Preliminary Investigation on Generating an Explicit GD&T Scheme From a Process Plan

Geometric Dimensioning and Tolerancing (GD&T) Standards have established a language for clear and concise specification of dimensional and geometric variations on manufactured parts. The language includes symbols for tolerance type, tolerance value, datum and reference frames, diameter and material condition modifiers and associativity with geometric entities. Designers use the standard to communicate their dimensional specifications to manufacturing and inspection personnel. However, process planners appear to be less formal in how tolerances are represented in process plans. Typically, they are shown only as dimensional plus/minus values. Datum Reference Frames (DRF) and geometric tolerance symbols are absent. It is believed that the latter are implicit in the set-up and fixturing prescribed in the plan. In this paper we explore how one might extract the implicit information systematically. The motivation for this effort is to verify the consistency of manufacturing tolerances with design specs and to be able to use the same tolerance analysis tools used in design. We discuss three research issues: extracting implied DRFs from set-ups and fixtures; converting plus/minus tolerances to appropriate geometric tolerances; and dealing with transient features — which are features that do not exist on the finished part used for GDT specs by the designer. We propose a new data structure, PCTF (process oriented constraint tolerance feature graph) to facilitate mapping between design and manufacturing tolerances.