Journal of Materials Processing Technology, 24 (1990) 115-124 115 Elsevier

FROM COMPUTER MODELLING TO THE MANUFACTURE OF DIES WITH SCULPTURED CONTOURS

Gordon Smith Senior Lecturer in CAE School of Science and Technology Division of Mechanical Engineering Teesside Polytechnic Mlddlesbrough Cleveland (UK)

SUMMARY

The application of CAD facilities to represent geometric definitions of components with sculptured contours is presented, and the development of a process planning system for the production of moulds and dies with free-form surfaces is discussed.

1. INTRODUCTION

The line of development of sculptured (doubly curved) surfaces can be

traced back to the design of ships hulls in the early part of the

twentieth century. More recent examples are found on component parts of

aircraft, cars and in other applications where smooth or aerodynamic

contours are needed. In addition to this, modern trends for designer

products and highly aesthetic consumer goods has led to a considerable

increase in interest in the design and manufacture of these surfaces

over the last decade. Consequently, sculptured surfaces are now found

on a wide range of household items ranging from detergent bottles to

electric kettles.

The increasing interest in goods with sculptured surfaces has led to an

increasing demand for moulds and dies to manufacture these goods.

Modern CAD/CAM systems facilitate the design and manufacture of dies

with complex surfaces, but much of the process planning and selection

work is still carried out by experienced tooling engineers. Hence, the

roles of design and manufacture for these dies is far from integrated.

A r t i f i c i a l I n t e l l i g e n c e [ A . I . ] t e c h n i q u e s i n g e n e r a l , and Knowledge Based

Systems [K.B.S.] in particular, are now being widely developed in an

attempt to close the gap between design and manufacture. Many of the

systems currently being developed are concerned with automated process

planning for families of parts consisting of cylindrical and/or

prismatic

0924-0136/90/$03.50 © 1990---Elsevier Science Publishers B.V.

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componen ts . However, d e s p i t e t h i s p r o l i f e r a t i o n of work in t h e f i e l d of

p r o c e s s p l a n n i n g , t h e r e a p p e a r s to be v e r y l i t t l e e f f o r t d i r e c t e d

t o w a r d s CAD/CAM i n t e g r a t i o n f o r more complex 3-D s u r f a c e s .

T h i s p a p e r d e s c r i b e s t he deve lopmen t o f a geome t r y i n t e r p r e t a t i o n module

and a knowledge based p l a n n i n g s y s t e m , and t h e i r s u b s e q u e n t e v a l u a t i o n

w i t h r e s p e c t to a u t o m a t e d p r o c e s s p l a n n i n g f o r t he p r o d u c t i o n o f d i e s

w i t h s c u l p t u r e d s u r f a c e ~ . Tbo Droblems of ge ome t r y i n t e r p r e t a t i o n , f o r

g e n e r t e d and f r e e - f o r m s u r f a c e s , a r e d i s c u s s e d and the deve lopmen t of a

PROLOG based e x p e r t s y s t e m f o r m a n u f a c t u r i n g d a t a g e n e r a t i o n i s

d e s c r i b e d . The r e l a t i o n s h i p be tween t h e s e modules and commerc ia l

s o f t w a r e f o r s u r f a c e d e s i g n and N.C. d a t a p r e p a r a t i o n i s a l s o p r e s e n t e d .

2. THE DESIGN OF SCULPTURED SURFACES

Most t r a d i t i o n a l methods used f o r t he d e s i g n o f s c u l p t u r e d s u r f a c e s a r e

based on t h e p r a c t i c e known as ' l o f t i n g ' , t h i s te rm d e r i v e s from t h e

s h i p b u i l d i n g i n d u s t r y where l a r g e l o f t a r e a s were used to l a y o u t s u r f a c e

d e s i g n s . Woodwark ( r e f 1} d e c r i b e s t he l o f t i n g p r o c e s s , u s i n g s u r f a c e

c r o s s s e c t i o n s , t o p roduce d e s i g n s f o r a f a i r e d s h i p s h u l l .

