Rapid Pro to Typing and Reverse Engineering

Rapid Prototyping and

Reverse Engineering

Rapid Prototyping and Reverse Engineering

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A rapid prototyping primer Part 1: Stereolithography ...........................................................4

Rapid Prototyping.............................................................................................................4 Stereolithography .............................................................................................................4

The process..................................................................................................................4 Supports .......................................................................................................................5 Post-processing............................................................................................................6 Speed ...........................................................................................................................6 Materials .......................................................................................................................7 Machine manufacturers ................................................................................................8 When to use stereolithography .....................................................................................8 Factors to consider when ordering ...............................................................................8

A rapid prototyping primer Part 2: laser sintering .............................................................9 The process..................................................................................................................9 Supports .....................................................................................................................10 Post-processing..........................................................................................................10 Machine makers .........................................................................................................10 Materials .....................................................................................................................11 Metal...........................................................................................................................12 Speed .........................................................................................................................12 When to use laser sintering ........................................................................................13 Factors to consider when ordering .............................................................................13

A rapid prototyping primer Part 3: fused deposition modeling ........................................13 Fused deposition modeling.........................................................................................14 Equipment manufacturers...........................................................................................15 Supports .....................................................................................................................16 Accuracy and surface finish........................................................................................16 Materials .....................................................................................................................16 Speed .........................................................................................................................17 When to use fused deposition modeling .....................................................................17 Factors to consider when ordering .............................................................................17

A rapid prototyping primer Part 4: Three-dimensional printing .......................................17 Three-dimensional printing .........................................................................................18 Speed .........................................................................................................................18 Materials .....................................................................................................................19 Post processing ..........................................................................................................20 Accuracy and surface finish........................................................................................20 Color ...........................................................................................................................20 When to use three-dimensional printing .....................................................................21 Factors to consider when ordering .............................................................................22

Rapid prototyping primer -- Part 5: Multijet modeling .....................................................23 Supports .....................................................................................................................24 Material.......................................................................................................................24 Accuracy and surface finish........................................................................................24 Speed .........................................................................................................................25 Post processing ..........................................................................................................25 When to use multijet modeling....................................................................................25 On the horizon -- Invision............................................................................................26

Preparing data for rapid prototyping ..................................................................................26 Service-bureau recommendations .................................................................................28

CATIA version four .....................................................................................................28


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Pro/Engineer...............................................................................................................28 SDRC I-DEAS ............................................................................................................29 SolidEdge ...................................................................................................................29 SolidWorks .................................................................................................................29

Rapid prototyping large objects -- from parts to art............................................................30 The Materialise Mammoths ............................................................................................30

Virtual medical modeling and rapid surgical templates ......................................................32 Reverse Engineering .........................................................................................................34

Capturing scan data .......................................................................................................34 Read scanned data.....................................................................................................34 Align point clouds .......................................................................................................34 Simplify the data .........................................................................................................36 Polygon meshing ........................................................................................................36 Polygon editing ...........................................................................................................37 Defining surface boundaries .......................................................................................37 Applying NURBS ........................................................................................................39 Exporting the data.......................................................................................................39 God is in the details ....................................................................................................39

Reverse-engineering update ..........................................................................................39 Much improved ...........................................................................................................40 Picking the right mix....................................................................................................41 Software .....................................................................................................................41 Caveat emptor ............................................................................................................44

UNDERSTANDING THREE-D SCANNERS AND DIGITIZERS — Part 1......................45 Coordinate-Measuring Machines................................................................................45 Scanners ....................................................................................................................46 Laser Scanners ..........................................................................................................47 White-Light Scanners .................................................................................................47 Finicky About Finishes................................................................................................48 Computed Tomography ..............................................................................................48 Destructive Cross Sectioning......................................................................................49


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A rapid prototyping primer Part 1: Stereolithography

Rapid Prototyping

Rapid prototyping is helping many companies to shave days, weeks, or even months off the time required to bring new products from concept to market. Moreover, rapid prototyping technologies have improved greatly in recent years. God is in the details, and rapid prototyping equipment makers have worked hard to get the details right. Today stereolithography, selective laser sintering, inkjet printing, and fused deposition modeling all produce good parts with relatively little hassle.

Each technology has strengths and weaknesses, and it’s a mistake to think that all rapid prototypes are created equal. Some are more durable than others. Some are more accurate. Some are more resistant to heat. Some are cheaper. In order to choose the rapid prototyping system best suited for each application, it’s necessary to understand the basics of each technology. Over the coming weeks, we’ll present primers on stereolithography, laser sintering, inkjet, and fused deposition modeling, and discuss how the mechanics of each affects characteristics of the parts it produces.

Stereolithography Stereolithography is the process most people picture when they think of rapid prototyping. Perhaps the most complex of all rapid prototyping technologies, it was the first to emerge as a commercial enterprise and is still the most widely used. Stereolithography builds three-dimensional objects by exposing and hardening thin layers of photocurable resin. It produces parts with good surface finish and detail but with limited physical properties. Parts won’t withstand excessive heat or moisture and tend to be brittle. They are, however, durable enough to be used as patterns for duplication processes such as silicon-rubber molding. Solid stereolithography parts don’t work well as investment-casting patterns because they expand during burnout and crack investment-casting shells. But parts built with a lattice-like internal structure work well as long as they contain no thin walls or delicate features.

The process The basic process of stereolithography begins by creating an STL file -- the standard input format for all rapid prototyping. (See Preparing Data for Rapid Prototyping) The STL file consists of a closed mesh of triangles generated from the surfaces of a CAD model. Software in the stereolithography system slices the STL file into a series of horizontal cross sections. These sections, solidified one on top of another, form a stereolithography prototype. Stereolithography machines build each layer by tracing the pattern of a cross-section on the surface of a vat of liquid resin. Wherever the laser touches, the resin absorbs light energy and polymerizes from liquid to solid. A platform suspended in the vat supports the part. After each layer is built, a mechanism lowers the platform into the vat by one layer’s thickness -- typically 0.001 to 0.005 inches (0.025mm to 0.125mm) to allow the next layer to be scanned. Between layers, a recoater blade sweeps across the surface of the vat to smooth liquid resin across the top of the growing part. This process is repeated -- solidifying a layer, lowering the platform, smoothing the surface -- until the entire part is built.


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Supports Because stereolithography builds parts by drawing on the surface of the liquid resin, overhanging areas that aren’t directly above a previous layer must be connected with support structures either to the part below or directly to the bottom of the building platform so that they won’t sag or float away. Stereolithography builds both a part and any necessary supports from the same material. After a part is finished the supports must be cut away.

A large stereolithography part built by a 3D Systems SLA-7000 stereolithography system. Note the supports connecting the bottom of the part to the building platform.


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Post-processing When a part is complete, the building platform is raised from the vat and the excess resin allowed to drain. At this point the part is not fully cured. After a hand-wiping to remove any of the viscous liquid resin that might still be on the surface, the part is placed in an agitated bath of solvent or wiped by hand with solvent to remove any additional uncured resin. It’s particularly important to make sure all such resin is removed from narrow passages. Otherwise it could solidify during post-curing. Technicians who clean the parts must wear gloves, masks, and eye protection to avoid contact with the uncured resin, which can cause allergic reactions. (See Cleaning stereolithography parts.)

Once a part is completed, it is carefully removed from the stereolithography system, washed with solvent to

remove any excess resin, and then post-cured with light and heat. After cleaning, the so-called “green part” must be post-cured to complete polymerization of the resin and fully harden it. In a special chamber called the post-curing apparatus (PCA), the part is exposed to light from ultraviolet lamps and sometimes, depending upon the resin, heat from infrared lamps. Post-curing takes between one and ten hours depending upon the part size. After post-curing, any supports can be cut away and the surface of the part finished by sanding or bead blasting. Parts built with thinner layers require less sanding to remove the stairsteps between layers. However, thinner layers increase part building time.

Speed Since a stereolithography machine’s laser solidifies each layer one point at a time, stereolithography builds parts more slowly than some rapid prototyping technologies, particularly those that deposit materials with multiple jets or build parts from sheets of material. Between each layer, the stereolithography machine must pause in order to let the liquid resin settle before building the next layer. The time needed to build a particular stereolithography part, therefore, is dependant upon the height of the part (the number of layers), the complexity of the part (the time required to scan each layer), and the wait time


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needed between layers to level the liquid resin (determined by the resin’s viscosity). Post-curing time is generally determined by the overall part size.

Materials The resins used in stereolithography -- exotic materials that harden when exposed to light energy -- both make stereolithography possible and also limit it. Perhaps the most impressive advancement in stereolithography technology over the last few years has been the improvement in the physical properties of resins. Unlike users of other rapid prototyping systems who are required to buy proprietary materials supplied by the machine manufacturers, stereolithography users may purchase resins from competing vendors including 3D Systems, DSM Somos, Vantico, and others. While the properties of proprietary materials used by other rapid prototyping technologies have remained fairly static over the years, competition among stereolithography resin manufacturers has resulted in a vast improvement in resin properties. Early stereolithography parts were so brittle that they rarely survived a design review meeting, and they often warped badly. But resins now deliver properties that approach those of such popular plastics as polypropylene, polystyrene, and polyethylene. Most resins build opaque parts, but some now make it possible to build parts that can be finished to optical clarity. (See 2002 Rapid Prototyping Materials Roundup.) Resin manufacturers often offer incentives to entice customers into switching resins. While stereolithography resins are still some of the most expensive materials used in rapid prototyping, competition has helped keep prices down.

