Art-to-part case studies by C. K. Chua, R. K. L. Gay, S. K. F. Cheong and H. B. Lee Nanyang Technological University, Singapore

The integration of a colour scanner, three-dimensional CAD/CAM system and stereolithography apparatus provided a powerful means of building art to part. Artwork such as Chinese characters, human faces or flowers can be scanned by a scanner; converted into 3D reliefs within a CAD/CAM system and built using the SLA. In this article, the effectiveness of such an integrated system is illustrated with three examples from the coin industry.

Introduction art-to-part process include the here are presently numerous following: commercially-availa ble software packages for product 1 scanning of artwork

design for a particular range of industries which include ceramics, elassware. bottle makine (both

2 generation of surfaces 3 generation of 3D relief 4 wrapping of relief on surfaces

T plastic anh glass], jeweKe;y, packaging, food processing, for moulded products and products produced from forming rolls, coins and badges, and embossing

All these industries share a common problem: most of their products have elements of complex engraving or low relief on them. Traditionally, such work is carried out by skilled engravers either in-house, or more often by a third-party subcontractor, working from 2D artwork. This process is costly, open to unwanted misinterpretation of the design by the engraver and, most importantly, lengthens the time of the design cycle.

The CAD/CAM revolution has boosted the production and the performance of many industries everywhere for the past 15 years; however, its applications in the above industries are still in their infancy. Prototyping is still very much a manual process which relies largely on the skills of an experienced craftsman who uses handtools such as a small chisel to carve and shape the model out of a plaster block. Little attention has been focused on the use of quick and accurate rapid- prototyping equipment for building prototypes in these industries.

The use of CAD/CAM and stereolithography apparatus (SLA) reduces the time required for design modifications and improvement of

5 convertine trianeular mesh files to U "

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. Fig. 1 shows the steps involved in the art-to-part process.

Scanning of artwork he function of scanning software is to automatically or T semi-automatically create a 2D

image from 2D artwork. It would normally be applied in cases where it would be too complicated and time consuming to model the part from a

scanning of artwork

I relief generation using ArlCAM 1-

yes

model buiiding by SLA

yes

final model 1 prototypes. The steps involved in the

COMPUTING & CONTROL ENGINEERING JOURNAL DECEMBER 1994 285

Fig. 1 Steps involved in the art-to-part process

Fig. 2 2D artwork

Fig. 3 Touched- up image

the 3D model. This is used as a base onto which the relief data is wrapped and later combined with the relief model to form the finished part.

Generation of 3D reliefs he next stage in creating the 3D relief is t o assign each T colour in the image a shape

profile. There are various fields which control the shape profile of the selected coloured region, namely, the overall general shape for the region, the curvatures of the profile (convex or concave), the maximum height, base height, angle and scale. Fig. 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. Fig. 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.

be given a maximum height. If the specified shape reaches this height, it will ‘plateau’ out at this height giving in effect a flat region with rounded or angled corners, depending on whether a round or square shape was selected for the overall profile (see Fig. 6).

The overall profile height which covers the respective region can be controlled by specifying the required angle of the profile which represents the taneent anele of the curve at the

The square and round profiles can

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drawing using existing CAD techniques.

The 2D artwork is first read into ArtCAM, the CAD/CAM system used for the project, using a Sharp JX A4 scanner. Fig. 2 shows the 2D artwork of a series of Chinese characters and

Generation of surfaces edge-of the region. Fig. 7 further illustrates the concept of the overall profile height. An alternative to control the overall profile height is to use the ‘scale’ function to flatten out or elevate the height of the shape orofile (see Fie. 71. The relief detail

he shape of a coin is generated to the required size in the CAD system for model building.

Fig. 4 shows the shape of a coin model generated. A triangular mesh file is Droduced automaticallv from

T a roaring dragon. This combination of hardware and software allows for the direct production of a standard image from the artwork, which can be reacfdirectly into ArtCAM. The 2D artwork in such instances represents the designs to be used on the face of the coin.

In the ArtCAM environment, the scanned image is first reduced from a colour image t o a monochrome image with the fully automatic ‘grey 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 and types of brushes or the automatic flood fill function. Fie. 3 illustrates the touched-ufimage

286

;an be examined ‘in a dynamic graphic window within the ArtCAM environment itself. Fig. 8 shows an illustration of the definition of shape profiles on different regions. Fig. 9 illustrates the 3D relief of an artwork.

