Kinematic and Elastic Characteristics of Couplings for Precision Fixturing

摘要

This paper addresses some of the issues involved in choosing a coupling system to allow for quick change fixturing on high precision lathes. Hirth couplings are shown to be too complex for closed form analytical evaluation of their performance. Based on manufacturers specifications, if used to hold a 254 mm (10 in.) hemispherical vacuum chuck, part wall thickness errors on the order of 3.1 mu m (120 mu in.) would result. In addition, the teeth are often too course for the principle of elastic averaging to outweigh the requirement for an extensive wear-in period. Thus their use would require all fixtures to be worn in simultaneously, and all chucks must be re-worn-in if a new chuck or machine is added to the group. A closed form analysis of a three groove Kinematic coupling is then developed and used to design a Kinematic coupling with 28.6 mm (1 1/6 in.) diameter balls and 6675 N (1500 lbf) preload. When subject to cutting loads on the order of 67 N (15 lbf), a 356 mm (14 in.) diameter coupling produces the following wall thickness errors in a 254 mm (10 in.) diameter hemispherical part: (1) a 0.13 mu m (5 mu in.) error from cutting force, and (2) a 0.38 mu m (15 mu in.) error from 4% preload error. If Silicon Nitride balls and grooves are used, fretting corrosion of the interface will not be a problem. The kinematic design is shown to be unaffected by film contamination and does not require an extensive wear-in period. 8 refs., 12 figs. (ERA citation 13:006234)