Computer Aided Manufacturing (CAD/CAM)Computer Hided and Integrated potx

Computer Aided Design / Computer Aided Manufacturing CAD/CAM Computer Hided and Integrated Manufacturing Systems fl S-Volume Set Cornelius T Leondes... Wrapping of relief on surfaces

Computer Aided Design / Computer Aided Manufacturing (CAD/CAM)

Computer Hided and

Integrated Manufacturing Systems

fl S-Volume Set

Cornelius T Leondes

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Published by

World Scientific Publishing Co Pte Ltd

5 Toh Tuck Link, Singapore 596224

USA office: Suite 202, 1060 Main Street, River Edge, NJ 07661

UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

COMPUTER AIDED AND INTEGRATED MANUFACTURING SYSTEMS

A 5-Volume Set

Volume 4: Computer Aided Design/Computer Aided Manufacturing (CAD/CAM)

Copyright © 2003 by World Scientific Publishing Co Pte Ltd

All rights reserved This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA In this case permission to photocopy is not required from the publisher

ISBN 981-238-339-5 (Set)

ISBN 981-238-980-6 (Vol 4)

Typeset by Stallion Press

Printed by Fulsland Offset Printing (S) Pte Ltd, Singapore

Preface

Computer Technology

This 5 volume MRW (Major Reference Work) is entitled "Computer Aided and Integrated Manufacturing Systems" A brief summary description of each of the

5 volumes will be noted in their respective PREFACES An MRW is normally on

a broad subject of major importance on the international scene Because of the breadth of a major subject area, an MRW will normally consist of an integrated set of distinctly titled and well-integrated volumes each of which occupies a major role in the broad subject of the MRW MRWs are normally required when a given major subject cannot be adequately treated in a single volume or, for that matter,

by a single author or coauthors

Normally, the individual chapter authors for the respective volumes of an MRW will be among the leading contributors on the international scene in the subject area of their chapter The great breadth and significance of the subject of this MRW evidently calls for treatment by means of an MRW

As will be noted later in this preface, the technology and techniques utilized in the methods of computer aided and integrated manufacturing systems have pro-duced and will, no doubt, continue to produce significant annual improvement in productivity — the goods and services produced from each hour of work In addi-tion, as will be noted later in this preface, the positive economic implications of constant annual improvements in productivity have very positive implications for national economies as, in fact, might be expected

Before getting into these matters, it is perhaps interesting to briefly touch on Moore's Law for integrated circuits because, while Moore's Law is in an entirely dif-ferent area, some significant and somewhat interesting parallels can be seen In 1965, Gordon Moore, cofounder of INTEL made the observation that the number of tran-sistors per square inch on integrated circuits could be expected to double every year for the foreseeable future In subsequent years, the pace slowed down a bit, but den-sity has doubled approximately every 18 months, and this is the current definition

of Moore's Law Currently, experts, including Moore himself, expect Moore's Law

to hold for at least another decade and a half This is impressive with many nificant implications in technology and economies on the international scene With these observations in mind, we now turn our attention to the greatly significant and broad subject area of this MRW

sig-Preface

"The Magic Elixir of Productivity" is the title of a significant editorial which

appeared in the Wall Street Journal While the focus in this editorial was on

produc-tivity trends in the United States and the significant positive implications for the economy in the United States, the issues addressed apply, in general, to developed economies on the international scene

Economists split productivity growth into two components: Capital ing which refers to expenditures in capital equipment, particularly IT (Informa-tion Technology) equipment: and what is called Multifactor Productivity Growth,

Deepen-in which existDeepen-ing resources of capital and labor are utilized more effectively It is observed by economists that Multifactor Productivity Growth is a better gauge of true productivity In fact, computer aided and integrated manufacturing systems are, in essence, Multifactor Productivity Growth in the hugely important manufac-turing sector of global economies Finally, in the United States, although there are various estimates by economists on what the annual growth in productivity might

be, Chairman of the Federal Reserve Board, Alan Greenspan — the one economist whose opinions actually count, remains an optimist that actual annual productivity gains can be expected to be close to 3% for the next 5 to 10 years Further, the Treasure Secretary in the President's Cabinet is of the view that the potential for productivity gains in the US economy is higher than we realize He observes that the penetration of good ideas suggests that we are still at the 20 to 30% level of what is possible

The economic implications of significant annual growth in productivity are huge

A half-percentage point rise in annual productivity adds $1.2 trillion to the federal budget revenues over a period of ten years This means, of course, that an annual growth rate of 2.5 to 3% in productivity over 10 years would generate anywhere from

$6 to $7 trillion in federal budget revenues over that time period and, of course, that is hugely significant Further, the faster productivity rises, the faster wages climb That is obviously good for workers, but it also means more taxes flowing into social security This, of course, strengthens the social security program Further, the annual productivity growth rate is a significant factor in controlling the growth rate of inflation This continuing annual growth in productivity can be compared with Moore's Law, both with huge implications for the economy

