卧式储罐下料制造工艺及夹具设计【说明书+CAD】
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Automated setup and fixture planning system for box-shaped partsAutomated setup and fixture planning system for box-shaped partsMichael StampferReceived: 18 September 2008 / Accepted: 16 February 2009 / Published online: 4 March 2009 # Springer-Verlag London Limited 2009AbstractThe topic of the research is related to the domain of computer-aided manufacturing process planning. This paper deals with the problem of setup and fixture planning for the machining of box-shaped parts on the horizontal machining centres. The setup and fixture planning involves the definition of setups, the setup sequence and conceptual design of fixtures for each setup. The central topic of this research is the automation of the conceptual design of fixtures. This topic is interconnected with the setup planning, and accordingly, the aim of the author has beenthe integrated handling of tasks of setup and fixture planning and the finding of solution in an integrated system. Based on the workpiece model, the developed system automatically determines the setup sequence, the content of setups and the conceptual solution of fixture for each setup. The paper presents the problems of fixturesolutions and the partial tasks of workpiece holding, the typical solution of partial tasks and the conditions of theirapplication and finally offers a new method, which makes the integrated handling of tasks of setup and fixture planning and finding solution in an integrated system possible.Keywords :Process planning . Fixture planning1 IntroductionThe technological planning can be broken down into several steps (Fig. 1): (1) setup planning and conceptual design of fixtures; (2) operation planning; (3) fixture configuration and design.The setup planning and conceptual design of fixtures is one of the most complex intellectual tasks in the process of industrial design and can be automated only with great difficulty. Human process planners very often find the solution relying on their experience and engineering intuition.This means that there is a small number of researchers who are engaged in the conceptual design of fixtures,especially in the case of prismatic workpieces. The reason is perhaps that the existing knowledge for the fixture solution is not available in explicit form as formulas, logical diagrams or well-defined processes. It restricts the development of the appropriate fixture planning methods in contrast with another design task.The attempt to automate fixture design activities is not a novel idea, nor is it the first attempt. Researchers have already recognised the necessity of the development of planning systems in order to solve the problem of conceptual design of fixtures. Some of the major achievements in this field are listed below.Boerma 1, 2 presented the FIXES system for setup and fixture planning for prismatic parts. In this system, first the features have to be selected, which are meant for candidate machining in one setup. Then, the suitable surfaces for locating and clamping are selected. The system searches clamping surfaces only on the opposite face to the plane locating face of the workpiece. It is a subsystem of the PART CAPP system, which is the first complete expert process planning system to be commercialised and covers most of the process planning functions. Giusti et al. 3 introduced the planning system KAPLAN. This is a knowledge-based approach to process planning of rotational parts. It selects the machine tools interactively, while the tools, the machining sequence and clamping devices are selected automatically. Quick Turnaround Cell 4 was developed by Chang and is an integrated planning system capable of designing, process planning, cell control and visual inspection. It is designed to create one-of-a-kind prismatic parts, which are machinable on vertical machining centres where the clamping device is a vice. Trappey etal. 5 set out to find the locating and clamping points. In this setup, the orientation of the workpiece is handled as input data. Their approach is based on an analysis of the workpiece projection onto the fixturing base element. In the first step, the algorithm selects the locator points on the base element. In the next step, it defines the coordinates of the two-point locating and finally the coordinates of the one-point locator. Two algorithms are reviewed for the clamping points definition, one for the clamping in the projection direction and one for the clamping in the perpendicular direction. Cai et al. 6 proposed a new method, called the Robust Fixture Configuration Design for fixture configuration, which minimises the locating errors. As input data, the following are used: the geometric model of workpiece, initial locator positions, orientation and value of the clamping force. The system keeps changing the initial locator positions until the locating errors are at the minimum. Joneja et al. 7 provided a short description of a Setup planner and a more detailed description of a fixture planning program. They are part of an integrated process planning system. The Setup planner first groups the surfaces to be machined into different setups, then defines several setup sequences and selects the one with the minimum number of setups. The Fixture planner first determines the clamping method (vise or modular fixture), selects the feasible features for locating, supporting and clamping, checks tool interference, then builds the entire assembly and finally checks stability. Ma