集成装置和变速箱壳体夹具规划系统.doc

此文档收集于网络，如有侵权，请联系网站删除Int J Adv Manuf Technol (2005) 26: 310318DOI 10.1007/s00170-003-1997-zORIGINAL ARTICLEM. StampferIntegrated set-up and xture planning system for gearbox casingsReceived: 23 March 2003 / Accepted: 27 May 2003 / Published online: 1 December 2004 Springer-Verlag London Limited 2004Abstract Briey reviewed in this paper is the development ofan expert system for the xture set-up and for gearbox casings tobe machined on machining centres. The set-up and xture plan-ning involves the denitions of operation sequence, workpieceorientation, locating type, locating surfaces, clamping type, andclamping surfaces. In this paper, the development of the systemand a more detailed description of the locating solution is pre-sented.Keywords Expert systems Fixture planning Locating Process planning1 Introduction此文档仅供学习与交流The conceptual design of xtures is one of the most complex in-tellectual tasks in the process of industrial design 1. This earlystage of design can be automated with the application of expertsystems.An expert system for set-up and clamping xture planning isdeveloped for machining of gearbox casings, which belong to theclass of prismatic workpieces.For machining prismatic workpieces, the most suitable ma-chine tool is the horizontal machining centre 6. Depending onthe workpiece complexity, the complete machining is feasible inone or two clamping operations. In the process of xture design,the technological process is reduced to operations, and generalsolutions for clamping xtures are generated for every clamping.The basic operation functions of clamping xture are the locat-ing and clamping of the workpiece 8. Locating can be dividedinto plane locating and side locating, and side locating is furtherreduced to endwise locating and guiding (Fig. 1).M. StampferDepartment of Mechanical Engineering,Pecs University, 7624 Pecs, Rokus u.2., HungaryE-mail: smihalywitch.pmmf.huTel.: +36-72-503650/3777Fax: +36-72-503650/3731Fig. 1. LocatingThe general solution for the xture requires the followingpoints to be dened: 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 xture planning2.1 Geometric modelA geometric model is formed by the principle of workpiece re-duction 11. The workpiece is rst reduced to six faces (top,bottom, left, right, front, back), which are further reduced to typi-cal forms (features). Features are dened by a series of attributes,which include dimensions, initial state, roughness, and dimen-sional accuracy.2.2 Functional characteristics of workpieceThe functional surfaces of a workpiece, besides the toleranceof features themselves, are “interconnected” with tolerances be-tween features. These relationships can be further divided into311Fig. 3. Interconnected surfaces with distance tolerancedistance tolerances and locating tolerances. The type of distanceand locating tolerances, which are used in gearbox casings arepresented in Fig. 2 and Fig. 3. The features whose location is de-termined by distance or locating tolerances are called “connectedfeatures”.2.2.1 Classication of functional characteristics with regard toxture solutionThe location of functional surfaces in the workpiece structureand the type of interconnections have a great inuence on theselection of the workpiece location in the machine workspaceand on the reduction of the technological process involved intoFig. 2. Interconnected surfaces with locating accuracyclamping operations. From a technological viewpoint, the ma-312chining of all the connected faces in one clamping operation isthe ideal case. However, there are cases when this is impossibleand in such cases one is forced to machine the functionally con-nected faces of the workpiece in separate clampings. In suchcases, a high accuracy of xture and a high accuracy of the lo-cating surfaces are needed to realize the prescribed workpieceaccuracy. Some of connecting types are relatively easy to accesswhen the connected faces are machined in separate clampings,but there are also types for which machining in separate clamp-ing is very difcult and these types need a xture with very highaccuracy.With respect to the realization difculty of the accuracy,when functionally connected surfaces are machined in separateclampings, the connecting types can be divided into two types:loosely and strictly tolerance-related connections (Fig. 2).The group of loosely tolerance-related connections consistsof accuracy-related requirements, which can be achieved rela-tively easily even in cases when the connected surfaces are ma-chined in two separate clampings. In the case of these functionalconnecting types the xture accuracy is critical only with respectto parallelism or perpendicularity to the machine table.The group of strictly tolerance-related connections consistsof accuracy-related requirements which can be achieved onlywith difculty in cases when the connected surfaces are ma-chined in two separate clampings. They need a high accuracy ofthe xture and of the locating elements of the workpiece. Forthese connecting types the connected surfaces need to be ma-chined in one clamping or, such surfaces should be machined intwo separate clampings, only when there is no other solution.The faces of a workpiece, depending on the surfaces theycontain, have appropriate attributes. The face is: “machined”when it contains a machined surface, it is “connected” whenit contains a connected surface, it is “strictly connected” whenit contains a strictly connected surface, and it is “loosely con-nected” when it contains a loosely connected surface.Fig. 5. Types of