Limitations in Primary Datum Plane Establishment Imposed by ISO Standard and a Proposal for an Improved Methodology

Abstract

According to the ISO Geometrical Product Specification (GPS), a primary common datum plane of a part with two parallel offset planes is established by two datum targets on one plane and one datum target on another parallel plane. This process cannot function theoretically, because the primary common datum plane is the bisector plane of these two parallel planes which cannot be established by only 3 datum targets on different planes. This deficit causes ambiguous datum establishing process for a part and leads an irreproducible measurement results in practice. This paper suggests a new approach for a function-oriented, explicitly defined primary datum plane establishing process and offers a corresponding clarified and simplified notation system of datums and datum system. This research is based on the analysis of physical behavior and theoretical alignment of a part during the alignment process by measuring. As a result, an improved methodology for a stable and reliable primary datum plane establishment was developed.