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Automated setup and fixture planning system for box-shaped parts
Automated setup and fixture planning system for box-shaped parts
Michael Stampfer
Received: 18 September 2008 / Accepted: 16 February 2009 / Published online: 4 March 2009 # Springer-Verlag London Limited 2009
Abstract
The 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 planning
1 Introduction
The 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. Horváth 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 [15–17]. 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 solutions
For 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 clamping
Besides 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 locating
The 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 locating
The 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 p2
According 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 clamping
The 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 [20]
4 The necessary inputs for setup and fixture planning
It 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 workpiece’s 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 T≥0.2 mm.
(b) Between the plane surface and axis if the tolerance zone is T≥0.2 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 T<0.2 mm.
(d) Between two plane surfaces if the tolerance zone is T<0.2 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 planning
Most 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 author’s 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 sequence
When 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.