When data transfer goes awry

IGES is as good as it is ever going to get. The Initial Graphics Exchange Specification went through its sixth, and final, revision last year. As the U.S. standard for exchanging data between dissimilar CAD systems, it has over time expanded to include most geometry concepts used for part modeling today. It is now rare to find a full-blown CAD system that doesn't support some version of IGES.

Does all this mean CAD drawings and models pass flawlessly between systems via IGES? Not quite. There are still issues that can either prevent drawings from translating at all or that cause problems in the translated drawing that weren't in the original.

The same can be said for the more comprehensive and still evolving STEP standard. The STandard for the Exchange of Product model data is an ISO standard (10303) describing not just part geometry, but also information about how the part is to be manufactured. It is becoming widely used as a means of transferring 3D models between dissimilar solid-modeling systems.

Data translation through IGES and STEP is becoming more important because of increased emphasis on collaborative projects involving multiple teams. The bad news is that translations still cause trouble. The good news is that the causes generally are well understood.

Processing pre and post
Exchanging data via either IGES or STEP requires two programs, one on each CAD system. Software on the first CAD system, called a preprocessor or writer, reads the CAD file and produces a version formatted in accordance with the IGES or STEP specification. The file moves to the second CAD system where a postprocessor program or reader creates a version in the format of the second CAD system.

IGES today is the predominant means of transferring 2D drawings between differing CAD systems. It also handles a lot of 3D work as well. Particularly for 3D, preprocessors have been notorious for putting out files that post-processor's on other systems couldn't read properly. But many of these problems are over says John Gray, a CAD manager at International TechneGroup Inc. in Milford, Ohio. ITI developed many of the IGES translators now used by major CAD vendors.

"CAD translator quality has improved, partly because the spec has settled down. You don't see people putting out bad data anymore," Gray says.

Difficulties in the past have arisen because IGES allows geometric entities and annotations to be represented in a variety of ways. When a postprocessor doesn't understand some of the entities a preprocessor supports, it may just ignore them. An unrecognized entity may even crash the postprocessor.

But common geometries usually come across fine, says Gray, with the exception of conic arcs and cross hatching. In both cases, IGES defines multiple methods of representation that not all post-processor's implement.

One problem that remains is that not all CAD users have up-to-date translators. Earlier versions of IGES did not support all of the 3D geometries constructed by widely used CAD systems. "Use of different mathematical representations has led to the need for crib sheets that tell you how to make IGES translations work," says Jim Rusk, MDA vice president at CAD supplier SDRC in Milford, Ohio. "For example, how you map a Nurbs surface into a Bezier surface can account for some of the struggle. That's why you might flavor an IGES translation so that it produces whatever the receiving side supports best."

Such difficulties are most pronounced when passing models between CAD systems that employ two different modeling approaches. Older systems, such as Catia V4, construct solids by manually creating trimmed surfaces, then sewing them into a closed volume. Newer systems, in contrast, derive solids via construction history. Models get built up bit by bit with features and parameters.

Leader lines and annotations are another matter. " You are more likely to run into translation problems for a dimension containing multiple lines of text, or with leaders having multiple segments," explains ITI's Gray. Ordinary text, leaders, and witness lines generally translate properly. But intelligence about what they refer to may be lost in translation.

The intelligence problem crops up in systems having associative dimensions. Here leader lines and callouts are attached to the geometry to which they refer. Change the length of a part, for example, and the leader lines change with it. These relationships can get lost in an IGES or STEP transfer, though. Change a dimension, and the leader lines stay back at the original spot.

"If the associativities are not written into the IGES file, it is a real trick to go back and guess what they are," says Mike Bomkamp, an independent contractor who works with file translation. "You have to look at every entity in the file and guess the relationship based on what it is near," he says. Bomkamp at one time was in charge of 3D IGES translation efforts for CAD vendor SDRC. He has experience working with IGES and STEP translators from a variety of CAD vendors and has noted some of the reasons that translation difficulties arise.

"CAD vendors are generally uninterested in having a good IGES or STEP preprocessor," he explains. "They don't particularly care whether their data gets out of their system 'nicely.' They will spend more time perfecting their IGES reader because the data is coming into their system. It has only been in the last few years that vendors realized they could close sales based on the quality of their IGES translator; prior to that, there were a lot of co-op students writing them."

History Buffs
Problems with associativities are among IGES' most notorious shortcoming at this point. "Where you have some intelligence, different CAD systems interpret the intelligence differently or try to do things automatically to fit a drafting standard," explains ITI's John Gray. "Typical problem areas are in dimensions that encompass multiple lines of text or that have leaders with multiple segments."

