Graph-based set-up planning and tolerance decomposition for computer-aidedfixture design

Set-up planning plays an important role in integrating design with process planning and fixture design. Its main task is to determine the number and s equence of set-ups, and select locating datum, machining features and opera tions in each setup. Computer-based set-up planning may be used to generate automatically a setup plan based on tolerance analysis for minimizing loca ting error, calculating machining error stack-up and improving CAD/CAPP/CAF D (Computer-aided Fixture Design) integration efficiency. Most of the curre nt set-up planning systems were based on empirical methods (such as rule an d knowledge base) and their applications background was plus/minus a dimens ioning and tolerancing (GD&T) scheme. Today, more and more industries are u sing a true positioning Geometric Dimensioning and Tolerancing (GD&T) schem e in design and manufacturing. To support this requirement, this paper pres ents an analytical set-up planning approach with three techniques, (1) an e xtended graph to describe a Feature and Tolerance Relationship Graph (FTG) and a Datum and Machining Feature Relationship Graph (DMG), which could be transferred to an analytical computer model; (2) seven set-up planning prin ciples to minimize machining error stack-up under a true positioning GD&T s cheme; and (3) tolerance decomposition models to partition a tolerance into interoperable machining errors, which could be used for locating error ana lysis or for feedback to design stage for design improvement. These techniq ues are implemented in a computer system and it is integrated with a commer cial CAD/CAM system to support an automobile manufacturing enterprise in fi xture design and production planning.