資源目錄里展示的全都有，所見即所得。下載后全都有,請(qǐng)放心下載。原稿可自行編輯修改=【QQ：401339828 或11970985 有疑問(wèn)可加】 Int J Adv Manuf Technol (1999) 15:171181 1999 Springer-Verlag London LimitedDevelopment of Automated Fixture Planning SystemsW. Ma, J. Li and Y. RongDepartment of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA, USAFixturing is an important manufacturing activity. The computer-aided fixture design technique is being rapidly developed toreduce the lead time involved in manufacturing planning. Anautomatedfixtureconfigurationdesignsystemhasbeendeveloped to select automatically modular fixture componentsand place them in position with satisfactory assembly relation-ships. In this paper, an automated fixturing planning systemis presented in which fixturing surfaces and points are auto-matically determined based on workpiece geometry and oper-ational information. Fixturing surface accessibility, featureaccuracy, and fixturing stability are the main concerns in thefixture planning. The system development, the fixture planningdecisionprocedure,andanimplementationexamplearepresented in the paper.Keywords: Accuracy; Clamping; Fixture planning; Locating1.IntroductionFixturing is an important manufacturing activity in the pro-duction cycle. A computer-aided (or automated) fixture design(CAFD) technique has been developed as part of CAD/CAMintegration 1. The development of CAFD contributes to thereduction of manufacturing lead time, the optimisation of manu-facturing operations, and the verification of manufacturing pro-cess designs 2. CAFD plays an important role in flexiblemanufacturing systems (FMS) and computer-integrated manu-facturing systems (CIMS) 3.Figure 1 outlines the activities for fixture design in manufac-turing systems which include three major aspects: set-up plan-ning, fixture planning, and fixture configuration design 4. Theobjective of set-up planning is to determine the number of set-ups, the position and orientation of the workpiece in each set-up, and also the machining surfaces in each set-up. Fixtureplanning determines the locating and clamping points on work-piece surfaces. The task of fixture configuration design is toselect fixture components and place them into a final configur-Correspondence and offprint requests to: Dr Kevin Rong, Departmentof Mechanical Engineering, Worcester Polytechnic Institute, Worcester,MA 01609-2280, USA. E-mail: rongKwpi.eduFig. 1. Fixture design in manufacturing systems.ation to fulfil the functions of locating and clamping theworkpiece. An automated modular fixture configuration designsystem has been developed in which, when fixturing surfacesand points are selected on the workpiece model, fixture unitsare automatically generated and placed into position with theassistance of fixture component assembly relationships 4,5.This paper deals with fixture planning when the fixturingsurfacesandpositionsontheworkpieceareselectedautomatically.Previous papers on fixture design analysis have been pub-lished, but a comprehensive fixture planning system which canbe used to generate fixture plans for industrial applications hasnot been developed. Previous work includes: a method for theautomated determination of fixture location and clampingderived from a mathematical model 6; an algorithm for theselection of locating and clamping positions which provide themaximum mechanical leverage 7; kinematic analysis basedfixture planning 8,9; a fixturing grade and dependency gradebased fixturability analysis 10; automated selection of set-ups172W. Ma and Y. Rongwith consideration of tolerance factors of orientation errors infixture design 11, and finally a geometric analysis based 2Dfixture planning system 12. In our previous research, fixturingfeatures 13, fixturing accuracy 14,15, geometric constraints16, and fixturing surface accessibility 17 have been studied.A framework has been developed for set-up planning andfixture design 18.In this paper, an automated fixture planning system, Fix-Planning, is presented where fixturing surfaces and points aredetermined when the workpiece model and set-up planninginformation is input to the system.2.Basic Requirements of FixturePlanningIn engineering practice, fixture planning is governed by anumber of factors, including workpiece geometric informationand tolerance; set-up planning information such as machiningfeatures, the machine tool and cutting tools to be used in eachset-up; initial and resulting forms of the workpiece in eachset-up; and available fixture components. To ensure that thefixture can hold the workpiece in an acceptable position sothat the manufacturing process can be carried out according tothe design specifications, the following conditions should besatisfied for a feasible fixture plan.1. The degrees of freedom (DOF) of the workpiece are totallyconstrained when the workpiece is located.2. Machining accuracy specifications can be ensured in thecurrent set-up.3. Fixture design is stable to resist any effects of externalforce and torque.4. Fixturing surfaces