About five seconds into your average conversation about CAD translation, someone will invariably mention one of the venerable and long-held neutral formats: STEP or IGES. Neutral CAD formats are the great equalizers of CAD interchange; the old stand-bys of an increasingly multi-CAD world. They provide the bridge to migrate geometry from one proprietary CAD system to another. While direct translation or newer direct modeling techniques now often preempt the use of neutral formats, chances are high you'll need to rely on them at one time or another. But a word of caution: while STEP and IGES are often grouped together in the same breath, they are certainly not equivalent. In fact, if formats were horses, it's probably time to send the IGES pony to the glue factory.
But first, a little about the two formats.
STEP
The Standard for the Exchange of Product model data (STEP) enables the exchange of data between all manner of CAD, CAM, CAE, PDM, and other systems across a variety of industries and applications. STEP is an international standard, ISO 10303 to be exact, and like any other ISO standard, is certainly not free. As you can probably imagine, the standard itself is a complex and labyrinthine bureaucrat's dream, and likely a cure for insomnia. But worry not, we'll give you the short, short version. STEP comes in a variety of flavors designed for specific purposes called Application Protocols (APs), and continues to be extended with new APs. The most relevant APs for CAD are:
- AP203 Configuration Controlled Design: Intended for the Aerospace industry, AP203 focuses on exchange data for 3D solid geometry of both individual parts as well as assembly structure.
- AP214 Core Data for Automotive Mechanical Design Processes: Intended for the automotive industry, AP214 is a superset of AP203, additionally providing interchange for colors, layers, Geometric Dimensioning and Tolerancing (GD&T), and certain forms of design intent on top of existing AP203 capabilities.
- AP203ed2 Configuration Controlled Design (2nd Edition): An update to the original AP203, added much of the capability of AP214 including parametric information and GD&T along other meta content, while also improving interoperability with other AP's like AP214. Be aware however, that many CAD systems still use the older implementation of AP203. Your mileage may vary.
- AP242 Managed Model Based 3D Engineering: The holy grail of standards, AP242 has been in development for precisely forever and due for publication any day now - but promises to unify the various flavors of AP203 and AP214 into a succinct whole.
IGES
The Initial Graphics Exchange Specification (IGES) started as a US Air Force project specifically for exchange of CAD geometry, which culminated in an ASCII-based exchange format adopted by the American National Standards Institute (ANSI). An aged, monolithic standard originally designed for wireframe and surface data, IGES underwent several revisions to extend its capability. Those revisions finally brought the standard into the realm of solid modeling. The chief advantage of IGES is that it is unencumbered by licensing, i.e. it's cheap as free. Which is probably why it's still everywhere after all these years. But all meaningful development of IGES stopped in 1996 at version 5.3. For perspective, that year Netscape Navigator 3.0 had 80% of the browser market, Windows 95 was having trouble convincing most of us to leave NT 3.X or MS-DOS 6.22, the first Blackberry was still 3 years away, and Phil Collins left Genesis to focus on his solo career. Yeah, that old.
Which brings us to the larger point. It's likely time to leave IGES behind for good. Especially in light of available direct translation, direct modeling, and the continued evolution of STEP, there's little excuse for holding on to IGES. After all:
- IGES is older than dirt. Regardless of how good it might have been in 1996, software has changed quite a bit since and will continue to change with new capabilities and requirements. For crying out loud, the IGES logo has tape reels in it. TAPE.
- IGES doesn't support product structure. No assembly models except as a monolithic import. Decimating product structure is generally a bad idea. How long have components been around?
- IGES is abandoned. No one's going to improve it. Ever.
- IGES solids capability is weak. Solids came late to IGES development. Consequently, translation of solids is more prone to geometric inconsistencies, resulting in unsewn/untrimmed surfaces. While IGES is still effective in wireframe translation, who uses wireframe anymore?
Despite these shortcomings, just about every modern CAD system still carries IGES translators, the result of 3 decades of inertia. So what's the harm in continuing to use IGES? IGES just can't keep up with regard to modern solid geometry, and will invariably cause geometry errors that will cost you real money down the line. Use a modern toolkit instead, be it the various flavors of STEP, refined direct translators, or direct modeling capabilities that can operate on foreign geometry. The next time you talk about neutral CAD formats, leave IGES out of it. Think about it, won't you?
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