The Changing Face of Surface Modeling

Nov. 18, 2010
Software helps support interoperability in the mold and die industries.

Large OEMs typically use complex supply chains that are five or six tiers deep and the data file that defines the part to be supplied is the main communication tool between these tiers. Ideally, all suppliers would have every CAD package so they could seamlessly accept and work with native data, obviously an impractical scenario. In practice, many supplier components are sent to suppliers via files translated into neutral formats such as IGES or STEP. Translation is rarely perfect, resulting in the need for substantial remodeling after the fact.

A key need is therefore not simply data transfer, but full interoperability, such that data from one system can be immediately reused in another.

Software such as PowerShape 2010 helps support interoperability, in part, because it uses Parasolid, an industry-standard geometric modeling kernel. Parasolid is used by more CAD packages than any other kernel. Also, many other CAD systems can import and export Parasolid’s native XT format. As such, these packages can exchange XT files without translation and the associated rework it usually entails. In addition, PowerShape lets users accept data via translations by manually or automatically fixing translation errors.

To see how the software helps foster interoperability, a brief refresher on the history of CAD is useful:

When “3D” CAD first arose in the early 1970s, computer-hardware limitations restricted it to narrow areas of design or manufacture. CAD design was initially limited to surface modeling, in which the underlying shape was defined by a network of wire frame over which the surfaces were draped. The first true solid-modeling system was introduced by Parametric Technology Corp. (PTC), Needham, Mass., during the 1980s.

Recall that surface modeling represents the exterior surfaces of products using definitions such as Coon Patch and Bezier curves. Examples include aerodynamic surfaces of aircraft or aesthetic surfaces of cars. Editing techniques let designers precisely control surface shapes and curvatures and combine them to create shells of products such as fuselages. Surface models are well suited to machining complex surfaces, and forming sheet metal, and for aerodynamic analysis. However, the method provides no definition of the substance of the design. Delcam’s Duct software, which later became PowerShape, was an early example of surface-modeling technology.

Solid modeling, on the other hand, is based on boundary representations (b-rep) which define an enclosed volume from which every aspect of a real object can be calculated including mass, volume, surface area, center of gravity, and moment of inertia. Topology defines connections between vertices and edges and so-called “edge loops” defines faces. Geometry is then attached such that curves define the shape of edges and surfaces define the shape of faces. The computational needs of early b-rep modelers restricted the software to analytical shapes such as combinations of cones, blocks, and spheres.

As computer power increased, the different technologies overlapped and general-purpose packages that let users build surfaces or solids, or both, arose. In developing PowerShape, Delcam, Salt Lake City, Utah, implemented Parasolid in addition to the existing kernel, creating a highly sophisticated surface and solid modeler.

Most PowerShape users are Tier 2 or Tier 3 suppliers in the mold and die industries. As such, they must create, import, and improve surfaces for manufacturing tooling, with the capacity to import design data from any source. Interoperability issues don’t always come from translation errors. Surface modeling in general is more forgiving than solid modeling where designers must follow stringent rules or modeling operations will fail. For example, in surface modeling, nothing restricts designers from building surfaces that overlap or have gaps between them. This leeway sometimes leads to problems later in the production cycle.

Parasolid has a number of strategies for sewing imported surfaces together to infer the topology and geometry needed to define an entirely closed, unambiguous and consistent volume. PowerShape includes a tool called Solid Doctor which examines solid models using Parasolid’s checking mechanisms. It reports inconsistencies in topology and geometry and then recommends how to fix models or does so automatically. In addition, Solid Doctor lets users extract affected faces or surfaces and repair them in isolation. When the surfaces are repaired, they are incorporated back into the solid model. This process continues until the user has resolved all inconsistencies and created a valid Parasolid model.

Under the hood of a solid-modeling kernel
Parasolid is built into CAD software via calls to the Parasolid application programming interface (API). The API comprises more than 850 functions developed over years of expert effort. The kernel supports many editing, querying, and model-creation functions and works to a precision of 10-8 micron. Parasolid lets developers easily add solid-modeling capabilities to applications.
3Dconnexion’s 3D mice work with Vectorworks
3D mice now support Vectorworks 2011 AEC/CAD design software. The 3D mice let users simultaneously pan, zoom, and rotate as if they are holding models in their hand. Programmable buttons provide quick access to commonly used Vectorworks commands. All of the company’s mice work with Vectorworks 2011 including the SpacePilot PRO ($399), SpaceExplorer ($299), SpaceNavigator ($99), and SpaceNavigator for Notebooks ($129).,

© 2010 Penton Media, Inc.

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