by Leslie Gordon, Machine Design, May 24, 2007
These neutral formats fare differently in collaborative design
Even when a geometry is valid and precise (has no gaps), it can be unrealistic. The blend on the right is offset from the one on the left, causing a sharp, unmanufacturable condition highlighted by CADIQ from ITI.
After a user has dragged files into DEXcenter's Web browser and selected a recipient, the software translates the files to other CAD formats using best-practice translation methods, then securely delivers the files electronically, according to ITI.
A view in STEP-NC Explorer software from STEP Tools Inc. displays machining information in a STEP-NC file. Toolpaths, feeds, and speeds are shown with workpiece and tool-shape information. Users can pan, roll, and zoom, and see tool motions step-by-step, or as a continuous animation.
Proficiency's Collaboration Gateway preserves implicit product knowledge between CAD systems, such as between a Catia 4 and Catia 5 system.
The ability to pass data between different CAD systems only gets more important with the increasing emphasis on collaborative model-based design. In such efforts, 3D solid models contain the information needed — such as geometry, notes, GD&T data, material properties, and the like — to completely define a product from concept to disposal.
This helps let companies automate digital-life-cycle processes and squeeze out costs. The venerable IGES and STEP standards were developed to provide neutral file formats for moving files between different CAD systems. Today, IGES and STEP each play into the model-based mix in different ways.
Developed in the late 1970s for exchanging geometry, IGES is a U.S. standard that has not changed much recently. The Initial Graphics Exchange Specification Version 5.3 (1996) is the last published specification, with Version 6.0 currently in the works. However, IGES is still in wide use, especially for transferring 2D drawings. It also supports free-form surfaces, wire frames, annotations, and, most recently, solid models.
STEP, or the STandard for the Exchange of Product data, on the other hand, is an ISO global standard (10303) developed in 1984 that continues to be updated. For example, it is now used to exchange all product information as well as part geometry. The larger and more-complex STEP includes application protocols (APs) that define how data for a particular purpose or domain gets transferred.
All major mechanical CAD systems include AP 203 and AP 214, each of which centers on designing mechanical parts. Additional APs in the standard include those for AEC, NC machining, shipbuilding, and factory-floor layout, among many others. STEP also includes what are known as Conformance Classes (CC), which provide guidelines for software developers implementing STEP.
It's helpful to note that all major 3D MCAD systems include an IGES and STEP reader, or preprocessor, and a writer, or postprocessor. The originating system's preprocessor reads the CAD file and generates a version formatted in accordance with the IGES or STEP protocol an end user has selected. The receiving system then postprocesses the results for further work.
WHEN A SINGLE KEY WON'T WORK
Translation problems between different CAD systems can arise because the standards allow different mathematical representations of the same geometry, says John Gray, a CAD manager at International TechneGroup Inc. (ITI), in Milford, Ohio. The firm developed many of the IGES and STEP translators being used today by major programs. "For instance, IGES represents surfaces as Nurbs surfaces, and as cylinders, a special type of surface. When a postprocessor doesn't understand some of the entities a preprocessor supports, it may just ignore them. And an unrecognized entity can even crash the postprocessor," he says.
Modeling complex 3D components and exporting them as IGES files to CAM systems can also cause problems. Unless the end user selects the correct translator, the resultant model could have large gaps between surfaces. When this happens, someone must try to fix the model based on their assumptions.
"Most average CAD users rely on default settings," says Gray. "In fact, many translation accidents happen because users don't get involved in data exchange unless it becomes an issue. Even for a translator that creates an IGES file from a drawing, there are probably 50 or more variables in terms of picking the correct flavor. Some relate to geometry, others to annotation," he says.
A good analogy, says Gray, comes from locks in a house. When they are all the same brand, a single key suffices. But when locks come from different companies, a single key won't work.
Problems also arise when moving models between MCAD systems that use different modeling approaches, says Doug Cheney, a CAD interoperability consultant with ITI. Older systems such as Catia 4 build surfaces and then sew them together to make 3D models. Newer systems, though, build solid models stepwise with features and parameters, and keep tabs of the steps in a history tree. Here, an IGES solid part can end up being a surface part because surfaces have become unsewn during translation.
Passing models between history and nonhistory-based systems also has issues, according to Cheney. History-based systems use implicit modeling, in which a piece of software defines the geometry, whereas IGES and STEP represent explicit geometry defined by mathematical equations. Thus, when files are exported from an implicit system to IGES, higher-order geometries must change to the lower-order, explicit type, which can cause problems. Poorly done conversions can introduce gaps in volumes so they are no longer closed.
