The development and implementation of process-control plans for the manufacture of threaded components is complex. Used in a wide variety of components from aerospace to automotive applications, the screw thread is one of the more difficult characteristics produced in general manufacturing operations. In many instances, the breakdown of process validation has its origin in the design phase. Incomplete product details and unrealistic tolerances are all too common.The net result increases production costs, extends delivery, and complicates the entire manufacturing process. As a manufacturer and designer of both standard and special inspection systems, our company is often required to manufacture special out-ofthe- box gaging solutions for unique requirements, when a simple design modification will permit use of standard off-theshelf gaging. Occasionally, complex designs coupled with excessively stringent dimensioning produce a part incapable of quality verification at any cost. The old adage “keep it simple” should be a design standard.



As products evolve from design to manufacture, the challenges increase. A first step in the development of a processcontrol plan is a complete understanding of the potential sources of variation. Unlike most manufacturing characteristics, the geometry defining the thread form defies easy solutions. Variation is not limited to change in the pitch diameter, but also to the interdependence of size and form variation as indicated by the functional size. Defined as the cumulative effect of profile variation, functional size includes the entire effect of thread-angle and thread-lead error on the pitch diameter size, and is related to the ability of a thread to assemble. In any control plan, it’s critical to control and minimize the difference between the functional and pitch-diameter size as a means of variability reduction.

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