Your CAD Models Are Busted

March 9, 2000
Bad CAD models and neutral-format geometry cost the automobile industry alone $1 billion per year. Here's how to spot and stamp out the problems.

Senior Editor

The best place to solve geometry problems is in the designer's computer. A Pro/E user, for example, has just run Prescient's DesignQA software on an unfinished model. The software tested the part for the design company's best modeling practices. Tables list the results of each test and a quality score. The highlighted item, Edges and face gaps, indicates what the software found and shows exactly where they are on the model. Gaps can be healed automatically or with user intervention. The software also works with Unigraphics, Catia and, soon, SDRC's I-DEAS.

Body healing is one of Cadkey 99's new interoperability tools. The models show before-and-after healing versions of the same part. Solid-body healing helps users repair less precise imported model geometry and common problems such as missing and duplicate surfaces, bad surface normals, and warped and self-intersecting surfaces. The tool works well for manufacturers and moldmakers who receive flawed data from clients, suppliers, and other CAD systems.

Red lines in the model behind the CADfix menus mean some edges are associated with only one face. If the crankshaft is to be a watertight volume, all edges must share two faces. The Repair stage can be set to run automatically, as it is here. The Progress Monitor shows software actions. The transform stage from the Wizard is an option. It lets users defeature or simplify models for finite-element analysis which might not need details such as small holes or fillets.

Geometry Fixer menu lets users repair items below the Select Problem label one by one. Red in the stop-light coding means the condition is uncorrected, green means it's fixed, and yellow indicates questionable conditions for items that might not matter. Integrity refers to closeness of edges. A solid or enclosed volume needs good connectivity and integrity. A step in the Topology section can orient all surface normals to point outward.

One way for Algor FEA users to avoid translation problems is with InCADPlus. The most recent version of the software reads part files from Solid Edge into the FEA program without loss of detail. Other versions work with SolidWorks, Pro/Engineer, and Mechanical Desktop. A filter in the software eliminates details not needed for analysis, such as small holes or fillets. Since companies have combinations of the mentioned CAD programs, low-cost plug-in modules let several CAD packages send flawless models to the developer's FEA program.

CAD/IQ from ITI, Milford, Ohio, is a CAD model quality testing tool developed especially for designers. It corrects model defects such as loop-orientation inconsistencies, and missing and selfintersecting geometry. It finds tolerancesensitive issues such as gaps between entities (adjacent vertices, edges, and faces). The software also looks for errors that make a model unmanufacturable such as cracks and voids, knife edges, and narrow steps. The software works with Pro/E, Catia, IDEAS, Parasolid, and STEP.

Complex models such as this automotive body shell are the class of models handled by Parasolid's PS/Bodyshop tool kit.

CADhealer from Theorem Solutions, Cincinnati, provides a data exchange and decision support tool. It can analyze native CAD models, STEP and IGES files, and report on problems associated with the data.

The software has analyzed a model for errors and found several on a nut. The Dialogue window shows the model flaws as four self-intersecting faces. These are highlighted on the model. The software also presents users with a range of options to repair and heal the data. The healer works on the geometry and topology of trimmed surface, solid models, and on wire-frame data.

The CAD models and IGES files you last sent to suppliers and clients are not the geometric gems you imagine. To name a few problems, the geometry probably contains gaps, duplicate edges, slivers, and missing surfaces. These flaws can cost the people who receive them a week or more of repair time before they begin real work.

Peter Bessey's experience with unusable CAD models, for example, is nottoo different from that of your downstream users. As a partner and designer with Hothouse Product Development Partners in the U.K., his teamdeals with files from almost everyimaginable CAD program. "One particularly annoying model assemblywas the product of several CAD systems and a translation. Its three majorparts were built with many complexorganic surfaces, but the final IGESfiles proved impossible to export andview successfully in any of severalavailable CAD programs," saysBessey. The model either refused toload, imported partially, or evencrashed the system. "We couldn't evencreate 2D drawings from some of thecomponent files," he says.

As a test, Bessey sent the problem file to, an online service that claims to fix unusable geometry. In less than a day it returned an SAT file of the model with about 85% of its flaws corrected, according to the accompanying report. What's more, the model could now be opened in several CAD programs or by using the Spatial Model Viewer which was downloaded from the site. "Finally, we could see the usable surfaces in it," he says. The report and the viewer also pinpointed what the model fixer could not repair, useful information for later work, he says.

Bessey's experience isn't unique. Arecent audit conducted by PrescientTechnologies Inc., Boston, of over3,000 separate product models fromseveral companies showed that only225 passed the standards set by theoriginating company. That is, 70% ofthe models failed standards that companies categorized as critical.

"The audit only scratched the surface of data quality in engineering," says Gavin Finn, president of Prescient. "The overwhelming rate at which data failed to meet defined standards was consistent across all audits." Results are not limited to company size or market, the problem spans the entire manufacturing industry. It probably comes from the increased pressure to use digital data throughout the automated product development process, says Finn.

