The Argument Against Model-Based Definition

Right from the time of the ancient pyramids and the classical Parthenon to the geodesic domes and space stations of today, technical drawings have been playing a fundamental role in constructing humankind’s progress. They carry clear and concise information without misinterpretation or assumption, remaining the traditional way of instructing what to make and how to make it. A set of technical drawings delivers all necessary information explicitly, requiring no further instructions.

However, with recent advances in technology to shorten product development time, trends are shaping up to revolutionize the tradition of communicating design and product manufacturing information (PMI). And model-based definition (MBD) is precisely the driver in this direction.

In MBD, the 3D product model is the sole carrier of comprehensive information, intended to eliminate the need for 2D drawings. The MBD dataset fundamentally consists of a 3D solid model with all geometry features as well as 3D annotations of required tolerances and manufacturing instructions, to specify a complete product definition.

The American Society of Mechanical Engineers issued the ASME Y14.41 standard for annotation of 3D models and utilization of 3D data as manufacturing information as early as 2003, in anticipation of 2D drawings being phased out. While MBD adoption does appear to be the next logical step toward achieving reduced time-to-market and higher product quality, MBD looks naïve in present conditions. Manufacturers are still struggling to transform their operations digitally. However, some intriguing conversations are happening within the engineering community on the demise of drawings, as best-in-class manufacturers shift their focus from a document-centric approach to a model-centric approach, which, again, is nothing but model-based definition.

[Learn more design hardware & software trends and developments at Pacific Design & Manufacturing, Feb. 9-11, at the Anaheim Convention Center.]

There are some successful stories about MBD, but the question about mass adoption of 3D PMI is still persistent. To some extent, there is MBD in the aerospace industry when looking at the examples set by leading companies such as Boeing, Northrop Grumman, and Lockheed, which turned their processes digital entirely. However, a trained draftsman at Boeing explains how MBD has added another layer of complexity over the already complex custom PLM system the company uses, in an attempt to make MBD work.

Why Isn’t it Working?

2D drawings actually work quite well. We still use them because they have remained effective in communicating the design intent clearly. A 3D PDF consisting of a model and engineering drawings serves the purpose efficiently, at least for the shop-floor people, where the actual manufacturing occurs. It is evident that as long as people are involved in the process, we are going to need drawings. Looking at the problem another way, prevalent instability issues, such as data integrity and software migration, reinforce the need to generate engineering drawings. Both 2D drawings and 3D models are needed today, to accurately communicate the PMI.

The biggest hurdle for MBD remains the cultural change required within and outside organizations. Individuals working with the same patterns and processes are more likely to continue working in the same lines, making it difficult to implement MBD. Also, the idea of developing an annotated model sounds quite impractical when you have a need for old parts in the design whose product definitions lie in drawing sets.

Authoring tools used today are also adding to the complexity of implementing the MBD approach. For it to work seamlessly, MBD datasets will have to adopt the properties of a document to assume all of the functions and roles of engineering drawings. Software tools also need to communicate with stakeholders in the respective languages of their domains. This demands domain-specific modeling languages and visualizations, in order to define the model unambiguously, so that every stakeholder associated with the lifecycle interprets the information correctly.

Shifting to a drawing-less environment is indeed a right step in the direction of achieving cost-effective manufacturing and product delivery objectives. Yet, the transformation process is more dramatic and painful, compared with what software vendors and MBD supporters are suggesting today. With the ASME Y14.41 standard, the movement does appear to be on the right track, but the completeness of these standards is still in question. Eliminating 2D drawings completely from the product development lifecycle seems quite unrealistic considering present situations where people are likely to resist the change. While several companies do have 3D PMI in place, they are still figuring out the benefits.

Nikunj Patel is a design engineer working with Hi-Tech CADD Services for the past four years. He loves designing specialized industrial equipment and can always be found in the lab discussing, brainstorming, and tweaking designs. He has also worked on architectural projects, with interest in every aspect of design and analysis.


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