MICAPP: A microcomputer-based process planning system

Journal of Mechanical Working Technology, 17 (1988) 21 - 31 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

21

MICAPP : A MICROCOMPUTER-BASED PROCESS PLANNING SYSTEM

S.H. Bok t and A.Y.C. Nee :~ t Research Assistant, CAE/CAD/CAM Centre, National University of Singapore Associate Professor, Department of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511, Republic of Singapore SUMMARY This paper first reviews some of the classical approaches to computer-aided process planning (CAPP). It then reports a microcomputer-based process planning program (MiCAPP) for planning the sequences of machining rotational parts, i.e. components turned on a lathe using a generative approach. The program was developed on an IBM PC using the popular AI language Turbo-PROLOG. The concept used in MiCAPP employs a state-space approach in visualising the machining operations. The key contribution in this approach is the capability to make inferences of the topological relationships between primitive geometric features to form "machinable envelopes". Using the concept of the inversed machining process, the correct machining sequence can be generaiised to an optimised search problem of the 'envelopes' of the hierachical tree within a given set of constraints. However, a heuristic approach has been adopted to produce a feasible sequence of 'envelopes' as the machining sequence. INTRODUCTION The modem manufacturing scenario is constantly under great pressure to cope with the steady decrease of product life cycles and a growing demand for shorter delivery times. This trend will continue as the society becomes more affluent and as a result of the growing tendency for the wealthier consumers to reject conformity in preference of product design variations. Manufacturing industries worldwide are presently looking towards computer-integrated manufacturing systems (CIMS) where the entire manufacturing process from the product engineer's detailed drawings to the shop assembly and inspection can be placed under computer management and control. CIMS answers to a large extent the growing needs of the manufacturing scene due to its flexibility, combined productivity and reliability. A major part of the CIMS activity involves computer-aided design (CAD) and computer-aided manufacturing (CAM). While CAD~CAM technology has successfully automated many areas of design, analysis and manufacturing, it seldom represents a smooth transition of information from the initial design stage to the final product. Process planning, for one, is difficult to be completely implemented on a CAD/CAM system. Traditionally, process planning has been regarded as a manual operation, usually carried out by qualified and experienced process planners or machinists. The creation of a good and optimum process plan is largely dependent on the individual skill of a plarmer and his aptitude for the planning task, his knowledge of manufacturing processes, equipment, materials and methods which are available in his production environment in particular. In addition to operation sequencing and operation selection, the correct choice of tooling and fixtures is also a major part of the process planning function. One of the problems often encountered with manual process planning is the difficulty in maintaining consistency of process plans created by different process planners, and to a certain extent, by the same individual. The reason for the inconsistency may be attributed to the complexity of process planning which requires the use of many disciplines including sequencing, machine selection, time and motion study, programming and material flow, etc. 0378 - 3804 / 88 / $ 03.50 ©1988 Elsevier Science Publishers B.V.