3D Systems SLA-7000 stereolithography system


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Machine manufacturers 3D Systems is the largest and most successful manufacturer of stereolithography equipment, although Japanese companies such as Teijin Seiki and Sony/D-MEC also make stereolithography systems, some of which soon may be available in the United States. (See 3D Systems settles with justice department.) 3D Systems’ machines range in capacity and price from the SLA Viper Si2, which starts at $179,000 and can build a part as large as a ten-inch cube (254mm), to the SLA-7000, which costs $799,000 and can build parts as large as 20 inches long, 20 inches wide, and 23.6 inches tall (508mm by 508mm by 600mm). (See 2002 U.S. Rapid Prototyping Equipment Roundup and Rapid Prototyping Outside the U.S.)

When to use stereolithography Stereolithography isn’t the fastest rapid prototyping technology, so it is not the best choice if you want the fastest possible delivery. Stereolithography uses expensive materials that are difficult to handle, so parts tend to be more costly. Photopolymers have limited physical properties. If you need high strength or high temperature resistance or you plan to submerge parts in water, stereolithography must be combined with secondary processes such as silicon-rubber molding or investment casting using 3D Systems’ Quickcast building style. Stereolithography remains the technology of choice for engineers seeking high accuracy and detail. For the most accurate, best looking parts that don’t need to survive rigorous physical testing, stereolithography is ideal.

Factors to consider when ordering When placing an order for stereolithography parts, here are factors to consider:

• Part size -- Can your part be built as a single piece in your service provider’s stereolithography machine? Large parts can be built in pieces and then glued together but this takes extra time and can degrade accuracy.

• Feature size -- Does your part have any features that are smaller than the layer thickness. If so, be sure and tell your service provider so that the part can be built with thinner layers.

• Type of resin -- There are a variety of resins with varying mechanical, water-resistant, and optical properties. See the 2002 Rapid Prototyping Materials Roundup.

• Support removal -- Be sure part supports can be positioned for removal and that stubs are accessible for sanding if necessary.

• Layer thickness -- Thinner layers can eliminate the need for hand-finishing, but they require more machine time, which is costly.

• Secondary processes -- Silicon-rubber molding or investment casting using 3D Systems’ Quickcast building style add cost and take more time but can provide very good mechanical or chemical properties.

Each rapid prototyping service company has unique policies and practices. Discuss your requirements and available materials with your rapid prototyping supplier before placing an order.


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A rapid prototyping primer Part 2: laser sintering Choosing the best rapid prototyping technology for a job can be confusing. Rapid prototyping methods are lumped together as a class because they build complex parts without either tools or the complex programs and machine setups required for numerically controlled milling. But various rapid prototyping technologies employ different methods and materials to fabricate parts. Stereolithography, the most widely used rapid prototyping technology, excels at building accurate parts with good surface finish and detail. However, stereolithography parts have limited physical properties. When more durable parts are required, laser sintering often can deliver better results.

The process Laser sintering is one of the most straightforward rapid prototyping technologies. Laser sintering builds three-dimensional objects by heating and fusing together grains of powder. Like most rapid prototyping processes, laser sintering builds parts in thin layers stacked on top of each other. Laser-sintering machines build each layer by first spreading a shallow layer of powder onto a building platform and then scanning a cross section of the part on the powder with a CO2 laser. Everywhere the laser touches, its energy heats the powder grains, causing them to adhere to their neighbors. Once a layer is complete, the building platform, which is mounted on a moveable piston, lowers by a single layer’s thickness � 0.004-0.006 inches (0.10-0.15mm) � and a new layer of loose powder is spread across the top of the growing part. As in stereolithography, the part cross-sections are derived from an STL file -- the standard input format for all rapid prototyping. (See Preparing Data for Rapid Prototyping.) The STL file consists of a closed mesh of triangles generated from the surfaces of a CAD model. Software in the laser-sintering system slices the STL file into a series of horizontal cross sections that define the laser’s pattern for each layer.

In order to produce accurate parts, it’s critical to control the amount of energy transferred as the laser scans the powder. If the powder grains soften too much and begin to flow like liquid, shrinkage may become unpredictable. If the powder isn’t heated enough, coherent layers won’t form. The building chamber is heated to just below the melting temperature of


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the powdered material so that only a slight amount of extra energy from the laser will sinter the powder grains. Heating the chamber also reduces the thermal shrinkage of the layers during fabrication. The chamber is filled with nitrogen to reduce the hazard of explosion that is present when handling flammable powder in air.

Supports Parts built by laser sintering don’t require supports. The unsintered powder surrounding previous layers supports isolated or overhanging areas of a layer as it is built. When finished, a sintered part is encased in loose powder.

Post-processing Sintered parts require no special post-processing. Once a sintered part is complete, the piston supporting the build platform rises and the unsintered powder surrounding the part can be scraped and brushed off and put back into the powder-supply bin for reuse. The warm part is then removed from the sintering system and transferred to a rack or table where it must be allowed to cool for several hours. After cooling, any remaining loose powder can be removed using compressed air, brushes, or other tools. Parts can be used as soon as they have cooled and the excess powder has been removed. Because they are constructed of a lattice of fused powder grains, however, sintered parts are porous and have a relatively rough surface finish. Parts can be infiltrated to improve strength and reduce surface roughness. Sanding and painting also can improve the smoothness of the surface of a sintered part.

Laser-sintered part. Note the rough surface. finish.

Machine makers Two companies manufacture laser-sintering equipment. In the United States, laser sintering was developed by the DTM Corporation, which was purchased by 3D Systems in 2001. 3D Systems sells the Vanguard laser-sintering system, which is capable of building parts as large as 15 inches by 13 inches by 18 inches (375mm by 330mm by 450mm) for $320,000 to $370,000. Electro Optical Systems GmbH (EOS) of Germany


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also sells several models of sintering machines. The smallest is the EOSINT P380, capable of building parts as large as 13 inches by 13 inches by 24 inches (340mm by 340mm by 620mm). It costs approximately $300,000. The largest is the dual-laser EOSINT P700, which can build parts as large as 28 inches by 15 inches by 23 inches (700mm by 380mm by 580mm) and costs $750,000 to $850,000. Recently EOS said it was planning to sell systems in the United States. To date, however, we have not heard that this is occurring.

3D Systems Vanguard system.

Materials Perhaps the greatest advantage of laser sintering is the broad range of potential materials. (See “2002 rapid prototyping materials roundup.”) Theoretically, any material that can be powdered and melted might be used. Commercially available materials include polycarbonate, nylon, glass-filled nylon, polyamide, acrylic-based polymer, elastomeric polymer, zirconium and silica sands, and polymer-coated steel and metal-alloy powders. In practice, the most commonly used materials are nylon and glass-filled nylon powders. While sintered nylon parts don’t have the same physical properties as those made by injection molding, they are sufficiently durable to be used for physical testing and some end-use applications. NASA, for example, has used sintered parts in spacecraft production (See “Selective laser sintering parts used directly in spacecraft production”), and Siemens is currently using laser sintering to manufacture custom hearing aids. (See “Laser Sintering makes custom hearing aids.”) Most laser-sintering materials are available only from the equipment manufacturers. Companies who have attempted to use second-source materials have been sued. Over the years, this has inhibited innovation in materials development and kept prices high. Laser- sintering system manufacturers have developed some innovative materials but have pursued only the most popular. Nevertheless, some specialty materials do exist. DSM Somos, the maker of popular stereolithography resins, offers a flexible sponge-like material for laser sintering. 3D Systems sells a polystyrene material called CastForm for


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making investment-casting patterns. And EOS sells binder-coated sands for building casting patterns.

Metal Both EOS and 3D Systems have developed methods to make metal parts with laser sintering. 3D Systems offers RapidSteel materials that consist of stainless-steel powders coated with binder. The binder coating on the powder grains melts during sinter to create a “green part.” After a part is built, the binder is burned away over several hours. With the binder gone, the part is delicate -- held together primarily by friction between the steel particles. In a second oven cycle, the part is placed in a crucible with bronze ingots. As the crucible is heated the bronze melts and flows into the part’s pores via capillary action. EOS has developed a direct-metal sintering material that combines metal powders of different melting temperatures but no organic binders. During sintering the components with lower melting temperatures melt and flow into the pores of the part. EOS direct-steel parts don’t require a separate infiltration step.

An EOS DirectMetal part.

Speed As in stereolithography, a sintering machine’s laser solidifies each layer one point at a time and thus builds parts more slowly than rapid prototyping technologies that deposit materials with multiple jets or build parts from sheets of material. Between each layer, the sintering machine must pause while the build platform lowers and a new layer of powder is spread. Building speed also is determined by the melting temperature of the material being used. Materials with lower melting temperatures are processed more rapidly than those with higher melting temperatures. The time needed to build a particular part, therefore, is dependant upon the height of the part (the number of layers), the complexity of the part


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(the time required to scan each layer), and the scanning speed of the laser (determined by the material’s melting temperature).

When to use laser sintering Laser sintering is best suited to applications requiring parts with superior material properties. Although laser sintering materials are more expensive than would seem necessary -- they are, after all, simply ground up plastic or metal -- sintered parts are strong and resist weather, chemicals, and heat. Because sintered parts don’t require supports that must be removed after building, it’s possible to create complex parts with inaccessible internal features that would be impossible to make with stereolithography. Laser sintering also is the only commercial rapid prototyping technology that can build complex metal parts, although the metals used are alloys that don’t deliver the same properties as cast, forged, or milled steel. Sintered parts, because they are made of powders, tend to have a grainy surface finish and need extra sanding and perhaps infiltration or painting if a smooth finish is required.

Factors to consider when ordering When placing an order for sintered parts, think about:

• Part size -- Can your part be built as a single piece in your service provider’s machine? Large parts can be built in pieces and glued together but this takes extra time and can degrade accuracy.

• Feature size -- Does your part have any features whose size approaches the building-layer thickness? Sintering systems can have problems producing very small features.

• Type of material -- Available materials include: polycarbonate, nylon, glass-filled nylon, polyamide, acrylic-based polymer, elastomeric polymer, zirconium and silica sands, and polymer-coated steel powder. For details about these, see the 2002 Rapid Prototyping Materials Roundup.