et al. 8 presented an automated fixture design system, in which the fixturing surfaces are automatically determined based on geometric and operational information. Horvth et al. 9 offered a method for setup planning and setup sequence optimising by applying genetic algorithms. Champati et al. 10 and Marefat et al. 11 applied casebased reasoning for process planning for prismatic parts where the clamping device is a vice. Cecil 12 selected the clamping areas by a geometrical approach to cutter accessibility in addition to the mechanical stability analysis. Paris et al. 13 developed a method, which deals with simultaneous grouping of machining operations in the setup and selects adapted fixturing solutions. The process plan is elaborated in three steps. The first step consists of formatting the basic data of process planning system. These are the set of machining features, the manufacturing facilities and the set of fixturing features. The fixturing features are defined as a combination of three locating features and at least one clamping feature. The fixture features are built interactively. The second step deals with the association of machining processes with machining features. The third step consists of organizing the global plan of the machining of the whole workpiece. Bansal et al. 14 presented an integrated setup and fixture planning system. The program reads the STEP file, reconstructs and modifies the model, slices it at different heights and tries to establish acceptable locating points, checks accessibility and stability and selects the solution that gives the list tolerance deviance. Several researchers have employed modular fixturing principles to generate fixture designs 1517. These systems are comprised by three modules: (1) module for selecting of modular fixture elements, (2) module for assembly of fixture elements and (3) module for interference checking. The conceptual design of fixture is handled as input data or it is solved interactively.Although numerous Computer Aided Fixture Design techniques have been proposed and implemented, fixture design still continues to be a major bottleneck in the integration of CAD and CAM activities 18.With the review of the new method for fixture planning, developed for box-shaped parts machined on four-axis horizontal machines, I wish to contribute to overcoming the difficulty.2 The basic function of fixture and the typified solutionsFor machining box-shaped workpieces, the most suitable machine tool is the horizontal machining centre. The major characteristics of horizontal machining centres are suitability for diverse manufacturing operations, tool-magazine and with the help of a revolving or NC-machine table, all the four faces of workpiece are machinable in one clamping. Depending on workpiece complexity, the manufacturing operation of complete machining can be done in one or two clampings.The basic functions of the clamping fixture are locating and clamping of the workpiece. Locating can be broken down into plane locating (supporting) and side locating, while side locating is further reduced to endwise locating and guiding (Fig. 2). Due to the great variability and complexity of prismatic parts, typifying the whole fixture is impossible. However, the solutions of partial fixture function can be typified.With the consideration of technological facilities of horizontal machining centres and analysis of existing clamping fixtures, according to the position of plane locating surface of workpiece, there are three types of plane locating established (Fig. 3): (1) horizontal (denoted with “pos1”), (2) vertical (“pos2”), (3) vertical with partial machining of the locating face (“pos3”). There are four basic types of side locating established (Fig. 4): (1) side locating by using surfaces, which are on the adjoining faces of the plane locating face; (2) side locating by using two boreholes on the plane locating face; (3) side locating by using one borehole on the plane locating face and a surface on one of the adjoining faces; (4) side locating by using two screws and threaded joints on the plane locating face. According to the direction of clamping forces, clamping can be perpendicular to the plane locating surface (type s1) or parallel with the plane locating surface (type s2). The basic type s1, depending on the location of clamping faces, can be further divided into subtypes s11, s12 and s13. In the case of s11, the clamping surfaces are on the adjoining sides of the plane locating face. In the case of s12, the clamping surface (or surfaces) is on the opposite side of the plane locating face, and with type s13, the clamping is on the opposite face and happens through the borehole. One of the specific ways of clamping is clamping by screws and threaded joints on the plane locating face and it is called type s3 (Fig. 5). The number of clamping points is also a very important attribute of clamping. According to the number of clamping points, we differentiate between clamping in one, two, three and four points. Adding these to the previous basic types, the possible clamping types are as follows: s11_2, s11_3, s11_4; s12_2, s12_3, s12_4; s13_1, s13_2; s2_1, s2_2; s3_2, s3_3, s3_4. In this list, the last number means the number of clamping points.3 Suitable surfaces of workpiece for locating and clampingBesides the presented systematisation of the solutions of partial tasks of workpiece holding, there are criteria established for determination of the workpiece surface suitability for supporting, side locating