22 Much effort and study has been directed at the development of a computer-aided process planning (CAPP) system where process plans can be generated automatically and consistently based on the geometrical and manufacturing attributes of the product and a suite of "expert rules" of how each operation should be planned. A REVIEW OF CAPP SYSTEMS CAPP systems can be classified into two broad categories: variant and generative. Variant process planning refers to the retrieval and editing of old plans usually aided by a classification and coding system. Generative process planning is the automatic creation of new plans based on decision logic, optimization formulae and other information programmed into the system. The earlier CAPP systems are mostly variant and have been criticised as such systems are liable to repeat errors made in the past and are difficult to upgrade when technology changes. Oassical CAPP svstems- Variant and Generative Early variant process planning systems include CAPP (CAM-I Automated Process Planning) developed by McDonnell Douglas Automation Company, 1976 [1], MIPLAN and MULTICAPP developed by TNO and OIR, 1980 [2], AUTOPLAN by Metcut Research Associates, 1981 [3] and a host of others. Generative process planning system examples are APPAS (Automated Process Planning and Selection) developed by Wysk at Purdue University, 1977 [4], for planning milling and hole cutting operations on machining centres; GENPLAN developed at Lockheed for planning turning operations on cylindrical parts, 1981 [5]; CMPP (Computer Managed Process Planning) by United Technologies Corporation in conjunction with the US Army Missile Command, 1983 [6]. Some twenty other systems were compared by Chang and Wysk [7] and more than 50 systems by Eversheim and Schulz [8]. Design strategies of ~enerative systems Steudel [9] reported two major strategies in computerising manufacturing planning logic and data. One approach uses flow charts to detail the process sequences of the members in a family. This is achieved by collecting and analyzing the pattern of a large number of parts in the family. A program developed with this technique, however, faces maintenance problems as technological changes occur in the manufacturing methods and rules may incur substantial changes in the program. A second method uses the process logic-oriented approach. In this technique, a universal geometry based language is used. The manufacturing processes are defined in terms of their capabilities to generate or modify geometric features or material properties. A turning process, in this case, will be defined in terms of the different geometric features or "macros" that can be generated at specified tolerances, Figure 1. If the geometric feature matches the processing capability, the process will be selected without reference to the type of the part. Hi-Mapp (Hierarchical and Intelligent Manufacturing Automated Process Planner) by Berenji and Khoshnevis [10] uses a set of form features to describe a part similar to the above technique. The features of a part are first identified and described as goal states. The desired attributes of those featrues are contained in the initial states. The production rules for specification of possible actions are the inferences in that system. Eversheim [11], in his AUTAP system, uses primitives to construct a part similar to a constructive solid geometry language. The process selection, sequencing and machine tool selection are based on an IF-THEN (condition-action) situation contained in a decision table logic. Decision tables, Table 1, have the advantage that the infomaation can be presented in a clear

manner which can be revised easily. MACRO FEATURE FEATURES MODIFIERS

L

DATUM

~1~ "~

.u,

1

"x

GROOVE

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ID /

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TEETH ._

I CYLINDRICITY

LCONCENTRICITY THREAD 14

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OUADRANTS

"~_.~., '"--~-~

Fig. 1. Use of geometric features as macros [9] Dia. < 0.5

T

T

T

T

F

F

F

F

F

F

F

F

0.5 < Dia. < 1.0

F

F

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F

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1.0 < Dia. < 2.0

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T

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T

0.01 < T.P.

T

Tol S 0.002

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0.002 < Tol < 0.01

F

.01 < Tol

T

Drill

1

Re4tm

F

F

T

F

T

F

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I

F

F

T

F

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FiniJh bore

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Rapid travel

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* T.P. -> True Position

Table 1 : Decision Table for a hole-making process [7] Wang and Wysk [12] described a Micm-GEPPS system that uses KK-3 as a coding scheme and decision trees to select the correct operations. Decision trees are used to describe or specify the various actions (decisions) associated with combinations of input conditions (rules), Figure 2. Similar in technique is an inference network used by Davies et al [13] in developing the Intelligent Knowledge Based system (EXCAP Y), Figure 3. Their knowledge base is expressed in terms of fuzzy rules of the type: IF

THEN .

24

141hDiSt" p t2OR] ld olGn"

R) TO THE TURNN6 ~m*~_ TREE

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i~t,~~ ~

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Fig. 2. Use of a decision tree [12]

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'Adcdtd ~dmce : E1 E2 E3 E~ Sub..hypolheses [,,,, ,Oodhypotw~s

: H1 H2 :H3W,

Fig. 3. An inference network similar to a decision tree [13] Liu and Allen [14] described their Synthetic, Interactive Process Planning System (SIPPS) where they used a geometric model generation technique consisting of vector symbols or a corresponding numeric code to define the shape. Once the component shape has been entered, it can be converted into a geometric model of inserting the dimensions of each feature prompted by the program. Their system is capable of describing shapes for turning, milling and sheet metal work. Van Houten [15], in his ROUND system for planning rotational parts, described a clampin~ module RNDFIX which is capable of determining the most feasible damping surfaces and the division of the volume to be removed into machining areas, Figure 4. Hinduja and Barrow [16] reported the deveAopmeot of a semi-generative TECHTURN system for producing rotational parts. Their system includes an optimisation module which automatically selects the optimum cutting conditions based on torque, power, force, tool deflection, etc.

25

.