• Layer thickness -- Thinner layers can reduce the need for hand-finishing, but they require more machine time, which is costly. At their best, laser-sintered parts won’t match the surface finish of those produced by stereolithography.

• Finishing -- Be sure to specify what kind of finish you want or are willing to accept. If you require a smooth surface, you may need to have sintered parts painted.


A rapid prototyping primer Part 3: fused deposition modeling Many companies see the advantages of bringing rapid prototyping technologies in house in order to produce parts for design review, fit testing, and functional prototypes. But as discussed in the first two parts of this series, stereolithography and laser sintering systems can’t be operated in offices. Stereolithography machines use messy liquid chemicals that must be handled with caution. (See “A rapid prototyping primer Part 1: stereolithography.”) Laser sintering employs fine-grained metal or plastic powders that can be messy. (See “A rapid prototyping primer Part 2: laser sintering.”) Both processes employ powerful lasers that can cause eye damage. These characteristics dictate that stereolithography and laser-sintering systems be installed in dedicated facilities apart from engineering offices.


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Fused deposition modeling For companies that want to produce their own rapid prototypes but don’t want the expense of setting up a dedicated rapid prototyping shop, fused deposition modeling (FDM) can provide a good alternative. Compared with stereolithography or laser sintering, fused deposition modeling seems downright low-tech. Instead of lasers and space-age materials, FDM builds by extruding molten plastic that hardens layer-by-layer to form a solid part. The building material, in the form of a thin solid filament, is fed from a spool to a movable head controlled by servomotors. When the filament reaches the head, heaters melt it. The molten material is then extruded through a nozzle onto the part surface.

As the molten material is extruded, it is flattened by the nozzle the way a plumber or painter uses the tip of a caulking gun to spread the material. The width of the road (in FDM parlance) can range between 0.0076 and 0.038 inches (0.193 and 0.965 mm) and is determined by the size of the extrusion nozzle. Nozzles can’t be changed during building, so the model resolution must be chosen in advance. As the molten material is dispensed, it quickly cools -- within a tenth of a second -- and solidifies. When a layer is completely covered, the building platform, mounted on a moveable stage, descends by one layer thickness, typically 0.005 inches to 0.010 inches (0.178mm to 0.356mm), and the process is repeated. As in stereolithography and laser sintering, FDM systems read STL files -- the standard input format for all rapid prototyping. (See Preparing Data for Rapid Prototyping.) The STL file consists of a closed mesh of triangles generated from the surfaces of a CAD model. Software in the FDM system slices the STL file into a series of horizontal cross sections that are traced by the material-extrusion head. In order to build accurate parts, it is critical to control the temperatures of both the building chamber and the growing part. The temperature of the chamber must be kept just below the melting temperature of the material so that only a small amount of heat is required to


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melt the filament sufficiently for extrusion and the growing part won’t sag or otherwise deform. The part must be kept cool enough that the molten material solidifies upon contact.

3 A close-up of the extrusion head in Stratasys' Maxum FDM machine. (Click image for a close-up.)

Equipment manufacturers The only maker of fused deposition modeling equipment is Stratasys of Minneapolis, Minnesota. Stratasys offers a range of FDM systems. The affordable Dimension (sold by a distributor network) builds parts as large as eight inches by eight inches by 12 inches (200mm by 200mm by 300mm) and costs $29,900. The giant FDM Maxum, which has a building capacity of 23.6 inches by 19.7 inches by 23.6 inches (600mm by 500mm by 600mm), costs $250,000. (See the “U.S. rapid prototyping equipment roundup 2002.”)

Stratasys’ $29,900 Dimension, like all FDM systems, can be operated in an office environment.


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Supports Overhanging or isolated features of FDM parts require temporary support during building. FDM machines use a second nozzle located next to the primary nozzle to extrude support material where necessary. Stratasys offers two types of supporting material: a wax-like material for its lower-priced machines and a water-soluble material for its more expensive ones. The wax supports can be broken away from a part but are difficult to remove from internal passages or small features. The water-soluble supports can be dissolved in an agitated bath. Other than support removal, FDM parts require no additional post-processing.

Accuracy and surface finish Stratasys claims its FDM systems deliver accuracy of +/- 0.005 inches (0.127mm). The surface finish of FDM parts is not as good as that of parts made with stereolithography but ranks with those produced by laser sintering. Whereas sintered parts have a grainy texture, FDM parts have a ribbed appearance because both the horizontal layers and the roads that make up the layers remain visible.

Materials Over the years, Stratasys has developed a number of materials for its FDM systems. The one that produces the highest-quality parts is acrylonitrile butadiene styrene (ABS). Other materials include wax for investment-casting patterns, polycarbonate for tough parts, and polyphenyl sulfones for high-temperature applications. FDM parts, because they are made of material that has been melted and solidified, exhibit physical properties close to those of parts made of similar materials by conventional methods. (See the “2002 rapid prototyping materials roundup.”)

Vacuum cleaner-maker Oreck uses FDM parts for extended physical testing of new designs.


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Speed Because FDM systems build parts by drawing a narrow bead of material, large or solid parts or those with thick walls can take a long time to complete. Small or thin-shelled parts can be built fairly rapidly. The time needed to make any part depends upon the deposition rate of the FDM system (determined by nozzle size), the height of the part (the number of layers), the gross size of the part (the time required to deposit each layer), and the number and complexity of supports required (supports for each layer are built in a separate step).

When to use fused deposition modeling FDM parts are tougher and more durable than those produced by stereolithography, but they have an inferior surface finish and sharpness. Stratasys’ ABS, polycarbonate, and polyphenyl sulfone materials are sufficiently resistant to heat, chemicals, and moisture that FDM parts can be used for limited to extensive functional testing, depending upon the application. FDM parts aren’t porous and don’t require secondary infiltration. Unlike stereolithography or laser sintering, FDM machines can be used in an office environment and FDM materials don’t require any special handling. Most FDM systems cost less than comparable stereolithography or sintering systems. For companies wanting to produce durable and accurate prototypes in house, FDM can be a good choice.

Factors to consider when ordering When placing an order for fused deposition modeling parts, consider:

• Part size -- Can the part be built as a single piece in your service provider’s machine? Large parts can be built in pieces and then glued together, but this takes extra time and can degrade accuracy.

• Support removal -- Will your part have any supports in inaccessible areas? If so, specify that water-soluble supports should be used if possible.

• Feature size -- Does your part have any features whose size approaches the building-layer thickness? FDM systems can have problems producing very small features.

• Layer thickness -- Thinner layers can reduce the need for hand-finishing, but they require more machine time, which is costly. Increasing bead size and layer thickness can speed the building process considerably but at the expense of part detail.

• Finishing -- Be sure to discuss with your service provider what kind of finish you want or are willing to accept. If you require a smooth surface, you may need to have FDM parts painted.


A rapid prototyping primer Part 4: Three-dimensional printing

October 31, 2002 -- In the first three articles of this series, we discussed the pros and cons of stereolithography, laser sintering, and fused deposition modeling. Each of these technologies has advantages and drawbacks: Stereolithography produces parts with high accuracy but limited durability. Sintering creates parts with good material properties but mediocre surface finish. Fused deposition modeling provides an economical way in an office environment to make parts that are durable but with limited accuracy. The ultimate aim of the aforementioned technologies is to make prototypes -- accurate parts that are


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sturdy enough to be used for at least limited functional testing. Although parts made by these technologies cost a fraction of similar prototypes made with traditional methods, they can still cost hundreds or thousands of dollars. At these prices, many companies don’t bring rapid prototyping into play until a design is nearly complete.

Concept modelers, on the other hand, produce relatively crude parts cheaply and quickly. Concept models are typically used early to evaluate a product’s design. Whereas a prototype simulates how a product will function, a concept model is the equivalent of a three-dimensional sketch. One industrial designer tells us that his concept models often have a useful life of only minutes before they hit the trashcan -- just long enough to tell him whether or not he is on the right track.

Three-dimensional printing The most popular concept-modeling systems are those based on three-dimensional printing (3DP), a process developed by the Massachusetts Institute of Technology and licensed to Z Corporation of Burlington, Massachusetts. Z Corporation offers a range of 3DP machines. The Z400, which costs $33,500 and can build parts as large as eight inches by 10 inches by eight inches (200mm by 250mm by 200mm) is the least expensive. The Z406, with the same build envelope, sells for $66,500 and offers greater speed. The Z810 with a maximum part size of 16 inches by 20 inches by 24 inches (400mm by 500mm by 600mm) and automated production features lists for $175,000. (See “A closer look at the Z810” for details.) As with other rapid prototyping systems, the input for three-dimensional printing is a CAD file that has been converted into the tessellated STL format. (See “Preparing data for rapid prototyping.”) Software in the 3DP system slices the STL model into a series of horizontal cross sections. 3DP machines build parts by printing the cross sections in liquid binder onto layers of powder. To start the process, a thin layer of powder -- between 0.003 inches and 0.010 inches thick (0.076-0.254mm)-- is spread by a roller onto a platform mounted on a piston in a building box. Multiple inkjets sweep across the surface of the layer and print the image of a single cross section onto the powder. Where the binder is printed, the powder is glued together. Loose powder surrounding the printed area remains in place and supports the part as it is built. Once a layer is complete, the piston lowers the build platform by one layer thickness and the process is repeated. The only time required between layers is the time needed to spread each new layer of powder. When a part is finished, the piston raises the build platform and any excess powder can be brushed or vacuumed off the part.