Wrapping of relief on surfaces he 3D relief is next wrapped onto the triangular mesh file T 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. 1 1 ). The triangular mesh files can be used to produce a 3D model suitable for colour shading and machinine. The two sets of trianeular

Fig. 4 Shape of a coin model mesh filer, of the relief and the coin

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mcanvex mcancave

max height 1001 base height 1 0 0 1 anale I 4 5 0 I I

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Fig. 5 Control panel for the shape profile

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).

Converting triangular mesh files to STL file

he STL format is originated by 3D System Inc. as the input T format to the SLA. and has

since been accepted as the de facto standard of input for layered- manufacturing sy~ tems .~ On 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 triangulated surfaces, however, of a slightly different format [see Fig. 14). Therefore, an interface program 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.

Building of model by the SlA alifornian company 3D System Inc., pioneered the layered- C manufacturing [LM) technology

when they released their commercial LM system in December 1988- the SLA-250 model of their stereolithography apparatus [SLA) . Stereolithography technology was first developed by Chuck Hall, 3D'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 solidifies when struck by a laser beam.

A flat profile square profile round profile

-base h e i g h t f i flat profile with round profile with

base height square profile with base helght base height

fi *=.ax height-

flat profile with base height and maximum height round profile with base height

and maximum heylht square profile wtth base height

and maximum height

Fig. 6 Various shapes of the 3D relief

The SLA Drocess chamber consists vectors as the cross-section of the 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 recoating blade wiper and a helium-cadmium or argon laser subsystem. At the start of the object building process, the platform is positioned at a depth of one layer's thickness below the resin level. The laser will trace over areas of the resin surface defined by

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 s to allow the resin level to settle and the recoating 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

X - cross-section of the region, x = 30 mm

angle = 60" Scale = 1 .O

angle = 20" scale ji 1 .O

angle = 20" scale 3 0.6

cross-section of the region, x - 30 mm angle = 60' scale=l.O

I\ angle = 20" scale = 1 .O

angle = 20" scale = 0.6

Fig. 7 Illustration of the overall profile height

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. square profile with angle 45" and base height 1 mm

round profile with angle 30' and 0 base height 1 mm

0 base height 1 mm and max height 2 mm

base height 1 mm

round profile with angle 30"

round profile with angle 20" and

Fig. 8 Illustration of the definition of shape profiles on different regions

COMPUTING 6( CONTROL ENGINEERING JOURNAL DECEMBER 1994

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Fig. 9 3D relief of an artwork

platform is raised above the vat of the resin to drain off the excess liquid resin that has adhered to the object. Fig. 16 illustrates the building of prototype using the SLA.

The object that comes out from the S U 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 post-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 i t has been completely built.

Fig. 10 3D relief wrapped onto coin surface

photopolymers with different properties suited to 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. The SLA is capable of a 0.125 mm minimum layer thickness and an accuracy of within 0.5%.

Advantages of the art-to-part process

he introduction of the scanner, the CAD/CAM system and the T SLA provides a list of specific

advantages to the art-to-part process as given below:

It saves time: The existing

The SLA makes use of a variety of

technique of hand-carving takes about two weeks to complete a plaster mould. However, relief can

Fig. 12 Two sets of triangular mesh files-relief and coin shape-are automatically combined

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Fig. 1 1 Wrapped relief converted into triangular mesh files

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, i t 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 q ~ i c k l y . ~ It is easy to amend: Very often, there is a need to amend the design of the prototype. Serious amendments will result in discarding of the plaster mould and doubling of the time needed to produce a model. The CAD/CAM system allows amendment to be done quickly and easily, and rebuilding of the model is also a simple task. It is 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.

Case studies hree case studies have been selected to illustrate the T significant advantages of using

the proposed art-to-part technique over the conventional tools and processes. These case studies are used to cover various types of artwork designs including animals, a

Fig:l3 Colour-shaded resultant model human face,floweE as as fi le Chinese characters, and at the same

time to show the feasibility of

COMPUTING 6( CONTROL ENGINEERING JOURNAL DECEMBER 1994

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.

The Chinese legend and tradition 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 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 is equivalent to 63% in time saving! The resin prototype which is painted in red is shown in Fig. 17.

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” 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 re-assigning a relief profile with a smaller angle, say 25”.

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.