L o f t i n g i n v o l v e s t h e p r o d u c t i o n o f many l i n e d r a w i n g s to d e s c r i b e a

s u r f a c e in t e rms o f a f a m i l y of p l a n a r c r o s s s e c t i o n s in t h r e e

p r o j e c t i o n p l a n e s . Curves on t h e s e c r o s s s e c t i o n s a r e g e n e r a t e d by

s p l i n i n g t h r o u g h a s e t o f c o - o r d i n a t e p o i n t s l y i n g on the d e s i g n

s u r f a c e .

When s u r f a c e s a r e p roduced m a n u a l l y , i t i s o~ly p o s s i b l e to draw a

l i m i t e d number o f c r o s s s e c t i o n s , t he s u r f a c e geome t r y be tween a d j a c e n t

s e c t i o n s must be i n t e r p o l a t e d f rom them, and i t i s n o t n e c e s s a r i l y t r u e

t h a t t he o v e r a l l s m o o t h n e s s of t h e f a i r e d s u r f a c e w i l l be c o n s i s t e n t

w i t h t h e s e s e c t i o n s .

There a r e many i n h e r e n t p r o b l e m s w i t h manual methods of s u r f a c e d e s i g n .

These s tem from the i n a b i l i t y o f two d i m e n s i o n a l c r o s s s e c t i o n s to

a c c u r a t e l y r e p r e s e n t n o n - p l a n a r s u r f a c e s .

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The increased interest in the design and manufacture of non-planar

surfaces has led to the development of a number of commercial software

packages with surface modelling capabilities.

Packages of this type allow designers to view three dimensional

representations of a surface as it is being developed; obviating many of

the ambiguities associated with surfaces defined by a series of two

dimensional cross sections.

There are two computer-assisted methods in common usage for the

geometric definition of surfaces: cross-sectioning and free-forming.

2.1 Cross-sectlon surfaces

The concept of developing geometric surface models from a series of

cross sections is based on the lofting technique described above.

However, when computers are used to generate numerical representations

of these surfaces there is no ambiguity about the profile between cross

sections. This is achieved by using spline equations to interpolate

between the sections in three dimensions.

Surfaces represented by interpolating in three dimensions between

cross-sections may be classified into three groups:

2.1.1 Generated Surfaces

Non-planar surfaces may be generated by constructing a single synthetic

boundary curve (consisting of points, lines, arcs and conics) and

subsequently translating or rotating this in 3-D space. The geometry of

the resulting surface being defined by the swept path of the original

boundary curve {figure 1).

2.1.2 Ruled Surfaces

A ruled surface is represented by a single family of straight lines

between two boundary cross-sections (figure 2). The geometry of a

ruledsurface is controlled by the shape of these cross-sections and the

paremetrisation of the ruling interpolation equations. Linear

interpolation gives the simplest form of surface with position

continuity along the rulings.

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i

Generated Surface

Fig. i

Ruled Surface

Fig. 2

Ducted Surface

Fig. 3

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2.1.3 Ducted Surfaces

Ducted surfaces are developed by arranging a number of surface

cross-sections along a space curve or spine. The space curve may be

constructed from a series of interconnecting arcs and lines lying in

different planes, or it may be a more complex curve. The surface

cross-sectlons are normally constrained to be perpendicular to the space

curve, blending and interpolating functions are then used to approximate

a fair surface between these sections. Figure S shows a surface

represented in this way.

Cross-sectioned surfaces have a wide range of applications, ranging from

simple cams to more complex shapes such as aerofoils and exhaust

manifolds. These allow designers to develop surfaces from 2-D drawings

and this makes them attractive from a conceptual point of view.

However, many shapes cannot be represented in this way and it is

therefore necessary to consider a less restrictive method of design for

these.

2 .2 F r e e - F o r m S u r f a c e s

S u r f a c e s w i t h no o b v i o u s a x i s o f symmetry a r e known as f r e e - f o r m

s u r f a c e s . These c a n n o t be g e n e r a t e d from c r o s s - s e c t i o n a l c u r v e s .

