The GD&T (Geometric Dimensioning & Tolerancing) protocol is rapidly becoming the standard for communicating engineering tolerances and allowable variation for product parts. It’s an elegant but not a simple system. Close to 50% of the GD&T drawings coming across my desk use the system incorrectly in whole or in part.
This is not meant to be a critical or accusatory statement. It’s simply an observation, as well as an indication that all of us in the machining and related industries need to take the time to learn the details of GD&T as tolerances get tighter, machining gets more precise, and competition gets fiercer.
Machining breakthroughs and software innovations can be overwhelming and it seems like we spend as much time learning techniques and processes as we do delivering for our customers. We all support numerous vertical markets around the country and the world. That’s why it’s more important than ever that we rely on a robust, common convention like GD&T for designing and modeling.
What are the most common problems I’ve seen with GD&T drawings coming in to Metalcraft?
The improper use of datums, such as orientation tolerances used without datums and datums listed without identified basic dimensions.
General notes on drawings that define electronic datasets to be the source of all basic dimensions, where the drawings have no or few X-Y dimensions.
A trend toward “partially defined drawings” (PDDs), which could indicate a positive trend toward using GD&T, but in many cases simply represents a failure to correctly apply GD&T fundamental drawing rules requiring completely defined, unambiguous tolerancing for every feature or surface on the drawing.
Position tolerances used in single feature control frames that list very tight tolerances which could be accomplished more easily by using multiple single segment or composite control frames.
Instances where parts with features that are tightly controlled for assembly purposes could be made more cost-effectively by the use of constraint notes knowing that the final products will be forced into their rigid configurations when assembled but fail a quality check in their free state, as per ASME-Y14.5Y.
Those are just a few. I’d love to hear your observations about the misapplication of GD&T.
After all, GD&T is not like coordinate or baseline tolerancing methods. It’s a whole new animal and we’re in a whole new era. It’s a complicated system of geometric symbology – a system that can describe, to any degree, the limits of form, fit, and function of an infinite set of physical circumstances.
But it’s not an intuitive system. GD&T is all about rules and defined symbols with specific translations. GD&T is “do it right or don’t do it.” It requires critical thinking skills to question drawing interpretations, before, during, and after parts make it through our shops and quality departments.
Starting with this blog, we hope to kick off a vigorous discussion about the implications of GD&T so that we all not only get a better feel for the system but can work together to promote its best and consistent use in our industry.
In upcoming blogs in this series, let’s collaborate to shed light on some of these problems and solutions. Specifically:
- What are some of the ways you see GD&T commonly misused?
- What are some of the solutions you’ve developed or observed for applying GD&T to especially complex situations?
If things go as I expect and hope, we’ll encounter disagreements and enjoy healthy, professional debate as we zero in on the best answers and practices. I’m looking forward to hosting this forum!
Chris Lindenberger has 30 years of experience as a process and project engineer and is the project engineer and estimator at Metalcraft Industries in Denver, Colorado. He teaches Precision Machining and Engineering at Front Range Community College. You can contact him at CAL@MetalCraftInd.com.