Predicting Process Capability with Variable Geometric Tolerance Specifications
Publication Date: 2002-Oct-17
The IP.com Prior Art Database
Current statistical formulas used to predict process capability from continuous data regard all dimensional specifications as having constant tolerance while many of them are variable. Geometric tolerances and datum feature designations followed by the tolerance modification symbols ? or ? permit a variable "bonus" or "datum-shift" tolerance based upon feature size. Process capability predictions from discreet data (measurement with attribute gauges) include the variable portion of tolerance in the gauge design but predictions from continuous data (measurement with variables gauges) ignore the variable portion of tolerance. The disparity betweent he capability prediction methods affect product acceptance and rejection, process precision requirements, gauge preferences, process optimization (targeting feature sizes), and design practices (designation of modifiers and the proportioning of variable tolerance between size and geometric form, orientation, or location). Since most quality control procedures require continuous data from variables gauges for manufacturing process analysis and control the problem is common throughout the manufacturing industry. This paper presents a solution to the problem of predicting the process capability of variable geometric tolerances. It also demonstrates the rationale for targeting feature size to optimize the process performance of geometric specifications and it cautions against the designation of tolerance modification symbols for reasons other than static fit, clearance, or boundary conditions (gauging preferences should not drive modifier selection).