If you still think the problem is insignificant, consider a March 1999 study from the National Institute of Standards and Technology ( that says interoperability snafus from data quality errors in the automotive supply chain alone could tally to $1 billion/yr.

The problems have several sources. Many flaws creep into models translated from a CAD system with wide tolerances to one with tighter tolerances. Others come from poorly constructed "join" functions that only place objects in proximity to each other rather than actually joining them. General translation software can disconnect surfaces, create slivers, and generally "shake" apart badly constructed models.

But take heart. A spate of recent software tools have the smarts to find and fix many drawing and model flaws usually left for data users. When the recent technology cannot fix the model, the programs can tell where problems remain so manual repairs are more efficient. Another rediscovered tactic uses direct translators to bypass altogether neutral files such as IGES and STEP.

The best place to solve modeling problems is at the designer's computer. Software such as CAD/IQ from ITI in Milford, Ohio, and DesignQA from Prescient provides several ways to minimize errors before models or drawings leave the design department. "For example, a configuration process in DesignQA lets managers define requirements for a department or just the project at hand," says Finn. What makes a good model is a subjective question that may vary from company to company. "So the software includes a configuration wizard with about 160 best practices we've gathered from clients. The wizard lets users add or remove rules as needed. These might describe what gap dimensions can be tolerated or what layer particular drawings elements should be on," he says. Rules can also be nongeometric, such as specific manufacturing techniques for an exotic material.

The next steps include detection, assessment, and correction. Detection is done at the touch of a button and should be performed perhaps several times a day, not just before a release to manufacturing. Running the detection portion lets the software examine the model just as a doctor would a patient. It compares the geometry to the company standards to spot details that are out of variance.

The software then judges or assesses the importance of an infraction.For example, a model should not include construction geometry if it'sheaded soon to a structural analyst.But it's OK if the model is only 25%complete. "The system is smartenough to know at what stage construction geometry is not critical," saysFinn. A report lists the errors andtheir gravity.

In a third phase, users can fix problems automatically, with intervention from the designer, or manually. In the automatic mode, gaps can be closed or drawing type fonts changed without intervention. In some cases, the software prompts the user for information such as recommended feeds and speeds for an unusual alloy. The software may even present the user with a table to choose from. In other cases, if two surfaces on a model do not meet, and closing them changes the geometry, the user may have to first modify several other features before the software can make repairs.

The software corrects more thanflaws. "What we also found," saysFinn, "was that solid models frequently cannot be modified by otherengineers. Modeling is a widely usedskill and the range of methods allowedby systems sometimes tolerates building features 20 different ways." So it'snot uncommon to rebuild parts ratherthan modify them because the modifying designer does not know how thepart was originally constructed.

The QA software helps by looking at layering conventions and keeping parent-child relationships in part trees in line with corporate standards. Customizable rules in the software can look at history trees for how parts and features are put together. Once a second engineer knows that, modifications are made more efficiently.

ITI's CAD/IQ also concentrates on the intended use of the model. The software analyzes native CAD files letting users detect and correct hidden problems that spell trouble for downstream users. "Our studies reveal that recipients of CAD models typically spend 20 to 70% of their time with nonvalue added tasks or reworking models," says Don Hemmelgarn, vice president and general manager of ITI's product data interoperability business.

Those receiving bad geometry, such as manufacturers or finite-element analysts, can find relief in several additional programs. CADfix from ITI provides technology that repairs and heals a range of flaws generated or aggravated by file transfers and sloppy modeling. from Spatial Technology Inc., Boulder, Colo., delivers an ondemand repair service. Users are charged per megabyte of healed model.

Unigraphics Solutions, Maryland Hts., Mo., developer ofthe Parasolid kernel, includes Tolerant Modeling functionsin the kernel. The company has also recently introducedPS/Bodyshop, an add-on application, that combines tolerant modeling with new, advanced healing capabilities thatwill let software developers build better translators. Andseveral solid modeling systems, such as Cadkey 99 fromCadkey Inc., Marlborough, Mass., and SolidDesigner fromCoCreate, Fort Collins, Colo., include built-in functions forhealing imperfect geometry. Each has advantages.

In a nutshell, a CADfix user would first tell the softwarethe kind of file to be imported. The software then automatically performs a series of operations that locates problemsand repairs the model. It can then be exported or manuallymanipulated to clean areas not entirely corrected by the automatic pass. Users can modify settings of the repair Wizard or let the program's intelligence set them.

Preliminary repair at the import stage works with an adjustable tolerance value. It lets the model fixer bring together points or edges that are within the tolerance, therebyeliminating duplicate geometry. "The initial tolerance isrecommended by the software after a brief analysis but it'suser adjustable," says Carl Izurieta, CAD/CAM interoperability specialist with ITI. "In many cases, this step repairsmost flaws in the model," he adds.

In the Repair stage, the software pulls gaps together. Inthe manual mode, the user might manipulate tolerances tomake changes that won't affect design intent. "The usermust get involved when repairs are needed after the automatic session," says Izurieta. "No one yet has a fully automatic solution." A report afterward tells what may not havebeen fixed.

