The Black Arts, aka GD&T

Why is it so misunderstood?

I have heard people say that geometric dimensioning and tolerancing (GD&T) is a black art, and I agree—it can almost seem that way sometimes. My experiences in coordinate metrology throughout the past 20 years demonstrate a widespread lack of understanding, confusion, and even resentment for this international engineering standard, which was meant to make things crystal clear and save everyone time and money.

Before I delve into that, let me dazzle you with a brief history lesson on the origins of GD&T, as I understand them.

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Comments

Useful resource for understandng GD&T

I have found the book "The Quality Technician's Handbook" by Gary K. Griffith sheed a loot of light in the GD&T "black magic". It has helped me understand a lot form the inspector point of view with very real world examples.

Full specifications

Quite often customers are not necessarily better, more educated, than their suppliers. Problem comes from failling to see that specification is not only from the drawing, like +/- 0.1 mm, but include the quality contract with creasy Cp/Cpk and Ppk targets. Starting by drawing spec and considering capability requirement, we can estimate the resolution of the measuring equipment needed just to realize that we do not have it or even sometimes it does not exist. Incompetent designers and buyers think that high capability requirement is good quality. It is mostly high cost.

GD&T

Very appropriate article, and looking forward to more to come. More designers contract out their designs to be manufactured coupled with being younger less experienced individuals. And use more standards for dimensioning and tolerancing. Sometimes near not manufacturable, and unnecessarily costly. Topped with 6 sigma expectations; expecting cost reduction. Would welcome suggestions on how to convey this to customers.

College Course that includes GD&T and metrology training

Bill, I thought your comment about offerings lacking GD&T training with metrology interesting. My college, Corning Community College, a SUNY school in upstate NY has a course MECH 1570 - Metrology that includes both components. The lecture component covers the 2009 ASME Y 14.5 standard and the lab teaches measuring characteristicas that have size (unrelated tolerance zones) as well as related tolerance zones using a CMM with PCdmis and several different types of physical datum feature simulators.

Our advisory board members find this education very valuable.

John Longwell

Professor and Technology Program Chair

Corning COmmunity College

Senior Technologist Y 14.5 - 1994