Tài liệu A Computer-Aided ppt

© 2001 by CRC Press LLC 4 A Computer-Aided and Integrated Machining System 4.1 Design Philosophy 4.2 User Interface 4.3 Feature Extraction 4.4 The Feature Recognizer AutoCad Data • Processor • Analyzer • Initial Feature Builder • Machining Feature Builder 4.5 The Knowledge Base 4.6 Factual Knowledge Data Content 4.7 Procedural Knowledge Selection of Operations • Sequencing of Operations • Selection of Tools • Selection of Machining Conditions • Generation of Tool Paths 4.8 Concluding Remarks In order to arrive at a suitable solution, be it for process planning or part programming, the pressing issues faced in an integrated manufacturing system are in the following. 1. Interpretation of CAD data for machining-based operations. 2. System integration in the activities involved in process planning, machinability data selection, and tool path generation. 3. Update information arising from progress in manufacturing technology. 4. Decision making relating to manufacturing tasks. 5. Optimization strategy suitable for CNC processing capabilities. The concept of an integrated machining system for the generation of production plans is introduced in this work. The development of methodologies to address the above issues is presented. This chapter expounds on the work reported by Yeo, Rahman, and Wong [1991] and Yeo [1995]. 4.1 Design Philosophy In this section, the concept of an integrated knowledge-based machining system is described. A software tool, GOLDWORKS TM [Gold Hill, 1989] with COMMON LISP as the programming platform, is used for the development work. The software tool requires Microsoft Windows ® in a PC which can be feasibly deployed in a wide range of industries. The system is aimed to provide a vital step to a totally integrated Swee-Hock Yeo Nanyang Technological University © 2001 by CRC Press LLC manufacturing environment. The integrated machining system (IMS) begins with the interpretation of a part drawing and transformation of the geometrical data into a frame structure of machining features. The system then assigns appropriate operations to all features identified. Automatic tool selection of holders and inserts follows before planning the entire operation sequence. For each operation, the machining conditions are determined. Finally, tool paths are generated for a selected CNC lathe. The assumption made in developing the proposed IMS is that component set-ups with suitable work- holding parameters are provided so that the solution generated by the IMS can be implemented. Proper set- up involves spatial reasoning, and it is a difficult task that an experienced machinist learns during an appren- ticeship. Varying methods of solution exist from simple rules-of-thumb, such as threshold ratio between length and diameter and analytical methods such as that of explained by Hinduja and Huang [1989]. The problem of specifying the appropriate set-up is further complicated by the variety of work-holding devices which include face plates and fixtures, mandrels, jaw-type chucks, step chucks, collets, and magnetic and vacuum chucks. The architecture of the system that has been developed in this research is illustrated in Figure 4.1. The system is comprised of a user-interface for interaction, a feature recognizer, and a knowledge base that contains facts and rules. Figure 4.2 shows the various types of approaches/strategies that have been used for the machining planning tasks. It also outlines the sections of this chapter for each of these tasks. The inferential strategy used for the decision-making process of the IMS is based on the goal directed forward chaining technique (which integrates both forward and backward chaining techniques) [Yeo, 1995]. FIGURE 4.1 Architecture of the integrated machining system. © 2001 by CRC Press LLC For a given goal, the chaining method uses rule sets to cluster forward-rules together, and each rule set is activated when an enabling pattern is matched and queried. The control structure consists of eight stages where each stage is made up to a rule set as depicted in the following with a brief description in the right column. The inferencing mechanism is fired by backward chaining via the THEN part of the control structure with the pattern named RUN-STATE SELECTION COMPLETED. Each rule set is activated sequentially when its enabling pattern (e.g., RUN-STATE FEATURE-RECOGNITION DONE) is matched and the inference mechanism performs forward chaining of all the rules related to the rule set. The benefits of having such methods are having one group of rules fired before another group and improving code efficiency of the matching process in which only appropriate rules for a particular state are used. Outputs obtained from the IMS are composed of two main parts namely (see Figure 4.1). 1. A process sheet for each set-up, which is a set of instructions consisting of the following. • Enumerated operations with lists of machining features. • Cutting tools (holders, inserts, drills, etc.) for the operations planned. • Positions of cutting tool mounted on the turret. 