The Prepare stage gets the model ready for the targetsystem. Preparing is "flavoring," for example, for SDRCIGES. The transform option simplifies surfaces for FEA andNC work. For instance, several small faces can be connectedinto one.

"CADfix may not repair everything 100% of the time," says Izurieta. "But in many instances, a model improved 80% might work well for the user. Regardless, model repairs that once took two weeks can be cut to a matter of hours."

Users of software built on the Parasolid kernel from Unigraphics Solutions already have built-in Tolerant Modeling features. Software such as SolidWorks, Solid Edge, IronCAD, and Unigraphics can use Parasolid's Tolerant Modeling technology to apply different tolerances to each edge of a model to accommodate anomalies in imported geometry. When imported trimmed surfaces do not match the kernel's precise accuracy, the surfaces can be sewn together by asking the kernel to calculate appropriate tolerances that will optimize downstream performance and reliability.

Parasolid users can also take advantage of the tools in PS/Bodyshop totighten loose models. This recently announced Parasolid application integrates new healing technology withParasolid's Tolerant Modeling technology. "PS/Bodyshop provides severalcomplementary approaches for movingmodel data from one application to another," says Graeme McBean, Unigraphics Solution's manager of operations and consulting for Parasolid.

By using the local precision for allmodeling operations on the importeddata, Parasolid guarantees not tochange design-critical geometry, andthereby preserves the design intent ofthe original model. Preserving geometry in this way is often an absolute requirement technically or legally. In addition to general model healing, the repair software can identify and repairinaccuracies at every stage of a translation process.

PS/Bodyshop can deal with all the common anomalies that occur when importing data in either trimmed surface (IGES) or B-rep form (STEP). This includes problems such as zero length curves, coincident curves, gaps, spikes, slivers, and illegal intersections. Several healing functions are provided for each phase of geometry translation. The emphasis is on healing data as early as possible in the translation process. After data is read in, further healing functions can be used to reduce the complexity of the imported model or to restore design constraints lost when the model was exported. Surfaces can be converted to simpler geometry or adjusted to ensure their edges meet tangentially. The repair software also provides tools for adjusting a Parasolid model that might be sent out to another system.

In addition, Unigraphics Solutionshas recently announced plans to release standards-based bidirectionaltranslator tool kits, includingPS/IGES, PS/STEP, and PS/VDA-FS,as well as direct translators for proprietary formats, including PS/Pro/Engineer, PS/Catia, PS/I-DEAS andPS/SAT. These translator tool kitstake advantage of PS/Bodyshop internally to significantly improve the quality of translated data.

The last two healing programs here have roots in Spatial Technology. The company has built its healing algorithms into an optional husk. Cadkey Inc. has been one of the first to provide the healing functions in Cadkey 99. The algorithms are also available at, Spatial's Webbased operation that repairs solid models. After registering, users can send IGES, STEP, Catia, or SAT models to the system. Algorithms automatically fix the models. Senders are notified by e-mail of completion.

Each of these last two systems has advantages. For instance, a mold shop might use the CAD program to repair incoming flawed models, without incurring additional expenses. Using the Webbased service, on the other hand, requires no initial outlay and it gets updated with the latest healing techniques. Users are charged only for the geometry healed.

The Spatial Technology system takes models through preprocessor, geometry simplifier, stitcher, and geometry-repair phases. Preprocessing performs an initial cleanup by removing inaccuracies such as zero-length edges, sliver and redundant faces, and duplicate vertices. Geometry simplification converts shapes to their corresponding analytic forms. Part geometry is analyzed here for an acceptable tolerance, but users can change the value. Stitching connects several smaller faces into a larger one or all of them into a solid body. This involves pairing the vertices and edges in the data.

By their own admission, developers say their model-fixing technology is not perfect and models heal without flaws about 10% of the time. Nevertheless, most models can be quickly improved to more usable states. That reason alone makes the technology an important tool for reclaiming part of the $1 billion spent each year on model repair.



STEP Tools Inc., Troy, N.Y., develops STEP software toolsets based on ISO standards. A recent ST-ACIS Library for programming and desktop tools provides bidirectional translations of STEP geometry to ACIS-SAT formats. The company also offers a no-cost translation service at, for translating files from STEP to X_T, SAT, and XML formats.

Tests conducted by NIST (National Institute of Standards and Testing) show that STEP performs better in most cases than IGES. The report mentioned earlier showed a comparison of STEP and IGES transfers conducted by the Automotive Industry Action Group. Sixty-one data exchanges between Catia, Cadds-5, Pro/E, and Unigraphics used both STEP and IGES translators.

Test criteria included file size, face count, surface count, and surface area. Results showed that, on average, STEP translators conveyed 80% of the surface area of the original model surface, whereas the IGES translator conveyed on average only 69%. In 60% of the cases, STEP provided a better exchange mechanism, in 32% of the cases, IGES performed better than STEP, and in 8% of the cases, the two systems worked equally well.

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