Tài liệu A Computer-Aided ppt

The integrated machining system IMS begins with the interpretation of apart drawing and transformation of the geometrical data into a frame structure of machining features.. For the exam

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A Computer-Aided and Integrated Machining

In order to arrive at a suitable solution, be it for process planning or part programming, the pressingissues 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, andtool 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 chapterexpounds 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 softwaretool, GOLDWORKSTM [Gold Hill, 1989] with COMMON LISP as the programming platform, is usedfor the development work The software tool requires Microsoft Windows® in a PC which can be feasiblydeployed in a wide range of industries The system is aimed to provide a vital step to a totally integratedSwee-Hock Yeo

Nanyang Technological University

manufacturing environment The integrated machining system (IMS) begins with the interpretation of apart drawing and transformation of the geometrical data into a frame structure of machining features Thesystem then assigns appropriate operations to all features identified Automatic tool selection of holders andinserts follows before planning the entire operation sequence For each operation, the machining conditionsare 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 holding parameters are provided so that the solution generated by the IMS can be implemented Proper set-

work-up involves spatial reasoning, and it is a difficult task that an experienced machinist learns during an ticeship Varying methods of solution exist from simple rules-of-thumb, such as threshold ratio between lengthand diameter and analytical methods such as that of explained by Hinduja and Huang [1989] The problem

appren-of specifying the appropriate set-up is further complicated by the variety appren-of work-holding devices which includeface 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 Thesystem is comprised of a user-interface for interaction, a feature recognizer, and a knowledge base thatcontains facts and rules Figure 4.2 shows the various types of approaches/strategies that have been usedfor 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 directedforward chaining technique (which integrates both forward and backward chaining techniques) [Yeo, 1995]

For a given goal, the chaining method uses rule sets to cluster forward-rules together, and each rule set isactivated when an enabling pattern is matched and queried The control structure consists of eight stages whereeach 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 structurewith the pattern named RUN-STATE SELECTION COMPLETED Each rule set is activated sequentiallywhen its enabling pattern (e.g., RUN-STATE FEATURE-RECOGNITION DONE) is matched and theinference mechanism performs forward chaining of all the rules related to the rule set The benefits ofhaving 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

(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

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 Errortrap 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, aplan view of the upper half cross-section of the part drawing of the workpiece will suffice The form ofthe profile is restricted to straight lines and arcs The feature recognizer requires access to part descriptionsstored in a CAD database The approach that has been adopted in this work is to develop a module thatinvolves inductive extraction of the geometrical data of a part created in a commercial CAD system ItsDXF (in AutoCAD®) facility provides the output format required by the IMS Before any operationplanning activity can take place, a set of features must be formulated for abstraction Since this work isdirected at metal removal processes, machinable features are used A machinable feature is any geometricsurface or combination of geometric surfaces which form a volume that can be shaped by cutting tooloperations The term “form features” is sometimes used

The necessary information about each feature is organized by means of a frame representation Thisknowledge 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 theirsignificance have been defined as shown in Figure 4.4 so that the CAD data can be used effectively in the nextstep All the entities representing the model and the stock size are exported using the AutoCad DXF format

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 whichdefines 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

((LINE (178.2 142.2) (221.3 165.2)) ; sub list for each

(ARC (141.7 151.7) 5.0 (180.0 270.0)))

easily understood The various arc types are illustrated in Figure 4.5 Their significance is used for toolselection For the example used in (“convex up”), the sub list resulting from this analysis gives theprimitive list data as follows:

Each sub list describes the types of primitive The entire primitive list data is sorted in order, startingfrom 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 listdata into embryonic objects (i.e., instances of frames) which are exclusively geometric The machiningfeatures of a component are represented in the knowledge base in the form of frames with hierarchical

((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)))

relationships The lattice structure consists of eight types of frame used for a rotationally symmetricpart, 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 withstraight lines

An example of a taper feature instance in the language syntax form is shown in Figure 4.7 The EXT6instance 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 alinked list For convenient identification, each instance is uniquely named and enumerated, for exampleEXT and INT imply external and internal feature-types respectively, and the numbers appended to themare 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 theknowledge 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 groovesrequires a more detailed geometrical treatment A groove (or recess) in a machining profile is generally

described as being confined by two adjacent boundaries This definition is very broad and might lead toinefficient automatic tool selection Grooves may be required in a wide variety of forms such as: circlipgrooving, 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 economicimplication 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 thebottom of the groove parallel to the Z-axis, the external tool must have an approach angle of more than90°, 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 mmsquare 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

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 producethis shape.