Speed Because the printheads sweep across the platform in a raster pattern and dispense binder from multiple jets simultaneously, three-dimensional printing is very fast, as much as 40 times faster than other rapid prototyping technologies. (In raster printing the printheads sweep back and forth rather than drawing patterns.) Every layer, regardless of complexity, takes the same amount of time to print. Building speed is approximately one layer every 30 seconds. According to Z Corporation, the Z810 can build an automotive manifold in about four hours and a full-sized engine block in about 16 hours. The Z406 can build six parts the size and shape of a soda can in less than two hours.


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Materials Z Corporation offers two material systems. The fastest and least expensive is a starch-based powder. Parts made of this material can be used as investment-casting patterns. More durable parts can be built using a plaster-based powder. (See “2002 rapid prototyping materials roundup.”) Total material cost is $0.30 to $1.65 per vertical inch of part.

Once a 3DP part is finished, the build platform is raised and excess powder brushed and vacuumed away.


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Post processing Because parts are surrounded by powder as they are built, no support structures are required. Starch parts, however, are very fragile and must be infiltrated with wax, cyanoacrylate, or two-part epoxy for added strength. Plaster parts are stronger but also require infiltration for extensive handling. The infiltration process can be messy. Parts are porous, and it is only necessary for the infiltrate to penetrate one-quarter inch to one-half inch in order to give the part a durable shell. Z Corporation sells an automated system for wax infiltration but cyanolacrylate and epoxy typically are applied with a cloth.

Accuracy and surface finish The principal advantages of three-dimensional printing are speed and low-cost materials. Accuracy and surface finish are inferior to stereolithography and laser sintering. Because they are made of layers of powder, 3DP parts, like sintered parts, have a grainy texture. Once infiltrated, however, they can be sanded to achieve a smoother surface.

3DP parts have a grainy surface finish

Color A unique capability of three-dimensional printing is the ability to produce multicolored parts. A part’s color is determined by dyes added to the liquid binder. Almost any color is possible. Each layer, in essence, is like a full-color image printed on a flat sheet of paper by an ink-jet printer.


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Multicolor 3DP parts can be built by mixing different colored inks in the liquid binder.

When to use three-dimensional printing Three-dimensional printing is best used when speed and price are more critical than accuracy, surface finish, and material properties. 3DP parts are most often used as concept models but, when infiltrated, also can be finished and used as patterns for duplication. Parts built with starch-based material can be used directly as investment-casting patterns. 3DP parts a have relatively poor surface finish. Small features and fine details are not replicated well. Thin walls can be problematic because all materials are brittle. A secondary use of three-dimensional printing is for making casting shells. Soligen Technologies of Northridge, California is a long-time M.I.T. licensee of 3DP for the production of ceramic investment-casting shells. Recently Soligen’s license with M.I.T. became nonexclusive, and Z Corporation is now testing methods of building casting shells from its plaster material. One challenge Soligen has faced in the production of ceramic casting shells has been that the poor interior surface finish of molds produced by 3DP is transferred during casting to the cast metal parts. Instead of making single-piece shells as Soligen does, Z Corporation suggests making molds in pieces that are put together along with cores prior to casting, much like sand-casting molds are assembled. That way, the casting surface of each mold piece can be sanded or filled to remove stair steps. Z Corporation says shells built with 3DP deliver a surface finish and accuracy similar to traditional sand casting in a fraction of the time.


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3DP casting shell produced by Z Corporation's Zcast process..

Factors to consider when ordering When placing an order for 3DP parts, consider:

• Part size -- Can the part be built as a single piece in your service provider’s machine? Large parts can be built in pieces and then glued together, but this takes extra time. Moreover, the low accuracy of 3DP can prevent part sections from fitting together well.

• Feature size -- Does your part have any small features or thin walls? 3DP systems can have problems producing very small features, and thin walls are sometimes impossible to build.

• Accuracy -- Three-D printed parts have low accuracy. Consequently, it is difficult (although not impossible) to assemble parts made with 3DP. Threads are not replicated.

• Joining -- Because materials are brittle, 3DP parts are difficult to drill and tap for joining with conventional fasteners.

• Material -- Be sure to specify what material and infiltrate you want. Plaster powder is more expensive than starch but delivers better strength, detail, and surface finish. If you want to use your parts as investment-casting patterns or just want to get parts as cheaply and quickly as possible, go for the starch dipped in wax. Changing materials in a 3DP system is messy and takes time. Not all service providers will offer both starch and plaster.

• Finishing -- Be sure to discuss with your service provider the type of finish you want or are willing to accept. For concept models, it may not be necessary to spend the extra time required to get pretty parts.


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Rapid prototyping primer -- Part 5: Multijet modeling

Rapid prototyping systems such as stereolithography, laser sintering, and fused deposition modeling are designed to create accurate parts that are durable enough for at least limited functional testing. Concept modelers, on the other hand, produce relatively fragile parts cheaply and quickly. Where rapid prototypes are often used to evaluate how a part will perform, concept models typically are used early to evaluate a product’s design.

Just as there are multiple rapid prototyping technologies, there are also several processes for making concept models. The characteristics of concept models vary widely. To get the most benefit from a concept model, choose a process that makes parts matching your requirements.

In our last installment, we discussed Z Corporation’s three-dimensional printing (3DP). (See “A rapid prototyping primer Part 4.”) Z Corporation parts are inexpensive but have relatively poor detail and surface finish. While 3DP is a good choice for building bulky parts, if you require small features or fine detail, 3D Systems’ multijet modeling might do a better job.

3D Systems’ ThermoJet multijet modeler builds parts as large as 10 inches by 7.5 inches by 8 inches (250mm by 190mm by 200mm). It costs $50,000. The ThermoJet builds parts by spraying a molten wax-like material in layers onto a platform using an eight-inch-wide print head containing 352 jets. Droplets from the jets measure just 0.003 inches (0.025 mm) in diameter) so they cool rapidly and harden upon striking the model surface. When each layer is complete, the build platform lowers by one layer thickness -- typically 0.0017 inches (0.042mm) -- and the next layer is sprayed.


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Supports

Features of ThermoJet parts that are isolated or overhanging require supports. 3D’s ThermoJet takes a unique approach to support-generation. Thin hair-like structures hold up all downward-facing surfaces. When a part is complete, the supports can be brushed away.

Material

The waxy materials used by the ThermoJet are not very strong. 3D doesn’t provide much data on material properties, but parts generally are tough enough to withstand handling and shipping if properly packed. Because the materials are wax-based, ThermoJet parts can be used directly as investment-casting patterns.

Accuracy and surface finish

The real strength of ThermoJet parts is their detail and surface finish. 3D claims that resolution of the system is 300 by 400 dots per inch along the horizontal X- and Y-axes and 600 dots per inch in the Z (vertical) direction. The smallest feature or wall that can be built measures about 0.003 inches (0.075mm) across. Surfaces where supports have been removed are quite rough but can be improved by polishing.


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Thermojet parts have good surface finish and detail. The material is available in a number of colors including gray as pictured here.

Speed

Because the ThermoJet builds by dispensing material through hundreds of jets simultaneously, the process is relatively fast. Z Corporation’s three-dimensional printers are faster, but ThermoJet systems can make even parts that fill the building volume in less than a day.

Post processing

Other than support removal, ThermoJet parts require no post processing and can be used as soon as they are built. Unlike porous parts produced by laser sintering or three-dimensional printing, they can’t be infiltrated with resins for additional strength.

When to use multijet modeling

MultiJet modeling is best for applications where part detail is more important than durability. It is also ideal for building parts to be used as investment-casting patterns. Because ThermoJets run cleanly and quietly in an office environment, the technology is ideal for small company use in-house. Material is loaded into the machine in cartridges. There is no messy powder or liquid. Support removal can be done over a wastebasket and requires no protective clothing.

Factors to consider when ordering When placing an order for multijet modeling parts, consider:

• Part size -- Can the part be built as a single piece in your service provider’s machine? It’s not easy to glue waxy Thermojet parts together.


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• Build orientation -- Downward facing surfaces on multijet modeling parts are uneven after supports have been removed. Though much of this roughnes can be smoothed with solvent, you should indicate which surfaces are most important so the operator can orient them facing upwards.

• Finishing -- Be sure to discuss with your service provider the type of finish you want or are willing to accept, especially on downward facing surfaces. For concept models, it may not be necessary to spend the extra time required to get perfect finishes on all surfaces.

Each rapid prototyping service company has unique policies and practices. Discuss your requirements with suppliers before ordering.

On the horizon -- Invision Earlier this year, 3D announced that is developing a new machine that combines multijet modeling with photopolymer technology. The Invision Si2 modeler will build parts by spraying acrylate photopolymer as a building material and using a wax similar to its current Thermojet material for supports. 3D says the new Invision system will include a separate finisher unit that will automatically remove the waxy supports. 3D has not yet announced a price for the Invision or when it will be available.

3D Systems demonstrated the new Invision at this summer's SIGGRAPH trade show.

Preparing data for rapid prototyping Rapid prototyping machines are often described as three-dimensional printers. Just as inkjet or laser printers produce physical copies of electronic documents, rapid prototyping systems make physical copies of three-dimensional CAD models. But making rapid prototypes is not as easy as printing documents. Whereas most two-dimensional printing is done through drivers installed with a computer’s operating system, rapid prototyping machines require that CAD models be converted into a special format called STL, a name derived from the first rapid prototyping process, stereolithography.


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Generating STL files is usually a fairly simple process. Virtually all modern CAD systems now include STL output as a standard feature and, for the most part, the files created are suitable for rapid prototyping. But the extra step of converting CAD models into STL format still presents a barrier to rapid prototyping novices or occasional users. Imagine how annoying it would be if you had to select parameters and convert word-processing documents or spreadsheets into a special file format before you could print them. Making STL files is further complicated by the fact that every CAD system uses different terms and parameters for defining the STL file’s resolution, requiring users to interpret such mystifying terms as chord height, absolute facet deviation, angle control, and adjacency tolerance. Such terms are needlessly confusing. The STL file format is simple. It’s not necessary to understand precisely what all the CAD system parameters mean in order to create useful files. An STL file is simply a mesh of triangles wrapped around a CAD model. CAD system settings specify how closely the STL mesh conforms to the actual surface geometry of a part. A mesh with triangles that are too large will create a small STL file, but the prototype made from it will have visible facets. A mesh with triangles approximately the size of the layers used by the rapid prototyping systems (typically about 0.003 inches or 0.075 mm) will produce a prototype with the best fidelity. A mesh with even smaller triangles will increase the size of the STL file and take longer to process, but it won’t increase prototype accuracy or resolution.

