Tuesday, September 16, 2008

Coexistence Strategy & Interoperability Best Practices for 3D CAD


In today’s product development environment, it is not uncommon to find multiple 3D CAD systems in place at the same company. This can be the result of acquisitions, process driven requirements or perhaps user preferences.
With this white paper we simply want to review some best practices for a coexistence strategy where two or more 3D CAD systems may exist in on environment or company.

Coexistence can mean different things to different people. It is very dependent on your process. In some cases coexistence can simply mean that data from different sources are being managed in the same database. While in other cases it may refer to data level interoperability between multiple systems. This paper is focused on the later.

Important Facts:

  • “History trees” or “feature trees” cannot be translated to other CAD systems. A feature tree contains things like 2d sketches, parameters on the sketches, parameters defining a 3D operation and the parent/child relationship of the operations or “features”. There is no industry standard for the translation of this type of data. Very specialized feature tree translators are available, but are very costly and marginally successful.
  • Today geometry is the only common data between 3D CAD systems. Translating geometry is common and possible through standards like IGES, STEP and a few others. Custom, or specialized geometry translators are also available that can improve the success rate of a translation.
  • To translate a 3D solid model, all edges must connect to form a “water-tight” solid model. Connectivity is critical. Without connectivity, a solid model cannot be formed and what is left is a set of unconnected surfaces and/or edges.
  • All edges have a start point and an end point. The start point of one edge must match the end point of another in order for it to be “connected”. The “accuracy level” of a 3D modeling system determines how close 2 points need to be before they can be considered one point, or “connected”.

Most CAD systems run at different accuracy settings. This fact greatly complicates the successful transfer of geometry. Some may run at 1e-06mm while others may run at 1e-04inch.

Best Practices and Suggestions:

Set a company standard for geometry accuracy. This is not a trivial task, but will be the most important and effective step to take in developing a coexistence strategy. It may involve raising the accuracy of one CAD system, or perhaps lowering the accuracy of another. Understanding the impact of accuracy settings is complex. Consider the following:

  • CAD systems that allow for uniting, subtracting and intersecting 3D parts with other 3D parts will require that all parts involved in the operation have the same accuracy setting. Many 3D modeling systems don’t allow this type of operation and as such utilizing geometry of a different accuracy level may not be an issue.
  • Consider downstream operations that leverage the 3D models. Such as tooling. What accuracy level is required to effectively communicate geometry to the CAM system.
  • Consider other outside suppliers and partners. Is there a common level accuracy that can be agreed to?
  • Increasing the accuracy with some CAD systems will greatly impact the robustness of the system. Many CAD systems run at a lower accuracy setting to increase the ability to perform complex geometry operations. A blend, or round, will work at the default lower accuracy setting, but the same blend may fail at a higher accuracy setting. Push your CAD vendor to make high accuracy modeling more robust. If a failure like this occurs, it is a defect.

Choose a standard format for geometry translation. The common choices are IGES, STEP. The ACIS SAT and Parasolid transmit file formats are also good. Also consider that for both IGES and STEP, there are many different configurations available. Each CAD system may better support a particular configuration over another.

  • IGES is the most simplistic geometry exchange standard. Because of the many different configurations possible in IGES, successful “connected” translations may be very challenging, especially if there are significant differences in geometry settings between the sending and receiving systems. It is very important to know what configurations the two systems are expecting.
  • STEP is by far more successful than IGES at capturing the connectivity of a solid model. There are different configurations such as AP203 and AP214, but it is much more rigid than IGES. STEP is also capable of exchanging much non-geometrical data such as assembly structures, part colors and other attributes, 3D dimensions, notes and parameters.
  • Custom translators will of course provide the highest rate of success in exchanging 3D connected geometry. They are still susceptible to variations in geometry accuracy but can be much more “forgiving” of inaccuracies in the geometry, and are tailored to the exchange of geometry between two specific CAD systems.

Most all CAD systems also provide the ability to “heal” geometry inaccuracies on import. Check with your CAD supplier and understand what the capabilities are and how they work.

Many CAD systems also allow direct user interaction with unconnected geometry in such a way that the user can effectively close gaps that cannot be automatically closed or “healed” during import. Be sure to understand what capabilities are provided by your CAD vender for doing this type of work.

With a common geometry accuracy level and a determined best exchange format, geometry exchange can be very robust. You can now consider automating the translation of part data based on process requirements.

  • Consider using PDM related automation technology to perform translations based on check-in, state-change, or some other manual or automatic trigger, regardless of which CAD system it comes from.
  • Notifications can be setup if required to keep the project team synchronized.
  • Use a PDM system to help keep all data organized properly.
  • It is best to use PDM capabilities that will allow for multiple documents (models, neutral files, …) to be related to a single part. This will allow you to associate multiple CAD native formats and the related neutral formats to a single part object within the PDM system.
  • The neutral files (IGES, STEP, x_t) can be linked to the part within the PDM system and accessed from any CAD system.

Other Challenges:

Geometry translations should be done at a part level, not an assembly level. Translation at an assembly level results in a single file that contains all parts that make up the assembly. As a result, coexistence at an assembly level, especially with formal data management, can be very challenging. It depends on your requirements for coexistence. Requirements for access control and revisioning/versioning will be critical to understand before defining an assembly level strategy. If it is desired to translate all parts within an assembly, it may be best to consider a batch translation of each individual part. If it is required to translate the assembly “structure”, you will need to define acceptable practices for the management of the results.

Most typical “history-based” modeling systems have very little ability to manipulate translated geometry as there will be no history, or feature tree. The translated geometry will come in as one feature with no parameters, rendering the model non-editable.


Keep in mind that a successful translation does not mean that the receiving system is superior to the sending system. It simply means that either the sending system is sending higher level accuracy data, or the receiving system is effectively healing the geometry. However, if the results on the receiving system are a collection of unconnected faces and edges, it is very likely that the sending system is sending geometry at such a drastically lower accuracy level than what the receiving system is expecting that the receiving system cannot heal the gaps without making unrealistic assumptions. All CAD systems have reasonably good geometry healing capabilities, but that can only go so far. Pay close attention to geometry accuracy.

Coexistence of different 3D CAD systems can be challenging, but it is possible with some careful planning in context with your product development process and requirements. Some trial-and-error will be required to best determine a suitable and acceptable accuracy level. The same is true when determining the translation format that produces the highest level of success.

For more information about CAD interoperability check out my article in Machine Design: http://machinedesign.com/ContentItem/72275/AnINTEROPERABILITYUpdate.aspx


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