The method most frequently used to overcome such difficulties is what's called a healing process. Human operators use a special healing program to examine IGES files and determine what is not translating properly. "Often we can determine what it is about the CAD system that causes it to put things in the wrong location. We can make modifications if we can understand why a receiving system is trying to change the IGES data coming in," says Gray.

STEP will address associative information and other data about relationships, but only in the future. It will someday incorporate an application protocol defined to support the transfer of data about design history or design intent — holes that are meant to go all the way through a part, say, even if the dimensions of the part grow. Those familiar with STEP's approval process see the standard encompassing an AP for transferring part history within the next few years.

"A lot of progress was made at the recent international STEP meeting and I think solutions may become available as early as 2002," says STEP Tools Inc. President Martin Hardwick. " The Japanese also are putting a big effort into implementing drawing data exchange via STEP for their building and construction industry." STEP Tools in Troy, N.Y., has developed STEP translators for major CAD vendors and also runs a STEP translation service.

Zero Tolerance
For at least the near future, the preferred method of handling inconsistencies caused by translation will be manual intervention. Recently, for example, ITI devised a software tool called CADfix for the task. CADfix is an aid not only for lost associativities, but also for problems from differing tolerances that are inherent to different brands of CAD systems.

Tolerances are perhaps the thorniest issue plaguing solid-model translation today. CAD systems from different vendors each employ differing base-line tolerance levels for such fundamental geometric considerations as whether or not two lines or end points coincide. Translating a model created with loose tolerances for use by a system with tighter tolerances generally causes problems, irrespective of whether or not the incoming model is in IGES or STEP format. Untoward gaps between parts and surfaces that are unbounded are the most common difficulties.

"Our own Master Series uses a tolerance of 0.01 mm, but we have found that the precision of data from other systems can be all over the map," says SDRC's Jim Rusk. " Both IGES and STEP adequately present data to support what the vendors provide. But models can lose precision when written out to IGES or STEP depending on how the data is represented in the CAD package."

Base-line tolerances can vary even in different parts of the same model. The problem is that designers may loosen tolerances as a way of speeding up the modeling process. Personnel at SDRC have noticed the phenomenon in models created by feature-based packages from other vendors. What seems to happen, they say, is that the designer starts creating features with a relatively tight tolerance. Features added or subtracted later on in the process tend to have looser tolerances, perhaps as designers approach a deadline.

One way to work around difficulties from differing CAD-system tolerances is to devise native-to-native translators. For example, "We have a translator that calls Pro/Engineer to output models in a special way," says SDRC's Rusk.

Problem is, feature definitions and mating relationships between parts get lost during translation, just as in STEP. There are other direct translation methods as well, but they also lose history data. Spatial Technology's ACIS kernel is a software library that CAD system writers can use to manage geometry and topology. Parasolid from Unigraphics is a similar library. Neither ACIS nor Parasolids handle drawings or nongeometric part attributes such as color.

Other translation difficulties can arise from misunderstandings about modeling practices. "Models built from trimmed surfaces and sewn together into solids can exhibit a class of problems similar to those on early modeling systems," explains Doug Cheney, a manager at International TechneGroup Inc. "You often see this in people who have become accustomed to using surface modelers. Their natural inclination is to simply complete their trim surface models by sewing surfaces together into a skin and then into a solid. The problem is that you bring low-level surface and curve quality issues. The sewing process does not improve the quality of curves and surfaces or their continuity. Just because you get a valid solid from sewn surfaces doesn't mean it will have as high a quality as a parametrically built solid."

The reason is that parametrically built solids are constructed with the help of high-level algorithms that control the flow of geometry across the faces of the model. This contrasts with what happens in the process of creating faces separately and stitching them together. "When you are building surfaces by themselves, you have a lot of flexibility and room for error," says Cheney. "When you build a solid model parametrically, the control of each face on the model is more removed from the designer."

Models built face-by-face tend to have problems with smooth tangency relationships and curvature between surfaces, says ITI. Ripples and surface waviness, termed low-level quality defects, are the result. These problems are absent in models created through newer parametric paradigms because low-level faces get created based on specified parameters and construction history.

Low-level model quality problems tend to cause problems in IGES and STEP translations. But what are called high-level model quality problems pass through translation unaffected. High-level model quality problems arise specifically in feature-based modelers. They are unintentional interactions among features that take the form of small cracks, knife edges, voids, and similar artifacts between features. "The geometry that results is good, but the interactions are unmanufacturable," explains ITI's Cheney.

Such effects cause downstream operations such as FEA and manufacturing software to break down. High-level defects become even more worrisome if they pass through a STEP or IGES transfer. The reason: The model loses all feature history during translation. "A model pushed through STEP or IGES has no parameters or history. The designer has no way to efficiently remove cracks or other defects other than by totally rebuilding that portion of the model," explains Cheney. "The source of the crack is the history, and it is the poor interaction of model features that must be tweaked slightly to remove the problem."

