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Supermodels get down to work

The second article in our two-part series on 3D modelling looks at some of the collaborative production applications

For architects, engineers and fabricators, digitally-based material-forming processes present exciting opportunities to create one-off prototypes, limited quantities of project-specific components, and freeform parts that would be difficult, prohibitively expensive or impossible to manufacture by conventional methods.

New computer-aided design and manufacturing (CAD/CAM) technologies allow digital models to be transformed into tangible, functional architectural components. This is done by using data from digital designs to instruct computer numerically controlled (CNC) machines to produce three-dimensional models direct from the screen.

The subtractive and additive modelling processes were described in some detail last week. To summarize, subtractive manufacturing processes use CNCmilling machines, routers or lathes to automatically 'carve' components or tools out of solid blocks of material. Generally, CNC mills and routers can move freely on a number of axes but only the five-axis machines can access undercuts and other complex features.

Additive manufacturing processes incrementally deposit, cure or bind very thin layers of material within the 'build chamber' of a machine, and produce parts by building them up layer by layer. These technologies are often referred to as rapid prototyping (RP) processes because they were first used commercially in product design to create prototypes quickly for evaluation and verification. Now, however, additive technologies are increasingly used to manufacture functional metal and plastic components directly or indirectly.

Most RP machines have small build chambers, but this constraint is often circumvented by subdividing a part and building it in sections. RP processes are also capable of making freeform parts that cannot otherwise be manufactured.

To use subtractive technologies, data is typically exported from CAD software in a neutral format such as IGES. This data is then imported into CAM software and used to create Gcode, the computer code containing the tool paths and other instructions necessary to direct material removal and operate a CNC machine. Popular CAM programs include MasterCAM, SurfCAM and X-CAM.

For additive technologies, surface data from computer models must be exported in STL format (named after stereolithography, the generic term for 3D modelling). STL files are then imported into software that permits the selection of an optimal build orientation and automatically 'slices' the component into very thin horizontal sections (layers). Their thickness is specified by the machine operator and determines the accuracy of external surfaces. Thicker layers result in faster formation and thinner layers in greater accuracy.

My own architectural practice designs organically-shaped metal frame structures with components such as the freeform column with an integral 'branching' member (pictured). I have found that the most inexpensive way to manufacture such large freeform tubular components accurately is to cast them using CNC-milled foam patterns.

The component (shown opposite) was designed in Pro/Engineer, an advanced parametric solid modelling program with the capacity to create freeform shapes. The computer model of this column was then divided into two interlocking subcomponents in order to accommodate locally available casting equipment. Each of these components was in turn subdivided into interlocking, self- registering pattern halves that were later glued together. These pattern component models were exported in IGES format and then imported into SurfCAM, which was used to create the G-Code.

The CNC milling machine automatically used a predetermined optimal variety of different shapes and sizes of bits as it 'carved' the pattern parts out of high density styrofoam. The completed foam patterns were used to investment-cast the freeform column components in stainless steel.

This freeform column represents an excellent example of the benefits of advanced solid modelling software.

Conventional architectural CAD programs create purely representational models that cannot function as useful manufacturing databases. CAD/ CAM processes can be engaged much more effectively by designers using parametric, dimensionally driven, feature-based modelling software such as Pro/Engineer, SolidWorks, or StudioWorks by Alias Wavefront, and these programs are no longer significantly more expensive than conventional architectural software.

In these programs, individual features of a model are created by inputting dimensional data or equations, and the three-dimensional geometry of these features can be changed automatically by simply editing dimensions.

The capacity of a computer model to evolve along with a design is extremely valuable in terms of obtaining collaborative technical input. The original design for the freeform column, for example, was based on aesthetic considerations and reasonable structural assumptions.

The software used to model this component made it easy to collaborate with both the foundry and structural engineer and then make modifications. The foundry analysed the computer models using MAGMASoft, a program for predicting the behaviour of molten alloys during casting, and this indicated the need to change some variations in wall thickness to ensure proper solidification and casting integrity.

Making the recommended changes to the model only required simple editing of the dimensions used to create the design. Technical input from the structural engineer was also readily accommodated, and the complex form of the component was revised and regenerated automatically.

Rapid prototyping can be particularly useful in architecture as a technology that enables the indirect manufacture of metal components.

Many readily available RP machines can be used to make expendable patterns for investment casting. DTM Corporation in the US and EOS in Germany manufacture selective laser sintering (SLS) machines and materials designed for use in foundries.

SLS machines produce very accurate patterns. The fused deposition modelling (FDM) machines made by Stratasys also make excellent patterns for investment casting. The Thermojet Solid Object Printer by 3D Systems uses multi-jet modelling (MJM) to make thermoplastic patterns that behave like wax, and Z Corporation's Z402 3D Printer (3DP) produces easily combustible patterns made of starch and wax.

TriPyramid Structures, a US company that designs and manufactures custom metal structural components, recently experimented with RP to make prototype castings for a structural connection in the Regional Performing Arts Center project in Philadelphia. This is being designed by Rafael Vinoly Architects and engineer Dewhurst Macfarlane.

TriPyramid collaborated with Harvard Design School's CAD/CAM lab, using its Z402 3D printer to make a mock-up of the assembly and its Thermojet machine to print patterns that were successfully used to investment-cast the components.

Additive technologies are especially useful for making small quantities of patterns with complex shapes. Portions of several different freeform architectural elements can be manufactured with connective or internal features that cannot easily be CNC milled. These may require composite RP processing. Similar components have successfully been cast in aluminum and stainless steel from patterns made using SLS, MJM, and 3DP.

Rapid prototyping can also be used very effectively with soft tooling to manufacture limited quantities of components inexpensively. This approach requires a strong, accurate and durable master pattern, and because of this stereolithography is often used. A soft mould is made by suspending a master pattern in a flask and filling it with a room temperature vulcanizing (RTV) material, typically silicon rubber. The master pattern is removed by cutting it out with a scalpel. The resulting mould components are then reassembled, and wax or urethane is injected into the mould to make replicas of the master pattern. These replicas make excellent investment casting patterns, and a typical RTV mould can produce at least 20 parts before wear diminishes accuracy.

IMI Rapid Prototyping in Birmingham has used this method extensively to make castings for the automotive industry, and used it to make the patterns, mould and structural castings for the Triumph Design Group's Daytona motorcycle.

It is easy to see how such methods could be adopted in architecture to make interesting and innovative components.

While it is now possible to use RP to manufacture architectural castings, it will eventually be possible to create metal components directly from CAD models. SLS is already used in some industries to make functional prototype parts from metal powder.

Professor Tom Childs and his students at Leeds University are conducting research in this area with my architectural input. Aerospace companies in the US and the UK are also developing additive technologies that will eventually be able to make very large and strong metal parts directly from CAD models.

Dr Kevin Rotheroe is an architect with Freeform, One Stuyvesant Oval Unit 9D, New York, e-mail krotheroe@ nyc. rr. com See classified pages 94 and 95 for modelmakers' information

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