and points can be accessed easily byavailable fixture components.5. There is no interference between the workpiece and thefixture, and between the cutter tool and the fixture.In this investigation, we focus on the first four requirements.Fixture planning is carried out based on the following consider-ations:1. Although the workpiece geometry can be complex in indus-trial production, in most fixture designs, planar and cylindri-cal surfaces (internal and external) are used as the locatingand clamping surfaces because of the ease of access andmeasurement of these features when the workpiece is fixed.In this investigation, planar and cylindrical surfaces are usedin fixture planning.2. Many CNC machines, especially machining centres, can beused to perform various operations within one set-up. Inmost cases, the cutting-tool axis of the machine tool isfixed. When considering fixturing stability, the locating sur-faces are preferably those with normal directions oppositeto, or perpendicular to, the cutting-tool axis. For clampingfeatures, the normal directions should be in line with, orperpendicular to, the cutting-tool axis, because, in fixturedesign, clamping forces should be against locators.3. For the surfaces to be machined, there should exist datumsurfaces which serve as position and orientation referencesfrom which other dimensions and tolerances are measured.In fixture planning, surfaces with high accuracy gradesshould be selected preferentially as locating surfaces so thatthe inherited machining error is minimised and the requiredtolerances of the machining features are easily attained.4. In fixture planning, more than one workpiece surface mustbe selected for the locating and clamping surfaces forrestricting the DOF of the workpiece in a set-up. Therefore,besides the conditions for individual surfaces, the combi-nation status of the available locating surfaces is alsoimportant for the accurate location of the workpiece.5. Since the locators and clamps are in contact with theworkpiece, the distribution of fixturing points plays a criticalrole in ensuring fixturing stability.6. For a feasible fixture design, the fixturing surfaces must beaccessible to the fixture components. The usable (effective)area of the fixturing surface should be large enough toaccommodate the functional surfaces of the locators andclamps. Besides considering a fixturing surface, the accessi-bility of potential fixturing points on the surface is alsoimportant for the determination of the final fixturing pointdistribution.3.Fixturing SurfacesThe concept of features has been widely used in design andmanufacturing. A workpiece to be machined can be viewed asa combination of features such as planes, steps, pockets, slots,and holes. In a particular operation set-up, features used forfixturing the workpiece can be defined as fixturing features orfixturing surfaces. In practice, most fixturing features are planarand cylindrical surfaces. According to the fixturing functions,the fixturing surfaces can be classified into locating, clamping,and supporting features. Unlike design and manufacturing fea-tures, fixturing surfaces are orientation-dependent. They do notplay the same role throughout the manufacturing processes. Aset of surfaces may serve as fixturing surfaces in a set-up, butmay not be used for fixturing or have different fixturingfunctions in another set-up.The concept of fixturing features allows the fixturing require-ments to be associated with the workpiece geometry. Featureinformation in a feature-based workpiece model can also beused directly for fixture design purposes. For manufacturingfeatures, the information necessary for describing a fixturingfeature contains geometric and non-geometric aspects. Theformer includes feature type, shape and dimensional parameters,and position and orientation of the workpiece. The latterincludes the surface finish, accuracy level and relationshipswith machining features, and surface accessibility.3.1Discretisation of Fixturing SurfacesIn most fixture designs, the fixturing features, especially thelocating surfaces, are planar and cylindrical surfaces. In ordertoevaluatefixturingsurfaceaccessibilityanddeterminelocating/clamping points on fixturing surfaces, a candidate fix-turing surface is sampled into grid-arrayed discrete points withDevelopment of Automated Fixture Planning Systems173equal interval T. If T is small enough, the discrete samplepoints will be almost continuous.In order to make the sampling algorithm generic, an outer-bounding rectangle on the surface is used as the samplingregion. Since in most cases, the primary locating surface isperpendicular to the other locating surfaces, especially in modu-lar fixture designs, the fixturing surfaces are considered asbottom-locating, top-clamping, side-locating, and side-clampingsurfaces. For a bottom-locating/top-clamping surface with anormal Z (or 2Z) direction, two edges of the outer-boundingrectangle must be parallel to the X-axis and two other