GD&T-A Beginning

Geometric Dimensions and Tolerances is a very broad and complicated subject to write about. There are complete books, websites, college courses, seminars, and high dollar consultants all dealing with GD&T. This column is dedicated to GD&T-an extremely important part of all mechanical design work, modern manufacturing, and inspection of parts. Of course, tooling is no exception and certainly more like the rule. This article will in no way cover all the aspects of GD&T-it won’t even try. It is just a beginning that will be developed over time.

History of GD&T:

GD&T was developed during the second world war in manufacturing facilities in England [1]. At the time parts using linear tolerances were being rejected as out of tolerance. It was determined however that these parts were still fully functional and that the position of the center of the holes were within a circle instead of a linear square.

 

Clearly, looking at the above picture you see that the parts are still good, usable, and completely functional; fit for purpose.  Since necessity  is the mother of invention -GD&T was created to help with manufacturing “errors” and “design intent.”

What is GD&T:

In the most basic sense GD&T is a language-a symbolic language. It is a language system that defines  and communicates engineering dimensions and tolerances. Like any language and system, there are rules, guidelines, and occasional reviews of those rules and guidelines. In fact, GD&T is a constantly evolving system that is becoming more and more refined over time due to ever changing technology changes in manufacturing.

There are several existing standard through out the world. The governing body in America is ASME which is the American Society of Mechanical Engineers. Their current standard is ASME Y14.5 – 2009; with the advantage of having this standard complete in one document addressing all of the issues within GD&T. There are ISO GD&T standards-however, they have a disadvantage to the end user because there are separate documents for the various areas of GD&T-which can lead to confusion and frustration when trying to interpret the various standards. Hopefully, someday the ISO standards will combine into a more manageable document.

What GD&T is Not:

Many times I have heard this phrase “our parts don’t need that type of precision that GD&T allows.” I think this shows are very poor understanding of what GD&T actually is and does for manufacturing tolerances. Let’s look at this statement:

-According to the ASME Y14.5-2009 standard, the purpose of geometric dimensioning and tolerancing (GD&T) is to describe the engineering intent of parts and assemblies. The datum reference frame can describe how the part fits or functions. GD&T can more accurately define the dimensional requirements for a part, allowing over 50% more tolerance zone than coordinate (or linear) dimensioning in some cases. Proper application of GD&T will ensure that the part defined on the drawing has the desired form, fit (within limits) and function with the largest possible tolerances. GD&T can add quality and reduce cost at the same time thru producability.-

 Is 50% more tolerance zone more precise than coordinate dimensions? It does not sound that way to me-to me it sounds like you will have a larger tolerance for a given part or assembly-which of course decreases costs, lead times, and manufacturing errors. It is also a basic fact of GD&T that using basic dimensions eliminates tolerance stack-ups and stack-up errors. This alone reduces assembly errors that frequently happen when using a non GD&T dimensioning scheme.

In Summary:

GD&T is a more robust and modern way of dimensioning parts for form, fit, and function. It is the standard that all modern engineering companies use and adhere to. The “trick” is to understand how the rules apply and when to use them in the right circumstances. With this knowledge, GD&T is a powerful tool in any engineer’s toolbox. We will learn it together in this column.

 

Ron Dietz is the Editor-in-Chief  for Tooling Magazine.
He has a BS in Mechanical Engineering and an MS in Engineering.
He has 20 years experience in metal manufacturing as a welder, machinist, tooling engineer, and machine designer.
His main interests are CNC machine designs, automated machinery, and complex tooling.

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