How Model-Based Definition Can Fix Your CAD Models

Engineers may waste a significant amount of time fixing CAD models at the cost of productive design efforts. Unfortunately, the current conditions of CAD models may be—or could be—roadblocks in your quest for successful design reuse.

Fortunately, an evolving solution for authoring product definition and design intent, sourced at the CAD models, is gaining recognition: model-based definition (MBD), a foundational element of a comprehensive initiative called model-based enterprise (MBE) that brings the benefits of good, complete and usable CAD models to an organization and its suppliers.

For this post, Jennifer Herron, CEO of Action Engineering, will share her expertise on changing drawing-based cultures with uncontrolled CAD models to complete and usable 3D models. Herron is the author of Re-Use Your CAD: The Model-Based CAD Handbook. She promotes using proper modeling techniques to achieve a return on MBE and reusing CAD models in as many downstream applications as possible.

Your company’s ongoing quest for successful design reuse could hinge on several potential roadblocks having to do with collaboration, culture and lack of usability, which will all be the focus of this article.

The need for complete, usable product definitions, be it 2D drawings or 3D models, is universal. As companies strive to be more competitive, productivity becomes a higher priority. Reexamining the usability of both 2D and 3D product definitions has been underway for more than 15 years. Yet, the discrete manufacturing industry has only now begun to require more rigorous standards for the creation, authoring and publication of 3D CAD data with all the dimensions and tolerances that fully describe the part being modeled.

Various standards have evolved to provide consistency in developing product definitions:

•  ASME Y14.41 “Digital Product Definition Data Practices” builds on the drawing-based standard ASME Y14.5 “Digital Product Definition” that includes geometric dimensioning and tolerancing to expand the rules, symbols and mathematical definitions from drawings to CAD models.
• ASME Y14.47 “Model Organization Schema Practices” tackles definitions for a “complete” model and the need for semantic (digitally associated) data geometry and tolerances. This standard addresses model-only definition without the hindrance of also accommodating for a 2D drawing.
• Quality information framework (QIF) describes how the data should capture and exchange key characteristics and critical safety items. It is an open-source, free American National Standards Institute standard supporting digital thread concepts in engineering applications, ranging from product design through manufacturing to quality inspection. Based on the XML Standard, it contains a library of XML schema ensuring both data integrity and data interoperability in MBE implementation.

These standards are the keys to building a complete, certified model, with an unambiguous definition that avoids human misinterpretation that causes scrap and rework.

Herron elaborates on the model shown in Figure 3:

“MBD is the practice of adding product manufacturing information (PMI) to 3D models for a ‘complete’ definition of annotations and attributes. By complete, I mean meeting key criteria, both technical and cultural, that include adhering to external standards, such as ASME and QIF. Another is complying with all internal design and modeling best-practice guides, such as how to setup CAD files, create CAD geometry and determine the amount of annotations displayed within the PMI. Of course, passing internal review/governance processes and gaining acceptance by all downstream users, internal and supply chain partners . . . all these processes are also critical.

Be aware that the trouble with those standards is the lack of agreement and what is a ‘complete’ product definition for MBD. ASME Y14.47, soon out for public comment, crafts language that will assist organizations to set expectations for upstream and downstream consumers of MBD data sets.

So, the introductory visual represents a typical unusable product definition. In contrast, note the completeness of the part shown in Figure 4. Adding full dimensions to facilitate human manual recreation of the part geometry is not necessary. Why? The 3D model geometry already defines the geometry that is basic, unless otherwise specified in the product definition. Manufacturing tolerance allowances are essential and key characteristics for first-article inspection or production-level inspection criteria.”

 According to Herron, the basic values of MBD that catalyze extensive benefits are clear, repeatable and unambiguous communication in a single source:

“Improving collaboration facilitates sharing the critical information among the stakeholders responsible for design, review, approval, sourcing and manufacture. Suppliers gain the 3D visualization with manufacturing requirements directly affixed to the affected part’s features. 

Also, good (usable) models reduce the wasted costs of preventable scrap and rework due to knowledge capture of design intent and to higher levels of verification before producing the parts. Let’s not forget the accepted axiom that changes to a work-in-process may be as high as 10 times the cost of prevention. For example, take complex geometry such as a lofted airfoil or a highly curved autobody. This is complex geometry and good models would reduce part count and assembly time. Benefits of MBD against the pitfalls of a drawing-centric process can and should be measured.

And recognize that you get increased ability to search the model data and save time in searching for the correct product and information about that product. When all components of an assembly are ‘completely’ defined, including appropriate metadata such as part number, description, design change, material and more, linear storage of these files is no longer necessary, increasing the value of the single-source database.

These everyday benefits certainly contribute to strategic benefits that interest your C-level personnel, such as shortening the design cycle with design reuse. But what about you? Are you one of those engineers forced to work late or on the weekend fixing design data that is not complete and usable?

We previously identified MBD as the “foundational element of a comprehensive initiative.” As the visual in Figure 3 implies, we must evaluate MBD in the context of MBE—the organizational environment in which MBD would be expected to flourish. 

Isn’t the essence setting standards and sticking to them? Herron responded:

“Bear in mind as you develop an initiative that they are simplistic at the moment and do not cover all situations. We still have big gaps between standards and instantiating measurable and repeatable design processes. Recent standard work is closing those gaps. 

But, we both know, as I expect many of your readers have experienced, cultural dynamics must be emphasized over technology. You do need to wrap your head around the technology and then plan a justification process to instantiate the benefits of MBD. Focus on what MBD can do, not what MBD can’t do in a 2D drawing land.

Consider and plan the critical activities that will ensure an accepting environment: assessing your current situation, developing test scenarios, investing in education and training, formulating a schema pertinent to your environment and building a roadmap.

In the long term, the benefits will outweigh the costs. Patience will be a virtue, as the benefits will take time to surface. Think of the current mantra about systems: ‘a journey, not a destination.’ Roles need to be well defined to ensure accountability. Continuous feedback will be critical to identify and modify standards and schema as needed. Lastly, ensure C-level backing of your project before starting the long journey.”