The respective volumes of this MRW "Computer Aided and Integrated facturing Systems" are entitled:

Manu-Volume 1: Computer Techniques

Volume 2: Intelligent Systems Technology

Volume 3: Optimization Methods

Volume 4: Computer Aided Design/Computer Aided Manufacturing (CAD/CAM) Volume 5: Manufacturing Process

A description of the contents of each of the volumes is included in the PREFACE for that respective volume

Preface vn

There is really very little doubt that all future manufacturing systems and cesses will utilize the methods of CAD/CAM (Computer Aided Design/Computer Aided Manufacturing), and this is the subject of Volume 4 Key to the processes

pro-of CAD/CAM is the generation pro-of three dimensional shapes, a subject treated at the beginning of this volume, 2D assembly drawings are what are generally utilized for conversion to 3D part drawings in the CAD process in order to generate three dimensional shapes for the CAM process, and this is treated in depth and rather comprehensively in this volume The evolution of a design process and product is often referred to as an adaptive growth representation in the CAD process and this receives necessary treatment in this volume Fixture designs for the manufacturing process utilize modular elements, and the CAD methods for this essential process are treated rather comprehensively in this volume Finite element techniques are becoming a way of life for CADS and CAE (Computer Aided Engineering) and rather powerful optimization techniques for processes involved here are also treated

in depth in this volume Rapid prototyping techniques are now a way of life in manufacturing systems, and CAD techniques for this are presented in this volume These and numerous other techniques are treated rather comprehensively in this volume

As noted earlier, this MRW (Major Reference Work) on "Computer Aided and Integrated Manufacturing Systems" consists of 5 distinctly titled and well-integrated volumes It is appropriate to mention that each of the volumes can be utilized indi-vidually The significance and the potential pervasiveness of the very broad subject

of this MRW certainly suggests the clear requirement of an MRW for a hensive treatment All the contributors to this MRW are to be highly commended for their splendid contributions that will provide a significant and unique reference source for students, research workers, practitioners, computer scientists and others,

compre-as well compre-as institutional libraries on the international scene for years to come

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C H A P T E R 1

G E N E R A T I O N OF T H R E E - D I M E N S I O N A L S H A P E S I N

C A D / C A M S Y S T E M S U S I N G ART-TO-PART T E C H N I Q U E

CHUA C K and CHOW K Y

School of Mechanical & Production Engineering, Nanyang Technological University,

50 Nanyang Avenue, Singapore 639798

In some industries, products have elements of complex engraving or low relief on them Traditionally, such work is carried out by skilled engravers working from 2D artwork manually This process is costly, open to unwanted misinterpretations and lengthens the design cycle This research presents the Art-to-Part technique which relies on computers and automation from the scanning of 2D artwork, to 3D surface and relief generation, and finally to the fabrication of the model by rapid prototyping The technique links design to manufacturing stages together and reduces the whole production time Furthermore, the quality is increased and reproducibility and reliability are ensured, as demonstrated in the 3 case studies

Keywords: 3D relief; Art-to-Part; CAD/CAM; rapid prototyping

1 I n t r o d u c t i o n

T h e r e are presently numerous commercially-available software for product design for

a particular range of industries which include ceramics, glassware, b o t t l e making,

b o t h plastic and glass, jewelry, packaging and food processing for molded ucts and products produced from forming rolls, coins and badges, and embossing

prod-r o l l e prod-r s 1 - 3 All of these industries share a common problem: most of their p r o d u c t s have elements of complex engraving or low relief on t h e m 4 Traditionally, such work

is carried out by skilled engravers either in-house or more often by a t h i r d - p a r t y sub-contractor, working from 2D artwork This process is costly, open t o unwanted misinterpretation of t h e design by t h e engraver and most importantly, lengthens

t h e t i m e of t h e design cycle

Advances in manufacturing technology allow m a n y industries t o u p g r a d e and change their usual p r o d u c t i o n practices from labor-intensive t o a u t o m a t e d and computerized m e t h o d s W i t h these changes, t h e production cycle t i m e and cost

1

2 Chua C K and Chow K Y

could be reduced tremendously with an improvement in the quality of the uct In recent years, computer-aided design and computer-aided manufacturing (CAD/CAM) have become very popular, especially in the manufacturing indus-tries It links the designing and manufacturing stages together and thus reduces the whole production time It is a significant step toward the design of the factory of the future.5

4 Wrapping of relief on surfaces

5 Converting triangular mesh files to STL file

6 Building of model by the SLA

The flow of this series of stages is illustrated using coin design as a case study Figure 1 shows the steps involved in the art-to-part process

In the ArtCAM environment, the scanned image is first reduced from a colour image to a monochrome image with the fully automatic "Gray Scale" function Alternatively, the number of colours in the image can be reduced using the "Reduce Colour" function A colour palette is provided for colour selection and the various areas of the images are coloured, either using different sizes or types of brushes or the automatic flood fill function Figure 3 illustrates the touched-up image