F r e e - f o r m s u r f a c e s a r e u s u a l l y r e p r e s e n t e d by a number o f p a r a m e t r i c

s u r f a c e p a t c h e s (Coons p a t c h e s ) . Each d e s i g n s u r f a c e b e i n g d e s c r i b e d by

a s e r i e s o f t h e s e p a t c h e s e x p r e s s e d in t e r m s o f p o s i t i o n , d i r e c t i o n and

t w i s t v e c t o r s . The Coons p a t c h e s a r e t w i s t e d and d i s t o r t e d in 3-D s p a c e

t o form complex , f r e e - f o r m , s u r f a c e s . B l e n d i n g and i n t e r p o l a t i o n

e q u a t i o n s a r e t h e n used to e n s u r e s l o p e c o n t i n u i t y a c r o s s t h e b o u n d a r i e s

of a d j a c e n t p a t c h e s ( f i g u r e 4 ) .

3 . EXPERT PLANNING SYSTEMS

Knowledge b a s e d s y s t e m s , " e x p e r t s y s t e m s " , a r e now employed in f i e l d s

such as med i ca l d i a g n o s i s , V . L . S . I . d e s i g n and p r o c e s s p l a n n i n g . The

g e n e r a l p r i n c i p l e i s t h a t e x p e r t s y s t e m s use a d a t a b a s e and i n f e r e n c e

p r o c e d u r e s to s o l v e p r o b l e m s which n o r m a l l y r e q u i r e human e x p e r t i s e and

knowledge .

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Free-form Surface Fig. 4

A MODULAR SYSTEM FOR MANUFACTURING DATA

~1 ~ " " " ~ ~ ~'-'~'~1- ~ , - , - , - - , o . I ' - - Generation of C L D A T A

o J I IMerpre t l t l on Of G ~ m e t r y | ~ ~ | -

EXP~.T SYSTEM ~ Knowladml ban . , . • me~dllte tO011

work r n e w l d s eutle~ ~ Of P:lO Carbide)

Infamnea nrnoadura l for. '- • ~ eeleolkm • ~ meterbl sekmtlon • nmelthte elmllqN

I pOST"nOCESSO"S I

Fig. 5

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Knowledge based systems offer a possible solution to process planning

problems by inferring data from design models and designers, analysing

this with respect to advanced manufacturing technology methods and

making the results available at the CAD/CAM workstation. Designs may

then be optimised for productivity before the production phase is

entered.

Expert planning systems are now being widely developed in an attempt to

solve process selection and planning problems (ref 2). Many of these

systems are concerned with automated process planning for families of

parts consisting of cylindrical and/or prismatic components. Facilities

such as automatic feature recognition, tool selection and cldata

generation have been developed and the flow of information from design

to production without human intervention has been demonstrated. Joshi,

et al (ref 3) have developed an expert process planning system

interfaced to a solid modeller, the object of which is to integrate

process planning with design using A.I. techniques. However, its

application is limited to components with prismatic features and

cylindrical holes. Altlng et al (ref 4) describe the development and

implementation of XPLAN, an interactive system used to generate process

plans for rotational components. There are many other examples of CAPP

systems for these types of parts.

Work is currently underway at Teesside Polytechnlc on the development of

a modular system which will address the problems of automated process

planning for complex surfaces; figure 5 gives an overview of this

system.

Commerc i a l s o f t w a r e p a c k a g e s a r e now a v a i l a b l e w i t h f a c i l i t i e s f o r

surface representation, and the generation and postprocesslng of cldata.

For the purposes of this research, it was decided to adopt the CAD

Centre's "3-D Surface", "GNC" and "CTAPE" software to perform these

functions. The major effort of the research project has therefore been

concentrated on surface geometry interpretation and manufacturing data

generation.

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4. GEOMETRY INTERPRETATION

The method of surface geometry interpretation depends on how the surface

is represented by the CAD system.

For cross-sectional surfaces, interpretation is concerned with the

geometry of the section curve, whereas the interpretation of free-form

surfaces is concerned with a matrix of nodes and vectors. In either

case, the software is concerned with the determination of maximum

cutting tool diameters, minimum blank sizes and minimum depth of

surfaces.

The geometry interpretation module is written in Fortran 77 and consists

of a menu driven user interface and a number of subroutines designed to

extract relevant characteristics from particular surface types.

