Tolerancing of Free Form Surfaces

Abstract

The ANSI Y14.5M -1982 and ANSI Y14.5.1M-1994 Standards define profile tolerance for surfaces as the outline of an object in a given plane (two-dimensional figure). Profiles are formed by projecting a three dimensional figure onto a plane. The ANSI Standard defines a profile tolerance zone as the volume or the area generated by offsetting each point on the nominal surface in the direction of the normal to the nominal surface at that point. The ISO standards defines a tolerance zone for the form tolerance of sculptured surface as the zone which is limited by two surface enveloping spheres of diameter t, the centers of which are situated on a surface having the true geometric form. During inspection of an engineering feature, it is always recommended that size tolerance be firstly inspected followed by position, orientation and finally form tolerances. In this paper, a systematic approach was followed to describe position, orientation and form tolerances for free form surfaces. Two different methods (Least squares and Arithmetic mean) are used to extract the position error vector. Based on the experimental and the theoretical analysis it was found that the arithmetic mean method is better in extracting the position error. Extracting orientation and form errors for free form surfaces was also described. Finally a comparison between the proposed calculated surface form errors (after position and orientation error extraction) and the measured form errors are presented to demonstrate the effectiveness of the technique.