edgesparallel to the Y-axis. For a side-locating/clamping surface,there must be two edges parallel to the Z-axis, while the othertwo edges must be perpendicular to the first two edges. Figure2 shows an example of sampled candidate fixturing surfaceswith the outer-bounding rectangle. With the assumption thatthe Z-axis is normal to the surface in the surface local coordi-nate system, the points within the outer-bounding rectangle canbe represented as:x = Xmin+ T u,u = 1, 2, %, Nuy = Ymin+ T v,v = 1, 2, %, Nv(1)where Nuand Nvare the numbers of points in the X- and Y-directions, respectively, which are: Nu= int (Xmax2 Xmin)/Tand Nv= int Ymax2 Ymin)/T.3.2Fixturing Surface AccessibilityFixturing surface accessibility is a measure of whether a candi-date fixturing surface is accessible to a regular fixture compo-nent. Three major factors must be taken into account:1. The geometry of the fixturing surface which involves theeffective area and shape of the surface.2. Possible obstruction of the workpiece geometry along thenormal direction and/or around the geometric region of thefixturing surface.3. The size and shape of the functional fixture components.In practical situations, it is possible that a planar surface ofthe workpiece has a complex shape and has a full/partialobstruction along its normal direction and/or around its geo-Fig. 2. Sampling of a candidate fixturing surface with an outer-bound-ing rectangle.metric region. It is thus required that the accessibility modelshould comprehensively reflect these facts so that a reasonablycomparable accessibility value can be applied for every candi-date fixturing surface.The surface accessibility is defined as a statistical valuebased on the point accessibility (PA) of every valid samplepoint on the surface, where PA consists of two parts: the pointself individual accessibility (SIA) and the point neighbourrelated accessibility (NRA). The SIA corresponds mainly to theisolated accessibility of the fixturing point, whereas the NRAreflects the extended accessibility of the fixturing point.The SIA of a sample point is defined on the basis of threeattribute tags. The tag s1is used to indicate whether the squaretest grid with its centre at the current sample point is inside,on, or outside the outer-loop of the fixturing surface. Threediscrete values are assigned to represent its status, i.e. 0, 1,and 2, respectively.If there exists obstructive workpiece geometry in the surfacenormal direction or surrounding the sample point, this affectsthe surface accessibility at the sample point. For example, asshown in Fig. 3(a), on a candidate bottom-locating surface ofa workpiece, sample point p1is not accessible because of theobstructive geometry of the workpiece along the bottom-locat-ing direction, and p2is not accessible either because of theobstructions surrounding it. To evaluate automatically whetheran obstruction exists in the surface normal direction, a virtualvolume is generated by extruding the square test grid to asolid entity in the surface normal direction. By employing atechnique for detecting the interference between two solidentities, the obstruction can be identified, as shown in Fig.3(b). The extruding method is a little different for the squareFig. 3. Obstruction checking at virtual sample points on a bottom-locating surface. (Kpimeans the extrusion is carried out at point pialong its accessible direction.)174W. Ma and Y. Rongtest grid on the side-locating/clamping surface, where thesquare test grid is first stretched along the bottom-locatingdirection, and then the stretched grid is extruded along theside-locating/clamping direction as illustrated in Fig. 4. Theattribute tag s2is used for recording the result of obstructionchecking at a sample point. When such an obstruction isdetected, s2= 1, otherwise, s2= 0.If the test grid at the sample point is found to be notobstructed, its individual accessibility is largely dependent onthe contact area between the test surface and the fixturecomponents, which is represented by the attribute tag s3. Thedefinition of s3iss3=Area1T2,s3P 0, 1(2)where AreaIis the contact area and T is the edge length ofthe test grid.On the basis of above three attribute tags, the SIA of asample point pu,vcan be given by a numerical value accordingto the following rules:if s1= OutsideOuterLoop, SIA = 21 (inaccessible);if s1 OutsideOuterLoop AND s2= Obstructed, SIA =2 1 (inaccessible);if bottom-locating/top-clamping AND S1 OutsideOut-erLoop AND s2= NotObstructed, SIA = s3;Fig. 4. Obstruction checking at sample points on a side-locating/ clamp-ing surface.if side-locating/clamping AND s1 OutsideOuterLoopAND s2= NotObstructed, SIA = 0.5vs3;where v reflects the height effect of the point in sidelocating/clamping.The accessibility in the surrounding area of the sample pointalso affects the accessibility of the point. On a fixturing surface,the positional relationship between the current sample pointand all the neighbouring sample points can be represented bya 3 3 map