Concurrent dimensioning for accuracy and cost

Abstract

The Cartesian coordinate system used for the geometrical, dimensional and accuracy description of a part is critical for programming CNC machining and achieving the required tolerances. Datum surfaces, fixture design, workpiece positioning and orientation in the machine tool workspace are directly influenced by the system which is adopted. On the other hand, constraints imposed by the available machining axes accuracy capabilities (i.e. accuracy of the available machine tools for the particular job) and the always present minimum machining cost requirement cannot be readily incorporated into the relevant considerations.

In mechanical design, dimensional chains are frequent and observation of the specified tolerances — in conjunction with the geometrical ones — guarantees the part functionality. Definition of the workpiece zero point and the Cartesian coordinate system for CNC machining and/or measurement (CMM) and tolerance transfer to this system of the functional tolerances under the constraints mentioned is, as the industrial practice has shown, a difficult, tedious, experience demanding and time consuming task.

In the paper an approach which has been developed for a systematic handling of this important engineering problem is presented. It is shown that the proposed algorithm and its CAD implementation constitute efficient tools for the analysis of the possible part dimensioning solutions. The designation of that zero workpiece point (ZWP) and Cartesian coordinate system which provide for minimum machining accuracy cost and for acceptable tolerances lying well within the machine tool accuracy capabilities is thus achieved. The complete methodology is further demonstrated and discussed in a 2 1/2 D case study and appropriate conclusions are drawn.