3D CAD: integration of reverse engineering

Paul Boughton

Both reverse engineering and three-dimensional CAD are established techniques, but using the two together has not always been as easy as it could be. Jon Severn reports on a development that combines solid, surface and triangle modelling to enable reverse engineering and CAD to be more closely integrated.

Not all new designs start with a 'clean sheet of paper' or, more likely today, a blank computer screen. New products are often adapted from existing ones for which no CAD data is available, and the creation of such data would be time-consuming and costly. Reverse engineering the existing product by using a three-dimensional scanner and appropriate software can be very cost-effective in such situations, though the resultant data is not necessarily as easy to use as a starting point for the new design as might be expected.

Another way in which scanned data plays a crucial role in the creation of new products is in the medical and dental fields. Yet again, however, the designer faces the problem of migrating the data from the reverse engineering software package to the design environment. The root of the problem in many cases is that scanners output data as triangle files whereas CAD systems use solid modelling, surface modelling or a hybrid combination of solids and surfaces.

Delcam says it is addressing this issue in a new version of its Copycad reverse engineering package. Copycad Pro is claimed to be the first product development system to offer what Delcam has called Tribrid Modelling, which adds triangle modelling to a hybrid surface/solid modeller. The Tribrid version of Copycad is said to improve the efficiency with which companies can re-engineer existing products to create improved or bespoke designs.

Chris Lawrie, Delcam's CAD product marketing manager, states: "The combination of solid modelling, surface modelling and triangle modelling provides a unique design system for these companies. Having all the different modelling techniques in a single package reduces the need to transfer data between multiple programs and so streamlines the whole product development process. Most existing reverse engineering software is limited to capturing data and creating duplicates. By combining all three modelling techniques in Copycad Pro, we can offer a complete re-engineering package in a single program."

Delcam has developed this new design approach through the integration of its Powershape surface and solid modelling functionality with the reverse engineering and triangle modelling options that were in earlier versions of Copycad. This means that data captured using reverse engineering techniques can be transferred into the design environment more easily, thereby saving time in the overall process of creating the new design. However, the Tribrid concept is not just a matter of bolting together existing functionality, rather it incorporates a wide range of enhancements to many of the key operations. Major improvements have been made to the sculpting and model repair tools previously available in Copycad for editing triangle files. These tools enable high-quality models to be produced from poor-quality reverse engineering data or from damaged or defective physical components. For example, uneven surfaces can be smoothed out, gaps in the data can be filled and extra points can be added in areas where only sparse data has been collected (Fig.1).

The availability of functionality from Powershape in Copycad Pro has enabled easier, faster and more accurate creation of CAD surfaces from triangle data. Users still retain control over the way the complete data set is divided into the component features and surfaces, but each area can now be converted into CAD data by generating a network of curves and then projecting it onto the mesh of triangles. A single surface is generated within the complete set of curves using the Smart Surfacing functionality from Powershape, which analyses the curves and determines the most appropriate way to create the surface.

Analysis tools are available in Copycad Pro to display the differences between the triangle mesh and the resulting surface (Fig.2). This is useful because it might, for example, show that a closer spacing between the curves would be needed in some areas of the model to keep the surfaces within the required tolerance.

The other key benefit of the integration into Copycad Pro of modelling options from Powershape is the ability to create 'perfect' geometry in areas where conventional reverse engineering systems would give only approximate results. For example, the design might include one extruded surface that could only be described by a series of individual surfaces in traditional reverse-engineering software. With the added modelling tools, Copycad Pro makes it easy to reproduce the original design intent by replacing the required area with a single surface. Similarly, a reverse-engineered fillet area can be replaced with a consistent, smooth fillet, rather than existing as a set of complex, free-form patches that may have varying radii.

Tribrid modelling also permits Boolean operations to be carried out between triangle models and either surfaces or solids. For many years, Powershape has been one of a limited number of CAD systems that enable Boolean operations to be performed between solids and surfaces. The ability to perform similar addition, subtraction and merging operations with triangles as well makes Copycad Pro even more versatile.

There are many situations where a combination of different techniques is needed to create a complete design. A typical example would be in the plastics industry, where initial hand-modelled prototypes are often produced in solid material. Reverse engineering from such models only produces the external surfaces of the component. However, by using Copycad Pro, these surfaces can be offset by the moulded material's wall thickness to generate the internal surfaces. Geometric features, such as reinforcing ribs and bosses for fixings, can then be added using the solid modelling tools to complete the design. The finished model can then be subtracted from a solid block to give the core and cavity shapes for the injection mould tool.

Consumers are moving away from mass-produced products, either because they want designs that are more individual or because they need an element of bespoke design, perhaps for medical reasons or for increased comfort. This can be achieved more efficiently and economically by integrating reverse engineering with computer-based design methods.

Tribrid Modelling in Copycad Pro enables users to create CAD models quickly with data from different sources. Delcam says Copycad Pro makes it easy to design the main outline of a product with CAD but to use hand models for complex details or decoration, or to capture specific data from a customer or patient. This extra data can be collected with a scanning device and combined with the main CAD model in the software.

The Tribrid modelling concept has also been applied in the latest version of Delcam's Powershape software, Powershape Pro, for which the main applications are expected to be in the medical area. This is because most data from medical scanners is generated as triangle files, both when creating internal prostheses and when manufacturing external customised aids such as orthotics. As with Copycad Pro, a key addition in Powershape Pro is the ability to undertake Boolean operations between pairs of triangle files, or between triangle models and solids or surfaces.

The new functionality in Powershape Pro expands considerably the previous ability to add triangle files of logos, branding, textures or other 3D decorations to the surfaces of CAD models. This effect is rather like sliding a transfer over the surface of an object, although it does give the added advantages that the decoration can be scaled or stretched to fit any given area and the result is fully three-dimensional. Tribrid Modelling enables all three forms to interact within any design, limited only by the user's imagination (Fig. 3).