Leveraging Model Based Definition (MBD) for Fast and Accurate Tolerance Analyses

Let’s face it – few mechanical engineers and designers enjoy performing tolerance analyses of their designs. It’s not that the calculations are necessarily difficult – in many cases the math involved is similar to balancing a checkbook, it’s just usually a tedious process. Understanding the parts involved in the stack-up so that you know what drawings are needed is the easy part. Actually being able to get all the drawings, especially if you’re using purchased components in the design, can be challenging. Understanding the logic of how features are controlled with dimensioning schemes defined by someone else is often difficult as well. The use of geometric tolerances via either GD&T or GPS controls does make understanding a bit easier, but then finding the referenced datum feature on a complex, multi-sheet drawing soon feels like helping your son find Waldo over and over again, though far less fun.

No, performing tolerance analyses is not what we envisioned would consume much of our professional lives as we sat in the thermodynamics, dynamics, fluid mechanics, advanced mathematics, and all the other courses ending in “ics” immersing ourselves in knowledge that would help us one day design products that would awe. In fact, as a new graduate with not only 4 years of collegiate academics under my belt but also 2 ½ years of work as a co-op student at a textile manufacturer, a national laboratory, and a defense contractor, I had never performed a tolerance analysis nor knew the language best suited for specifying permissible variation on part features, GD&T/GPS, upon starting my engineering career.

Many, many engineers would say the same. Few learn the methods for performing tolerance analyses as part of a mechanical engineering curriculum despite the fact that most will need to perform such analyses during their careers. Informal surveys of people interested in tolerance analyses solutions yield some interesting statistics:

  • Over 90% perform tolerance analyses at least monthly, and ½ of that group do at least weekly
  • Over 70% use only spreadsheets for analyzing tolerances
  • Over 60% specify tolerance information only on the drawings
  • The top 3 reasons for looking into a tolerance analysis solution are:
    1. Improve the efficiency of analyzing tolerances (> 90%)
    2. Reduce manufacturing problems caused by tolerance issues (>80%)
    3. Improve the integration of the tolerance analysis tool and CAD data (>75%)

Clearly tolerance analysis is an important aspect of product design despite the lack of emphasis placed on it by most universities. People recognize that it is directly related to the ease with which products can be manufactured and hence impacting the products’ profitability to the company. The data also suggests using spreadsheets with tolerances only documented on drawings isn’t working well for many. Fortunately about 75% of the respondents work for companies that have initiatives to move to Model Based Definition (MBD) within the next 5 years, so a better approach is on the horizon.

It’s important to note what MBD is and what it isn’t. Many think of MBD as a drawingless environment in which all information is transferred and displayed electronically. While that is the long-term vision of the Model Based Enterprise (MBE) which utilizes MBD, much like the aspirations of a paperless work environment described many years ago, the pragmatic implementation of MBD is to transition from an environment where drawings are the master definition of the design to one in which the model file is the master, and drawings are created as needed for specific purposes. In this environment, though, drawings need not contain ALL information pertinent to the design but instead just information sufficient to meet a specific need. Multiple different drawings may be generated from the same CAD model for different purposes. Many companies today are already on the verge of transitioning to an MBD environment by having drawings with just a few specific, critical pieces of information shown and a note indicating that for all else the model should be used.

How does MBD help with the tasks of performing tolerance analyses? A MBD technical data package includes all of the part requirements and assembly definitions (i.e. how the parts are assembled) needed by manufacturing to generate the product. It includes not only the definitions of the nominal geometries, but many other things such as permissible deviation from “perfection” on all of the features, the materials to be used, information useful for tracking the parts throughout the product life cycle, descriptions of how the parts are combined into assemblies, critical parameters to monitor during production closely tied to the functional requirements of the product, and much more. It is not only defined electronically, but the relationships are also maintained. For example, the tolerance for a feature must be “linked” to the feature within the MBD definition so that the downstream processes can programmatically relate the two. It is this very detailed definition of the permissible variation, surface relationships, and part relationships that can be leveraged by tolerance analysis solutions to automate the analyses.

The video below compares performing a tolerance analysis using a spreadsheet with drawings and using CETOL 6σ, an add-on tolerance analysis solution for SOLIDWORKS, with models that contain functional mate definitions and tolerance information. The video shows that even in this simple analysis consisting of only six contributing tolerances the CETOL 6σ approach is significantly faster than using a spreadsheet. Moreover, when differences exist between the two results, only CETOL 6σ has the additional tools to help the user understand the results of the study visually. Not only was the analysis completed faster using CETOL 6σ, but unlike the spreadsheet the results were accurate, and visual tools help the user understand why.

Like other historical, significant changes to design processes the transition to MBD will involve many challenges, but there are numerous benefits motivating companies to transform. The ability to automate tolerance analyses, a task companies struggle with today, is just one, but a substantial one. Of course engineers don’t have to wait until their companies fully embrace MBD before benefiting from a solution like CETOL 6σ. Companies have been using it for many years to understand the impact of tolerances only defined on their drawings and gaining a much deeper understanding about their designs than what is possible using spreadsheets.


Stephen Werst

Stephen Werst

Stephen Werst is the Product Marketing Manager for Sigmetrix, LLC, Global Experts in GD&T and Mechanical Variation. His earlier work at Texas Instruments and Raytheon Systems as a mechanical engineer in the Tolerancing Center for Excellence laid the technical foundation for his expertise in the fields of variation analysis, GD&T, and Design for Six Sigma. His later work at Hewlett Packard as a software Product Manager established skills he uses today to help companies understand both the technical and business benefits achieved by the software Sigmetrix provides: CETOL 6σ, GD&T Advisor, and Tolerance Analysis Extension.