Ill II I II 1

I I i I

I I

.-.LL.-.J..--.__I

i

I

,'I

,~

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TURNING

FACING

Fig. 4. Idea of machining areas for a clamping module [15] Maior difficulties Converting manual process planning procedures into computerised sequences is not a simple matter as the information embodied in the manual process is often fuzzy, ill-defined and incomplete, typically in an ambiguous narrative form. Information must be re-structured in a format which can be used to check for the completeness as well as easy coding of the program. Groppetti and Semeraro [17] reviewed a good number of CAPP papers and concluded that the major difficulties to the implementation of a generative process planning system are related to the difficulties to identify, understand, extract and collect the process planning logic, and to the lack of a unique and systematic method to perform process planning. They believed that a fully generative process planning system with self learning capability is still not yet available.

AN OVERVIEW OF MICAPP This section sets out to present a technique similar to the process logic-oriented approach in the development of a micro-computer based generative system at the National University of Singapore, MiCA_PP. This approach uses 'envelopes' for the reduction of the workspace, defined by the part geometry and the shape of the raw material (say, round bar stock), into manageable work areas. The accent of this part of the paper is therefore to underscore the design principles and methodology of MiCAPP. It should be pointed out that MiCAPP is based more on the geometric attributes rather than manufacturing attributes of the product. In this way, a case study approach will be used for explanation. Process planning can be treated as an AI planning problem, thus MiCAPP regards as two boundary states : an initial state that corresponds to the part geometry and a goal state that corresponds to the raw material shape. These two states are chosen so that the inverse machining process can be simulated. DESIGN METHODOLOGY OF MiCAPP Part ~eometrv renresentation The representation of the part geometry is critical in enabling MiCAPP to infer relevant knowledge or facts of the geometry for higher level manipulation. Therefore the approach to perceiving geometric entities is highly relevant.

26 The part geometry can be considered as an assembly of shape dements. Therefore, a pan can also be 'disassembled' into elements that are integral to the overall shape. The concept of shape elements is not new, Plummet and Hannam had used it [18]. They can be classified according to characteristics typical of topological considerations despite geometry-related values of sizing and location. Presently, a basic set of shape elements has been identified for turning operations. Therefore these elements must be compatible with the process capability of turning operations. The elements are "primitive" in that they are either radiused to the centre of rotation or they are straight. The common feature is simply that they are of circular cross-section. "Composite" shapes such as slots and grooves are also possible. These "composites" can best be treated as secondary to the previous elements. The aim is to crystallise some basic geometry-related ideas that provide the reasoning processes in MiCAPP. In addition, it can be said that a large class of pan geometries can be formed by these basic shape elements. As a case study approach has been chosen, Figure 5 shows a typical part geometry using basic shape elements.

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.

.

.

.

.

.

.

.r.r' I. Fig. 5. A typical part geometry for turning Hence a planner uses a simple interface within MiCAPP to design a part by identifying and assembling the shape elements about the centre of rotation. This is easy as the "shape understanding" is already native to him. Together with dimensions that are prompted for by the system, MiCAPP is capable of inferring topological/adjacency facts of the part geometry's initial states. This is done by supplying dimensions with respect to datum points which are local to the shape element as well as global to a reference taken at the intersection of the centre of rotation with the face of the bar stock. In addition, the dimensions are used to characterise the shape of the element chosen. The direction of each shape element can then be verified internally without the specification of the planner. The objective of these design features is to allow the planner to concentrate on just the design of the shape of the part geometry. Hence both topological "truth" and dimensional constraints are checked and maintained. The planner is always placed in the position to correct the errors detected by the system. For example, it is topologically unsound to have a rightwards facing taper/cone followed by a left facing slot. Similarly, a cylinder followed by another is questioned in the system and rectified by the planner. Specifying a taper, but giving dimensions that do not fulfil the definition of a taper is also an error. Another error would be to fail to guarantee adjacency between consecutive features. Therefore, adjacency relationships are verified as well, a direct result of these relationships is the inference of "markers" that are essential to the subsequent enveloping process. "Markers" typified locations with explicit direction attributes in the part geometry. The maintenance of each shape's integral definition as well as correct topological relationships reflects some degree of intelligence in the system.