5. THE GENERATION OF ~tNUFACTURING DATA

The manufacturing module consists of three knowledge bases and

associated inference procedures to generate manufacturing procedures.

Salford PROLOG was chosen as the development tool for this module

because it allows the use of both integer and real arithmetic, tberby

simplifying the coding of machining parameter calculations.

Process planning for the production of moulds and dies with complex

surfaces involves the following operations:

5.1 Machine Selection

The problems associated with selecting appropriate machines to produce a

given component are enormous. For the purposes of complex surface

production, the machine type was limited to 3 axis CNC milling machines

using ball-nosed cutters. Machine selection was then based on the power

and capacity needed to manufacture a given surface. A knowledge base

has been developed and this is used to store the parameters of available

CNC machine tools.

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5.2 Machining StateLy

The s t r a t e g y a d o p t e d f o r t h e p r o d u c t i o n o f comp lex s u r f a c e s was t o

emp loy t h e p r i n c i p l e o f s u c c e s s i v e l e v e l c o n t r o l , w i t h u p p e r and lower Z

limits, for roughing cuts followed by a single finishing cut over the

entire surface. This strategy reduces non-productive time by ensuring

that areas machined in one roughing pass are not machined in subsequent

roughing passes.

When ball-nosed cutters are used to machine a surface, it is normal to

feed the cutter to an appropriate depth then progress along the surface

keeping Y (or X) constant. This process is repeated until the whole

area has been machined. The depth may then be incremented (z shift) and

the whole process will again be repeated until the full surface depth is

achieved. In order to generate detailed instructions for these cuts it

was necessary to develop knowledge bases and rules for materials and

cutting tools.

5.3 Machining Parameters

The next step in the process planning sequence was to determine

machining parameters such as tool size and material, spindle speeds,

tool feeds, depth of cuts and tool stepover for roughing and finishing

operations. The current version of the KBS expects data relating to

work materials and surface finish to be entered interactively.

The g e o m e t r y i n t e r p r e t a t i o n modu le s u p p l i e s d a t a r e l a t i n g t o minimum

b l a n k s i z e , d e p t h o f s u r f a c e and maximum c u t t e r s i z e d i r e c t l y f rom t h e

s u r f a c e m o d e l l i n g s o f t w a r e .

I n f e r e n c e p r o c e d u r e s a r e t h e n u s e d t o g e n e r a t e m a c h i n i n g p a r a m e t e r s f o r

r o u g h i n g and f i n i s h i n g o p e r a t i o n s .

I n f e r e n c e p r o c e d u r e s f o r s u c c e s s i v e l e v e l c o n t r o l , f i n i s h i n g a l l o w a n c e ,

m a c h i n e t o o l and c u t t e r s e l e c t i o n h a v e a l s o b e e n i n c o r p o r a t e d i n t o t h e

s o f t w a r e .

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GENERATION OF CLDATA AND PART PROGRAMMES

The current process planner is equipped with data on eight machine

types, twenty one work materials and twenty five cutting tools (standard

and long-series high speed steel and P30 grade carbide tipped ball-nosed

s l o t d r i l l s ) .

An interactive session with the planning system will generate much of

the data normally provided by experienced process planning engineers.

This data may subsequently be entered into an N.C. preparation package

(GNC) together with component geometry definitions. Thus facilitating

the generation of cldata and CNC part programmes.

The system described has successfully been used to generate part

programmes for 2-axis CNC milling machines. These have been down-loaded

to the Polytechnic's machine tools and a number of demonstration surface

have been manufactured.

REFERENCES

1.

2.

3.

4.

Woodwark.J. 'Computing Shape'. Butterworths, London (UK). 1986.

Davies.B.J. and Darbyshire. I.L. 'The Use of Expert Systems in Process Planning'. Annals of CIRP. 33/1/84 1984. pp303-306.

Joshi.S.et al 'Expert Process Planning with Solid Model Interface' Int Journal of Production Research.1988.

Alting. L. et al. 'XPLAN - An Expert Process Planning system and Its further Development. Proceedings of the 27th Matador Conference. Macmillan Education, London (UK) 1988.