and clamping.3.1 Suitable surfaces for plane locatingThe suitability of surfaces for plane locating depends on the shape and dimension of the surface. Based on their shape, the following surfaces are suitable for plane locating: planar surfaces, intermittent planar surfaces, a group of planar surfaces in the same plane, a group of planar surfaces in two different planes, cylindrical surfaces (with parallel axes), a combination of cylindrical and planar surfaces.The surfaces (features) listed so far are not equally suitable. The suitability decreases in the order of enumeration. In the expert system prototype, only the first three features are built-in.Apart from having an appropriate shape (feature), the plane locating surface must be sufficient in size in order to be applicable for plane locating. The dimensions of candidate features for plane locating must be compared with the three overall dimensions of the workpiece (Fig. 6). 3.2 Surfaces suitable for side locatingThe suitable features are established by each side locating type. Side locating can be divided into guide locating and endwise locating. Hence, suitability tests are to be performed separately for guiding and for endwise locating.Suitability of guiding must be tested from three aspects, i.e. according to shape of the surface, dimension and position of the surface.Suitability for endwise locating must be tested from two aspects, i.e. according to shape of the surface and position of the surface.3.2.2 Suitable surfaces for side locating type p2According to the shape of the surface, there should be two holes on the plane locating face. The typical dimension is the distance between holes, and they must not be less than 35% of the longest side of the plane locating face.Suitable surfaces for other side locating types are defined in a similar way.3.3 Suitable surfaces for clampingThe suitability of surfaces for clamping must be tested from four aspects, i.e. according to shape of the surface, position of the surface, clamping force flow and dimension of the surface. Suitable clamping surfaces have been established for every clamping type 204 The necessary inputs for setup and fixture planningIt seems that the feature-based workpiece model is unavoidable by the technological process planning. For the conceptual design of fixtures, however, besides the local data of features, the workpiece model must contain the global structure of the workpiece and must be given the possibility of the description of relationships between features as locating tolerances and dimensional tolerances. This problem is solved in a way that the whole workpiece is first reduced to six faces (top, bottom, left, right, front, back), and to each face, there is one or more equidistant plane designated, the position of which is determined by the distance to the workpieces zero point (Fig. 7). Each feature has a reference point the position of which is defined with two coordinates. That way, each feature position is defined to the workpiece zero point (and hereby also to each other). The distance tolerances and locating tolerances of the features themselves (Fig. 8) are vital parts of the workpiece of the model. The tolerances between features (which “interconnect” features) can be divided into loosely and strictly tolerance-related connections. The group of loosely tolerance-related connections consists of accuracy-related requirements, which can be actualised 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. In this group, the following types of tolerances have been determined:1. Locating tolerance(a) Parallelism or perpendicularity between two plane surfaces.(b) Parallelism or perpendicularity between the plane surface and axis.2. Distance tolerance(a) Between two plane surfaces if the tolerance zone is T0.2 mm.(b) Between the plane surface and axis if the tolerance zone is T0.2 mm.The group of strictly tolerance-related connections consists of accuracy-related requirements, which can be actualised with difficulty in cases when the connected surfaces are machined in two separate clampings, respectively; they need high accuracy of fixture and locating elements of the workpiece. This group contains the following types of tolerances:1. Locating tolerance(a) Parallelism, concentricity or perpendicularity between two axis.2. Distance tolerance(a) Between two parallel axis.(b) Between skew axis.(c) Between the plane surface and axis if the tolerance zone is T0.2 mm.(d) Between two plane surfaces if the tolerance zone is T0.2 mm.With these connecting types, the connected surfaces need to be machined in one clamping or rather such surfaces are machined in two separate clampings, only when there is no other solution. These attributes are extended also to the workpiece faces, so the faces of the workpiece which contains strictly connected features are strictly connected, and faces which contain only loosely connected features are loosely connected faces.The feature-based workpiece model is created primarily by recognition and has to be extended by an expert.5 Setup and fixture planningMost researchers divide the fixture planning process into several steps: (1) setup planning, (2) conceptual design of fixture, (3) fixture configuration 8, 15, 21.In the authors opinion, the setup planning and the conceptual design of fixture are so interconnected that the practical planning tasks cannot be divided into separate fixture or rather setup planning tasks. The new method makes the integrated handling of tasks of setup and fixture planning