2. A CNC part program consisting of machine codes. FIGURE 4.2 Decisions modes in the integrated machining system. (DEFINE-RULE SELECTION-CONTROL ; production rule with a series of enabling patterns (RUN-STATE FEATURE-RECOGNITION DONE) ; define machinable features, stock size, etc. (RUN-STATE INTERNAL-OPERATION DONE) ; selection of internal operations (RUN-STATE EXTERNAL-OPERATION DONE) ; selection of external operations (RUN-STATE OPERATION-SEQUENCING DONE) ; assignment of operation sequence (RUN-STATE TOOLHOLDER-SELECTION DONE) ; selection of toolholders (RUN-STATE INSERT-SELECTION DONE) ; selection of inserts (RUN-STATE MACHINING-CONDITION DONE) ; selection of machining conditions (RUN-STATE TOOL-PATH-GENERATION DONE) ; generation of tool paths THEN (RUN-STATE SELECTION COMPLETED) ; query of the attempt to fire the rule © 2001 by CRC Press LLC 4.2 User Interface The purpose of this user interface is to provide interaction between the user and the system, data input, and data modification during the machining planning process. The user interface is driven by means of a series of menu-driven facilities and graphic images. These include the following. 1. A screen layout with a row of commands for pull-down menus. 2. A window with a set of buttons for performing machining planning tasks. 3. A window for schematic display of a task. 4. A status bar window to monitor the progress of the machining planning tasks. 5. A window for the textual explanation of the problem solving procedures. 6. Pop-up menus for the data input. In addition, there is a help facility for obtaining information regarding tooling, work materials, etc. Error trap facilities are also incorporated in the system, for example, input of a null value or an erroneous value. 4.3 Feature Extraction Since this research deals with 2-D operations for producing rotationally symmetric parts on lathes, a plan view of the upper half cross-section of the part drawing of the workpiece will suffice. The form of the profile is restricted to straight lines and arcs. The feature recognizer requires access to part descriptions stored in a CAD database. The approach that has been adopted in this work is to develop a module that involves inductive extraction of the geometrical data of a part created in a commercial CAD system. Its DXF (in AutoCAD®) facility provides the output format required by the IMS. Before any operation planning activity can take place, a set of features must be formulated for abstraction. Since this work is directed at metal removal processes, machinable features are used. A machinable feature is any geometric surface or combination of geometric surfaces which form a volume that can be shaped by cutting tool operations. The term “form features” is sometimes used. The necessary information about each feature is organized by means of a frame representation. This knowledge representation provides modularity and convenient accessibility of the knowledge base. 4.4 The Feature Recognizer The feature recognizer module has five steps which are shown in Figure 4.3 as described: AutoCad Data A section view of the top half of a part is created using AutoCAD®. Line-types of each entity and their significance have been defined as shown in Figure 4.4 so that the CAD data can be used effectively in the next step. All the entities representing the model and the stock size are exported using the AutoCad DXF format. Processor The Processor accepts a DXF data file and processes it into a form acceptable to the analyzer. The COMMON LISP list-data structure has been used to store the DXF data into a temporary file. An example of the temporary file is as follows: (0 SECTION 2 ENTITIES 0 POINT 8 0 LINE 8 0 ARC 8 0 ENDSEC 0 EOF) © 2001 by CRC Press LLC The temporary list file is then processed further by removing integer notations and group entities into a more refined list data file as follows: Each entity type and geometrical data is contained in a sub list. Analyzer The analyzer determines the nature of each entity and assigns to each entity a primitive feature which defines functionality. The primitive features include vertical_line, horizontal_line, slant_line, threaded_line, convex_up, convex_down, concave_up, and concave_down. The various line types are FIGURE 4.3 Structure of the feature recognizer. ((LINE (178.2 142.2) (221.3 165.2)) ; sub list for each ; entity type (ARC (141.7 151.7) 5.0 (180.0 270.0)) ) © 2001 by CRC Press LLC easily understood. The various arc types are illustrated in Figure 4.5. Their significance is used for tool selection. For the example used in (“convex up”), the sub list resulting from this analysis gives the primitive list data as follows: Each sub list describes the types of primitive. The entire primitive list data is sorted in order, starting from the left-most primitive of a part. Initial Feature Builder Using a frame lattice structure shown in Figure 4.6, the