If an STL file is set with too loose a tolerance, facets will show on the prototype (left). Ideally, the triangle size

should be close to the layer thickness used by the rapid prototyping system. It would make sense for the rapid prototyping machine manufacturers to work with the CAD software folks to develop a set of standard drivers that would eliminate the need for users to select STL output parameters. Whenever the idea has been suggested to 3D


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Systems or Stratasys, however, executives have brushed it off, saying they’ll get around to it sooner or later.

Service-bureau recommendations We’ve surveyed rapid prototyping services and find that they generally agree on the STL settings for various CAD systems that will produce the best rapid prototypes. Regardless of the parameters that any CAD system uses to define model resolution, service bureaus say the chord height of an STL file (the maximum distance a point on a triangle can deviate from the true surface of the part) should be between 0.001 and 0.003 inches (0.025 and 0.075 mm). Whenever possible, STL files should be in binary format. (In the past, some CAD systems did such a poor job of STL creation that developers included an ASCII STL option as a debugging tool. ASCII STL files tend to be huge, but they can be edited with a text editor.) Some CAD systems will generate error messages during STL conversion saying that some part geometry is outside of the positive X, Y, Z quadrant or is in negative space. These messages can be ignored, as the STL file will be located in the rapid prototyping machine’s build space and oriented for optimal surface finish and build speed by the system software. If you’re using a solid-modeling CAD system listed below, the following settings should produce good STL files. If you’re using a surface modeler or another system not listed, we’d suggest discussing file preparation with your service provider or in-house prototyping shop. Surface-modeling CAD systems describe part geometry using mathematical patches with no thickness. In order to create good STL files, all of these surfaces must be joined so that there are no gaps or overlaps. This stitching or sewing, as it is frequently called, can be a tedious process, and it’s easy to miss small flaws that can crash a rapid prototyping system.

CATIA version four

• Select STL command. • Set Maximum Sag to 0.0005 inches or 0.0125 mm. • Pick the model to be converted to STL. • Select Yes to generate triangles. • Select Export, type a file name, and click OK to output an STL file.

Note: CATIA has a utility for checking and analyzing STL files. It is a good practice to check the direction of normal vectors (they should point outward) and verify that the STL surface is closed. Parts created as CATIA solids should be no problem. Parts created as volumes from surfaces are more likely to have flaws.

Pro/Engineer

• Open your model and from the File menu choose Export/Model. • Select the file type STL. • In the Export STL dialog box, set Format to Binary. • Set the Chord Height to 0. The field will be replaced by a minimum

acceptable value for the geometry of the model. • Set Angle Control to 1. • Name the file and click the OK button. Pro/Engineer will save your STL

file.


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SDRC I-DEAS

• From the File menu, select Export/Rapid Prototype File. • Select the file to be exported. • Select Prototype Device to SLA500.dat. • Set Part Positioning to Centered. • Set Absolute Facet Deviation to 0.000395 inches. • Select Binary as file type. • Name and Save your STL file.

SolidEdge

• Open your model and from the File menu choose Save As. • Select the file type STL. • Click on the Options button in the Save As dialog box. • Set the Conversion Tolerance to 0.001 inches or 0.0254 mm. • Set the Surface Plane Angle to 45o. • Select the Binary radio button and click the OK button. • Name and Save your STL file.

SolidWorks

• Open your model and from the File menu choose Save As. • Set the file type to STL. • Click on the Options button at the bottom of the Save dialog box. • Select Output as Binary and select the desired units (inches or

millimeters). • Set Quality to Fine. • If you want to preview the STL model before saving, check the “Show

STL info before saving” box • Name and Save your STL file.

STL files can be created from SolidWorks parts or assemblies. If you wish to create an STL file of an assembly, there is a check box at the bottom of the Export options dialog box to save the assembly as one STL file or individual STL files for each part. SolidWorks also allows users to use custom triangle deviation settings. With our test file, using SolidWork’s coarse settings resulted in a 244-kilobyte STL file with visible facets. The default fine settings (triangle deviation 0.006 inches) produced a smooth file of 836 kilobytes. Setting a custom deviation of 0.003 inches made an 1,828-kilobyte file. An ASCII STL file generated with the fine settings was 7,894 kilobytes.


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Unigraphics

• Open your model and from the File menu choose Export/Rapid Prototyping.

• Set Output Type to Binary. • Set Triangle Tolerance to 0.0025. • Set Adjacency Tolerance to 0.12. • Set Auto Normal Gen to On. • Set Normal Display to Off. • Set Triangle Display to On. • Name and Save your STL file.

Rapid prototyping large objects -- from parts to art Most companies use technologies such as stereolithography, selective laser sintering, and fused deposition modeling to make parts for such small consumer products as telephones, toasters, and track shoes. A typical rapid prototyping part is less than ten inches in length. Because most rapid systems build parts by depositing, solidifying, or sintering material point-by-point, making larger objects takes a long time, and in many cases, large objects won’t fit the fabber’s building chamber. For instance, the largest part that a 3D Systems’ SLA-7000 stereolithography machine or a Stratasys FDM Titan can make is only 24 inches long (600mm). (See “2002 U.S. Rapid Prototyping Equipment Roundup.”) But what if you need to build models larger than two feet across? The approach taken by most companies is to build larger parts in sections and then glue them together. But building multiple pieces takes time, and assembling them can be tricky.

The Materialise Mammoths To meet customer demand for larger parts, Materialise of Belgium has built three custom large-format stereolithography machines. The Mammoth I has a building capacity of 2,100mm by 640mm by 490mm (82.6 inches by 25.2 inches by 19.2 inches). The Mammoth II can build parts as large as 2,150mm by 620mm by 500mm (84.6 inches by 24.4 inches by 19.7 inches). And the newest, the Mammoth III, can build parts 2,100mm by 650mm by 600mm (82.6 inches by 25.6 inches by 23.6 inches). Each Mammoth employs multiple lasers and uses a formulation of DSM’s Somos 9100 resin that it calls PolyPox. Materialise demonstrated the capabilities of the Mammoths recently, as part of a project sponsored by the European Union to explore the possibility of using waste powder created in marble quarrying to manufacture replicas of statues. The idea behind the project is to use rapid prototyping models as masters for duplication by powder-injection molding with a marble-powder composite. For the demonstration, Materialise built a full-sized copy of a Greek kouros on the Mammoth II. Kouroi are life-sized statues of idealized young men, sculpted in ancient Greece between 650 B.C. and 500 B.C. to mark graves or as gifts in temples to the gods. Only a few dozen of these statues remain, and their stylistic progression from idealized to more realistic representation helps art historians delineate the transition from the Archaic to the Classical period of Greek art. The Volomandra Kouros, on display in the National Archeological Museum in Athens, stands almost six feet tall and is dated to 550-570 B.C. For the EU project, the Greek company Geo-Analysis S.A. of Thessaloniki, scanned the statue and sent the files to Materialise in Leuven, Belgium.


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Materialise converted the scan data to STL format and built the giant stereolithography replica. In all, it took 99 hours and 23 minutes to build the kouros. Sixteen kilograms of stereolithography resin were used. To save resin and minimize building time, Materialise built the statue as a shell with a wall thickness of five millimeters. The resulting copy makes it clear that the real challenge to duplicating art lies not in the limitations of rapid prototyping but rather in the difficulty of collecting good scan data. The technicians from Geo-Analysis faced precisely the same problems met by companies doing full-body scans of people to create computer games or action figures. While it’s easy to scan geometric shapes, the human form, with all its curves, is more difficult to capture. The stereolithography statue duplicated flaws in the scan data along with the features of the original. Building statuary, however, won’t be the primary mission for the Mammoths. Materialise says it has experienced growing demand, particularly from automotive companies, for single-piece prototypes of such large parts as instrument panels, bumpers, and interior trim items. Contact: Materialise NV, Technologielaan 15, 3001 Leuven Belgium Telephone: (32) 16 39 66 11 Fax: (32) 16 39 66 00 Web site: http:// www.materialise.be.

The original Volomandra Kouros and thestereolithography replica.

After 24 hours of building, the shape of the kouros began to emerge.

Lifting the kouros out of the Mammoth.


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Virtual medical modeling and rapid surgical templates

In recent years, forward-thinking surgeons have used rapid prototyping models to help plan difficult operations. From scan data collected by computed tomography (CT) or magnetic resonance imaging (MRI), it is possible to build models of biological structures such as damaged jaws or joints. With biomodels in hand, surgeons can practice operations and build custom surgical appliances prior to actual operations. These techniques can drastically reduce the time required for operations and in some cases eliminate the need for preliminary exploratory surgery. Doctors have even used stereolithography models to help plan the separation of conjoined twins. (See “Rapid prototyping helps separate conjoined twins.”).

The logical next step is to dispense with the physical models and rehearse surgeries in a virtual world. Even the speediest rapid prototyping systems can take hours or days to build medical models, and the relatively high cost of rapid prototypes limits their use. Virtual models, by contrast, can be created in just a few hours and cost nothing except the time required to process scan data. A team of maxillofacial surgeons at the Morriston Hospital in Swansea, United Kingdom, is exploring how virtual models can shorten the time and costs involved in planning surgery.

