Quality Blog

Quality Remix: GD&T - A Contributor to U.S. Manufacturing's Decline?

September 30, 2009
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There should be some training and explanations of proper GD&T evaluation. I wouldn't say it is THE cause of manufacturing's decline, but possibly a contributor.

I believe there should be some training and explanations of proper GD&T evaluation. It is the language shared between engineering, manufacturing and quality, yet is one of the languages that many struggle with. Why isn't this taught at the academic level? No required courses for engineering students? More standardized training for the industry? Has improper use of GD&T caused part of manufacturing's decline in the U.S.?"

I wouldn't say it is THE cause of manufacturing's decline, but possibly a contributor. Not GD&T itself, yet misunderstanding of what the standards are stating and what the symbols actually mean in design, evaluation and manufacturing.

I'll preface my statements with: I am not a GD&T expert. The more I study and learn, the more I realize what I don't know and study some more. I know enough to be “dangerous," yet I understand when to go to relevant sources before a quality or manufacturing decision is made.

I have had the opportunity to work with young customer engineers on various projects. They are brilliant engineers and can do calculations beyond my level of expertise, however, what I find is a common lacking flaw is communicating their brilliant designs onto a blueprint. In addition, I run across many engineers who don't understand the importance of a tolerance stack up analysis and just put arbitrary tolerances on dimensions. Sure the parts may work just fine, but could more open tolerances also work?

If manufacturing is required to maintain a tighter tolerance, the parts are more expensive. If U.S. manufacturers are striving to maintain the engineered tolerance, the U.S. shops cannot compete with the overseas companies who have lesser quality and cheaper costs. However, what is considered "lesser" quality? If a part is not to the print, is the part defective? Not necessarily.

Design must allow as much tolerance that is functionally allowed for the products to work. The more tolerance, the easier to produce, and the cheaper the costs. Of course, the U.S. cannot compete with overseas labor costs, but more tolerance equates into good functioning product, faster turnaround and better deliveries. If there are quality issues, the problems are resolved quicker because of the logistics of the manufacturer.

As my GD&T instructor so adamantly explained, “Functionality dictates design."

On the flip side of not enough tolerance is too much tolerance, or improper GD&T datum callouts on print. On many prints, I have seen people measure this feature to say.....umm......here, use this datum. But that datum may have absolutely no functional bearing on the measurement evaluation. I've seen parts that meet all the print requirements, yet fail in the functional states. Manufacturers who do not assembly the parts only have the print as the guide. The shop could be making "good ‘ parts all day long that meet the print, yet won't work.

It appears that I could be picking on engineering, but manufacturing and quality hold as much responsibility. The designs could be flawless-prints calling out the proper datum structures-and parts still fail. It comes down to the manufacturing and inspection departments’ interpretation of the drawing. It isn't necessarily interpretation, yet a misunderstanding of what the language is. Sure you can have wonderful automated and inspection equipment that produce excellent printouts. Your SPC charts could always show in control, yet parts aren't working. Is the GD&T "decoded" properly? How was your inspection equipment programmed?

Equipment such as CMMs allow for multiple evaluation methods….even four answers for one bore size. CMMs and other inspection equipment are tools, nothing more. A good printout is just a piece of paper. Bad parts could be made to look good if one doesn't understand to ask the CMM for the right evaluations.

There are MANY factors that are contributing to the industry’s difficulties, but while this is just a small factor, it’s a factor that could provide some small answers and turn this industry around.

How is it turned around? How are these changes implemented? The engineering academic level is one place. Should these subjects be covered more extensively in trade publications such as Quality Magazine?

So, how do we improve? Do quality management systems require true knowledge of the subject? No, but maybe it should. My company is ISO certified, yet there is nothing in the QMS about industry technical knowledge-perhaps there should be. Sure we have the best calibration system, or our gages are always in excellent condition and accurate, but what if we can't understand the language telling us what is required and how to evaluate properly? The best machine shop with the best equipment means nothing if the shop doesn't understand what true position is really asking for. The most brilliant engineer will not get the parts he or she asked for if the universal language isn't understood.

Even globally, there are multiple standards: ASME, ISO, etc. Why isn't there one standard? Why does concentricity and symmetry per the ASME standard mean something different than the rest of the world?

Even scarier, how many in the industry are even aware of what the GD&T symbols mean? Where is it learned? When I went through a technical college, we had blueprint reading, yet GD&T was not discussed. Why? The information needs to be out there.

Nathan Corliss is assistant quality manager at Seiler Instruments and Manufacturing in St. Louis.
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GD&T can certainly HELP

Norm Crawford
October 4, 2009
This is a good article addressing one key issue often over looked by organizations. That issue is that the proper use and understanding of GD&T and gaining real value in return relies on cross-functional knowledge. All to often engineering takes the training in GD&T, applies it, and then manufacturing or inspection request changes for "their" better understanding or application. AND, at that time in the product cycle the cost in dollars for any delay is huge and so these organizations often get their way without appropriate design impact studies of such change requests. So yes, improper "use" of GD&T can certainly be a problem and no GD&T is worse! But "use" of the language has to be defined as everyone who "uses" the language through out the entire product development lifecycle.

I agree

October 15, 2009
I have an MSME, and pride myself in understanding the GDT language - but more importantly, the rationale and usefulness of it. When I got my BSME, I went to work with a company on a military project. I did all the design work and made all the drawings. I *thought* I knew how to make a drawing, until I presented my drawings at the official design review. I learned of GDT that day. I was furious that I had gotten a BSME from a VERY prestigous ABET school - a mandatory co-op school at that, but had never even heard of GDT.

Yes and Know

November 4, 2009
I agree with Mr. Corliss than many design engineers I have encountered do not understand GD&T. Many of them also do not know the capabilities of their production equipment. Designing the part, and then just"throwing it over the wall" to Production and Quality should not be tolerated by management, but sadly, it still is in many places.

GD&T is a "factor that could provide some small answers and turn this industry around"

Phil Johnson
February 24, 2010
I am a Dimensional Engineer (DE), and GD&T is the language that I speak. I really enjoyed this article, as a GD&T expert we sometimes need to reaffirm, and maintain focus, in the GD&T drawings we are producing will be correctly read & utilized further on down the path. It is one of the few articles I have ever read, that elaborates on the true lifecycle of the GD&T drawing. There is a considerable savings that can be gained when the GD&T is followed properly. My last point is this, and perhaps the only item missed in this article, and that is who is the person following the GD&T drawing and validating the other engineers interpretation of that GD&T. I'm sure you see where I am going with this, and the only answer is a Dimensional Engineer. A DE will establish or update your product's GD&T attributes, build variation models for predicting the effect of tolerances and assembly sequences on critical characteristics of assemblies or to study the relationship of assembly process, tolerances and assembly to eliminate unnecessary manufacturing costs. If you ever have any questions, feel free to contact me at the DCS office. Just 'Google' us. Thanks again for the article.




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