initial feature builder converts the primitive list data into embryonic objects (i.e., instances of frames) which are exclusively geometric. The machining features of a component are represented in the knowledge base in the form of frames with hierarchical FIGURE 4.4 Significance of the line type. FIGURE 4.5 Element library of arc types. ((LINE (178.2 142.2) (221.3 165.2) (PRIMITIVE SLANT_LINE)) (ARC (141.7 151.7) 5.0 (180.0 270.0) (PRIMITIVE CONVEX_UP)) ) © 2001 by CRC Press LLC relationships. The lattice structure consists of eight types of frame used for a rotationally symmetric part, which are: plane, horizontal, vertical, taper, arc, chamfer, groove, and thread. Seven of these types of frame may have associated instances. Two slots in the feature frame are used to provide connectivity to adjacent features. The plane-type frame groups three other frames which are all concerned with straight lines. An example of a taper feature instance in the language syntax form is shown in Figure 4.7. The EXT6 instance is related to the taper frame which is a child frame of the plane frame. Beside the representation of feature types, each instance is linked to its adjacent features (i.e., EXT5 and EXT7), thus forming a linked list. For convenient identification, each instance is uniquely named and enumerated, for example EXT and INT imply external and internal feature-types respectively, and the numbers appended to them are arranged in order starting from the left-most feature of the part. Machining Feature Builder The machining feature builder makes a complete set of all the machinable features of a part for the knowledge base. Production rules are used in the pattern matching process. The rules include recognition of grooves, chamfers, and threads. Surface roughness and geometric tolerances can be added manually to the attributes of a feature as appropriate. While the recognition of chamfers and threads are easily implemented, the recognition of grooves requires a more detailed geometrical treatment. A groove (or recess) in a machining profile is generally FIGURE 4.6 Frame lattice structure of the machining features for a rotationally symmetric part. © 2001 by CRC Press LLC described as being confined by two adjacent boundaries. This definition is very broad and might lead to inefficient automatic tool selection. Grooves may be required in a wide variety of forms such as: circlip grooving, O-ring grooving, face grooving, deep grooving, wide grooving, and undercutting. In this work, the recognition of groove is tool-oriented. There are various possibilities for machining a groove feature consisting of a horizontal element bounded by two vertical elements as illustrated in Figure 4.8. The use of two tools, that is, two longitudinal tools (Figure 4.8a) or a longitudinal tool and a grooving tool (Figure 4.8b), requires two tool positions on the turret; thus there is an economic implication. One grooving tool (Figure 4.8c) could be used to solve the problem of limited turret capacity. It may not be economical if a large number of passes is required, i.e., for a wide groove. To produce a rectangular groove feature, i.e., both the vertical elements parallel to the X -axis and the bottom of the groove parallel to the Z -axis, the external tool must have an approach angle of more than 90°, and the largest possible trailing edge angle. An insert shape of 35° included angle would be suitable. Figure 4.9 shows the best choice among the standard tools available for external operations. Using a 25 mm square shank toolholder, the groove depth (l) should not be more than 42 mm and width ( w ) at least 64 mm, depending on the clearance provided, hence, the critical w/l ratio is equal to 1.524. FIGURE 4.7 An example of a taper feature instance. © 2001 by CRC Press LLC As compared with this tool-type, a standard 25 mm square shank grooving tool has a w/l ratio of 1.63 (see Figure 4.10). Though the w/l ratio for grooving tools is marginally less than that of the external tool, the size of the groove can be as deep as 16 mm with 26.2 mm width. The former tool cannot produce this shape. FIGURE 4.8 Three possibilities for machining a groove feature. © 2001 by CRC Press LLC The machining feature builder is designed in such a manner that modification to the knowledge base can be done easily. Thus, the production rule for a groove can be extended to provide for a wide groove, a narrow groove, and a deep groove. Other form tools for cutting specific grooves or recesses are not considered in the present work. An example of a rule to recognize a groove-feature is as follows: Using the example given in Figure 4.7, Features EXT2, EXT3, and EXT4 satisfy the groove-feature rule and are combined into a groove feature; thus, a frame-based approach to formulate a lattice structure of machining features in a rotationally symmetric part has provided an efficient means to represent a generic model. The organization of the lattice structure is suitable for efficient pattern matching of rules. The methodology has been used for a wide range of machining features in a part, and provides the flexibility to change feature definitions with minimal effort. FIGURE 4.9 Geometrical constraint of an external cutting tool to machining a recess. FIGURE 4.10 External grooving tool. IF surface, F1 is adjacent to horizontal surface, F2, and horizontal surface F2 is adjacent to surface, F3, F1 & F3 are 90° to F2, and F2 with width of less than 26.2 mm and F1 with depth of less than 16 mm THEN surfaces F1, F2, F3 form a feature GROOVE [...]