27 External shape features have been chosen for the present implementation of MiCAPP since it is surmised that their identical internal opposites can be treated similarly by the process of enveloping except of course, that the resulting tool selection will differ. The use of hierachieal menus facilitates a logical and sequenced design process. Enveloping process The enveloping process relies on the inferred knowledge of "markers" in the part geometry to further define significant work areas of the workspace, called envelopes. The workspace concept is useful in directing the focus towards the areas to be "filled-in" in an inverse machining process. In this respect, the definition of envelopes can be visualised in Figure 6.

Al

ll

Fig. 6. The result of the enveloping process using "markers" An important interpretation of these envelopes can be obtained by examining the interface boundaries at the envelopes. These boundaries can be visualised as intermediate machinable surfaces which are used as sub-goal states to the ultimate goal state of inverse machining to the raw material shape. These boundaries are also convenient in demarcating as well as identifying each envelope. In terms of graph representation, the general knowledge construction process will have to relate these envelopes as nodes in the graph. As far as MiCAPP is concerned, these nodes represent significant potential considerations for merging or changing of tools, and prevention of shallow cuts. They also represent convenient areas for follow-up splitting in a problem-reduction approach. This approach will be mentioned again. The approach within MiCAPP does not strictly implement this graph as it leads to a "tree-search" situation which may be time-consuming for complex parts, however it employs heuristics that are based on criteria an experienced planner would probably use. Therefore MiCAPP simulates an initial abstraction of a plan that is credible to an experienced planner. The result of this abstraction is a global machining sequence of the envelopes. In conjunction with the subdivision of envelopes into "layers" which correspond to the turning process, this abstract plan also simulates the human process of hierachical planning. Layering is the task for the computation of machining parameters module. Envelopes are obtained by first using each marker to radiate out either leftwards or rightwards depending on the directional attribute of each marker. The result of this radiating is an intersection with opposite surfaces. The intersection is performed in a forwards or backwards search of compatible surfaces in the part geometry. The direction of each surface is thus used for possible testing of intersection. It is noted that multiple intersections of a surface are possible. The result of intersections

28 is that "child" surfaces and hence "child" envelopes are gmerated. These "child" surfaces(envelopes) are easily visualised in a graph tree represemtion. Each intersection has the implication that integrity of identification of each pair of "child" surfaces(envelopes) must be maintained. Since each surface in the part geometry would have been indexed, a method of consistently and quickly identifying each child surface and determining its "generation" or level in the graph tree has been devised. As far as identification and indexing are concerned, the method requires the appending of one of a pair of suitable digits to the "parent" number. Each member of the pair effectively follows a convention of identifying the location of the child surface as to whether it is above the intersection point. All primary surfaces of the part geometry are identified via integers. Following a decimal point will be the sequence of digits that determine the particular child surface. The workspace is effectively transformed to a number of elementary envelopes whose surfaces or "child" surfaces are similarly identified as in the starting set of primary surfaces for the part geometry. Once all markers have been exhausted, a process takes over to generate integral envelopes since the intersection process results in overlapping envelopes. This process relies on first detecting the lowest intermediate surface(identified with the lowest envelope) in the part geometry. The envelope with which this intermediate surface is identified is then used as a " s e e d " for set-intersection with all other envelopes. The boundary condition for this process is that each envelope must ultimately have a three-member surface list which describes the envelope's adjacent sides, besides the intermediate surface used to identify that envelope. The terminating condition is also that the " o p e n " intermediate surfaces describing the limits of the raw material shape are described with envelopes containing the three-member lists. The middle member of that list automatically identifies the intermediate surface of a "child" envelope or ultimately a primary surface at the contour of the part geometry. Implicit in this process is the required provision that 'child' surfaces must be correctly identified. This permits automatic selection of the "sibling"/"child" surface for membership in the three-member list. It can also be pointed out that the construction of envelopes effectively demonstrates the possiblity of sequencing the envelopes in an inverse machining order. Process seouencin~ This process heuristically supplies a sequence of the envelopes. The heuristic rules are qualitatively based on surmising that envelopes that are more "hidden" and embedded in the workspace are to be inverse-machined first. It is assumed that such an approach is feasible for a human process planner. This is feasible as a result of the enveloping process. A general note can be made in that due to the topology of the part geometry using the shape elements, the envelopes sequenced at the beginning of an inverse-machining order are geometrically more complex and hence more stringent in tool geometry requirements. This point is useful subsequently for tool selection. At the other end of the inverse-machining sequence are envelopes that would be simpler and more "open' '. A minimal set of tools is then possible. Tool selection In what follows, the various informational support for tool selection is described. They are peripheral in that they are only necessary for the generation of a more complete process plan once the enveloping process is successful. A tool database is necessarily used here. Information is dichotomised into toolholders and inserts. The holders are represented such that access is quick for cheeks on rigidity, reach, accessibility, contouring ability, compatible insert types, effective cutting edge length and cutting direction. Information on inserts is structured for access to dimensions and preferred grades. A process capability database depicting the usage of holders for facing in, facing out and turning is also

