Chapter 2 LITERATURE REVIEW

Transcription

1 11 Chapter 2 LITERATURE REVIEW 2.1 INTRODUCTION Industrial world has witnessed significant improvements in product design and manufacturing since the advent of computeraided design (CAD) and computer-aided manufacturing (CAM) technologies. Although CAD and CAM have been significantly developed over the last three decades, they have traditionally been treated as separate activities. Many designers use CAD with little understanding of CAM. This sometimes results in design of nonmachinable components or use of expensive tools and difficult operations to machine non-crucial geometries. In many cases, design must be modified several times, resulting in increased machining lead times and cost. Therefore, great savings in machining times and costs can be achieved if designers can solve machining problems of the products at the design stage. This can only be achieved through the use of fully integrated CAD/CAM systems. The need to integrate CAD and CAM has long been recognised and many systems have been developed. Kalta and Davies [1994] developed an IGES pre-processor to integrate CAD and CAPP for turned components. In a different work, a rule-based system was developed by Jacobs et al., [1991], for converting engineering drawings into numerically controlled machine programs for two dimensional

2 12 punch objects. Lye and Yeo [1992] and Thakar et al., [1993] developed integrated CAD/CAM systems for design and manufacture of turned components. The fact is that CAD/CAM has close to five decades of history stretching almost as far back as the computer. CAD/CAM, like the digital computer, had its inception in the military. In the mid-1950s the U.S. Air Force began testing an air defence system known as SAGE (Semi-Automatic Ground Environment) to graphically depict data received on radar systems. The first computer actually rendering a program, SAGE was conceived at the Massachusetts Institute of Technology (MIT). In 1960, computer scientists at MIT produced yet another project called Sketchpad, an application that is now considered to be the first design program with industrial use. A similar program sprouted up at General Motors soon after. At that time, mainframes were still large enough to take up entire rooms. During the 1960's CAD/CAM technology continued to evolve and spread to other areas. Automotive companies were the first to adopt the technology, and used it primarily to design automobile bodies. It then quickly spread to other sectors of industry, which were only too eager to abandon traditional pen and paper methods of drafting. By 1973, CAD/CAM was being used to design industrial tools. Midway through the decade, the 19-inch monitor came out, which meant that drawings could be viewed larger than the previous standard of 11 inches. In the last half of the 70's, solid modeling software became available. It allowed users to take "geometric

3 13 primitives" (basic geometric shapes such as boxes and cones) and combine them using Boolean operations. In 1982, Autodesk made CAD/CAM history when it released the first version of AutoCAD, which soon became the premiere software platform for automobile design. The need to integrate CAD and CAM has long been recognised and many systems have been developed. However, fully CAD/CAM integration is not yet achieved. The process of integration of CAD and CAM has been relatively slow in comparison with the developments made in each of these technologies. Researchers believe that the slowness of the integration over the recent decades is essentially attributable to the incompatibility of database formats and the lack of common languages. The developed systems can only be considered as islands of integration and still there is a missing link fully integrating these two technologies. In recent years, many researchers have been contributing their part towards the integration of CAD/CAM and this chapter presents a review of their work. 2.2 COMPUTER AIDED DESIGN (CAD) Computer- aided design (CAD) is the technology concerned with the use of computer systems to assist in the creation, modification, analysis and optimization of design [Groover M.P]. Thus any computer program facilitating engineering functions in the design process is classified as CAD software. In other words, CAD tools can vary from geometric tools for manipulating shapes at one extreme, to customized

4 14 application programs, such as those for analysis and optimization, at the other extreme [Zeid.I 1998]. Encarracao [1987] had presented a survey paper on CAD systems and applications. CAD has revolutionized the function of computer technology in terms of processing design, documentation and implementation. In the case of CAD, there were a number of people and institutions working on the idea of computer graphics for design. But the individual generally accorded to have been first with the most is Dr. Ivan Sutherland, who did his seminal work with a system called Sketchpad at the Massachusetts Institute of Technology(MIT). Parallel work was being done at the General Motors Research Laboratories. One of the people involved with the General Motors CAD project was Dr. Patrick Hanratty, who later left GM to form his own company and develop the first commercially available software for mechanical drafting. Dr. Hanratty's software was used as the basis for nearly a dozen start-up companies selling turnkey CAD programs. Today, an estimated 90% of commercial drafting software can trace its roots back to Dr. Hanratty's original program, called ADAM (Automated Drafting And Machining). CAD has long been integrated in most of our societal affairs. It is mostly applied in automotive, architecture and other important social and mechanical industries.

