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1 Introduction The conceptual design of fixtures is one of the most complex intellectual tasks in the process of industrial design [1]. This early stage of design can be automated with the application of expert systems.35323
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An expert system for set-up and clamping fixture planning is developed for machining of gearbox casings, which belong to the class of prismatic workpieces.
For machining prismatic workpieces, the most suitable machine tool is the horizontal machining centre [6]. Depending on the workpiece complexity, the complete machining is feasible in one or two clamping operations. In the process of fixture design,the technological process is reduced to operations, and general solutions for clamping fixtures are generated for every clamping. The basic operation functions of clamping fixture are the locating and clamping of the workpiece [8]. Locating can be pided
into plane locating and side locating, and side locating is further reduced to endwise locating and guiding (Fig. 1).
Fig. 1. Locating
The general solution for the fixture requires the following points to be defined: clamping sequence, plane locating type, plane locating face, plane locating surface, type of side locating, locating surfaces, clamping type, and clamping surfaces.
2 Inputs to system for set-up and fixture planning
2.1 Geometric model
A geometric model is formed by the principle of workpiece reduction [11]. The workpiece is first reduced to six faces (top, bottom, left, right, front, back), which are further reduced to typical forms (features). Features are defined by a series of attributes,
which include dimensions, initial state, roughness, and dimensional accuracy.
2.2 Functional characteristics of workpiece
The functional surfaces of a workpiece, besides the tolerance of features themselves, are “interconnected” with tolerances between features. These relationships can be further pided into distance tolerances and locating tolerances. The type of distance and locating tolerances, which are used in gearbox casings are presented in Fig. 2 and Fig. 3. The features whose location is determined by distance or locating tolerances are called “connected features”.
Fig. 2. Interconnected surfaces with locating accuracy
Fig. 3. Interconnected surfaces with distance tolerance
2.2.1 Classification of functional characteristics with regard to fixture solution
The location of functional surfaces in the workpiece structure and the type of interconnections have a great influence on the selection of the workpiece location in the machine workspace and on the reduction of the technological process involved into
clamping operations. From a technological viewpoint, the ma chining of all the connected faces in one clamping operation is the ideal case. However, there are cases when this is impossible and in such cases one is forced to machine the functionally connected faces of the workpiece in separate clampings. In such cases, a high accuracy of fixture and a high accuracy of the locating surfaces are needed to realize the prescribed workpiece accuracy. Some of connecting types are relatively easy to access when the connected faces are machined in separate clampings, but there are also types for which machining in separate clamping is very difficult and these types need a fixture with very high accuracy.
With respect to the realization difficulty of the accuracy, when functionally connected surfaces are machined in separate clampings, the connecting types can be pided into two types: loosely and strictly tolerance-related connections (Fig. 2).
The group of loosely tolerance-related connections consists of accuracy-related requirements, which can be achieved relatively easily even in cases when the connected surfaces are machined in two separate clampings. In the case of these functional connecting types the fixture accuracy is critical only with respect to parallelism or perpendicularity to the machine table.