Parlez-vous GD&T: A Quick Intro to Geometric Dimensioning & Tolerancing [Interview with Chris Wilkes]

Written By: Cat McClintock
  • 6/25/2017

In general, when engineers create or modify a design, they do much more than show lines, holes, curves, or even solids. That’s because geometry alone can’t communicate everything needed to turn a design on paper (or on the screen) into a real-world object.

So, traditionally, engineers create a 2D drawing with notes (or annotations) showing dimensions, materials, tolerances, etc. Then, anyone who needs to make or assemble the parts can find all the information they need with the geometry. 

Of course, these days it’s a bit tedious if you designed your part in a 3D CAD system to then generate annotated 2D drawings. So, now design engineers often add those same annotations directly to the 3D model—getting rid of 2D drawings all together. This is often at the heart of what they call “model-based design.” But we’re getting ahead of ourselves.

The Language of GD&T

If you’re new to all this, here’s a quick introduction to how engineers communicate information with their drawings and models. They use a symbolic language called geometric dimensioning and tolerancing (GD&T).

What is GD&T exactly?

Chris Wilkes, President and CEO of Sigmetrix, recently answered that very question. Watch:

GD&T communicates the “allowable amount of variation” in a model dimension says Wilkes—something every manufacturer needs to know before they set up machines and start production.

Here’s why it matters:

In short, GD&T determines the precision (and ultimately the expense—it’s usually a tradeoff) of the physical part. How much variation can you allow in that gear tooth? 5 millimeters? 5 nanometers? GD&T can communicate your answers, even when you’re not around.

GD&T Advisor new in Creo 4.0

While GD&T seems like a good idea, the truth is that manually adding and updating GD&T annotations can take time and introduce errors. With the release of Creo 4.0, Sigmetrix and PTC partnered to offer GD&T Advisor. The tool automates and verifies your work, creating a faster, easier, more accurate way of adding annotations to 3D models. We’ll talk more about it in upcoming blog articles. Until then check out our new e-book…

Ready to Learn More?

It’s easier to create, maintain and share product information when you’re using one model as the authority throughout product development. That’s why MBD is quickly becoming the preferred design approach. Download the free e-book from PTC to learn more about how MBD can help you save time and reduce errors.


  • CAD
  • Retail and Consumer Products
  • Connected Devices

About the Author

Cat McClintock

Cat McClintock edits the Creo and Mathcad blogs for PTC.  She has been a writer and editor for 15+ years,  working for CAD, PDM, ERP, and CRM software companies. Prior to that, she edited science journals for an academic publisher and aligned optical assemblies for a medical device manufacturer. She holds degrees in Technical Journalism, Classics, and Electro-Optics. She loves talking to PTC customers and learning about the interesting work they're doing and the innovative ways they use the software.