and finding a solution in an integrated system possible.It is assumed that the machining is carried out on the horizontal machining centres. At best, four faces of the workpiece are machinable on the horizontal machining centres in a one setup. Since one box-shaped workpiece has six faces, so it is almost always machinable in two setups. The question is which faces must be machined in one setup or to paraphrase it, how does one select the workpiece position in the workspace of machine tool? These questions can be answered only by analysing the accuracy requirements of the workpiece. It is obvious that the most straightforward way to the realisation of the prescribed tolerances is to machine those feature in one setup, which are interconnected by tolerance.The definition of the workpiece position in the workspace of machine tool, and the setup definition must be done according to the location of the functional (connected) features in the structure of the workpiece. However, the workpiece position selected in this way can be accepted only when it is suitable for fixture solution (namely suitable for supporting, locating and clamping). This fact necessitates the integrated approach of setup planning and the conceptual design of fixture.The setup, i.e. clamping, in which the functional surfaces or most of them are machined is denoted by main clamping, while the setup in which the rest of the surfaces are machined is called additional clamping. Regardless of the clamping sequence, it is the main clamping, which is solved prior to the additional clamping.Based on the above statements and restrictions, the setup and fixture solution consists of the main clamping and additional clamping solution (Fig. 9).6 General solution concept of main clamping fixture and operation sequenceWhen solving the main clamping, one must try to find a position of the workpiece in the workspace of machining centres in which the machining of all connected faces is possible. In this way, we can reach great accuracy of the workpiece, and at the same time, the accuracy requirement and the complexity of fixture are minimal. This position of the workpiece is denoted by the “technologically ideal workpiece position”. The fixture received this way is the best possible fixture solution.However, in several cases, the disposition of connected faces is such that the workpiece cannot be held in a technologically ideal workpiece position. In this case, one has to aim at finding a position of the workpieces in which at least the machining of strictly connected faces is possible in one clamping. In other words, the loosely connected faces in this stage are disregarded. This way, the fixture solution is still “fair”, but the accuracy requirements refer only to the parallelism or rather to the perpendicularity of any fixture surfaces (see Section 4).If this attempt is not successful, then one must give up the idea of machining all of the strictly connected faces in one clamping. At the same time, there are cases when not the entire face but the strictly connected features (holes) on the face are workable in one clamping with other strictly connected faces (see Section 6.3).Lastly, when none of enumerated attempts are successful, one is forced to machine the strictly tolerance-related features in separate clampings. In this case, the accuracy requirements of the fixture are very high.Taking into consideration the above-mentioned aspects, four strategies are devised for solving the main clamping (Fig. 9):& Fixture solution for technologically ideal position of the workpiece& Fixture solution based on disregarding the loosely tolerance-related faces& Fixture solution based on the reduction of a strictly tolerance-related side into loosely tolerance-related surfaces and strictly tolerance-related surfaces& Fixture solution by disintegration of strict functional relationshipsThe strategies stated so far do not yield solutions equally suitable for the fixture. The best results are obtained by applying the first strategy. The second strategy is applied only in cases when the first strategy fails to produce a solution, etc.6.1 Main clamping fixture solution for technologically ideal position of the workpieceThis strategy is aimed at finding the position of the workpiece in the machine workspace, which would allow the machining of all connected faces in one clamping. The potential position of the workpiece in the machine workspace is determined by the requirement that all connected faces should be machined in one clamping. Depending on the plane locating type, this can be formulated as follows:& In the case of the horizontal plane locating typepos1the plane locating face and its opposite face must not be tolerance-related faces because it is not possible to machine them together with other faces in the same clamping.