Identifying tolerance chains with a surface-chain model in tolerance charting

A tolerance chart is an effective tool used to determine the working dimensions and tolerances in process planning. Its kernel are the tolerance chains among the machining cuts through which the component will be manufactured. Identifying tolerance chains is a painstaking job. In this paper, a coordinate system is set up for the component in question so that all surfaces of the component are expressed with their coordinates and the tolerance chart is parameterised. Then an object-oriented model is developed so that each blueprint dimension is represented as a blueprint object while each working dimension is expressed as a machining-cut object. Each object has a special attribute, the surface chain, which is made up of all surfaces involved in the tolerance chart. The surface-chain model is very helpful in identifying the tolerance chains. By establishing the linear equations of the working dimensions, the stock removals and the blueprint dimensions, the tolerance chains among the machining cuts are derived automatically. The working dimensions and tolerances then can be determined easily once the tolerance chains are identified.