where Pcis the current sample point with adiscrete position of (u, v), P1| P8are 8-neighbour samplepoints, and their locations are all labelled in Fig. 5. The NRAat sample point pu,vcan be calculated using the equation:NRA(u, v) =O8k=1Fk8(3)where Fkis the related-access factor of kth neighbour, whichcan be determined based on the SIA as well as its measure(s1, s2, s3).For bottom-locating/top-clamping,F9k=521,s2(pk) = 10,s1(pk) = 2and s2(pk) = 0IA(pk), s1(pk) 2 and s2(pk) = 0(4)Fk=5F9k,k = 1, 3, 5, 7F9k,k = 2, 4, 6, 8, F9k21$ 0and F9k+1$ 021, k = 2, 4, 6, 8, F9k21= 2 1 or F9k+1= 2 1(5)For side-locating/clamping,F9k=521,s2(pk) = 1, k = 1, 5, 6, 820.5,s2(pk) = 1, k = 2, 3, 40,s1(pk) = 2and s2(pk) = 0SIA(Pk), s1(pk) 2 and s2(pk) = 0(6)Fk=5F9k,k = 1, 3, 5, 6, 7, 8F9k,k = 2, 4,F93$ 020.5, k = 2, 4,F93= 20.5(7)For a valid sample point, once the SIA and NRA are obtained,the PA can also be calculated according to the equation:Fig. 5. 3 3 position map of current point Pcand 8-neighbour samplepoints P1| P8.Development of Automated Fixture Planning Systems175PA = SIA + NRA,if PA , 0,then PA = 0(8)From the definitions of SIA and NRA, SIA is in the rangeof 0 | 1 and NRA is in the range of 21|1. Therefore, PAmust be in the range of 21|2. When the value of PA is lessthan zero, the sample point is severely obstructed and is nota feasible fixturing point. The overall accessibility (OA) of thefixturing surface is defined as the sum of the PA values at allvalid sample points, i.e.,OA =ONvalidPAu,v,sample point pu,vis tested valid(9)As OA is statistically measured by the overall effect of theaccessibility of the sample points on the surface, the infor-mation about the effective area and shape complexity of thesurface is represented in the model. Generally, the modelsatisfies the criterion that the surface with the larger OA ismore accessible than the one with the smaller OA.3.3Generalised Accuracy of the Fixturing FeaturesOne of the most important tasks for fixture planing is toguarantee that the tolerance requirements are met when theworkpiece is machined. The accuracy of features can be charac-terised by their tolerance and surface finish, and the tolerancebetween features. Generally, the tolerance of features can beclassified into two types: dimensional tolerance and geometrictolerance. The magnitude of the dimensional tolerance mayexpress the relationship between two features on the workpiece.If there is a feature with a tight dimensional tolerance withrespect to a machining feature, this implies that the featuremay be used potentially as an operational datum, i.e. a locatingsurface in the set-up. Based on whether a datum feature isneeded, the geometric tolerance can be further divided intoform tolerance and positional/orientation tolerance. The formtolerance is associated only with the feature itself, whichspecifies the allowable geometric variation of individual fea-tures. The form tolerance, e.g. surface finish, of a featureaffects the suitability of the feature to be the fixturing datum.The positional/orientation tolerance is of the same importanceas the dimensional tolerance for fixture planning since it alsorepresents a relationship between features. In order to evaluatethe accuracy of a feature and use it efficiently in fixtureplanning, a generalised feature accuracy grade is applied inthis investigation, which is defined as:Tg= (w1Td+ w2Tp) * (w3Tf+ w4Tr)(10)where Td, Tpand Tfare the dimensional tolerance grade,positional tolerance grade and form tolerance grade, respect-ively; Tris the tolerance grade equivalent to the surface finishof the feature. w1, w2, w3, and w4are the weight factors. Themultiple operation “*” represents a dominant relationship wherea zero value can contribute to the final result, while theoperation “+” represents a relatively weak relationship withpreferences. Td, Tp, Tf, and Trcan be obtained by applying thealgorithms described in 11,18.4.Development of Automated FixturePlanning SystemsAn overview of the automated fixture planning system is shownin Fig. 6. The procedure for fixture planning can be dividedinto five stages, i.e. input, analysis, planning, verification, andoutput. The input data includes a workpiece CAD modelcontaining the geometric and tolerance information of thefeatures of the workpiece, and set-up planning informationincluding the features to be machined and the machine tooltype for the specific set-up. The data can be extracted eitherfrom a CAD database or entered interactively by the user ina CAD system.Analysis involves the extraction of the candidate fixturingfeatures with related accuracy information and an evaluationof the accessibility of the fixturing features. In this study, planarand cylindrical surfaces are considered for fixturing purposes.The task of planning is to determine automatically theprimarylocatingdirectionandtoselecttheoptimallocating/clamping surfaces and points in the current set-up.Algorithms are developed for the planning of the bottom (top)and side locating/clampin