If you are looking at Model-Based Definition (MBD), you must also consider Geometric Dimensioning and Tolerancing (GD&T) and how your organization will incorporate it.
What is GD&T? It is a set of symbology representing the mathematical definition that identifies the allowable variations of the actual part. Quality engineers and inspectors use this language to compare the engineering requirements against the actual fabricated parts’ measurements and determine if they are in or out of specification.
I have not taught an MBD class yet where a student does not ask about GD&T and how it works using CAD models.
If you are a GD&T expert, then you realize the GD&T symbolic language has evolved to capture part variation (tolerance) in 3-dimensions. Some want to get rid of GD&T, some want to add GD&T, and most are caught in the middle.
Here are a few tips for sorting out the puzzle.
Why do GD&T and MBD go together?
MBD and GD&T go together because we think in 3D, we design in 3D, and GD&T is a 3D method to document allowable tolerances. Why do you need to consider allowable tolerances? Because if a bolt goes through two parts, and the holes don’t line up, then the bolt can’t bolt those two parts together, and now those parts are scrap. Some industries have up to 40% scrap rates. We can do better! Enter MBD.
When you add GD&T using annotations to the CAD model and ensure those annotations are digitally connected to the representative features on a part or assembly, then the mathematical tolerance definition becomes automatable. Digitizing the allowable tolerance through GD&T enables powerful design simulations such as tolerance stack-up analysis, statistical variation for production optimization, and inspection.
With drawings, designers, fabricators, and inspectors must read, interpret, and re-enter data from the drawing into their respective analysis systems. That method is very time-consuming and error-prone. If the digital data persists in 3D, then that data empowers tolerance stack-up, statistical analysis, and inspection, thereby increasing the accuracy of data exchange and removing ambiguity. MBD with GD&T is a better method.
Can you do MBD without doing GD&T?
Yes, but it depends on your parts. If you do First Article Inspections (FAI) and Production Part Approval Process (PPAP), then GD&T is your best friend because it is the unambiguous method to digitally encoded tolerance requirements into your model. That digital data enables you to achieve 80% time savings during FAI and PPAP. If you stick with ambiguous plus/minus dimensioning, you are stuck manually interpreting a drawing and missing out on the automation potential.
What about my basic dimensions?
Often basic dimensions are not displayed on the 3D Viewable. With MBD, the CAD model (native or neutral) becomes the basic dimensions. That 3D geometric data is consumed directly into fabrication machines like CNC. If you still need a basic dimension to assist a manual inspection process better, then show it. Otherwise, leave off displaying the basic dimensions to save you time during product definition authoring.
If my drawings use plus/minus dimensions today, why can’t I put them in the model?
The purpose of GD&T is to define relationships between features in different parts. It is a 3-dimensional language meant to define how parts fit and interact together. You don’t build parts, you build assemblies, so it’s crucial to define tolerances to make sure the parts will fit together once assembled.
Plus/minus dimensioning is often referred to as two-point dimensioning. Inspectors pick two points on a part with calipers to measure the size of things. It’s great for measuring the sizes and locations of one element, but it doesn’t give you the full unambiguous definition that relates an entire feature to another entire feature. The geometric tolerancing method establishes a datum reference frame that creates an origin for the tolerances to reference. This robust origin provides a solid foundation to perform tolerance stack-ups.
A robust datum reference frame and unambiguous tolerance definition open the opportunity to perform a 3-dimensional tolerance analysis, which provides a more accurate, reliable, and trustworthy allowance for variation. Manufacturing processes don’t make perfect parts every time; they make parts with variation in that manufacturing process. Unambiguous definition variation allowance enables maximum allowance for manufacturing variability. Whether you’re making a lot of cheap parts numbered in the billions, or if you’re making a handful of parts, digitally represented geometric tolerances reduce scrap rate and increase yield. Allowing for more manufacturing variation, and understanding the relationship between your parts in an assembly and the features within those parts, opens up an enormous amount of opportunity to increase your production yields. All this is because of the 3D GD&T language.
How do the American and International GD&T standards work together?
A survey shows that greater than 65% of companies use the ASME Y14.5 GD&T standard over the ISO GPS suite of standards. (citations: https://www.gdandtbasics.com/iso-vs-asme-standards/ and https://krulikowskiconsulting.com/survey-results-on-the-usage-of-iso-and-asme-y14-5-standards/). But many companies have to deal with both. Here are some great survey results about who uses ASME and who uses GPS.
The good news, there isn’t much difference between ASME and GPS. The principles are very similar, and if you can author ASME GD&T, then authoring in ISO GPS will take some study but is doable. As with any task, the more you do, the better you get.
Most CAD authoring platforms support both ASME GD&T and ISO GPS, and the symbology used is almost identical. From an MBD standpoint, there is not any real difference.
I hope you consider authoring MBD for your components and assemblies where your existing drawing-based documentation methods give you a lot of trouble today.
Founder & CEO
Jennifer Herron, CEO of Action Engineering, spearheads a Women-Owned Small Business empowering 3D Data adoption and cultivating a future-focused workforce. With her adept team, she's counseled 180+ organizations since 2008, offering holistic coaching and training. A driving force behind Model-Based Enterprise (MBE) transitions through Model-Based Definition (MBD), Jennifer actively shapes industry evolution by engaging in leading standards bodies. Notably, she contributes to the Digital Metrology Standards Consortium (DMSC) Board, shaping QIF and ISO 10303 for digital twins. Possessing extensive expertise in hardware design for spaceflight and military systems, she's also the holder of a snake propulsion mechanism patent. Leveraging her diverse career, she's developed CAD modeling best practices, having worked across platforms like Creo, NX, SOLIDWORKS, and Inventor. Her pragmatic insights culminated in "Re-Use Your CAD: The Model-Based CAD Handbook," the singular guide of its kind.