Bal-tec™ Home A Typical Kinematic Application
The three balls sit in three vees that are formed by the six large diameter tungsten carbide cylinders. One surface of these precision lapped spheres and cylinders was diamond ground flat. A good example of a Kinematic coupling that doesn't restrict all six degrees of freedom is the Cantilevered Ball Bar. This device is used to evaluate the performance of Coordinate Measuring Machines. A Ball Bar consists of two very round spheres of exactly the same size securely attached to the opposite ends of a rather long rigid bar. The Cantilever Ball Bar design has additional support spheres just behind the master evaluation spheres. These two support spheres are held in Kinematic couplings that form an extremely rigid non-constraining coupling. The first support sphere sits in the Kinematic cup formed by three spheres. The second one sits in the cradle formed by two precision cylinders.
Just how accurately will a Kinematic coupling repeat? We attempted to answer this question once and for all. If we accept the inter-atomic dimensions of a steel molecule at about 3 Angstroms, the outcome of our test places the number at about ten molecular dimensions or one nanometer, but like so many other state of the art attempts there are some funny things going on. The absolute measurement is time dependent, indicating that the surface of the steel part is viscoelastic. A simplistic explanation might be that the atomic binding forces are lower at the surface so it is more compliant i.e. it actually has a much lower modulus of elasticity.
Another explanation might be that the non contact capacitance gage used for the test is causing local heating of the test part, thus causing a local swelling of the surface. The sinking is linear with time until it stops so the heating explanation doesn't seem to fit. The kinematic mount used in this test consisted of two spherical diamond contacts of .125 inch (3.2mm) radius placed at a 120 degree angle to each other. The third contact was a flat gold plate. The sensor was a capacitance gage placed between the two diamond spheres. The test sphere was a hardened and lapped 1.00 inch (25.4mm) diameter stainless steel ball with a surface texture of .26 microinches (about 7 nm) Ra (Arithmetic Average). The sphere was raised up out of and lowered back down into the Kinematic coupling by a robot.