Generation of 3D Shapes in CAD/CAM Systems

Scanning of artwork

_^_

Generation of surfaces (eg.coin shape)

4 Chna C K and Chow K Y

Generation of 3D Shapes in CAD/CAM Systems 5

Fig 4 Shape of a coin model

•

•

Selected colour J Plane • Round • Square | Convex • Concave | Max height:

Base height:

Angle : Scale:

Heij Calculate

Fig 5 Control pane! for the shape profile

the profile (convex or concave), the maximum height, base height, angle and scale Figure 5 shows the control panel for the shape profile

There are three possibilities for the overall general shape; a plane shape profile will appear completely flat, whereas a round shape profile will have a rounded cross section and lastly, the square shape profile will have straight angled sides Figure 6 illustrates the various shapes of the 3D reliefs For each of these shapes, there is an option to define the profile as either convex or concave

6 Chua C K and Chow K Y

The square and round profiles can be given a maximum height If the specified shape reaches this height? it will 'plateau' out at this height giving in effect a l a t region with rounded or angled corners, depending on whether a round or square shape was selected for the overall prolle respectively (see Fig 6)

The overall prolle height, which covers the respective region, can be controlled

by specifying the required angle of the profile which represents the tangent angle

of the curve at the edge of the region Figure 7 further illustrates the concept

of the overall profile height An alternative to control the overall profile height is

to use the 'scale5 function to flatten out or elevate the height of the shape profile

&r*d flft&3s**£t*?8 ftfci|ghl nod mzk^m^m my$%

Fig 6 Various shapes of t h e 3D relief

Generation of 3D Shapes in CAD/CAM Systems 7

• « fT£$ &3&& Insight 1 mm

r i round profila *w$h angle 30 s*sd

U t^&& h^gSht 1 mm asd max tw§M ^ mm

r - r^yr^d proiiifc w#H £^$8 W and

Fig 8 An illustration of t h e definition of shape profiles on different regions

Fig 9 3D relief of an artwork

(see Fig 7) The relief detail can be examined in a dynamic Graphic Window within the ArtCAM environment itself Figure 8 shows an illustration of the definition of shape profiles on different regions Figure 9 illustrates the 3D relief of an artwork

2*4 Wrapping of relief on surfaces

The 3D relief is next wrapped onto the triangular mesh file generated from the coin surfaces using the command Wrap (see Fig 10) This is a true surface wrap and not

a simple projection The wrapped relief is also converted into triangular mesh files (see Fig 11) The triangular mesh files can be used to produce a 3D model suitable for colour shading and machining The two sets of triangular mesh files, of the relief and the coin shape, are automatically combined (see Fig 12) The resultant model file can be colour-shaded and used by the SLA to build the prototype (see Fig 13)

2.5* Converting of triangular mesh file into STL file

The STL format is originated by 3D System Inc as the input format to the SLA,

and has since been accepted as the de facto standard of input for Rapid Prototyping

8 Chua C K and Chow K Y

Fig 10 3D relief wrapped onto coin surface

Fig 11 Wrapped relief converted Into triangular mesh lies

(EP) systems.6"8 Upon conversion to STL, the object's surfaces are triangulated, which means that the STL format essentially consists of a description of inter-joining triangles that enclose the object's volume The triangular mesh files are also trian-gulated surfaces, however, of a slightly different format (see Fig 14) Therefore, an interface programme written in Turbo-C language was developed for the purpose

of conversion The converted triangular file adheres to the standard STL format as

in Fig 15 It has the capability of handling triangular files of huge memory size

2.6 Building of model by SLA

Californian company 3D System Inc pioneered the Rapid Prototyping (EP) nology when they released their commercial E P system in December 1938 — the

tech-Generation of 3D Shapes in CAD/CAM Systems 9

Fig 12 2 sets of triangular mesh iiles •- relief and coin shapes are automatically combined

Fig 13 Colour-shaded resultant model file

SLA-250 model of their Stereolithography Apparatus (SLA).6'7 Stereolithography

technology was Irst developed by Chuck Hall, SD's founding president, in 1982

Stereolithography works by using a low-power Helium-Cadmium laser or an Argon

laser to scan the surface of a vat of liquid photopolymer which solidiies when struck

by a laser beam

The SLA process chamber consists of a vat containing liquid photopolymer resin,

a platform on which the object is to be built and whose height is controlled by an

elevator mechanism, a re-coating blade wiper and a Helium-Cadmium or Argon

laser subsystem At the start of the object building process, the platform is

posi-tioned at a depth of one layer's thickness below the resin level The laser will trace

over areas of the resin surface defined by vectors as the cross-section of the first

layer The area where the resin is struck by the laser beam solidifies to form the

first layer of the object Subsequently, the platform is lowered by a distance equal

to the layer thickness, pauses for about 15 seconds to allow the resin level to settle

and the re-coating blade wipes over the resin surface to prepare the construction of

the next layer as the process repeats itself When the object has been completely

built, the platform is raised above the vat of the resin to drain off the excess liquid

resin that has adhered to the object Figure 16 illustrates the building of prototype

using the SLA

10 Chua C K and Chow K Y

DUCT 5.2 TRIANGLE BLOCK P

0 0 0.00000 0.00000 0.00000 0.00000 1.00000 0.00000 1.00000 20.00000 1.00000 20.00000 1.00000