side locating3 Identication of the locating and clamping entities3.1 Plane locating types of the workpieceDetermining of the plane locating types was with the help ofthe analysis of technological facilities of horizontal machiningcenters and existing clamping xtures. There are three types ofplane locations (Fig. 4): horizontal (denoted with “pos1”), verti-cal (“pos2”), and vertical with partial machining of the locatingface (“pos3”).The selection of a plane locating face with a given plane lo-cating type determines the set of workable faces.3.2 Side locating typesFrom an analysis of existing clamping xtures and the corres-ponding literature, there are four basic types of side location(Fig. 5) 8, 10:(1) Side location by using surfaces, which are on the adjoin-ing faces of the plane locating face(2) Side location using two inside diameter on the plane lo-cating face(3) Side location by using one inside diameter on the planelocating face and a face on one of the adjoining facesFig. 4. Plane locating types313(4) Side location by using two threaded joints on the planelocating face3.3 Clamping typesAccording to the direction of clamping forces, the clamping canbe perpendicular to the plane locating surface (type s1) or paral-lel with the plane locating surface (type s2). The basic type s1,depending on the location of clamping faces, can be further di-vided into subtypes s11, s12, and s13. In the case of s11, theclamping surfaces are on the adjoining sides of the plane locatingface. In the case of s12, the clamping surface (or surfaces) is onthe opposite side of the plane locating face, and in type s13, theclamping is on the through hole (Fig. 6).One of the specic ways of clamping is clamping by screwsand threaded joints on the plane locating face which is calledtype s3.The number of clamping points is also a very important char-acteristic of clamping. According to the number of clampingpoints, clamping is differentiated in one, two, three, and fourpoints. Adding these to the previous basic types, the possibleclamping types are: s11_2, s11_3, s11_4; s12_2, s12_3, s12_4;s13_1, s13_2; s2_1, s2_2; s3_2, s3_3, and s3_4.In this notation, the last number means the number of clamp-ing points.3.4 Suitable surfaces of the workpiece for locating and clamping3.4.1 Suitable surfaces for plane locatingThe suitability of surfaces for plane locating depends on theshape and dimension of the surface.Fig. 6. Types of clampingAccording to the shape, the following surfaces are suitablefor plane locating: planar surfaces, intermittent planar surfaces,group of planar surfaces in same plane, group of planar surfacesin two different planes, cylindrical surfaces (with parallel axes),and combination of cylindrical and planar surfaces.The surfaces (features) so far identied are not equally suit-able. The suitability decreases in the order of enumeration. In theexpert system prototype, only the rst three features are built-in. Next to the fair shape (feature), the plane locating surfacemust be sufciently large in order to be usable for plane locating.Prismatic workpieces, according to their global structure, can bedivided into: cubical, at and long workpieces.Not all the faces of a workpiece are suitable for plane locat-ing. In the case of at workpieces, only the large faces, and in thecase of long workpieces, only the long faces are suitable.The dimensions of candidate features for plane locating mustbe compared with the three overall dimensions of the workpiece.3.4.2 Surfaces suitable for side locatingSuitable features are established by each side locating type.a) Suitable surfaces for side locating type p1. Side locating canbe divided into guide locating and endwise locating. Hence, suit-ability tests are to perform separately for guiding and for endwiselocating.Suitability for guiding must be tested from three aspects, i.e.,shape of surface, dimension, and position of surface. According to the shape of the guide locating element the use-able faces are: planar face, two planar faces, two cylindricalfaces, combination of cylindrical and planar surfaces, andsingle cylindrical surface.314 The typical dimension of a surface must not be less than 35%of the longest dimension of the plane locating face. According to the position of the guide locating element, theymust belong to adjoining faces of the plane locating face.Suitability for endwise locating must be tested from two aspects,i.e., according to shape of surface and position of surface. According to the shape of the surface, planar or cylindricalsurface is useable for endwise locating. According to the position of the endwise locating elements,they must be on the face, which is the adjoining face of boththe plane locating face and guiding face.b) Suitable surfaces for side locating type p2. According to theshape of surface there should be two holes on the plane locat-ing face. The distance between the holes must not be less than35% of the greatest length of the plane locating face. Suitablesurfaces for other side locating types are dened in a similarway.3.4.3 Suitable surfaces for clampingSuitable clamping surfaces have been established for everyclamping type.4 General solution concept of operation sequenceand clamping xturesRectangular castings can almost always be machined on a ho-rizontal machining centre in two operations. The operation inwhich the functional surfaces, or most of them, are machined, iscalled the main clamping, while the operation in which the rest ofthe surfaces are machined is called additional clamping. Fixturesfor both the main and additional