High-level defects arise inadvertently through the course of modeling. Say, for example, a part contains a linear extrusion feature such as a rib next to a thin wall. One of the last modeling operations may be the application of a draft angle to the thin wall for molding purposes. Adding a draft angle can create a thin crack between the rib and the wall. " We found cracks like this all through one of our customers' injection-molded parts," says Cheney. "We went back through their PDM system and found this defect had been an issue from the first day the part had been created."

Moreover, high-level defects should not be treated with healing software used to cure low-level defects. "If a healer tries to do something about a crack between features, it will affect the model in ways that may change the design intent," says Cheney.

The way to keep high-level defects from getting worse after a translation is to prevent them in the first place. ITI recommends that designers run a diagnostic tool as they create models. Such a tool, which ITI markets, notices cracks and related defects as they arise. It then queries the designer about whether or not the interaction it notices was intended.

Some STEP translators can heal modeling problems themselves, rather than depend on a separate program. Says STEP Tools' Martin Hardwick, "Some translators can detect the fact that features won't exist because they are below the system tolerance. The translator can use topology to heal the model, sometimes with the assistance of the user, sometimes automatically."

STEP Tools personnel have also noted some common difficulties that crop up during conversions.

"It is coming from neutral format back into native format where you really separate the men from the boys," says Hardwick. "That is where you are faced with tolerance issues and the need for healing. There are some good STEP translators and some poor ones. Good ones include those that go with Pro/Engineer, Unigraphics, Catia, and Cadkey."

Hardwick seconds the motion that models most likely to cause trouble are those done hurriedly. "It can be in your interest to lower the CAD system's tolerance level if you are trying to work fast, so the parametrics evaluate quickly," he says. "Such practices can lead to a low-quality model that doesn't get detected until it goes to another CAD system. Unfortunately the messenger often gets the blame for the problem in such cases, and the messenger is STEP."

Hardwick thinks more engineering departments are wising up to model quality problems — only about 1% of all STEP translations done at STEP Tools experience difficulties. And most of these glitches stem from STEP visualization functions which are relatively new to the standard.

Conversions between B-rep and non-B-rep solids are less of a problem in STEP because the standard supports more classes of geometries than IGES, claims Hardwick. "You can translate for Class 4, a collection of surfaces with topology, if you want to go to a surface-only CAD system. You can use Class 6 for going between two solid modelers, or Class 5, basically facets, if you are just interested in presentation," he says.

What has not been addressed in STEP are callouts, annotations, and leader lines. "Standards committees have been focused on the geometry of solid models and assemblies. Only now are people beginning to address dimensions and annotations, partly because many people think the job IGES is doing in this area is good enough," says Hardwick.

Alternatives
There are some moves afoot to completely eliminate any need to translate models between CAD systems. One of these initiatives is cosponsored by this magazine along with SDRC and PartSolutions LLC. MDcybercad.com is a Web site that provides free part models in a variety of native CAD formats. The models never go through any sort of native-to-native translation. Instead, they are stored in a single neutral format. This base information is encoded on demand into a native CAD format, then e-mailed to the person requesting it. The service is free to end users.

Another alternative to data translation is called CADScript and comes from International TechneGroup. It is basically a middleware program that provides a consistent method of accessing multiple CAD APIs, for the purpose of sending CAD data to downstream applications such as analysis.

ITI developers got the idea for CADScript by looking at the reasons why CAD models were being translated into and out of STEP and IGES. In many cases, it was to get into a format compatible with a particular downstream application. Code written once with CADScript can be used against any API, allowing access to CAD data without translation. Point projections, queries, and so forth return the same results the CAD system itself would.

Imagecom Inc. of Arlington, Tex., recently debuted an Internet portal called ASPire3D.com that provides services aimed at eliminating the need for IGES. The site converts 2D drawings to 3D native CAD models on a pay-per-use basis. The conversion algorithm, called e-FlexiDesign, is said to automatically recognize features on 2D drawings. When the conversion is complete, users get an e-mail notification to pick up their file.

Finally, there are several firms now marketing direct CAD-to-CAD conversion products, either via Web site services or through sales of conversion software. One in this category is Elysium Co. Ltd. of Hamamatsu, Shizuoka, Japan, with offices in Torrance, Calif.

Elysium has written a variety of translators that convert one native CAD format into another. It recently formed a relationship with CoCreate Software Inc. that allows CoCreate OneSpace to work with the native environments of several major CAD systems, including SolidDesigner, I-DEAS Master Series, Pro/Engineer, UG, and Catia. Through an Elysium interface, files from several CAD vendors can all work in a OneSpace collaborative session without going through a translation step. OneSpace session participants can view, mark up, and change geometries on any of these CAD files.