& For a vertical plane locating typepos2the plane locating face, the face which is facing the machine table and its opposite face, must not be tolerance-related faces.Since the horizontal plane locating (pos.1) allows machining of four faces and the fixture construction is simpler, the system first attempts to generate a fixture solution for the horizontal plane locating type. If that attempt fails, a second attempt is made at solving the fixture for the vertical plane locating type. In order to finally adopt the selected workpiece position, it should be suitable for the workpiece fixture solution. A certain position of the workpiece is suitable when it is easy for plane and side locating and clamping (Fig. 10). in the next step, a suitability check for side locating is performed. The selected workpiece position is suitable for side locating if for some of the possible types of side locating there are suitable surfaces on the appropriate workpiece face. In the next step, the suitability for clamping is checked.If, for some of the possible types of clamping, there are suitable surfaces on the appropriate workpiece face (or faces), then the supposed workpiece position is considered to be suitable for clamping. Since suitability for plane locating and side locating has already been checked for in the previous stages, the fixture solution has been reached from the point of technologically ideal workpiece position. If the supposed workpiece position is not suitable for clamping, the position is eliminated, and a new position is selected, which is again checked for plane locating, side locating and clamping suitability.Considering that the workpiece is six-sided and that, in principle, any one of the faces can be chosen for the plane locating face, the workpiece has six possible positions in the machine workspace for each of the plane locating types, pos1 and pos2. If none of the possible workpiece positions meet the discussed requirements, the fixture solution strategy based on the ideal workpiece position cannot yield solutions and should be abandoned.6.2 Fixture solution based on disregarding the loosely tolerance-related facesSince the first strategy, namely the assumption that all connected faces must be machined in one setup, has failed to produce a fixture solution, one is forced to reduce this requirement and be content with the workpiece position where in the machining of all strictly tolerance-related faces is possible in one setup. This strategy is similar to the previous one, with the exception that the loosely tolerance related faces are omitted during the process of selection of the workpiece position in the machine workspace. This can be formulated in the following manner:& In the case of the horizontal plane locating type, the plane locating face and its opposite face must not be strictly connected faces.& For vertical plane locating, the plane locating face, the face which is facing the machine table and its opposite face, must not be strictly connected faces.In order for the assumed workpiece position to be adopted according to the previous requirement, it has to be suitable for plane locating, side locating and clamping. With this strategy, main clamping is always the second one, which implies that only finished surfaces can be used for locating. Suitability of a workpiece position is checked in the same manner as with the first strategy.6.3 Fixture solution based on the reduction of a strictly tolerance-related side into loosely tolerance-related surfaces and strictly tolerance-related surfacesThis strategy is resorted to in cases when the first two strategies fail. The main point of this strategy is to find such a fixture solution, which enables the machining of at least all strictly connected features in one setup, if the machining of all strictly connected faces of the workpiece is not possible. It employs the analysis of strictly tolerance-related faces in order to find such a face, which, besides strictly tolerance-related holes, also includes tolerance-unrelated or loosely tolerance-related faces, which are suitable for locating and clamping. If such a face exists, then all the included surfaces, except the strictly tolerance-related ones, are machined in additional clamping. That face will be the plane locating face in the main clamping where all the strictly tolerance-related workpiece surfaces, including these on the plane locating face, will be machined. Since in this case some surfaces on the plane locating face are machined, the plane locating type is always vertical with partial machining of the locating face (pos3). A more precise definition of the workpiece position follows from the condition that the face facing the machine table and its opposite face must not be strictly tolerance-related. The logic behind this strategy is portrayed with the ANDOR graph in Fig. 11. This fixture solution allows machining accuracy without special requirements in respect to fixture accuracy but nevertheless increases fixture complexity. 6.4 Fixture solution by disintegration of strict functional relationshipsShould none of the discussed strategies succeed in generating the desired solution, one is forced to resort to machining the strictly tolerance-related surfaces in separate clampings. For this reason, the fixture for the second clamping must be highly accurate, which often presents a key issue in the process of meeting workpiece accuracy demands. By now, one needs to analyse only the workpiece positions where the plane locating face is one of the strictly connected faces. Namely, the other possible positions of the workpiece are already analysed through the application of the first three strategies, but no solution has been found. The first step involves an attempt to find by horizontal plane locating such a workpiece position in the machine workspace, which ensures that the face opposite the plane locating face is not strictly tolerance-related. If this attempt fails, then both the plane locating face and its opposite face are allowed to be strictly tolerance-related. The selection of type of side locating must be done with special attention, performed on the basis of the type of strict face connection. This strategy is described in