CAD data exchange

CAD data exchange involves a number of software technologies and methods to translate data from one Computer-aided design system to another CAD file format. This PLM technology is required to facilitate collaborative work (CPD) between OEMs and their suppliers.

The main topic is with the translation of geometry (wireframe, surface and solid) but also of importance is other data such as attributes; metadata, assembly structure and feature data.

Methods of translation

There are basically three methods of transferring data from one CAD system to another.

Direct internal

Some CAD systems can directly read and/or write other CAD formats, simply by using file open and file save as options. As most CAD file formats are not open, this option is limited to either systems owned by the same company or via hacking of competitor's file format.

Direct external

There are a number of companies that specialize in CAD data translation software, providing software that can read one system and write the information in another CAD system format. These systems have their own proprietary intermediate format some of which will allow reviewing the data during translation. Some of these translators work stand-alone while others require one or both of the CAD packages installed on the translation machine as they use code (APIs) from these systems to read/write the data.

Data translation formats

A common method of translation is via an intermediary format. The sending CAD system exports out to this format and the receiving CAD system reads in this format. Some formats are independent of the CAD vendors being defined by standards organisations while others, although owned by a company, are widely used and are regarded as quasi industry standards. It is becoming increasingly common for companies owning these quasi industry standards to further the use of their formats by openly publishing these data formats.

Example formats:

A number of CAD data exchange methods are described by recent academic studies. The neutral modeling command (NMC) method,[1] proposed by Zhejiang University, is an example of these methods.

Level of information detail translated.

As each CAD system has its own method of describing geometry, both mathematically and structurally, there is always some loss of information when translating data from one CAD data format to another. The intermediate file formats are also limited in what they can describe, and they can be interpreted differently by both the sending and receiving systems.

It is therefore important when transferring data between systems to identify what needs to be translated.

If only the 3D model is required for the downstream process, then only the model description needs to be transferred. However, there are levels of detail. For example: is the data wireframe, surface, or solid; is the topology (BREP) information required; must the face and edge identifications be preserved on subsequent modification; must the feature information and history be preserved between systems; and is PMI annotation to be transferred.

With product models, retaining the assembly structure may be required.

If drawings need to be translated, the wireframe geometry is normally not an issue; however text, dimensions and other annotation can be an issue, particularly fonts and formats.

No matter what data is to be translated, there is also a need to preserve attributes (such as color and layer of graphical objects) and text information stored within the files.

Sometimes, however, there is a problem caused by too much information being preserved. An example are the constraints placed on designers arising out of the design intent-history captured in parametric design systems. The receiving system must provide designers with the design freedom to modify geometry without having to understand the history of, or undo, the design tree.

Some translation methods are more successful than others at translating data between CAD systems.

MultiCAD Digital Mockups

Two CAD/CAM/CAE PLM trends have been driving CAD Data Exchange technology. One is the need for close interaction throughout today’s extended multiCAD enterprises. The other is the increased reliance on digital mockups to permit visualization, design in context, simulation and analysis of large scale assemblies prior to the actual manufacture of the physical product. Ongoing advances in data exchange technology have enabled significant fulfillment of those needs.

The ability to visualize medium if not large scale assemblies was one of the early successes of these CAD translation formats. Hardware improvements and the development of lightweight formats supported larger scale assemblies.

Current advances now allow an “Active Mockup.” This technology allows design in context with simulations such as dynamic clearance analysis and automatic generation of motion envelopes. Active mockups allow the edit of components from directly within the multi-CAD assembly. Multiple level-of-detail displays support interactive performance even in huge assemblies.

CAD to CAM Data Exchange

NC programming typically requires that the geometry received from a CAD system, whether in wireframe, surface, solid or combined formats, be free from any irregularities and inconsistencies that may have occurred in the CAD phase of geometry creation. Data exchange from CAD to CAM must therefore include tools for identifying and repairing those inconsistencies. These tools are typically included in the data exchange software of each CAM solution-set.

In a true PLM environment, CAD to CAM data exchange must provide for more than the transfer of geometry. Product Manufacturing Information, whether generated by the designer for use by manufacturing, or generated by the manufacturing organization for use by design, must be a part of the data exchange system. STEP-NC was designed to carry GD&T and other PMI through CAD and CAM into a CNC.

Bibliography

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