1 4

18 AUG 1993 21.43.28

0 0.00000 0.00000 1.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000

20.00000 0.00000 1.00000 1.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000

Fig 14 T h e original triangular file format

2.00000e+01 0.00000e+00 2.00000e+01

facet normal 0.00000e+00 2.00000e+02 0.00000e+00

Fig 15 The converted triangular file to follow t h e STL format

Generation of 3D Shapes in CAD/CAM Systems 11

Fig 16 Building of prototype using the SLA

The object that comes out from the SLA's process chamber is approximately 96% solidified and there are minute gaps between the laser's cross-hatching vectors

in between the top and bottom layers which hold uncured, liquid resin A curing apparatus, which comes in the form of an oven containing ultraviolet lamps and a rotating turntable, is used to post-cure the object to make it totally solid Support structures are required at the base of the object so that it does not adhere directly to the platform They are also needed to support overhanging features of the object to prevent them from collapsing during the building process These support structures are removed from the object when it has been completely built

post-The SLA makes use of a variety of photopolymers with different properties suited for different requirements The properties of the cured photopolymers should allow SLA prototypes to be used for making soft tools like rubber moulds for mass production Research has shown that feasible rubber moulds can be made from SLA-produced jewellery rings.9 The SLA is capable of a 0.125 mm minimum layer thickness and an accuracy of within 0.5%

3 Advantages of Art-to-Part Process

The introduction of the scanner, the CAD/CAM system and the SLA provides a list

of specific advantages to the art-to-part process: (1) save time, (2) easy to amend and (3) easy to master and apply

3.1 Save time

The existing technique of hand-carving takes about two weeks to complete a plaster mould However, relief can be created in the CAD/CAM system in two hours' time and the prototype will be ready for examination in the next morning after going through the SLA Most companies that manufacture a product invest considerable time and money in developing a prototype or model Typically, it is common that the prototyping process could take weeks or months The time to market has become a competitive issue in the need to prototype quickly.10,11 Besides SLA, other methods are also available for R P

12 Chua C K and Chow K Y

3.3 Easy to master and apply

The whole package is relatively user friendly and the procedures for generating relief are short and simple The fear of making mistakes in the design becomes an unjustified worry There is also a high potential in further extending the application into other areas such as the jewellery and ceramics tableware industries

4 Development of STL File Interface

ArtCAM is a 3D CNC engraving software produced by Delcam International and

is used to convert a two-dimensional picture into a three-dimensional relief mat The two-dimensional picture can be a scanned picture in bitmap format, any picture file in graphics format like BMP, TIFF, GIF, JPEG or PCX format The software converts this picture into a three-dimensional format (file extension *.rlf)

for-by colouring the picture and assigning different colours to each part of the picture The colours are then given an altitude or height so that when a relief is calculated and displayed each of the colours is transformed into a relief and the whole image

is viewable as a three-dimensional format called the relief The output relief mat (*.rlf) is specific to the ArtCAM software which is used for CNC engraving However the relief format is not suitable for RP Systems In order to create the 3D part using RP technology, it is required to transform the relief format into a STL file

for-Rapid prototyping (RP) is a key technology of the 1990s More than two dozen

RP techniques have emerged since the first RP technique, stereolithography, was commercialised in 1988.15 The most commonly used input to a RP system is the

de facto stereolithography file (STL) All vendors of RP systems accept this format

and practically all major suppliers of CAD/CAM systems today provide an interface between their CAD model and the STL file

4.1 Format of relief and STL files

The formats of the relief and STL file, which are the input and the output files, are respectively discussed in detail The structure of their internal detail is explained with the help of figures

Generation of 3D Shapes in CAD/CAM Systems 13

4.1.1 Format of relief file (input file)

A relief file consists of 3D image in x, y and z coordinates The x and y coordinates represent the in-plane data and the z coordinate represents height measurements

in pixels The 3D relief image is bounded by a rectangular frame The height of the

pixel gives the z coordinates, which is the most important part of the relief The

coordinates represent the internal structure of the relief The file extension is rlf The contents of the relief file can be binary or ASCII

A relief is represented internally as a 2 dimensional array of 16 bit signed gers along with a scaling factor used to transform the integer value into a floating point height This representation halves the memory requirements of a relief when compared to storing the values as floats

inte-4.1.2 Format of STL file (output file)

The STL file format is the most commonly used file format for input in rapid

prototyping technologies and is also the de facto industry standard The STL file

defines the surface of an object as a set of interfacing triangles or facet

Each facet as shown in Fig 17 is defined with three vertices and a normal, which identifies which side faces out and which side faces in In the STL file, solid models are represented as an unordered collection of facets and each facet has an outward directed facet normal associated with it.16