5 FEATURE RECOGNITION SYSTEM Feature recognition is the important link in the process of Integrating CAD and CAM. It forms the basis for extracting the design features from the CAD files and correlating them to the CAM in a more meaningful way. Bojan Babic et al., [2008], has made an excellent survey on extant literature on automated feature recognition with rule-based pattern recognition. They present detailed classification of developed Automatic Feature Recognition (AFR) systems with their potentials and limitations. Ismail et al., [ ], in a series of papers presented Edge Boundary Classification (EBC) approach, considering the use of spatial addressability information of solid models that identifies the solid and void sides of a boundary entity. Their approach can be applied for recognition of some features in parts to be produced on 2 Dimensional NC machines; pockets, slots and steps and also for cylindrical and truncated conical features, both in prismatic and rotational parts. One of the earliest works in feature recognition is by Woo [1984]. He proposed a technique known as Alternating Sum of Volumes (ASV), expressions for extracting features from the boundary representation of 2-D or 3-D parts. Kakazu [1984] presented an approach that can generate group technology codes for axial geometry parts (rotational) from a Constructive Solid Geometry (CSG) model. The generated GT code is

6 16 made up of three kinds of code systems; unique geometry code, macro geometry code and semantic code. Joshi and Chang (1987) described a method for identifying features. The method uses a graph called the Attributes Adjacency Graph (AAG) from a BREP database. Faces form the nodes of this graph and edges form the arcs, the arcs being labeled as convex or as concave. Rules are defined to recognize certain configurations in the graph. Since this technique requires an exhaustive search, a heuristic method is adopted to limit the search. Rong-Kwei Li [1988] proposed a part feature recognition algorithm for rotational parts. This system serves as the intelligent part feature recognition system which extracts part definition data from CAD systems using IGES standard data format. This system is implemented in PROLOG language. The algorithm is based on the occurrence of a string of certain primitives. However, the feature recognized is constrained by the type of pattern defined. Somashekar Subrahmanyam and Michael Wozny [1995] presented a comprehensive survey paper on feature recognition techniques for CAPP. They discussed on various methods used in feature recognition like cell division, cavity volume, convex hull, laminae slicing and other methods. The authors stressed upon the future needs or scope, to strengthen the link between CAD and CAM. Carlos Rico et al., [1997] proposed a methodology to recognize the part geometry based on a combination between an automatic identification of 2D profiles and a feature- based method. The authors

7 17 established the minimum number of grippings (based on the part shape), the blank type and the geometrical tolerances to determine the machining operations. Nafis Ahmed and A.F.M Anwarul Haque [2001] proposed manufacturing feature recognition of a rotational component using DXF file. They used a C/C++ program to extract the information from the DXF file. The authors could extract the information only when the profiles are drawn with Polyline. They classified the features as horizontal, vertical, inclined and curved. Gibson et al., [ ] describe the structure and operation of a prototype feature recognition system ipfr (Interactive Programmable Feature Recognisor). The system proposed by them can accept entity descriptions from CAD systems in the form of DXF files through a DXF translator. H.C.W.Lau et al., [2005] in their paper made an attempt to introduce a computer-integrated system for design feature recognition in order to achieve automatic process planning. They proposed two modules; (1) STEP feature recognition module to extract a solid design model, which is converted to group technology (GT) code that can be used as input data to recognize feature patterns. (2) Rule-based process planning module to map the features to the appropriate operations. Aslan. E et al., [1999], proposed a feature extraction technique, for rotational parts, based on step-wise examination of geometric data and gradual identification of basic meaningful features from DXF files.

8 18 Zuraini and Habiollah [2002] presented a framework of feature extraction from standard DXF file for the prismatic parts. Their work focused on code classification algorithm to read and classify DXF code and mapping algorithm to get the dimensional information necessary for the selection of machining parameter in milling machine. B.S. Prabhu et al., [2001] presented a system for automatic extraction of geometric and non- geometric part information from Engineering drawings created using CADD tools. The authors used a heuristic search to interpret characteristic attributes of dimension sets that denote linear, diametrical, radial and angular dimensions. They used Natural Language Processing (NLP) techniques to process textual callouts. Jain P.K and Kumar S [1998] proposed a system that takes a wire frame part representation model, imported from AutoCAD (*.dxf file). Their system provides the recognition of several form features for prismatic parts: hole, step, slot and protrusions with orthogonal boundary faces. In extension to their work, graphs are translated in strings and then the strings are matched with the patterns in a knowledge base. Ismail et al., [2002] in their paper focused on Edge boundary Classification (EBC), using the spatial addressability information of solid models that identifies the solid and void sides of a boundary entity, for syntactic pattern recognition.