29 included. In addition, geometric compatibility is revealed in the tool geometry in terms of the side cutting edge and end cutting edge angles. The process capability of inserts is also constructed. It is based on the recommendations of insert choices against their machining behaviour. A weightage system is used which accounts for the workpiece material type, the chipbrealdng ability of each insert, machining style and vibration tendencies of the insert. Tool selection for individual envelopes is easily conducted on the basis of tool geometry compatibility with envelope geometry. Envelope geometry merely requires the tool to access within a certain limit of "shadow region". Failure to do this means that "remnant" envelopes are left behind leading to unnecessary tracking of this type of envelopes. " O p e n " envelopes have the most relaxed geometric test for tool geometry. Operations m-ouping This module recommends the minimum number for tools for the maximum possible number of envelopes. The tool selection process would have supplied sets of feasible toolholders for each envelope. By following the envelope sequence and attempting to propagate the common use of a toolholder, a minimisation of toolholders over the series of envelopes is possible. This propagation relies on two conditions namely, the common membership of a toolholder being considered in any two consecutive envelopes and the geometric satisfaction of tool geometry against combined envelope pair geometry. The latter condition stipulates the possibility of actually accessing the envelope pair without necessitating a toolchange. Additional checks on tool clearance with part geometry are vital in order to avoid accidents. The result is a recommendation of a feasible toolholder for the greatest number of envelopes. Computation of machining parameters To provide relevant information for proper computation of machining parameters, a cutting parameters database has been constructed. It contains the use of Taylor's tool life exponents adjusted for the type of tool insert chosen. The key task in this module is to produce the least number of layers required for turning. In doing this, layer mergings in between two envelopes are tested. Certain configurations of envelope pairs effectively prevent layer merging considerations and hence force the result of an extra layer. Table 2 depicts the machining parameter results or facts inferred as layers for roughing the part shown in Figure 7.

Fig. 7. A layered model of the part geometry

~0

MACHINING PARAMETER RESULTS Roughing

Envelope

Index

Cutting Speed

Depth

Direction

Nee

Nos

Speed

of Cut

Feed Rate

(m/min)

(nun)

(mmRev)

114.2

4

0.2

5

0.6

RIGHT

-6

1,2,3

LEFT

-6

4,5

RIGHT

-5,-4

6,7,8,9

4

0.2

LEFT

-5,-4

10,11.12.13,14,15

4

0.4

LEFT

-7,- 1

16,17

4

0.2

Index

Envelope

Diameter ¢~"Cut

Spindle Speed

Cutting Time

Cutting Length

Layer Start

No

No

(ram)

(rev/min)

(rain)

(ram)

(ram)