THE NEXT STEP
To get exact mathematical models, STEP developers first built tools to define the standard with a language called Express, says David Loffredo, vice president of STEP Tools Inc. in Troy, N.Y., a firm which has developed STEP translators for major CAD developers.
"How to handle the different internal accuracies of disparate CAD systems was a problem developers solved early on. They have been steadily improving the STEP standard ever since. Also, for about 15 years, all major MCAD companies have been working on STEP as well. In fact, companies are involved in international forums in the U.S. and Germany, moving CAD data back and forth to test the standard and their individual implementations of it," he says.
According to Loffredo, the Conformance Classes mentioned earlier let CAD developers implement APs in stages. As mentioned earlier, all major MCAD systems include AP 203. It is grouped around the types of geometric representations a system can exchange. Thus, CC1, the first CC developed in AP 203, defines just PDM information, not geometry. Next came CC2, which defines wire frames and adds surfaces floating in space. Other CCs followed. The most complex, CC 6, added B-reps. All major MCAD systems today use CC6.
Another example comes from the AP 238 STEP-NC standard for NC machining. Here, CC1 simply defines toolpaths, speeds, and feeds. CC2 adds product, fixture, and tool geometries. CC3 extends this with CAM information such as features, machining strategies, and operation parameters. CC4 for GD&T, which includes information that lets end users perform collision checking and take measurements knowing tolerance ranges, completes the picture. Loffredo says software from STEP Tools called ST-Machine combines all the necessary data sources such as CAD models, CAM files, and G and M-code files.
In addition, other translation problems between CAD systems stem from formats that do not transfer all of the history tree or built-in design intelligence, says Alex Zavorski, director of Technical Marketing, Proficiency Inc., in Westborough, Mass. His firm provides product knowledge integration (PKI) between OEMs and suppliers, and belongs to ProSTEP, the European forum for testing STEP. Zavorski says the standards committee is currently working on an AP for design history, although it is not yet in place.
HANDLING BAD CAD
ITI has expanded its past definition of bad CAD to better encompass translation problems, says Cheney. Invalid geometry, or geometry with gaps in it is at the top of the problem list. The company's CADfix healing software ensures data gets into CAD as a closed volume. The software refits curve and surface data so it is within tolerances accepted by the receiving system. It also lets users defeature models for FEA.
"Next on the bad-CAD list is unrealistic features," says Cheney. "For example, geometry can be valid but might have, say, sharp edges where two blends are not really connected. Even if the model is modified, customers might have a hard time with simulating or manufacturing it." Some of the company's software lets users compare translated files side by side with the originals. This comes in handy for checking supposedly complete models, making sure they are not missing features.
Next on the list comes unacceptable changes. According to Cheney, many companies send legacy models to India and Russia for manual rebuilding or what is called remastering. However, firms should check results because a lot of models come back with mistakes. Other bad-CAD problems the company's software handles include undocumented changes, which happen when models are changed and notes are incomplete, and incomplete changes, in which someone says they changed the model, but didn't.
Gray says there is software that lets users customize the flavoring of IGES and STEP files as well as direct translators so customers always get ready-to-use models. Translation is done and verified in the background. FTP processes are automated, which saves time dealing with Indian and Chinese suppliers.
Gray says there are movements afoot towards other formats for exchanging data. One example is JT, developed by UGS Corp., in Plano, Tex. "There are different levels of content in a JT file, from simple visualization, to models that can be sent to suppliers for quotes, to mathematically complete representations," he says. "The same is true with the Adobe 3D format. It has different levels of intelligence and content that can be put into models."
The history tree contains intellectual property in addition to design intent, says Proficiency, so firms need a way beyond STEP to control which parts of their IPs get transferred outside of firewalls. The company's software queries CAD system APIs for model history and features. It then extracts knowledge from models to what are called Universal Product Representations. A UPR contains the complete product definition, including features, parameters, history, manufacturing attributes, and associations. This information can work with Catia V4 and V5, Unigraphics, I-deas, Pro/Engineer, IGES, STEP, and JT files.
Lastly, Loffredo says the STEP standards committee has recently standardized an XML-exchange form that STEP Tools uses in its STEP-NC software. "This lets end users exchange XML data that is described by STEP. A big advantage to using XML is it ties IT more closely to engineering. Until recently, these have been two totally separate worlds. Now IT can use a familiar tool to place engineering information in databases and data-retrieval systems."