Working with Dr. Richard Bibb, head of medical applications at the University of Wales’ National Centre for Product Design and Development Research (PDR) in Cardiff, doctors recently decided to see if virtual modeling could be

used to plan an operation to insert implants for attaching a prosthetic ear. In similar cases, says Bibb, the team has used stereolithography models built by PDR to help assess the depth of bone and select drilling sites for implants. While the physicians credit the rapid prototypes with improved surgical accuracy, they say the stereolithography parts often break during practice drilling. And the cost of rapid prototyping makes it impractical to build multiple models to evaluate different surgical options.

More typically, the Mammoths are used to build large automotive parts such as this instrument panel.

Using a mirror-image of the scan data of a patient’s healthy ear, doctors created a virtual model of a prosthetic ear.


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For the test case, the doctors took CT and MRI scans of a patient’s skull and soft head tissue. Bibb used Materialise’s Mimics software to convert the scan data into virtual models in STL format. He says the process took a few hours but with practice might be done in as little as 30 minutes.

Bibb created a three-dimensional mirror image of the patient’s healthy ear. Bibb and the surgeons aligned the new virtual ear with the virtual model of the patient’s skull to determine the best way to attach it. Cylinders representing implant screws were placed through the virtual prosthetic into the virtual skull. Bibb says the doctors were able to determine the bone depth at potential implant sites by viewing cross-sections of the skull model. Then they optimized the position of the implants. When the doctors were satisfied with the location of the implants, Bibb created a virtual block around the cylinders that fit exactly against the patient’s skull. Then he subtracted the cylinders from the block in a Boolean operation to create guide holes for drilling. Bibb used an STL file of this block, with the guide holes, to build a stereolithography part with Vantico’s RenShape SL H-C 9100R resin. This is the only stereolithography material that has been t

During the subsequent implantation operation, the surgeons exposed the patient’s skull. By placing the sterilized stereolithography template against it, they were able to drill holes precisely where they had planned. Bibb says PDR has since worked with the Morriston maxillofacial doctors on several similar cases. Where it used to take days and cost hundreds of dollars to have anatomical SLA models made, plan an operation, and then hand-make templates from the models, surgical planning now can be done in a matter of minutes and costs very little.ested and approved to a recognized standard (FDA USP 23 Class VI) for medical use.

The doctors were able to plan the location of the implant screws and create a virtual drilling template.

A stereolithography model of thedrilling template was used duringsurgery to align the implant holes.


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Reverse Engineering

Capturing scan data

Converting scanned data to CAD models is a complex but straightforward process. Unlike design, which starts with only vague requirements and entails frequent revisions, reverse engineering can be reduced to a recipe. Here’s how it goes.

Read scanned data

Scanners typically write out point data in proprietary formats that must be converted to the format of the reverse-engineering software. Better scan-processing programs read directly the formats of a variety of scanners. Some scanning software also reads data directly from the scanner interface.

After each data set is imported, it is helpful to remove stray data points that are obviously not part of the cloud of points that define the product’s surfaces. Some scan-processing software has filters that will recognize the stray points and select them for deletion.

The reverse-engineering process begins with a point cloud, generated by a high speed optical or x-ray scanner.

Align point clouds

Rarely can a product model be captured in one shot from a scanner. More typically, overlapping scans are taken from multiple directions. These are frequently saved in multiple data files that must be aligned and merged to create a single point cloud covering the entire product surface.


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Multiple scans of the Venus de Milo capture different portions of the statue’s face.

Until recently, scanner operators frequently glued targets, such as ping-pong balls, to the surface of the physical product before scanning it. These targets aided operators in the painstaking alignment of scan sets. Today scan-processing software can align multiple point clouds in semi-automated fashion without the aid of targets.

Three scans registered to form a single surface.


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Simplify the data

Today’s scanners typically gather more data than is needed to make accurate CAD models. Operators generally crank up the scanner resolution, because setups can be costly, and the operators don’t want to risk having to reshoot a product. Redundant data also occurs in areas where multiple scans overlap.

Good reverse-engineering software contains a variety of routines for finding and removing unnecessary data points. Some algorithms filter out points in areas where scans overlap. In general, more accurate measurements are made on surfaces that are normal to the eye of the scanner.

All measurements are inaccurate, and point clouds are no exception. Most points deviate from the true surface by some amount that varies according to a statistical distribution. Some systems enable users to find and delete points that vary from mean values by more than two or three standard deviations.

Most software also has routines for sampling points at a uniform density over the model removing some of them. Point density also can be further reduced in areas of the model where the radius of curvature is large. In more rounded regions or near sharp bends, higher data density is needed to maintain accuracy. Some algorithms can recognize regions of low curvature in the point cloud and automatically remove points from them.

Polygon meshing

The next step is to connect the remaining points in the cloud with straight lines to produce a polygon mesh of triangles. This polygonal surface, which resembles the stereolithography file or the faceted solid models of CATIA version four’s solid-M type, is easier to work with than a set of disconnected points.

Getting a good polygon surface over a large point cloud may take some work. Different settings may be required for scans of varying point density. It may be necessary to remesh certain sections to get good results.

Close-up of a polygon mesh derived from data scanned from a folded air bag. A CAD model of this part would be useful for designing an airbag enclosure.


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Polygon editing

Editing the polygon surface enables users to improve the quality of scan data in a number of ways. Indeed, it is the ability to create and manipulate polygon data that sets modern reverse-engineering software apart from its forebears.

Modern systems enable operators to smooth the surface data globally or to smooth local areas with tools that sand or cut away rough spots. Once the surface is smoothed, the polygon count can again be reduced in areas of low curvature to reduce the file size.

Other tools enable users to repair holes in surfaces that scanners missed. These tools also enable users to cut out regions, such as a manufacturer’s ID or serial number cast into a part, and repair the cutout. Users also can trim rough surface edges using smooth curves drawn on the surfaces.

Polygon editing tools enable users of scan-processing software to fill holes caused by spots that the scanner missed.

Polygon editing tools enable multiple surfaces to be merged into completely enclosed volumes, enabling them to be used for rapid prototyping or imported into solid models. Polygonal models can be scaled or sculpted to change them from the original design.

If the end use of the scanned data is to create rapid prototypes, the process can stop here. Polygon models can be exported in the STL format used by rapid prototyping systems without having to set tolerance parameters. (See “Preparing data for rapid prototyping” for a description of the STL format.)

Defining surface boundaries

If scanned data’s ultimate destination is a CAD or CAM system, then polygonal surfaces must be converted to parametric surfaces, specifically non-uniform rational b-splines (NURBS) that have become the building blocks of most commercial CAD software. In principle, numerically controlled mills could be programmed to cut polygonal surfaces. However, people familiar with the process say that machines work faster when tool paths are based on spline surfaces instead of more randomly oriented polygons.


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Orange lines denote boundaries of surfaces drawn by the operator. Black lines show boundaries of individual

NURBS patches generated by the reverse-engineering software.

NURBs have four-sided boundaries. Some CAD systems group these four-sided patches into larger surfaces that can be manipulated as one object. However, it’s not possible at this time to apply NURBS to arbitrary surfaces without human intervention.

Modern reverse-engineering software enables operators to define the boundaries of multi-patch surfaces and control the arrangement of patches in each surface. Such control reduces the size of CAD models and assures that small splinter surfaces don’t impede the functioning of finite-element or tool-path generating programs.

Operators define surface boundaries by drawing curves on the polygon surface that divide it into manageable regions. In most cases, the curves follow ridgelines, valleys or sharp edges that form the natural features of the part. Some programs detect such contours and draw boundary regions automatically. The operator can change the curve locations if the computer makes poor choices.


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Operators may craft boundaries of patches interactively, if necessary, to satisfy special requirements.

Applying NURBS

After surface boundaries have been created, the operator applies surfaces to the region. Most systems allow the operator to control the patch size and orientation. Better systems enable users to define how surfaces blend into one another. All systems that produce data suitable for CAD assure that adjacent surfaces are continuous and, if desired, tangent. So-called class A scan-capture software (used primarily for automobile design) lets users assure that the derivative of tangency (called C2 in computer graphics parlance) and the second derivative (C3) also are continuous, if desired.

After the system has generated parametric surfaces, operators can edit them interactively. Such editing is painstaking work, as splines may bulge or sink in unexpected places. When editing, operators can use the original polygonal surface as a guide for correcting such errors.

Exporting the data

After the surfaces are shaped satisfactorily, data can be exported as either surfaces or solid models, as the user chooses. Most CAD software, from SolidWorks to Unigraphics, can import surfaces and incorporate them in solid models.

Better systems have direct translators to the leading CAD programs including Pro/Engineer, Parasolid (for Unigraphics, SolidWorks, and Solid Edge), and CATIA. Most systems also can write data in STEP or IGES formats when direct translators aren’t available.

The end product of reverse-engineering software is a solid or surface model without dimension-driven form features. Developers of higher end products are working to add limited feature-creation capabilities that will enable operators to produce round holes and cylinders, for example.

God is in the details

The process described above is greatly simplified. Most reverse-engineering software has a variety of sophisticated functions that have been added to satisfy specific customer needs. The art of learning reverse-engineering software involves knowing which functions and parameters to employ for your company’s classes of problems. Some systems have programming interfaces that enable certain functions to be selected automatically, enabling work to be performed by less highly skilled operators.

For more, see "Reverse engineering Update."

Reverse-engineering update

Most of the articles on CADCAMNet are about creating computer models and turning them into physical products or tools. Sometimes, however, companies have physical products that they need to turn into CAD models. The reasons people might need to do this are varied.


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No CAD model may exist. Some manufacturers need models of production parts or subsystems to incorporate into a new product model. Existing automobile engines and transmissions, for example, are frequently reused in new models with only slight modifications. Lack of CAD models may occur for a couple of reasons. The product may originally have been designed with a combination of draftings and physical modeling techniques. In some cases, CAD models may have been lost due to poor data management.

