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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.