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Development of a Decision Support System for Fixture Design Manisha Yadav and Suhas Mohite Mechanical Engineering Department, Govt. College of Engineering, Karad, PIN - 415124, Dist- Satara, (M. S.) [email protected]

Abstract. A comprehensive decision support system is developed to design fixtures for machining centers. The CAD interface provided to the system enables it to exchange drawings and data with other commercial CAD software tools. A set of structured queries incorporated in the preprocessor prompts the designer to extract qualitative and quantitative part features. The database, rule base and knowledge base built into the design module assist the designer to select an appropriate fixture body, position on to it various fixture elements, calculate clamping forces, decide number and types of clamps with their locations and orientations, etc. The post processor, finally, generates the bill of material, part drawings and assembly drawing of the designed fixture. The software is implemented in a fixture manufacturing industry. It is observed that, the design lead time for fixture is reduced from a few days to a few hours with fractional efforts and expertise of the designer. Keywords: fixture design, feature extraction, decision support system.

1 Introduction Fixture is a device used in machining, inspection, assembly, welding, and other manufacturing operations to locate and hold a work piece firmly in position. Fixture plays an important role in ensuring production quality, shortening production cycle time and reducing production cost. Fixture design, fabrication, and testing consume a substantial portion of the product development time. Traditionally, fixture design is performed manually for the selection of locators, rest-pads, work-supports, fixtureplate and the design of clamps. In a consumer driven market environment, the product mix has large variety and small batches which also means a correspondingly similar need for fixtures. Therefore, there is a need for automating the fixture design activity to ensure efficient and effective fixture design. The computer-based automation of the fixture design activities is commonly referred to as computer aided fixture design (CAFD) [1]. In the next session we will discuss the need of CAFD. 1.1 Need of CAFD Design of a fixture is one of the important tasks. The design and manufacturing of fixtures requires a few weeks. Of these, the manufacturing lead-time for fixtures has K. Shah, V.R. Lakshmi Gorty, and A. Phirke (Eds.): ICTSM 2011, CCIS 145, pp. 182–189, 2011. © Springer-Verlag Berlin Heidelberg 2011

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been considerably reduced due to the availability of standard parts such as standard base plates, support plates, locators, clamps and other accessories, which form the basic building blocks of fixtures. So, once a fixture is conceptualized, manufacturing the fixture is only making the assembly of the modular parts. However, designing fixture requires expertise and knowledge of experienced designer and hence, it is highly human dependent. Availability of sophisticated software such as FIXES, FIXTURENET, etc. is addressing this issue; but these software are not affordable to small scale industries. Also, the needs of these industries are specific and limited to their scope of design and manufacturing activities. This necessitates having simpler and cheaper software tailored for the specific needs of these industries. In this paper, fixture design software developed with Visual Basic 6.0 as a frontend tool is presented. The database is organized in MS-Access as a back-end tool.A drawing library of fixturing elements drawn in Pro-E is linked to the database [2]. The detailed structure of the software of the developed system is discussed in the next section. 1.2 Structure of the CAFD Software The CAFD software consists of three modules, viz., preprocessor, processor and postprocessor as shown in fig. 1. The information contained in the part drawing is an input to this system. Extracting this is really a challenging task. The most important step in

Fig. 1. The structure of CAFD, showing preprocessor, processor and postprocessor stages

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fixture design, viz., understanding the part drawing in Pro-E or AutoCAD, is accomplished by a query of the part drawing prompted by questionnaires in the preprocessor. The system is developed in an algorithmic manner in such a way that at every stage it navigates the designer to extract qualitative part features such as shape, material, operations, etc. by selecting from the menu items. Alongside, a database of locators, clamps, work support, etc. integrated with the software provides various options to the design engineer to choose from. Next, decision making or processing task involves identifying appropriate locating surfaces and choosing suitable locators to constrain maximum number of degrees of freedom of the component. This is followed by computation of the clamping forces and decision on the number and location of clamps, work supports and the base plate. At every stage a series of checkpoints are provided to remove the discrepancies in selection of fixturing elements. The checkpoints are the series of questionnaire provided to foolproof the selection of elements. In the post processor task, the output is generated as a bill of material (BOM) and drawings of the elements of the fixture. Various case studies have been carried out to assess the effectiveness of the system developed. One such case study is reported in the next section.

2 Case Study Here, we present a case study on fixture design for a gear-box housing carried out using the CAFD software at Datta Tools Pvt. Ltd. Karad, a tool room solution provider. Figure 2 shows the solid model of the gearbox housing in front and rear view. The following three subsections briefly describe the automated fixture design procedure leading to the bill of material and the assembly drawing. 2.1 Preprocessor Task In the preprocessor task, details regarding component, machine tool and operation are extracted from 2D CAD drawing interactively as prompted by the software. 2.1.1 Component Details In this, important physical features of the component are extracted. To enable systematic storage and retrieval of the fixture design case, the drawing is coded for various attributes using group technology guidelines. Table 1. Component details selected from the menu options provided by the software Component details Drawing code Material Raw material form Shape Size Weight Axis symmetry Other information

Specifications 02B2Q4V3JG1017 Cast iron As cast 2D box 700mm x795mm x136mm 90 kg Symmetric e.g., specific constraints, etc. given by the customer