Another reason manufacturers want to make CAD models from physical products is because as-built parts may deviate from original designs. During product development and tooling design, it's common for manufacturing engineers to change part geometries in subtle ways. If these changes cause parts to fail, manufacturers may need to apply finite element analysis to the as-built parts to determine why they failed. It such cases, it's important to faithfully copy the failed geometry. If the as-built geometry is satisfactory, designers may want models of it to use in derivative product designs.

A third reason for reverse engineering is that in some industries, stylists still prefer to work in physical media such as clay, wood, or foam board. CAD systems are great for modeling regularly shaped parts such as round shafts or sheet-metal enclosures. They are much harder to use for products with arbitrary curved surfaces or fine details. Capturing artists' models enables manufacturers to use techniques such as numerically controlled milling and rapid prototyping to make production tools. And engineers can use CAD software to add internal details such as stiffening ribs and bosses for fasteners.

Much improved

Reverse engineering of physical models used to be a slow, costly process. Technology improvements have made it faster and easier than it was only a few years ago. Scanners operate faster and hold more data, thanks to increases in semiconductor speed and reductions in memory-chip costs. Today's visual light scanners can capture millions of points in seconds with the ease of photographic cameras. CT (computed tomography) scanners can produce accurate cross sections of internal details that white-light or laser scanners can't reach.

Computer workstations have doubled in speed and memory capacity in the past 18 months, enabling them to process larger quantities of data than ever. It takes 12 bytes of data to store one coordinate from a three-D scanner, according to Ping Fu, chief executive of Raindrop Geomagic, a maker of reverse-engineering software. That means a desktop system with 512 megabytes of RAM can handle up to 1.5 million data points (including allowances for the operating system and application software). Systems with two gigabytes of RAM can manage up to 20 million scanned points.

Lastly software for reverse engineering has improved steadily along with hardware. Modern products are easier to use and more capable than ever of automating the conversion of scanned data to CAD models.


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Integer performance of Intel Pentium processors as a function of clock speed.

Picking the right mix

To convert physical models to computer models successfully and efficiently, engineering managers must have the right technologies available. These include scanners, software for converting scanned data to CAD or rapid prototyping models, and workstations for running the conversion software. Workers who use these technologies also must receive the right training.

Whether shopping either for scanners or scanning services, it's important to know the degree of accuracy your work requires. White-light scanners are fast and easy to set up and use, but they are the least accurate. Coordinate-measuring machines (CMMs) are slow and highly accurate. For details, see “Understanding three-D scanners and digitizers” in the February and March 2000 CAD Reports.

Software

As with scanners, there is a large variety of software for converting scan data into CAD models. Some industrial designers employ plain CAD software. Speaking at the 2002 SolidWorks World conference, Edward Eaton explained how he used SolidWorks with no supplemental software to create accurate models of cars and computer joysticks from data scanned from physical models. Such techniques work well for relatively simple products where some deviation from the scanned geometry might be okay. These techniques also may require extraordinary operator skill and resourcefulness, characteristics that aren’t universal.


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Inus Technology’s Rapidform software converts clouds of points captured by high-speed scanners into NURBS surface models suitable for CAD software.

Some companies employ add-in software to supplement their basic CAD systems. Some of these products, such as Parametric Technology’s Pro/Scantools for Pro/Engineer and Dassault Systèmes’ Reverse Engineering Configuration for CATIA version five, are made by the CAD software makers themselves. Others, such as RevWorks for SolidWorks or HighRES’ Sphinx for Pro/Engineer, come from third parties.

Built-in reverse-engineering software has advantages. It may be less costly than the stand-alone applications described below. It also requires CAD-system operators to learn and memorize fewer commands because functions such as creating, opening, or closing files and positioning three-D models on the display are common to both the reverse-engineering software and the CAD program.

But built-in reverse-engineering systems also have limitations. First, they are best suited for forms that need to capture models for only one CAD system. Exporting models to other systems requires the use of data-translation software. Models optimized for one CAD system might be less than ideal for another. Second, software that works within CAD programs may be limited in the number of scanned points it can handle. CAD systems can work with tens or even hundreds of thousands of points. But their designers didn’t envision the millions of points that modern scanners can capture. Lastly, built-in packages from the CAD companies don’t sell well, so they tend to get lower priority from sales, service, and development people than do stand-alone packages made by smaller companies.

Stand-alone software is the other choice for converting scan data to CAD models. At this writing, these products can be divided into two broad classes: those for what are called class A or class 1 automobile surfaces and those intended for other than class A work. Class A reverse-engineering software is characterized by the ability to control the continuity of curvature (called C2) and the derivative of curvature (C3) across surface


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boundaries. Class A software also gives stylists special features needed for auto body design. One example is control over the radius of curvature as surfaces approach other surface boundaries.

ICEM Technologies’ ICEM Surf and EDS’ Imageware are the two best known products for Class A reverse engineering. Both products have fallen on hard times. In August, PTC sold ICEM Technologies to a group of its managers led by Lee Cureton, acting general manager of ICEM, and financed by Indigo Capital Limited, a UK-based investment firm. The company is currently called ICEM Limited and is headquartered in Southhampton, U.K.

Imageware was originally founded by University of Michigan graduates for the purpose of converting scanned data from automotive clay models into class A surfaces. Over the years, the software has gotten mixed reviews from users: some loved it and some hated it. In 1998 SDRC bought the company to bolster the surfacing capabilities of its I-DEAS software, following the winning of a $200 million CAD-software contract from Ford. However, Ford continues to use ICEM Surf. Imageware now appears to have fallen between the cracks of the merger of EDS and Unigraphics Solutions. According to one customer who uses the software, EDS allowed more than a year to pass without providing an upgrade release.

EDS is continuing to sell Imageware as part of the Unigraphics software family. Dassault Systèmes has developed a new class A surfacing product for CATIA version five.

Outside the rarefied world of car-body design, the two main companies sell general-purpose software for converting scan data to CAD models: Raindrop Geomagic Studio and Inus Technology’s Rapidform. Automakers and other industries use these products to reverse-engineer parts that don’t have class A surfaces such as power train and suspension components and interior body panels. For details, see “Geomagic and Rapidform.”

Raindrop Geomagic’s Studio software automatically aligns clouds of points from multiple scans to make a

complete part envelope.


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The market for reverse-engineering software is small and the technology complex. So reverse-engineering products cost more than most CAD systems.

Many software products can get the job done. To maximize efficiency, potential buyers need to match their requirements with the capabilities of each software product. The right one depends on the frequency, complexity, and accuracy of your reverse-engineering tasks.

Buyers shouldn’t rely only on demonstrations and vendor-run benchmarks to make a final selection. Obtain an evaluation license, send at least one user for training, and attempt to use the software before writing a purchase order. Only hands-on experience can reveal cumbersome operating procedures, poor training or reference manuals, or bugs that can make scan-capture software even harder to use than it otherwise would be.

Caveat emptor

As reverse-engineering software becomes more powerful, some companies may be tempted to use it as a substitute for good engineering practices. It pays to resist temptation.

Reverse engineering is still an arduous process. It’s cheaper to care for your original CAD data. Companies need to organize CAD models for long-term storage, fast retrieval, and reuse. Parts suppliers should not be allowed to deviate from CAD-model geometry if strength, safety, performance, and reliability are important to your company’s product. Changes required for manufacturing reasons should be made to the master CAD model, approved, and reanalyzed by the customer’s engineers before they’re permitted. They should not be made by unsupervised toolmakers to shave costs.

Scan-capture software is complex, and operators need to learn how to use it properly. Training in software functions is important. So is understanding the uses to which the data will be put. For example, if parts are to be milled or used in FEA, the presence of tiny faces on the surface of reverse-engineered parts can trouble these applications. Deciding how surface boundaries are defined and the level of accuracy required can affect model quality.

Finally, reverse engineering with large numbers of scanned points is a computationally demanding activity. Budget for powerful desktop systems with adequate memory. Compared to the cost of most software, computers are cheap.

For a detailed description of the reverse-engineering process, see “Capturing scan data.”


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For most engineering uses, you won't need an instrument as big as this Lambda, an extremely large gantry-type coordinate- measuring machine from Browne & Sharpe, North Kingstown, Rhode Island.

UNDERSTANDING THREE-D SCANNERS AND DIGITIZERS — Part 1

Last July, we wrote about techniques for developing CAD models from coordinate data captured by three-D scanners or digitizers. This reverse engineering is painstaking and time- consuming, but interest in it continues to grow as more companies employ three-D CAD in their engineering processes. Moreover, the software for working with point clouds is improving; more engineers are gaining experience with the techniques, and the digitizing hardware is improving as well. Thanks to these changes, more engineering managers are evaluating whether to integrate scan data into their design and manufacturing workflow.

This month, we review the major classes of hardware used to capture point data, including how each type works and some of its strengths and weaknesses. Next month, we'll look at how to choose from the various types of commercial systems.

Three-D capture hardware can be installed in-house through use of purchased or leased equipment or obtained on a job-by-job basis from a service bureau. In either case, a basic understanding of the available capture technology will help you pick the right hardware for each of your applications.

Coordinate-Measuring Machines

Three-D data acquisition in the manufacturing trades remains rooted in industrial metrology and inspection. The coordinate-measuring machine (CMM), the mainstay of metrology, still is the most common tool used by firms for capturing data from existing parts, even in reverse engineering, rapid prototyping, and related disciplines. In part, this is due to availability. CMMs are widely distributed in workplaces, typically in large-company quality-assurance departments. The technology is proven. Portable CMMs also are cheaper than optical scanners, which have emerged as their main rival in automobile and


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aerospace design offices. Beyond inertia and price, CMMs have technical advantages that should keep them relevant for a long time.