The generation of these facets depends on the information contained in the STL file It should be noted that in the format of the STL file, the coordinates

of the vertices are ordered according to the right hand screw rule That is in an

V2

Edge y j \ /

14 Chua C K and Chow K Y

anti-clockwise direction such that the normal of the facet is being directed away from the model as shown in the Fig 18

Another important information that can be derived from the STL file is that for every facet edge, there must be another facet and only that facet sharing the same edge Since the vertices of a facet are ordered, the direction on one facet's edge is exactly opposite to another facet sharing the same edge, this necessary condition is also known as Mobius rule as shown in Fig 19

A facet can reference the three edges which bound it.17 Each edge can reference the two vertices which define it Vertex points can contain the connectivity infor-mation to all edges or faces which share it The STL format only contains facets with minimum information necessary to define the image or solid object

Fig 18 Right hand screw rule

Fig 19 Mobius rule = Edge shared by two facets

Generation of 3D Shapes in CAD/CAM Systems 15

For each vertex that is present in the STL file, it is absolutely necessary to calculate the normal in order to determine which way the facet is facing, whether inwards or outwards To calculate the normal, the essential information required is the vertices which bound the facet The normal is calculated by the cross product

of the vertices

In order to understand the format of a STL file, it is important to know the basic internal structure of a STL file There are two different formats of STL files One

is the ASCII format which is human readable, and the other is the binary which

is totally unreadable During the developmental stages of this project, the ASCII format was used for debugging purposes

The binary format is used only in the final release version The reason for using the binary file format is because of its compactness To illustrate with the example

of the bear, the size of the STL file in ASCII is 12 MB whereas the size STL file in binary is only 900 kilobytes The size of the file differentiates these two formats

4.2 STL conversion

To convert the relief file into STL output, a number of problems must be overcome They are mainly related to the size of the image file being converted The major concerns that will affect the image size are the resolution of the relief image (input file), the size of the output file with respect to testing, and the reduction of triangles which are directly dependent on the resolution of the image Each problem will be explained in detail as follows:

4.2.1 Limitation of the STL format

In order for the file size not to be too large, the ASCII version of the STL file was used only for verification purposes and the binary version was used for testing and

in the final release version The size of the converted STL file (binary) should not exceed a size greater than 50 MB This is a limitation of the STL format amongst several other disadvantages of the STL format This is one of the major problems affecting the testing of the file because if the file could not be tested, it would not

be possible to detect the errors This puts a major constraint on the image size of the relief file The example used for testing purposes was that of the face of a bear The output of the STL ASCII file size was 12 MB whereas the size of the binary file was 900 kilobytes

4.2.2 Resolution of the relief image

The resolution of the relief image also affected the output If the resolution of the relief image was higher, then the STL output would grow in direct proportion to the resolution of the relief image The primary reason being that, for a high-resolution image the number of pixels used to describe the image would be far greater than required Hence in order to keep the size of the file under control, it is necessary to

16 Chin C K and Chow K Y

keep the resolution of the bitmap image used as input to the ArtCAM software to

an acceptable level of visibility

The acceptable level of visibility means that the bitmap image can be viewed without raising the resolution measured in dots per inch (DPI) The DPI has to

be as minimum as possible and at the same time the image detail should be clear Once this has been done the relief obtained from the bitmap image would also be of reasonable resolution This would help in reducing the size of the output STL file

An example of a relief with low resolution and high resolution is shown in Fig 20

Fig 20 Low and high resolution relief

Generation of 3D Shapes in CAD/CAM Systems 17

4.2.3 Triangle reduction

The next factor affecting the size of the output file is the number of triangles used in describing the STL output The technique used in reducing the number of triangles was by searching for triangles having the same pixel height Then, these large trian-gles with the same pixel height will be grouped in an orderly manner and reduced

to only two triangles An algorithm in the next section explains the implementation

of this step Thus by reducing the STL output file to fewer triangles, it would result

in fewer points in the STL output file, thereby reducing its overall size

4.3 Conversion algorithm

The algorithm has the following steps:

1 Converting each set of points on the relief into triangles Checking for points with same pixel height Formation of triangles with same pixel heights

2 Checking for gaps in the relief image.18

3 Formation of box so that the STL output is closed with a variable height The relief is first treated as a whole with rectangular mapping of points as shown in Fig 21 like a XY graph The points on the XY graph represent the pixels

(Z plane) Each pixel may or may not have a height depending on its location on the

image The direction of its traversal path starts from the origin zero The following

steps are carried out using the notation (x,y):

Start with the first point (0, 0) and its counterpart (0, 1) which is on top left These two points are considered as a set of points for comparison

