MCAD Modeling Methods: Data Exchange Tips and Tricks14 Sep, 2004 By: Don LaCourse
Web site collects advice for 3D CAD users.
This month's focus on data exchange provides an excellent opportunity to share some of the 3D modeling information I've collected for my new Web site: www.3DCADTips.com (figure 1). During my 25 years of experience, first as a 3D CAD designer and then as a writer, I've accumulated hundreds of technology tips. Some are useful regardless of what application you use. Others are specific to products such as Alias Studio Tools, Alibre Design, Pro/ENGINEER, Solid Edge, thinkdesign, and VX CAD/CAM.
Figure 1. 3DCADTips.com contains more than 270 tips on topics such as implementation, construction and editing techniques, performance issues, PDM, design collaboration, data exchange, and documentation.
You can register and submit your own tips for the next update.
Figure 2. Each CAD system supports a subset of the IGES standard. The entity types supported by both systems are mapped from the sending system to the receiving system. Other entities within the subset may be ignored.
How Data Loss OccursNeutral-format data exchange standards such as IGES and STEP are extensive in structure and scope. This is because they attempt to support a varied field of disciplines—mechanical design and AEC are only two among many. During the exchange process, a 3D model file is translated from one native CAD format (sending system) to an IGES or STEP file. This file is then translated into another native CAD format (receiving system).
Because of the size and scope of the standard, mechanical CAD systems support only a subset of the standard. While System A may support entities a, b, c, d, and e, System B may support only entities a, c, and e. So, you ask, what happens to entities b and d? System B ignores those entities because it doesn't support them. This is one way in which data can be lost (figure 2). See www.3DCADTips.com for more about data loss during 3D CAD data exchange.
Can Data Accuracy Change?Accuracy can change in many ways. Starting from the native sending system, what you see on the screen isn't necessarily what's in the 3D database. 3D modeling kernels can force geometry to connect or treat elements as though they connect even though they don't. Neutral-format data exchange can expose these inconsistencies.
DATA EXCHANGE CHECKLISTS
IGES File Format FactsDavid Mattei contributed these interesting facts about the IGES neutral file format.
- 1. The number of entities in the IGES file equals the number of DE (directory entry) records divided by two.
- 2. You can identify an entity by its DE number.
- 3. An entity's DE number is always an odd number.
- 4. The DE and PD (parameter data) for an entity contain pointers to the entity's location in each section of the IGES file.
- 5. In general, you can delete an entity by replacing its entity type number with type zero in both the DE and PD sections. Do this only as a last resort. Editing IGES files by hand is typically not recommended. The file format and syntax may become corrupt and render the file unusable. I recommend that you always use an IGES utility if you attempt to edit IGES files.
Figure 3. 3D modeling kernels can force geometry to connect even though it does not. Note that the conditions portrayed in this figure are enhanced for better understanding.
ASP DATA EXCHANGE TIPSAlways test any ASP-based (application service provider) data translation service before using it in a production environment. The test should include files from actual transfer participants such as customers and vendors. Scope the test to those geometry types that are similar to what you expect to encounter in production (don't try to test all data types). The tests should also take into account the ASP's policies and procedures.
Customer and Vendor TestsTest the ASP with each customer and vendor you plan to exchange data with and with each different 3D CAD system. A single customer or vendor may expect to exchange data with more than one 3D CAD system. Be sure to exchange a paper print with dimensions along with the 3D CAD data so that you can independently verify accuracy.
asp-based data exchange sites
Measuring ResultsAfter establishing the test requirements, determine how you'll measure the results. Allow for a certain number of errors if they occur in areas that are not critical to the form, fit, or function of the part.
Test Case CharacteristicsConsider these characteristics and issues when you select or develop a test case. Be sure that it:
- 1. is relevant to your business,
- 2. is developed and used on systems consistent with those you normally use,
- 3. is developed with design methods similar to those used in your business,
- 4. covers many and ideally all of the entities and constructs you normally use or anticipate using, and
- 5. takes advantage of the capabilities of your systems to influence and record configuration and switch settings.
Also be sure to record your test results and save them for future reference.