The machining feature builder is designed in such a manner that modification to the knowledge basecan 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 notconsidered 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 andare combined into a groove feature; thus, a frame-based approach to formulate a lattice structure of machiningfeatures in a rotationally symmetric part has provided an efficient means to represent a generic model Theorganization of the lattice structure is suitable for efficient pattern matching of rules The methodologyhas been used for a wide range of machining features in a part, and provides the flexibility to change featuredefinitions with minimal effort

IFsurface, F1 is adjacent to horizontal surface, F2, andhorizontal surface F2 is adjacent to surface, F3,F1 & F3 are 90° to F2, and

F2 with width of less than 26.2 mmand

F1 with depth of less than 16 mmTHEN

surfaces F1, F2, F3 form a feature GROOVE

4.5 The Knowledge Base

The types of knowledge acquired are a combination of factual (or declarative) knowledge and proceduralknowledge, which involves selection of: operations, operation sequencing, selection of tools, selection ofmachining conditions, and generation of tool paths

4.6 Factual Knowledge

A sizable amount of data is required for machining decision making Specifications of work materials,toolholders, inserts, etc., must be stored in the files of the 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 beemployed for the retrieval of relevant data External data base files, where data searching must be donesequentially on the index key until the search key field is matched, must be accessed by the interfacewthisway, besides improving the computational time, the data base management system can also be used toserve other organizational functions, such as tool management which is important to improve overallproductivity The crucial issues, namely system integration, ease of data accessibility and system flexibilityare thus taken into account

Data Content

The data base files for recommending machining data required by the knowledge-based system have beendescribed by Yeo, Wong, and Rahman [1991] In order to search and retrieve data from the work material database files, the dbase-action frame structure shown in Figure 4.12 serves as an interface to the external files

For example, a data base file is opened by using the function:

The variable interface is an instance of dbase-action frame The action slot-name of ‘:open-file’ simply opens

a data base file when the go slot-name is ‘:yes.’ A further action named ‘:find-record’ searches the file untilthe search-key slot-name is matched and the result (i.e., record) is asserted in 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 tothe relevant slots using the above interface procedure External data accessing is done quickly with easeeven with a large data file size If numerous data queries are made on the same file, for example thecutting tool data files used for checking tool availability and tool selection, it would be convenient toconvert the data into a frame system A particular machine tool with its attributes is represented as aninstance of the machine tool frame THE-MACHINE-INFO in Figure 4.14

Data base structure for cutting tools can amount to more than 20,000 combinations of tool modules[Evershiem, Jacobs, and Wienand, 1987] It is important to provide efficiently in the data base structurefor the following

1 Ease of maintenance

2 Complete and concise attributes of tools

3 Linkages between holders, inserts, etc

A tool database structure consisting of 21 data fields has been formalized and is used for external andinternal holders as well as for drills Examples of a tool file containing three types of tool are shown in Table 4.1

(Defun open dbase (interface)(setf (slot value interface ‘action) :open file)(setf (slot value interface ‘go) :yes))

The purpose of each data field is as follows.

1 Identification code.This field is used to designate a unique identification for a tool holder or a drill

2 Insert clamping method.This field is used for four types of clamping systems which are designated

in the ISO standard, i.e., C, M, P, and S types The C type provides a positive rake which is usuallypreferred for finishing operations, while the M type provides rigidity and permits use of doublesided inserts The P type offers unobstructed chip flow in addition to the advantages for M type.The S type is used for finishing operations and provides a larger variety of insert shapes The C type

is largely used in ceramic inserts

3 Coupling (shank) size This field is used for the specification of shank height, shank width, andtool length, which affect the stiffness of the tool Its size is restricted by the turret tooling capability