1 Select the next point along the x-axis (1, 0) and its counterpart (1, 1) which is

on top right These two points are considered the next set of points

2 Check if the heights of the first and second sets of points along with their terparts are equal

coun-There are three possible cases that arise from the comparison

a If the heights of the points are equal, then there are two possibilities to be considered The first possibility is the heights of the pixel for these points is zero Triangles are not formed for this possibility because this part of the image does not contain the relief points The second possibility is that the heights are equal but the heights of the pixel are greater than zero Once the heights are equal and greater than zero, a comparison with the next point in sequence is made Here the next point is the third point A comparison is made between the first and the third set of points A continuous comparison is made until the end

of the x-axis is reached or when the heights are not equal

b If the heights are not equal then triangles are formed as shown in the Fig 22

c Once a set of triangles is created between two sets of points A {(0,0), (0,1)} and B {(1,0), (1,1)} as shown in Fig 22, then the second set of points in this

18 Chua C K and Chow K Y

Formation of triangles when the heights of points are

equal until the end of the x~axis

Fig 21 Case A traversal p a t h showing formation of triangles when heights of two consecutive

points are equal

case B {(1,0), (1,1)} is used as the reference origin from which the heights are compared with the next set of points

3 This is done for the entire image, covering the entire x and y-axis

The next step involves detection of gaps for those areas of images where the surface of the image protrudes outwards with its upper surface not connected to the base of the relief on all sides One side of the surface is open whereas the other side is connected to the relief as shown in Fig 23 The above algorithm does not handle this kind of problem and so the entire relief image is scanned for such gaps and if they exist, these gaps are covered by formation of triangles using the triangle reduction technique

The inal step is the formation of a box Giving a fixed height measurement to the i n a l image does this The image is enclosed by a rectangular boundary based

on the relief image in pixels An exact shaped boundary is provided at the bottom with enclosures on all the four sides The final image is a box with fixed width and the STL image located on the top surface of the box Figures 24 and 25 show the

Generation of 3D Shapes in GAD/CAM Systems 19

(X=Maximum, Y=Maximum)

I (1,0)

Origin (x,y) (0,0)

(2,0) Starting point after triangles

have been formed with previous points

(End of x axis, y=l)

(End of x axis, y=0)

Formation of triangles when the heights of the points are not equal and so triangles is formed with these points The process starts again with the last point being starting point as shown above

Fig 22 Case B traversal path showing formation of triangles when heights of two consecutive points is not equal

Gap or uncovered portion of

image

Fig 23 Gaps in relief image

20 Chua C K and Chow K Y

Fig 24 Image of t h e sculptor shown in STL format without rectangular box

Sculptor model before and after the formation of the box respectively This is done

so that the whole relief image is closed and no openings are shown

Generation of 3D Shapes in CAD/CAM Systems 21

Fig 25 Shaded image of the sculptor converted to STL file enclosed by a rectangular box

5, C a s e S t u d i e s

Three case studies are selected to illustrate the significant advantages of using the proposed art-to-part technique over the conventional tools and processes These case studies are done to cover various types of artwork designs including animals,

a human face, flowers as well as Chinese characters, and at the same time to show the feasibility of replacing the current plaster mould prototype with the resin model prototype Alongside the advantages obtained in adopting the use of the proposed prototyping technique, the case studies also revealed shortcomings which provide scope for future work

5.1 Chinese Legend mnd Tpaditi&n

The Chinese Legend and Tradition is a coin design consisting of a series of Chinese characters and a roaring dragon In the conventional method, a trained craftsman would require 30-40 hours to make a plaster mould prototype of diameter 200 mm (the size of the plaster mould should be made sufficiently large for easy carving to

be carried out) In the new prototyping technique, the diameter and thickness of the resin model can be reduced substantially to save production cost using the zooming function in the software The total processing time from scanning in of the artwork

up to building of the model using the SLA takes about 13 hours to complete, which

22 Chum C K and Chow K Y

Fig 26 Resin prototype of Chinese characters and roaring dragon

is equivalent to 63% in time saving! The resin prototype which is painted in red is shown in Fig 26

The reliefs of the Chinese characters and the border surrounding the dragon are designed to form a plane surface with the base height of 1.0 mm Whereas the dragon is assigned with a convex relief at a starting angle of 35 degrees and base height of 0.5 mm After examining the resin model built from the SLA, it is found that all the relief details turned out as defined except for the body of the dragon which is slightly higher than the desirable convex profile This can be overcome by changing the colour of that portion and reassigning a relief profile with a smaller angle, say 25 degrees

Steps are observed on the dragon profile, particularly on the slope, due to the constraint of the minimum layer size obtainable on the SLA (0.125 mm) In this case, the craftsman has to spend a little time on removing the stepped appearance of the surfaces for a smoother, better-looking finish However, this compares favourably because the new prototyping technique is faster and less tedious than carving the prototype from a plaster block

as compared to that of the black and white dragon image Nevertheless, the total processing time of 14 hours is far shorter than the conventional prototyping method

Generation of 3D Shapes in CAD/CAM Systems 23

which requires §6 hours to produce a human facial feature plaster mould prototype The edited image is shown in Fig 27

