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In the area of AFCD, relatively less literature can be found. Given locating and clamping points on workpiece surfaces, fixture elements can be selected to hold the workpiece based on CAD graphic function [14]. A 2-D modular fixture synthesis algorithm was developed for polygonal parts [15]. Whybrew and Ngoi presented a method to automatically design the configuration of the T-slot based modular fixturing elements [16]. The key feature of the system was the development of a matrix spatial representation technique which permitted the program to search and identify both objects and object intersections. It was also able to determine the position of objects during the design process. However, the limitation of the method was that only the blocks whose edges were parallel or perpendicular to each other could be represented. Therefore, the design system could only layout the fixture elements in such a way that all the edges of fixture elements were parallel or perpendicular to each other. It only presented a detailed research on selecting the fixed point between baseplate and bottom modular fixture elements and did not describe the rule to select the suitable modular fixture elements and the way to combine them together. The fixture design methodology in the case of 3-2-1 fixture layout method was applied in the study.
The major problems involved in AFCD include: selection of locators and clamps which make contact with the workpiece , determination of the heights of these units to hold the workpiece , placement of locating and clamping units around the workpiece and on the baseplate, determination of connections between fixture elements, and interference checking among fixture units and with the workpiece and maching envelope. In this paper,t he fundamental structure of dowel-pin based modular fixture and fixturing characteristics of commonly used modular fixture elements are first investigated. A Modular Fixture Element Assembly Relationship Graph (MFEARG) is introduced to represent basic combination relationships between modular fixture elements. Based on MFEARG, algorithms are implemented to choose all suitable fixturing unit candidates. Algorithms of mounting fixture units on baseplates are also discussed in this paper. The input of the system is workpiece representation, workpiece orientation, fixture planning and machining envelope .This information is extracted from a CAD model of workpieces with process planning information. Its output is a fixture assembly drawing displayed on the computer screen or plotted as a hard-copy, and a list of modular fixture elements as well as their position coordinates and orientations.
2 Fixture Design and Modular Fixtures
A fixture is a device to accurately locate and firmly hold a workpiece in manufacturing processes. Generally, a fixture body is composed of three basic types of elements : locators clamps and supports, which are mounted on a baseplate. Locators are used to properly position the workpiece with accuracy and repeatability in machining operations. The workpiece needs to be restricted in all six degrees of freedom through the layout of locators, Supports are sometimes applied to decrease the deflection of the workpiece caused by clamping forces , machining force and its own weight. Supports are needed especially for thin workpieces .Clamps are for holding workpieces firmly against the locators and supports to secure the position accuracy of the workpieces during the operation .Machining forces and their directions, and the positions of locators and supports are taken into account in the layout of clamps. In the fixture design process, determination of a locating method is the dominant task because the major purpose of fixture design is to provide fixture configurations that can ensure the accuracy in machining processes. The selection of clamping method is dominated by the locating method because the clamping force should be directed against the locators.