Design of quasi-kinematic couplings

A quasi-kinematic coupling (QKC) is a fixturing device that can be used to make low-cost assemblies with sub-micron precision and/or sealing contact. Unlike kinematic couplings that form small-area contacts between mating balls in v-grooves, QKCs are based on arc contacts formed by mating three balls with three axisymmetric grooves. Though a QKC is technically not an exact constraint coupling, proper design of the contacts can produce a weakly over constrained coupling that emulates an exact constraint coupling. This paper covers the practical design of QKCs and derives the theory that predicts QKC stiffness. A metric used to minimize over constraint in QKCs is presented. Experimental results are provided to show that QKCs can provide repeatability (1/4 μm) that is comparable to that of kinematic couplings.