The human face This case study illustrates the

formation of a human face using the relief generating method and building of the resin model using the SLA. The scanned-in image is originally a full coloured picture

The reliefs of the Chinese

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Fig. 15 Converted triangular file to follow the STL format

resin

Fig. 16 Building of prototype using the SLA

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Fig. 17 Resin prototype of Chinese characters and roaring dragon consisting of more than 100 different colours. Reducing this huge amount of colours to eight colours representing different parts of the face seems tedious. However, the ‘reduce colour’ function helps to simplify the job. The 40 minutes spent on editing the colours is relatively long 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 which requires 9 6 hours to produce a human facial feature plaster mould prototype. The edited image is shown in Fig. 18.

The complex shape of the human facial expression, however, presents two main problems when it comes to building the prototypes using the SLA. First, as mentioned earlier, steps are clearly seen on the relief at almost every portion of the face. There are two factors contributing to this shortcoming, the first 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. 19) are enhanced in this case where the convex profiles cover a large area

Fig. on 1

19 Steps the face

Fig. 18 Edited image of a human face

[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 100 mm diameter coin surface). The second contribution to the distinct step appearance is the gradual slope (ranging from 5 to 25”) 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. Fig. 20 shows the resin prototype, painted in blue. This is largely dependent on the skill and experience of the designer in manipulating the relief as close to the ideal situation as possible.

The orchid 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, 2 1 ) of this orchid flower consists of more than 100 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. 22) diverging unacceptably from the original design, after editing work has been carried out.

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 building depends largely on the number of layers required and the size of the model.

The level of complexity in the orchid design is less than that of the human facial design. The reliefs produced are close t o the pattern done by the craftsman. However, steps are 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.

flower pattern appears to be slightly higher than the ideal size: however, this can be easily amended by re- defining the angle of the relief in the ArtCAM environment, which is actually the main advantage of using this technique in prototyping. A new model can be rebuilt within 10 hours.

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. 2.3) has the advantage of avoiding undesirable steps due to its plane surface nature.

The convex curvature of the

Texts of all font types can be

290 COMPUTING & CONTROL ENGINEERING JOURNAL DECEMBER 1994

Fig. 20 Resin prototype of the human face

Fig. 22 Edited image of the scanned-in orchid

Conclusion he CAD/CAM software allows the formation of complicated T and time consuming reliefs on

models such as jewellery, ceramics, tableware, pewter ware, coin dies etc. to be semi-automatically or automatically created. The software provides a realistic viewing function to see the colour shaded final model and permits amendments t o be made easily.

Experiments on building models using the S L A have been carried out to study the application of the relief generating software system. Three models, the Chinese legend and tradition, the human face and the orchid, were built and examined. It was found that a substantial amount of polishing work is needed to improve the surface finish of the resin models. Steps were observed on the slope of the relief profile,

Fig. 21 Original photograph of the orchid

Fig. 23 Resin prototype of the orchid

which was caused mainly by the minimum layer size obtainable on the SLA, which is not small enough to produce a smooth surface finish. The major advantage of this prototyping technique is the ability to create more prototypes for less time and cost.

Ac knowledgrnent he authors acknowledge the industrial sponsorship of T Delcam (Singapore] for this

project. Contributions were also made by Ms Lye Sau Lin from Delcam (Singapore).

References 1 CHUA, C. K., HOHEISEL, W., KELLER,

C., and WERLINC, E.: 'Adapting decorative patterns for ceramic tableware', Computing h Controrol Engjneerjng Journal, 1993. 4, ( 5 ) . pp.209-217

2 LEE HAN BOON, MICHAEL S. H. KO,

ROBERT K. L. GAY LEONC KAH FA1 and CHUA CHEE KAl: 'Using computer- based tools and technology to improve jewellery design and manufacturing', International Journal of Computer Applications in Technology, 1992, 5,

3 MUELLER, I.: 'Stereolithography-a rapid way to prototype die castings', Die Casting Engineer; May-June 1992, 36, (31

4 ULERICH, F? L.: 'Rapid prototyping- cornerstone of the new design process', Proceedings of the 1992 ASME International Computers in Engineering Exposition, Computers in Engineering, ASME, 1992

( I ) , pp.72-80

0 IEE: 1994 Chua C. K. and Cheong S. K. F. are with the School of Mechanical and Production Engineering and Gay R. K. L. and Lee H. B. are with the ClNTlC Institute of Manufacturing Technology, Nanyang Technological University. Nanyang Avenue, Singapore 2263.

29 1 COMPUTING 6( CONTROL ENGINEERING JOURNAL DECEMBER 1994