The complex shape of the human facial expression, however, presents two main problems when it comes to building the prototypes using the SLA Firstly, as men-tioned earlier, steps are clearly seen on the relief at almost every portion of the face There are two factors contributing to this shortcoming, the Irst Is the large area occupied by the artwork on the coin surface Flaws can be easily detected in a large object, likewise, the steps on the face (see Fig 28) Is enhanced in this case where the

S^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ™ ^

Fig 27 Edited image of human face

Fig 28 Steps on the face

24 Chum C K and Chow K Y

Pig 29 Resin prototype of the human face

convex profiles ccwer a large area (relief area of 120 x 120 mm2 on a 180 mm diameter coin surface) as compared to that of the dragon's body (relief area of 60 x 50 mm2

on a 100mm diameter coin surface) The second contribution to the distinct step appearance is the gradual slope (ranging from 5 to 25 degrees) of the relief defined The more gently the slope is defined, the further is the distance between subsequent layer edges, in order for the relief to fit into the curvature Therefore, it is strongly advisable to define a higher relief angle or to scale down the diameter of the coin surface so that the relief profile covers a smaller area

The second main problem is the reality of the facial expressions, especially the eyes, that can be brought out by the resin prototype Figure 29 shows the resin prototype This is largely dependent on the skill and experience of the designer in manipulating the relief as close to the ideal situation as possible

5,3 Omhid

Flowers are one of the most popular features used in designing coins The orchid, being the national flower of Singapore, is thus selected as a case study in prototype making Similar to that of the human face image, the original photograph (see Fig 30) of this orchid lower consists of more than a hundred colours, which poses quite a tedious job in reducing the number of colours on the scanned-in image and raising the possibility of the final image (see Fig 31) diverging unacceptably from the original design, after editing work has been carried out

Generation of 3D Shapes in CAD/CAM Systems 25

Fig 31 Edit image of the scanned-in orchid

The number of hours spent on building the orchid model using the SLA

is close to that for the human face model The duration of model ing depends largely on the number of layers required and the size of the model

build-26 Chua C K and Chow K Y

Fig 32 Resin prototype of the orchid

The level of complexity in the orchid design is less than that of the human facial design The reliefs produced are close to the pattern done by the craftsman However, steps axe seen on the slope of the convex profile, resulting in a substantial amount of polishing work needed to be carried out to improve the surface finish The convex curvature of the flower pattern appears to be slightly higher than the ideal size, however, this can be easily amended by redefining the angle of the relief in the Art CAM environment, which is actually the main advantage of using this technique in prototyping A new model can be rebuilt within 10 hours

Texts of all font types can be added to the design by scanning the required texts together with the main design The reliefs' of the texts are usually defined as a plane surface The CAD/CAM system has the full capability of generating such reliefs The resulting model built by the SLA (see Fig 32) has the advantage of avoiding undesirable steps due to its plane surface nature

6 A p p l i c a t i o n of A r t - t o » P a r t T e c h n i q u e In M i n t I n d u s t r y

The mint industry has traditionally been regarded as very labour-intensive and craft-based It relies primarily on the skills of trained craftsmen At present, automation in this industry has been restricted to the use of machines at certain individual stages of the manufacturing process Several of these stages are not linked

up and thus slow down the whole process The flexibility of CAD/CAM enables the modelling and manufacturing of working dies needed for stamping coins It is able

to link up the design and die-making stages and to provide a common database

Generation of 3D Shapes in CAD/CAM Systems 27

6.1 Current practices

This section briefly outlines the various processes involved in the design and ufacturing cycle of coin items (circulating and noncirculating) Based on the dis-cussion held with a coin manufacturer as well as some literature,19'20 it is found that advanced technologies have been implemented in upgrading the production speed and quality of the coin manufacturing processes, especially in the designing stage whereby artworks are prepared using computer graphics software However, the prototyping of a coin model is still as traditional as one hundred years ago, which is done by hand-carving a plaster mould

man-This section focuses on the suitability of applying CAD/CAM techniques to replace the conventional practices in prototyping coin models Therefore, the dis-cussion on the coin manufacturing processes will concentrate on the initial stages up

to the making of the working die The design and manufacturing cycle can be broken down into the following stages: design, plaster mould engraving, making of rubber mould, making of epoxy mould, making of master die and making of working die

6.1.1 Design

At the first stage of the coin production cycle, the designer prepares a 2D artwork based on the Aldus Freehand 3.1 graphical software package which accepts scanned images as input Majority of the work is spent on touching up the scanned-in picture and editing the text which may not be found in the original image The output

is directed to a laser printer, the resolution of the print-out can be improved by camera shooting it, developed into a photograph, rescanned into Aldus Freehand environment and edit to yield a better image It is essential to do a few iterations

to produce a piece of high resolution artwork so as to reduce the probability of the craftsman's misinterpretation of the artwork while making the plaster mould The designer's concern is concentrated on the aesthetic issues of the design rather than the mundane issues of precise geometry and the dimensions of the design