9 19 Henderson and Anderson [1984] introduced the logic approach, on B-rep solid modelers. A set of production rules, defining form features, provide the patterns for automatic feature recognition. Woo [1984] used the geometry-decomposition approach, for feature recognition, where features are viewed as volumes to be removed by a machine operation. He introduced the concept of cavity features, i.e., features, as primitives, are the material to be removed or machined. Given a CSG tree of primitive volumes and the Boolean operators used in building a part, the cavity types were deduced from the spatial relationships of the primitive volume faces. Vosniakos and Davies [1993] proposed a shape feature recognition framework and its application to holes in prismatic parts. They discuss a system consisting of a feature definition part and a feature matching part, operating on a B-rep model. Vandenbrande and Requicha [1990] presented a hint- based method for recognition of machinable features. The major claim of this method is that the concept of generating hints for features and further checking the validity of these hints is more efficient than procedural rule-base techniques. Some of the limitations as stated in this work are that the method applies to 2.5 D swept features only. The decompositions produced by their algorithm are not controlled. Wong and Wong [1995] paper describe the development of an object- oriented feature-based design system for integrating CAD and CAPP. The authors have used AME (Advanced Modelling Extension) of the AutoCAD as the geometric modeller. The design information of a

10 20 prismatic part is transformed into the manufacturing instructions with the knowledge provided in the rule-based feature mapping system. Kim et al., [1996] in their prototype Feature Based Automated Process Planning (FBAPP) system proposed to recognize the features from the removable volume point of view rather than from the design part point of view. A feature based approach for cylindrical surface machining process is developed with a provision of generating precedence relations between the machining volumes. Ismail et al., [ ] discuss the extraction of cylindrical based features from a neutral data format, namely STEP (Standard for the Exchange of Product Model Data) file produced by any CAD system. The authors developed a rule-based algorithm for the extraction of the cylindrical features. Bandarkar and Ragi [2000] developed feature extraction system takes STEP file as input and to define the geometry and topology of a part. In addition, the system generates STEP file as output with form feature information is AP224 format for form feature process planning. 2.4 CAPP / CAM Systems Shu-Chu Liu and Shih-Yaug Liu [1996] proposed a heuristic approach for Automated Process Sequencing in CAPP. Their work was focused on minimization of setup time. Constraints are used to determine which machine, fixture, and tool setup should be machined first. Then the constraints are used to sequence the order in the same tool setup.

11 21 Pande and Walvaker (1989) reported, the design and implementation of a computer assisted process planning system (PC- CAPP) for prismatic components used in the batch production of portable electric tools. The system proposed by them has various modules for component feature representation; automatic machine, tooling and process parameter selection. The system was more of a feature based process planning and demands much user intervention. Andrew Kusiak and Ranker Vujosevic [1992 ] presented an objectoriented approach for selection of machinable volumes to be included in a process plan. Their approach uses a feature-based part representation for prismatic parts. Volumes of the material to be removed are generated in parallel with definition of form features of the part. Later, geometric reasoning is used for grouping these volumes into a set of machinable volumes. Precedence constraints among the machinable volumes are established to generate a sequence of machinable volumes. Nafis Ahmad and Haque [2001] have proposed a system for the optimization of process parameters for rotational components by GA approach. The optimization function was to reduce the machining time under the constraints of maximum allowable force on the cutting tool and also maximum power limit of the machine tool. But the system was limited to roughing operations

12 22 H. B. Marri et al., [1998] presented a comprehensive review of literature on CAPP. They made a thorough insight into the design and implementation of different CAPP systems. They discuss the advantages and disadvantages of various CAPP systems and indicated the future research directions. Horris C. [1996] had classified the literature on Computer aided process planning into twelve areas of interest, starting from general references/reviews/surveys to emerging AI techniques. The study was also made on parallel machining, Petri nets and object oriented approach in CAPP. Yakup Yildiz et al., [2006] discuss automatic feature recognition and tool path planning for rotational parts, using Delphi 7 programming language. The authors have focused on File reading system, feature recognition system and tool path planning. Zhao et al., [2001] paper describes a novel concept for the integration of a CAD system and a knowledge based system for the selection of cutting tools. IGES neutral format is used as an input for the feature recognition. They presented an integrated system (CADEXCATS) which is a modular based system containing three main programmes designed to process the IGES data for roughing and finishing operations and finally to merge the output files into one single file that represents the final output.