greater detail in 19, 20.6.5 Definition of the content of setupsUpon solving the main clamping fixture, the following facts have become known: plane locating type, plane locating face of the workpiece in main clamping, plane locating surfaces, side locating type, guiding and endwise surface, clamping type and clamping surfaces. Based on these facts, it is possible to reduce technological operations to main and additional clamping. Thus, it has to be defined which features are machined in the main clamping and which in the additional clamping. This task can be relatively easy when the plane locating type in main clamping is horizontal or vertical. In these cases, it is sufficient to define the faces (and certainly all features which belong to some face), which will be machined in the main clamping or rather in the additional clamping. When the plane locating type is vertical with partial machining of the plane locating face, then the reduction of technological operation to main and additional clamping presents a more complex task due to the fact that the main clamping plane locating face has to be partially machined in both main and additional clamping. For this reason, it should be precisely defined which surfaces on the plane locating face should be machined in the main clamping and which should be machined in the additional clamping (Fig. 12). Sequentially ordered, with the exception of one case, the additional clamping takes precedence. The main clamping takes precedence in that case when the fixture solution is done in a technologically ideal workpiece position but in this position, the workpiece has only rough surfaces suitable for plane locating.7 Conceptual solution of fixture for additional clampingThe conceptual solution of fixture for additional clamping follows after the reduction of the technological operations to main and additional clamping. At first, one must have a look at the necessity of additional clamping because such situations may occur, when the workpiece contains only four or less faces for machining, and they can be machined in the main clamping. The workpiece position within the machine workspace is selected in such a way as to allow machining of those workpiece faces, which are scheduled for machining in additional clamping. With auxiliary clamping, horizontal and vertical plane locating types may be applied. The assumed workpiece position has to meet plane locating, side locating and clamping requirements.8 Implementation and test examplesThe Integrated Process Planning and Fixture Planning System are made up of several modules presented in Fig. 1. The module for Setup and Fixture Planning is implemented in Visual Prolog programming language. The feature-based workpiece model, which is created in module for CAD model post-processing, is the input of the Fixture Planning System. Based on this input, the system generates an acceptable solution for fixtures and setup definition. Search for a fixture solution is carried out automatically, but in certain cases, the solution offered by the system must be approved by the user by pressing the “Y” key or rejected by pressing “N”. This may primarily be necessary regarding the selection of the clamping points, as it may happen that the clamping element obstructs the machining, and at its current level of development, the system is unable to detect inference.8.1 Test exampleThe approach described above has been tested on industrial parts. Presented in Fig. 13 is an example of a typical workpiece. The surfaces (features) of the workpiece are marked with numbers on the drawing. With geometrical and tolerance analysis of the workpiece, the following facts are known: Strong connected features of the workpiece are the features marked with the numbers 2, 3, 12 and 13. Loose connected features are marked 1 and 11. Respectively, the front and the back faces are strong connected faces and top , bottom , left and right faces are free faces. In the first step, the system tries to solve the main clamping, applying the first strategy, namely the main clamping fixture solution for the technologically ideal position of the workpiece. The workpiece can be placed in the workspace of the machine tool in the technologically ideal workpiece position if, for example, the plane locating face is at the bottom because the bottom face and top faces are not tolerance-related faces, but the bottom face is not suitable for plane locating (see Fig. 10). Hence, this attempt will fail. The same problem occurs by supposing the plane locating face is the top, left or right face. That is why this strategy must be skipped and the second strategy be applied.By using the second strategy, namely the Fixture solution based on disregarding the loosely tolerancerelated faces, the same problems are likely to occur as with the first strategy.The test results of the expert system are presented in Table 1 in the form of facts. With the help of the post-processor, this output in the form of facts can be presented in another form, as well.9 ConclusionThe definition of the typical partial task of the fixture, the definition of suitable surfaces for locating and for clamping, the discovered logical connection, practical relationships, and the presented new method for setup and fixture planning make the automation of the conceptual design of fixtures and the integrated handling of tasks of setup and fixture planning possible.References1 Veilleux Raymond F, Petro Louis W. Tool and manufactu
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