In a typical installation, CMMs find use as contact devices (also called digitizers), acquiring data upon hitting the target surface. CMMs normally are tipped with a touch probe, a small metal or jeweled instrument shaped like a ball or pin. The probe is deployed like a finger at the end of an articulated arm, with segments that rotate around various axes named for their human analogues: shoulder, elbow, and wrist. Encoders track the movements of the joints, allowing width, length, and depth to be specified as values along all three Cartesian planes, `X,' `Y,' and `Z.'

Some CMMs are built into extended platforms, their arms hanging from gantries that can slide over working volumes large enough to envelop big parts, such as an entire automobile. These behemoths, stabilized by granite bases, can provide for unrivaled accuracy, measured in microns. The CMM is also available in portable versions, notably those made by Faro Technologies and Romer, Inc. They can be mounted on heavy-duty tripods, tables with threaded holes, even onto a fuselage or truck body. Portable CMMs can be packed up and trucked to remote locations, though their precision is measured at best in thousandths of an inch.

A plus for CMMs is that data from a touch probe tends to be sparse and is relatively easy to structure (as splines), particularly when an engineer who knows what to look for operates the apparatus. The biggest drawback to CMMs is that harvesting data one point at a time is a tedious way to copy curving, free-form surfaces. And touch-probe accuracy can be skewed when acquiring data from surfaces that deflect on contact, such as the fabric cover of a convertible or a leather seat cushion. Though soft-tipped devices are available, touch probes are not recommended when targets are prone to scoring. Touch probes also aren't designed to obtain features buried beneath the surface of a part, unless the part is broken open. And depending on the probe head, CMMs may not be able to obtain data from recesses or tight curves, such as those that might be found on a bumper or automobile grille.

Scanners

Hoping to capitalize on CMM deficiencies, vendors for more than a decade have been bringing to market many different types of machinery capable of copying multiple points simultaneously. The market for scanners, as these bulk-data acquirers are sometimes called, has of late begun to take off.

The enabler has been the growing power of relatively cheap Intel-based computers, says Dr. Anshuman Razdan. Razdan is technical director of Arizona State University's Partnership for Research in Stereo Modeling, and he tracks reverse-engineering technology closely. Computers once took forever to cull useful features from the volumes of unstructured coordinates, called point clouds, collected by scanners. This reality sustained the CMM because in comparison to scanners the data CMMs acquire is easier for computers to handle. For this reason, the new computer power is welcome to scanner manufacturers. A related momentum booster is that the work of culling usable features from a point cloud can be readily distributed to multiple processors. This means point-cloud processing is one of the rare desktop applications that will benefit from Intel's campaign to seed the technical world with workstations equipped with multiple microprocessors (see the April 1998 CAD Report).


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Bulk-data scanners have gained an impulse from yet another techno-commercial development, says Razdan. The cost of chips carrying charge-coupled diodes, a key element for the sensors used in most of today's scan heads, has dropped as CCDs have found their way into consumer products such as digital cameras.

Most scanners acquire data by interpreting the interactions of target volumes with various forms of energy, including white light, laser beams, and x-rays. Laser and white-light scanners derive range data via a technique called triangulation. Z-axis values are calculated using trigonometric functions. Broadly speaking, this is similar to how humans and other beasts blessed with binocular vision achieve depth perception, reconciling discrepant views captured by our two eyes.

Laser Scanners

Laser-based systems use a low-intensity red laser to capture data as either points or ribbons of coordinates. In laser scanning, Z-axis values are calculated on a matrix of X and Y coordinates like so many holes in a pegboard. Laser light is shot at the grid like a pawn progressing across a chessboard one square at a time. The laser light approaches its target along a line of sight, fixed at between 30 and 45 degrees with respect to the receiving sensor. If an outcropping of the target object sits between the reflected beam and the sensor, this area will remain in shadow.

A laser beam can also be spread into a fan of light. Upon hitting the object, the line will follow the surface's contour, as if it were a ribbon. Various profiles of a solid object are collected as the laser fan sweeps across the face of the target.

The laser market is populated by a dizzying array of products and companies. Among the top ones are Digibotics's Digibot point scanners, popular for capturing small to medium-sized objects, and Mensi's SOISIC-brand scanners for capturing very large targets, such as wall-sized structures in power plants or underground mines.

Cyberware manufactures laser scanners that work quickly, making them popular for obtaining models of entertainers or medical patients. The latter includes burn victims for whom face maps need to be generated to make therapeutic masks. Cyberware also markets a variety of products aimed at industrial reverse engineering.

Polhemus sells portable scan heads operated like hand-held magic wands. These collect data when they are waved over the volume to be copied. An electromagnetic tracker monitors the wand's location, making the device useless for capturing metal parts. But Polhemus scanners can capture plastic or fabric and live subjects. Several vendors also manufacture laser scan heads that replace touch probes on the ends of CMMs. Among these are ribbon scanners from Perceptron, Hymarc, and Kreon.

White-Light Scanners

Optical scanners can be thought of as three-D cameras. They develop models by reading the play of white light over the surface of a target object. An optical scan head contains a projector assembly, including a light source (usually a halogen bulb) and one or two digital cameras.


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Typically, white-light scanners combine triangulation with interferometry, a technique for generating interference patterns, to gather information about the target surface. A moiré (a wavy pattern) is produced when two geometrically regular patterns are overlapped at acute angles. Flat surfaces provoke little deformation in the pattern, while contours produce more distortion.

On the plus side, optical scanners are portable. They can be mounted on tripods or sometimes portable CMMs. Optical scanners can acquire large data volumes quickly, because they harvest many points in parallel. The combination of interference and triangulation also makes for excellent levels of accuracy, as good as portable CMMs.

On the downside, optical scanners will pick up only surfaces that are visible along their line of sight. Nooks and crannies or parts that are blocked by other parts may end up in shadow, a problem if you're trying to capture a turbine or pump assembly with closely overlapping blades. As with laser devices, sometimes this drawback can be overcome by shooting the target from multiple angles, though this may make the downstream work of postprocessing the image more time-consuming.

Two rival optical scanner product lines, both originating with German companies, have become popular in automobile design studios and aerospace firms. These are the ATOS scanners, developed by GOM GmbH, and the Comet series scanners from Steinbichler GmbH. Minolta and EOIS also have offerings based on white-light scanning technology.

Finicky About Finishes

White-light and laser scanners can both be extra sensitive to target surface finishes. Objects that are very shiny, such as chrome parts, will generate too many erratic reflections resulting in point clouds too full of noise or garbage data. The reverse also is true: A flat black surface will absorb so much light that not enough data will be acquired. If the surfaces can be prepared, shiny or dark surfaces can be sprayed with chalky powders or paint to obtain the dull-gray matte finish preferred by optical scanners.

Ambient lighting conditions also have to be controlled. We were told by a service bureau professional about the results of an optical scanning session that were thrown off when sunlight, filtering in from an open window, was covered by a passing cloud. Optical scanning sessions are best conducted in dim ambient light. Red-laser scanners don't like sun or incandescent light because both carry plenty of red in their spectrums. Fluorescent ambient lighting is less objectionable for laser scanning sessions. Mensi reports that it has come up with a way of filtering out ambient light so that its scanners will work out of doors.

Computed Tomography

Computed tomography (the term comes from the Greek word for section) is familiar to anybody who has been forced to lay down in one of those large, tube-shaped devices and undergo a medical CT scan. As in medicine, industrial CT scanners are adept at obtaining information about internal features without having to open the surface. CT scanners are maximum strength versions of their medical counterparts, powered up to nine mega electron volts.

“Parts don't complain if you radiate them with lots of energy,” said Simon Huss, a research scientist at Aracor, a manufacturer and vendor of industrial CT scanners. That's good


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news, because the greater the x-ray flux, the better the resulting image. When more energy is used, x-ray images have sharper contrasts along edges, and less noise is produced.

The raison d'etre of CT scanners is that they can obtain internal geometry without destroying a part. But they have other advantages. They don't care about surface finishes, and they come in sizes large enough to easily scan parts as big as automobile engines. With only a few exceptions (a lead object being one), x-rays will penetrate just about anything, even the hardest of metals used in jet turbines. They also can be used to report details about the composition of parts. They can distinguish between plastic and metal, and even between different kinds of metal — say aluminum and steel.

Cost is their biggest disadvantage. Lower- powered devices start at around $500,000, and the high-end ones cost $2,000,000. They are owned outright only by the largest of industrial concerns in aerospace and the automobile industry. Even on a contract basis, individual scans are expensive.

Destructive Cross Sectioning

Cross-sectional scanning was developed by Crump Group International, which does business as Capture Geometry Inside. The company's devices are usually just called CGI scanners. The main advantage of cross-sectional scanners is that, like CT scanners, they readily obtain point clouds from parts with complex internal geometry. The machines range in cost from $90,000 to $225,000. A CGI scanner can digitize any object that can be milled, including steel or aluminum parts.

A part to be digitized by a CGI scanner is placed in a mold and liquid epoxy is poured around it. The epoxy is chosen so that upon drying it will contrast in color with the part. The mold is put in a pressurized vacuum system to extract bubbles from the epoxy and push the glue into all internal channels and cavities. The goo dries within two to six hours, depending on how much was poured. The hardened block that results is then ready for the cut-and-scan process. A mill grinds down the block. Layers range in thickness from 0.0010 inch to 0.0005 inch. A digital camera captures an image of each layer, and a computer captures these cross sections, ready for reverse engineering.

The biggest drawback is that any part scanned will be obliterated — literally turned to powder. It also takes a relatively long time for the scanner to chew through the parts. Scans, however, are automated, so shops that use the system generally load multiple components and let the machines run overnight.

Next month, we'll discuss how to match scanning technologies to the problems you need to solve. Sometimes it takes more than one type of hardware to get the job done.

Documents

Rapid Pro to Typing and Reverse Engineering