6.1.2 Plaster mould engraving

The creation of coin prototypes from a circular plaster block using simple tools such as small chisels involves a high level of skill and experience The designer's artwork of a coin piece is interpreted by the craftsman who builds a prototype from this interpretation The craftsman is responsible for dimensioning the various parts of the design based on the proportions provided by the designer, who does not dimension his artwork These interpretations are greatly influenced by his skills and experience, as a direct consequence, misinterpretations often result Generally,

a number of iterations is carried out within the first two stages (i.e design and prototyping) before a design is approved for manufacturing The designer assesses the prototype and suggests modifications to be made to the prototype if it does not turn out in the expected form The quintessential aspect of prototype building is

Chua C K and Chow K Y

Fig 33 Engraving of plaster mould by a craftsman

the skill of the craftsman, which ultimately determines the quality of the prototype, and consequently, that of the finished product It takes about one to two weeks for the craftsman to complete one piece of plaster mould prototype Any crack or mistake on the mould will result in a great deal of amendment work and a more serious case will require the whole work-piece to be discarded Figure 33 illustrates the engraving of plaster mould prototype by a craftsman

6.1.3 Making of rubber mould

This is a negative side of the plaster mould produced by pouring liquid rubber over the surface of the plaster mould The rubber mould is removed by peeling off from the plaster mould when solidified The time taken for a rubber mould to solidify varies from 5 to 8 hours, depending on the amount of hardener mixed in the solution

6.1.4 Making of epoxy mould

The epoxy mould is similar to the plaster mould in terms of its shape and size, except that the epoxy mould is made up of a harder material The epoxy mould

is produced by solidifying liquid epoxy over the rubber mould for about one day

It will be used for making the master die

Generation of 3D Shapes in CAD/CAM Systems 29

6.1.5 Making of master die

Steel is employed as the working material and processed by an instrument called the pantograph The pantograph, an instrument for the mechanical copying of a drawing

or diagram on the same, an enlarged or a reduced scale, is used for the engraving

of coin dies It consists of an arm that is used to trace an enlarged design at one end and at the other end, a revolving engraving tool simultaneously cuts an exact reduction of the original, to produce a hub from which working dies are made.19

The whole process takes about two days to complete, after which it undergoes a polishing and touching up treatment

6.1.6 Making of working die

The final working die, a negative face, is produced by the hobbing process using the master die The hobbing process takes one day to complete, excluding the annealing process which requires another 8 hours of the time Polishing of the working die is necessary between hobbing processes

The same procedures are carried when processing the reverse surfaces of the working die of a coin Flow A of Fig 34 illustrates the steps involved in making coin prototype by the traditional method

6.2 Use of CAD modeling and CNC machining

Once the client has given the requirements (usually in the form of artwork or tographs such as in Fig 35), the designer would scan in the design and store as

pho-an image file in the workstation called Silicon Graphics Iris The surface modeller, DUCT5.2, is used to map this 2D pattern onto 3D surfaces The method used to create surfaces is to form four boundaries for each surface needed using the scanned image in the background as a guide for outlining By doing so, the whole design would be made of many surfaces but is still 2D Flow B of Fig 34 gives a brief out-line of the process Flow C of Fig 34 outlines the art-to-part process as described

in Sec 2 for coin manufacturing

Using the same software, the surfaces could be manipulated in all directions so

as to edit on the shape, size (as in Fig 36) and most important of all, give them a third dimension Each surface is made up of many meshes The number of meshes is proportional to the flexibility in manipulating the surfaces The intersections of the meshes are known as points DUCT5.2 defines meshes using laterals and longitudes which are perpendicular to each other at the points DUCT5.2 uses NURBS to define surfaces Thus, the control of each point is local, that is, moving the position of a point would not affect the rest of the points, even those that are near to that point After some manipulation of the 3D surfaces, the design on the top surface of the coin is then ready for addition of text After the top surface has been completed, the shape of the coin is then modelled

Chua C K and Chow K Y

with surfaces (real

time) using DUCT

Surface painting

to visualise the

actual model of

the coin before

producing the die

Relief Genertion using ArtCAM

WrapArtwork with reliefs created onto 3D Coin

Surface apinting to visualise the actual model of the coin before producing the prototype

Create prototype using Stereolithography

Making of Epoxy Mould

Pentograph Machining of Master Die

(?) Create Machining Path

Fine Touching up and Polishing of Master Die

(?)

CNC Machining

Hobbing Process to produce Working Die

(?)

Yes

T ->f Final Polishing

= ^ _

Working Die

(?)

->- Ready to Manufacture Coin

Fig 34 Current and proposed coining processes

After the coin has been completely modelled, surface painting is done to visualise the actual model of the coin before producing the die Lighting on the model could be done to enhance the appearance of the model Up till this stage, the soft prototype has been produced on the workstation

Before investing substantial sum of money in making the mould, the manager could actually see the prototype of the coin and comment on it Alterations could

be made and money would not be spent unnecessarily in making moulds