... interface for data acquisition of factual knowledge from external data base files has been elaborated The data structure and data contents of cutting tools, work materials, etc., have been presented Application programs for data retrieval and data search have been developed 5 A methodology using feature relationships for the selection of operations has been adopted Tooling types, tooling availability, and... computational time, the data base management system can also be used to serve other organizational functions, such as tool management which is important to improve overall productivity The crucial issues, namely system integration, ease of data accessibility and system flexibility are thus taken into account Data Content The data base files for recommending machining data required by the knowledge-based... data base management system, with maintenance of these files being carried out as necessary As indicated in Figure 4.11 an interface between the knowledge base and the data base files must be employed for the retrieval of relevant data External data base files, where data searching must be done sequentially on the index key until the search key field is matched, must be accessed by the interfacewthis way,... of human intervention are significantly reduced The developed IMS has been able to satisfy the following important considerations • Modularity, expandability, and adaptability to different operating environments • Interactive prompting routines that lead a user to retrieve information in an orderly manner • Data base of updatable files containing standard information, proprietary information, and product-facility... feature approach © 2001 by CRC Press LLC FIGURE 4.18 Operation planning for each feature FIGURE 4.19 Planning based on operation capability There are two aspects to the selection of operation, namely (a) types of operation, and (b) types of feature Variables are designated by a name preceded with “?” The variable ‘?FEATURE’ refers to an instance of the frame-type representing the feature being planned and... strategy is that feature relations are not considered; the tools used for each feature may require numerous tool changes on the machine tool and the individual cutter paths may have to be modified before they are merged Instead of selecting the operations separately for each feature, a production rule may be used to determine which features can be produced by a particular operation An example of a rule... knowledge-based system have been described by Yeo, Wong, and Rahman [1991] In order to search and retrieve data from the work material data base files, the dbase-action frame structure shown in Figure 4.12 serves as an interface to the external files FIGURE 4.11 Data acquisition system for the knowledge base © 2001 by CRC Press LLC FIGURE 4.12 Frame structure of a dbase-action For example, a data base file is opened... of tool paths for the Okuma lathe An example of a ‘nc-for-facing’ rule is given by: (Define-rule nc-for-facing () (instance ?op1 is operation-data with operation-in-sequence rough_facing_op) (or (instance ?op2 is operation-data with operation-in-sequence semi_finish_facing_op) (instance ?op2 is operation-data with operation-in-sequence finish_facing_op)) (generate-nc-data lap-facing) (bind ?instance (gentemp... the value slot-name The part description of an instance named THE-PART-INFO is shown in Figure 4.13 The assertion of the stock size is obtained from the DXF file The material specifications are retrieved and asserted to the relevant slots using the above interface procedure External data accessing is done quickly with ease even with a large data file size If numerous data queries are made on the same file,... to all the external grooves 5 Thread the external threads 6 Apply rough boring to internal stepped profile by a longitudinal/radial canned cycle 7 Apply finishing/semi-finishing to internal stepped profile by a longitudinal/radial canned cycle 8 Apply grooving operations to all internal grooves 9 Thread all internal threads 10 Apply part-off operation if workpiece has a free end The logical sequence utilizes . interpretation of a part drawing and transformation of the geometrical data into a frame structure of machining features. The system then assigns appropriate. groove feature; thus, a frame-based approach to formulate a lattice structure of machining features in a rotationally symmetric part has provided an efficient