1

-6

54.0

673.0

0.13

11.4

50.0

62.0

586.0

0.07

5.7

50.0

68.0

534,7

0.02

1.4

50.0

55.0

501.0

0.07

13.6

60.7

65.0

423.9

0.17

29.5

53.6

54.0

673.0

0.09

7.7

28

62.0

586.4

0.08

6.0

24

80,5

448.0

0.03

1.9

14.8

81.0

447.0

0.02

I,I

14.5

54.0

668.0

0.02

4.3

35.7

11

62.0

581.8

0.06

10.0

30.0

12

72.0

501.0

0.13

17.7

23.3 21.4

2 3 4

-6

5 6

-5

7 8

-4

9 10

-5

114.2

74.0

487.0

0.53

69.4

14

82.0

439.9

0.70

82.1

15.7

15

88.0

409.9

0.84

91.6

11.4

13

-4

16

-7

87.0

417.9

1.83

102.1

0

17

-I

94.0

386.8

2.25

116

0

Table 2 : Machining parameter results for Fig. 7 The total machining time without considering toolchanging time is approximately 7 minutes. CONCLUSIONS The extant program is by no means complete. Therefore MiCAPP is being developed to include additional capabilities such as a clamping module and handling more 'secondary' features such as fillets, curved surfaces and internal surfaces. At present, the concept of 'envelopes' and its potential generalisation to include surfaces mentioned above has been presented. Its feasibility is demonstrated in terms of intermediate surface generation, process sequencing, operations grouping and machining parameter computation. It must be said that improvements are necessary with respect to the design and use of the declarative knowledge of the problem as well as the procedural knowledge (the control structure or decision logic) for handling a greater number of variant situations such as tool limitations, multiple setups and higher order surfaces. With regard to this, future considerations must account for manufacturing-oriented rather than the present largely geometric-oriented attributes.

31 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

Link, C.H. "CAPP - CAM-I Automated Process Planning System", Proc., 13th Numerical Control Society Annual Meeting and Technical Conference, Ohio, March 1976. Schaffer, G., "GT via Automated Process Planning", American Machinist, May 1980, pp.119122. Vogel, S.A. and E.J. Adlard, "The AUTOPLAN Process Planning System," Proc. 18th Numerical Control Society Annual Meeting and Technical Conference, Texas, May 1981, pp. 422-429. Wysk, R.A., "An Automated Process Planning and Selection Program: APPAS", PhD Thesis, Purdue University, 1977. Tulkoff, J., "Lockheed's GENPLAN", Proc., 18th Numerical Control Society Annual Meeting and Technical Conference, Texas, May 1981, pp. 417-421. Waldman, H., "Process Planning at Sikorsky", CAD/CAM Technology, 1983:No.13. Chang, T-C., and R.A. Wysk, "An Introduction to Automated Process Planning Systems", Prentice-Hall, Inc., 1985. Eversheim, W. and J. Schulz, "Survey of Computer Aided Process Planning Systems", Annals of the CIRP, Vol.34/2/1985, pp.607-613. Steudel, H.J., "Computer-aided process planning: past, present and future", Int. J. of Prod. Res. Vol. 22, No.2, 1984, pp. 3-14. Berenji, H.R. and B. Khoshnevis, "Use of artificial intelligence in automated process planning", Computers in Mechanical Engineering, September 1986, pp. 47-55. Eversheim, W., H. Fuchs and K.H. Zons, "Automatic Process Planning with Regard to Production by Application of the System AUTAP for Control Problems", Computer Graphics in Manufacturing systems, 12thCIRP Int. Seminar on Manufacturing systems, 1980. Wang, H.P. and R.A. Wysk, "Applications of Microcomputers in Automated Process Planning", J. of Manuf. Sys., Vol.5, No.2 (1986), pp.103-111. Davies, B.J., I.L. Darbyshire and A.J. Wright, "The integration of process planning with CAD CAM including the use of expert systems , Proc. Int. Conf. Computer Aided Production Engineering, Edingburgh, 1986, pp.35-40. Liu, Y.S. and R. Alien, "Aproposed synthetic, interactive process planning system", Proc. Int. Conf. Computer Aided Production Engineering, Edingburgh, 1986, pp.201-208. Van Houten, F . J . M . , "Strategy in generative planning of turning processes", Annals of the CIRP, Vol.35/1/1986, pp.331-535. Hinduja, S. and G. Barrow, "TECHTURN: a technologically oriented system for turned components", lbid, pp.255-260. Gr0ppetti, R. and Q. Semeraro, "Generative approach to computer aided process planning", Ibid, pp.179-189. Plummer, C. S. and Hannam, R.G., Design for mam!facture usinga CAD~CAM system - a methodology for turned parts", Proc. Inst. of Mech. Engineers, PartB, v197B, 1983, pp 187 195.