13 23 M. Tolouei-Rad and G. Payeganeh [2006] presented a hybrid approach for automatic generation of NC programs. They discussed about the two expert systems. The first system communicates with the CAD system for recognizing machining features using an interface program developed in LISP. The second expert system requires extensive communications with several databases for retrieving tooling and machining information to develop Machining Data File (MDF). MDF is used as an input to a conventional CAM system to produce the required NC code. The proposed system works as a interface between conventional CAD and CAM packages, but lacks integrity. Joe G Chow and Robert L Sakal [1995] proposed a system to integrate AutoCAD and Mastercam using generative process planning interface programs written in Autolisp. Authors present turning and milling modules in their work. The turning interface program uses a 2-D drawing of the axisymmetric part and user interface to determine the machining operations to machine the part in Mastercam. The system proposed by them does not generate the toolpaths or NC code by itself, but interfaces directly with Mastercam. Thus it needs CAM software for the generation of NC codes. George Vosniakos [1998] made an attempt to generate automatic NC part programs for machined components of 2-1/2 dimensions from a wireframe CAD model. An IGES post-processor was developed to interface the system to any CAD system, drawing annotation information was allocated to solid model entities through pattern

14 24 matching techniques in Prolog. In his work, operation selection uses the feature model as well as process planning knowledge structured into a hierarchy of rules. The resulting operation sequence is communicated to a module that produces NC code. 2.5 CURRENT RESEARCH WORK COMPARE AND CONTRAST This research is different from the extant literature discussed earlier in this chapter in the following ways: 1. In this research work, the focus is on the extraction of design features from the DXF file format. As can be seen in the literature, few methodologies were proposed to extract the features, but they were limited to the profiles that were drawn either by basic line and arc commands or by polyline commands. Yakup Yildiz [2006], Chow J.G and Sakal R.L [1994]) and others worked on the extraction of the basic line and arc features. Whereas, Aslan et.al.[1999] and Nafis Ahmed [2001] had worked on the methodologies for the extraction of polyline features. The methodologies proposed by them wouldn t be able to extract the features, when the components are drawn with a combination of line, arc and polyline commands. In this work, a novel methodology is proposed to extract the features even when the profiles are drawn by a combination of those commands and irrespective of their sequence (Para 4.2.1).

15 25 2. In respect of CAD/CAM integration, there are mainly two approaches in using form-features, a) Feature recognition and b) Feature based design. a) Feature recognition endeavors to automatically recognize and extract appropriate features from a geometrical model. Several different approaches and techniques have been tried in various feature recognition systems. For example Woo[1984] used the geometry- decomposition approach where features are viewed as volumes to be removed, Joshi and Chang [1987] suggested an AAG (Attributed Adjacency Graph) approach, while Henderson [1984] worked on B-rep solid modelers. In this work, predefined pattern primitives were developed based on the adjacency relation among the surfaces of the features (Para 5.1.2). Pre-defined pattern primitives have been classified as External and internal predefined features. b) The feature based design approach, also known as designby-features or feature-based modeling, provides the designer with a set of features in the features library (Wong.T.N [1995]). A designer, applying the appropriate modeling operators such as Boolean set operators in the Constructive Solid Geometry (CSG) modellers, can build the product model with the predefined features. In this model, the designer can choose the manufacturing processes whilst working on the design. As a product is

16 26 being developed, the designer can invoke down-stream applications easily. In this model the user is limited to build the component by selecting the features in an sequential order (normally from left to right) with continuity in selection of features. But, in real time situations, different designers may adopt different sequential order in drawing the same given profile, i.e., they may draw the profile from left to right or from right to left or by other ways. In doing so the entity details are stored in their respective dxf files as per the happenings. Thus for same profiles drawn in different sequences the dxf files will be different. The proposed system can accommodate that differences and is capable of developing the CNC code irrespective of the way in which the profiles are generated. Its first output gives the entity details in the same sequence as they were used (Table 4.2) while in the second stage they are arranged in an order (Table 5.2), so that they can be used for the down stream applications. 3) Another salient feature of this research work is that, in the feature recognition module, in addition to the step turning, taper turning and profile turning algorithms, new algorithms for the extraction of groove / recess elements have been developed and the corresponding strings for syntactic representation have been developed so as to map them to the

17 27 manufacturing features along with the geometrical details. Aslan.E [1999] had made an attempt for the identification of angled recess by binary decision tree and to prepare a Features and Machining Parameters Array (FMPA). In this work algorithms were developed to extract the coordinate values of the groove features and subsequently map the details to the manufacturing features. 4) The proposed integration system is novel in its way and is not discussed in extant literature. Thus the current research outcome may provide vital contribution in the area of feature recognition and integration of feature recognition and manufacturing.