clamping make a clamping x-ture set. The additional clamping takes precedence in most cases.Regardless of the clamping sequence, it is the main clamping thatis solved prior to the additional clamping.4.1 General solution concept of main clamping xtureloosely connected faces in this stage are left out of consideration.In this way, the xture solution is still “fair”, but the accuracyrequirements refer only to the parallelism, or rather to the per-pendicularity of any xture surfaces.If this attempt is not successful, then one must give up theidea of machining all of strictly connected faces in one clamp-ing. There are also cases when not the entire face, but only thestrictly connected features (holes) on the face are workable withother strictly connected faces in one clamping (see Sect. 4.1.3).Lastly, when none of these attempts is successful the strictlytolerance-related features must be machined in separate clamp-ings. In this case, the accuracy requirements of the xture arevery high.Taking into consideration the abovementioned aspects, fourstrategies are devised for solving the main clamping.4.1.1 Technologically ideal clampingThis strategy is aimed at nding the position of the workpiecein the machine workspace, which would allow machining of allthe connected faces in one clamping. The potential position ofa workpiece in the machine workspace is determined from therequirement that all connected faces should be machined in oneclamping. Depending on the plane locating type, this can be for-mulated in the following manner: In the case of horizontal plane locating type pos1, the planelocating face and its opposite face must not be tolerance-related faces, because it is not possible to machine themtogether with other faces in the same clamping. For a vertical plane locating type pos2, the plane locatingface, the face which is facing the machine table, and its op-posite face, must not be tolerance-related faces.In order to select the workpiece position, it should be satisfactoryfor both plane and side locating and clamping (Fig. 7). Since thehorizontal plane locating (pos1) allows machining of four faces(while pos2 allows only three faces to be machined) and the x-ture construction is simpler, the system rst attempts to generatea xture solution for horizontal plane locating type. If that at-tempt fails, a second attempt is made at solving the xture forvertical plane locating.On a horizontal machining centre in one clamping, a maximumof four faces of the workpiece are workable. When solving themain clamping, one must try to nd a position for the workpiecein the workspace a of machining centre in which the machiningof all the connected faces is possible. In this way the greatestaccuracy of the workpiece can be obtained and the accuracy re-quirement and the complexity of the xture are the lowest. Thisposition of the workpiece is called the “technologically idealworkpiece position”. The xture determined in this way is thebest possible xture solution.However, in several cases the disposition of connected facesmeans that the workpiece can not be held in a technologicallyideal workpiece position. In this case, it is necesary to nd a pos-ition of the workpieces in which at least the machining of strictlyconnected faces is possible in one clamping. In other words, theSuitability for plane locating. The selected workpiece positionis suitable for plane locating if the plane locating face containssurfaces which are suitable for plane locating. If the workpieceposition is suitable for plane locating, then in the next step,a check for the suitability for side locating is performed.Suitability for side locating. The clamping sequence mainly de-termines the applicability of certain types of side locating: if thesecond clamping is the main clamping, then the side locatingtype can be either p2.2 or p4, and if the rst clamping is the mainclamping, then the plane locating type can be p1 or, alternatively,p3.If, for some of the possible types of side locating, there aresuitable surfaces on the appropriate workpiece face, then the315the plane locating types, pos1 and pos2. If none of the possibleworkpiece positions meet the requirements, the xture solutionstrategy based on the ideal workpiece position cannot yield a re-sult and should be abandoned.4.1.2 Fixture solution based on the overlook of the looselytolerance-related facesThis strategy is used in cases when the rst one fails to producea xture solution. The eventual workpiece position is selectedunder the condition that all strictly tolerance-related surfacesmust be machined in one clamping. This strategy is similar tothe previous one, with the exception that the loosely tolerance-related faces are omitted during the process of selection of theworkpiece position in the machine workspace.In order for the assumed workpiece position to be adoptedaccording to the previous requirement, it has to be suitable forlocating and clamping. With this strategy, the main clampingis always the second one, which implies that only nished sur-faces can be used for locating. The suitability of a workpieceposition is checked as for the rst strategy. This strategy still gen-erates a fairly acceptable xture solution, considering that xtureaccuracy is critical only with respect to the parallelism or per-pendicularity of certain surfaces of the main clamping xture.In cases when this strategy fails to produce a solution, anotherstrategy should be applied.Fig. 7. A part of AND-OR graph for xture solution in a technologicallyideal workpiece position4.1.3 Fixture solution based on the r