Finite element study of a workpiece in a machining fixture

Abstract

The design of a machining fixture involves more than the selection and positioning of fixturing elements to conform to the rules for geometric, dimensional and mechanical control of the workpiece. A workpiece under restraint is subjected to active forces (the weight of the workpiece, clamping forces, cutting forces) and reactive forces (reactions at locators and supports). These tend to deflect or deform the workpiece which could lead to dimensional errors and possibly rejects. Supports should ideally be positioned at the points of maximum deflection to minimise the errors. This paper presents a finite elements (FE) study to determine the elastic behaviour of a workpiece in a fixture and to locate the points of maximum deflection. Based on the use of the I-Deas FE analysis package by SDRC, the study takes into consideration the restraint set, the force set and the geometry and properties of the material of the workpiece. The information gained from the FE modelling is fed to an expert system for designing jigs and fixtures. The expert system requires this information to decide on the placement of supports for a given set of positions of locators and clamps or to recommend new positions for them if the calculated deflections prove to be excessive. The paper begins with a short summary of the applications of FE modelling in mechanical design. The FE analysis technique is reviewed next. This is followed by a detailed description of the application of FE analysis to improve fixture design.