4 Hole diameter.This field is used to ensure that a boring bar is able to function without collidinginto the workpiece For example, a 25 mm bar diameter should have a minimum hole diameter

of 32 mm when the bar is placed at the centre of rotation [Sandvik Coromant, 1990] In the case

of drilling, the data field refers to the drill diameter

5 Tool function.This field is used for the classification of operations (sometimes referred to as processclassification) The hand of tool (i.e., left, right, or neutral) specified in the ISO standard is insuf-ficient to describe the full capability of a tool such as that illustrated in Figure 4.15 The right-handtool, PDJNR, shown in Figure 4.15a is used for profiling while PCLNR, shown in Figure 4.15b isused for longitudinal turning and out-facing, as well as for in-facing

THE-PART-INFO.

6 Entering angle This field is used as part of the tool specification defined in ISO 3002/1-1977 Theeffects of this geometry characteristic are well established in machining processes For example, asmaller entering angle increases the ability of a tool to take a larger feed On the other hand, alarger entering angle is able to reduce tool chatter and vibration

7 Trailing angle This field is used to check for tool collision, particularly important in an in-copyingsituation

8 Inclination angle This field is used as part of the tool specification in which its primarily influence

is the chip flow direction

9 Normal rake angle This field is used as part of the tool specification which influences the powerconsumed in a machining process.

10 Insert group This field is used to group inserts which are applicable to particular types of toolholder, i.e., for inserts that fall in a group named C12P, a tool holder ‘PCLNR 2525 M12’ can

be used

11 Cost This field specifies the cost of the holder

12 Adaptor This field specifies appropriate adaptors to be used in conjunction with holders such asboring bars and drills

13 Factors.This field specifies merits of three factors influencing the tool selection process, namely:entering angle, insert clamping system, and shank size A weightage which is dependent on acutting situation is applied to the merits of a tool to determine its suitability, i.e., in a roughingoperation, insert clamping system of P-type is most suitable

14 Groove range.This field gives a range of groove-width which is dependent on the capability of itsholder

15 Z/X dimensions.This field gives the Z- and X-dimensions relating to the overall tool size

16 Maximum cutting length This field is used for drilling and grooving tools For drilling tools, thisrefers to the cutting length of its body, while for grooving, it implies the groove cut-depth as shown

19 Drill tip angle This field specifies the drill tip angle, i.e., a 114° tip angle is used for U-drills

20 Material grade This field specifies the material grade for solid cutting tools such as delta drills

21 Hand.This field is used to identify right hand, left hand, or neutral tools

and a Drill

No Description External Holders Internal Holders Drill

1 Identification code PCLNR_2525M_12 S20S-SDUCR_11-M R416.1-0180-20-05

2 Insert clamping method T-MAX P T-MAX U T-MAX U

5 Tool function (face-in-turn

axial-turn face-out-axial-turn)

(profile-bore) (drilling)

9 Normal rake angle (deg)  6  6 nil

15 Z/X dimensions (mm) (32 150) (250 13) nil

16 Maximum cutting length

(mm) (or drill cut length;

groove cut depth)

Similarly, information about inserts is stored in an insert file database structure where each record consists

of nine data fields An example of an insert file containing three types of insert is shown in Table 4.2.The purpose of each data field is as follows:

1 Identification code This field is used to identify a unique insert type

2 Insert group This field is used to categorize inserts into various groups, such as T12P For example,

an insert identity of TNMG_16_04_08-MF_GC415 is classified as insert group named T12P

3 Insert shape types This field is used to describe insert shapes according to letter symbols (as inISO 1832: 1985) For example, round inserts, square inserts, and triangular inserts have symbols

of R, S, and T, respectively

4 Nose radius/pitch/width This field is used to specify a nose radius, pitch, or width of a tool tip.The tool tip may be for turning, threading, or grooving

5 Grade This field gives the nominal material grade specified by the tool manufacturer, such as GC415

6 ISO application area This field provides the equivalent carbide grade according to ISO 513:1975

7 Chipbreaker This field refers to the chip breaker type used in the insert

8 Number of cutting edges per insert This field specifies the number of edges in an insert