dimensionally stable during the evaluation sequence. This dimensional stability must be physical stability. By this we mean that the ball bar must not distort mechanically when the test probe actually makes physical contact with the calibration object.

This mechanical stiffness must be maintained on these rather large devices but at the same time they must still be light weight enough for portability in the field and so that a single technician can manipulate them through the 20 to 35 positions required to meet the ANSI B89.4. 1-1997 specification for the "Performance Evaluation of Coordinate Measuring Machines".

This rather daunting problem is further complicated by the fact that large objects are very sensitive to small variations in temperature. An eight foot (96-inch) [two point four meter] long aluminum bar will expand more than one thousandths of an inch (.025mm) for each degree F. of temperature rise. For steel it is over one half of a thousandths of an inch (.013mm) per degree F.

The same eight-foot (two-point four-meter) structure made of Invar® will only expand sixty-five microinches (1.7 micro meters) per degree F.

The realization of lightweight, good rigidity and low temperature sensitivity are all achieved by building a composite structure of a series of round tubular Invar™ members, clamped together by a number of robust bulkheads. The tubular structure provides the stiffness and the lightweight while the ultra low expansion properties of Invar¨ solves the temperature sensitivity problem.

This large device must be supported on a metrology platform with the same rigidity, light weight and low temperature sensitivity as the artifact itself, so we have devised a Metrology Tripod with the very same design features as the Portable Arm C.M.M. Calibrator, see our Technical Data Sheet CMM-24. The large tubular Invar¨ structure is supported on the tripod by a ball bearing turret that allows it to be easily positioned at any angle from vertical to horizontal.

There is another unique problem in evaluating the performance of a portable arm C.M.M. When using a hard probe it is difficult to make a single only contact with a small calibration artifact. This is even more difficult, when probing a sphere, which is the standard A.N.S.I. artifact. The machine has difficulty identifying one only point on an infinitely varying target. A unique way to cope with this problem while still using a spherical target has been developed. A very precision sphere of rather large diameter between 0.59-1.00 inch ( 15mm to 25mm ) is used as the measuring probe. A Three Ball Kinematic Coupling is rigidity mounted on the face of each of the four bulkheads that hold the Invar™ tubing together and a fifth is mounted on the top center of the turret. By placing the large probe sphere in the Three Ball Kinematic Coupling, a single only point in three-dimensional space is described. In order to make the hit more positive the Three Ball Kinematic Coupling has a light magnetic preloading.

The Kinematic Coupling is made up of three high grade, five sixteenths of an inch ( 5/16", 0.3125", 7.9 mm ) diameter tungsten carbide balls. Each of these balls have a deep hole drilled in them. At assembly they are glued, over a high shear strength pin, into individual spherical cavities in the face of the bulkheads. This design provides maximum strength with great shear resistance and a very thin glue line that prevents hygroscopy due to moisture absorption.