(a) Front side

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(b) Rear side

Fig. 2. Solid model of the gear-box housing showing location holes

2.1.2 Machine Details In this, technical specifications of the machine tool on which the component is to be machined are listed. These usually are given by the customer. The fixture must be designed within the constraints and limitations posed by these specifications. Table 2. Machine specifications fed to the CAFD system as provided by the customer Machine Parameters Type of machine Power Table size Pallet Loading capacity Hydraulic hose ATC tools Max. tool dia. x length Max. weight of tool

Specifications Vertical Machining Centre (VMC) 11/15 kW 1500mm x 810mm Rectangular 2000kg Not available 24 No. ø100 mm x 250mm 8 kg

2.1.3 Operation Details In this, various operations that are already performed and required to be performed on the component are extracted along with the dimensional and the form tolerances [3]. In Table 3, important operations have been listed with dimensions and tolerances. Thus, all the relevant information regarding the component, machine tool and operations necessary for fixture design are fed to the system. With this, the preprocessor task ends. Next, the design (processor) task is performed for the component with the CAFD software. Table 3. Operations to be performed on components with dimensions and tolerances Sr. No. 1. 2.

Operation

Geometric Tolerance Ø 75.0mm Boss face-milling Ø 15.8mm Drilling and tapping (5/8 BSW) -

Dimensional Tolerance 100 μm 50 μm

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2.2 Preprocessor Task In the processor, the system assists the designer to take decisions on the selection of the fixture elements with the help of the knowledge and rules incorporated in it. The selection of important fixture elements is described in the next subsection. 2.2.1 Locator Selection The software helps the designer in locator selection by evaluating errors for different alternatives available among the locating surfaces and selecting the one with minimum error of location [4]. Two location holes L1 and L2, ø6.3H7 diameter are selected for location (see, fig. 2). Figure 3 shows 2D part drawing on a digitized window accessed by the CAFD software. The locators L1 and L2, their types, coordinates, locating surface and orientation are displayed on the screen [5], [6]. Next, we see the clamp selection.

Fig. 3. Locator selection form showing CAD interface and details of location and orientation

2.2.2 Clamp Selection The clamp selection is made in the software through the links provided to the operation details input. All the values from major operation are selected for the calculation of forces, cutting power and number of clamps. The cutting force is 527 N. Four hydraulic swing clamps provide the clamping force of 4x131.84= 527 N for face milling


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operation. Therefore, the clamping force is equal to the machining force. Fig. 4 shows 2D part drawing on a digitized window accessed by the CAFD software. The clamps CL1, CL2, CL3 and CL4, their direction, position, type, number of clamp, and machining and clamping force values are displayed on the screen. Next, we see the clamp selection. Next, we see the rest pad and work support selection.

Fig. 4. Clamp selection form showing clamp positions on component drawing (CL1, CL2, CL3, and CL4), type and number of clamps, machining and clamping forces

2.2.3 Rest Pad and Work Support Selection The software subsequently helps the designer for selection of the rest pads and work supports. The digitized display shows the coordinates of the position of the rest pads and work supports. As major operation of milling is carried on the two bosses of diameter 75 mm, there is only one work support used between them. In the next subsection, the fixture plate selection process is discussed. 2.2.4 Fixture Plate Selection The software helps to select the fixture plate through the links provided to component details and clamp selection form. The thickness of the fixture plate depends on the weight of the component, its volume and the magnitude of the cutting force. In Fig. 5, the 2D drawing of the component shows the positions of all the fixturing elements on the component along with the selection of the fixture plate. Standard parameterized parts are selected from the database are marked on the base plate with different colours. Next, we discuss the post processor task.

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Fig. 5. Base plate selection and final assembly drawing as developed using CAFD system

2.3 Post Processor The post processor generates the output in the form of the bill of material as listed in Table 4 and the part drawings and the assembly drawing prepared by the designer. Table 4. The bill of material generated by CAFD software for the designed fixture Sr. No. 1. 2. 3. 4. 5. 6.

Fixture Elements Fixture plate Cylindrical locator Diamond locator Swing clamps Work-support Rest-pads

Specifications CI / 835x800x60 EN-24 / Ø 13.5g6 EN-24 / Ø 13.5g6 Model no. 41-5233-21/22 Model no. 41-0050-02 EN24/Ø66x18.5/HRC50

Qty 1 1 1 4 7 4

3 Implementation and Conclusions The computer assisted fixture design software, CAFD, has proved to be a quick and effective tool for feature extraction and efficient decision making for the selection of fixture elements finally leading to a comprehensive solution in the form of fixture assembly and the bill of material. The CAFD is implemented at Datta Tools Pvt. Ltd.,


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Fig. 6. Gear box housing with the component and fixturing elements assembled on fixture plate

Karad, a tool room solution provider. Fig.6 is the final photograph of the manufactured fixture with all elements along with the work piece assembled on to it. The software assists the designer at every stage of the design by taking into account all input parameters and helps in selecting the major fixturing elements accordingly. The design lead time is reduced from a few days to a few hours. Further, a major part of the repetitive design procedure has become independent of an individual designer thereby relieving the designer to spend more time on creative aspects of the design.

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