Right behind Stonehenge and an improbability drive, Geometric Dimensioning and Tolerancing (GD&T) is somewhat of an engineering enigma, if there ever was one. Developed as a language for precisely communicating design intent, the ASME standard has been in existence for over half a century. The benefits of GD&T are very real, providing a reliable and verifiable means to ensure parts function and interface as intended, while reducing scrap rates normally driven by unnecessarily restrictive rectangular tolerancing schemes. Yet, at the same time, GD&T is largely not institutionalized in engineering curriculums, nor is training widely delivered to engineers at companies of all shapes and sizes. Some companies hold an utter disregard, and don’t utilize GD&T at all in their engineering. Model Based Engineering (MBE) promising to annotate GD&T directly into 3D models as Product Manufacturing Information (PMI), continues to suffer serious adoption problems. Certainly, something is wrong.
Sometimes it’s easy to characterize GD&T as an absolute, the fanfare can resemble the Sarge coming out of hypersleep: Every day in GD&T is like a day on the farm. Every feature control frame is a banquet! Every cost savings a fortune! Every virtual condition a parade! I love GD&T! But such enthusiasm is not misplaced. Reducing design intent miscommunication is paramount when dealing with global suppliers. Part interchangeability or scrappage problems can be quite costly, especially in higher volume environments. GD&T is specifically tailored to alleviate these issues.
However, there’s certainly an issue of complexity. Even with substantial and frequent training the concepts involved are fundamentally straight forward, but ultimately arise to rather complex situations in practice. You can still have furious chair-tossing debates over whether that tertiary datum should have been referenced at max material condition (depending on which version of ASME Y14.5 the company has adopted). Such arguments could result in questionable references to parentage and unwelcome advice on where to place your datums. In many ways it’s quite literally a system developed by rocket scientists to be used by ordinary humans. As fundamentally perfect as the system is, it’s not terribly accessible for many and that’s a problem. Most experts are apt to react negatively to such criticism, pointing out that the standard by its very nature is necessarily complicated. Tolerancing complex parts can never be “dumbed down” and to even attempt such is an unacceptable concession. In other words, if you can’t deal with complexity, then the problem is you. Look into my eye.
So if the system is perfect, then the problem must be those pesky humans. Too many of them don’t know what they are doing. What do we do? Resort to genetically-engineered border collies? Non-adversarial Cylons? We could try training the humans and vaporize them if they complain too much. If their management doesn’t want to pay for said training, then we’ll vaporize their management. What could possi-bly go wrong?
Most engineering curriculums are limited to a single engineering documentation course – many of which have been more aptly focused what little time they do have on Computer Aided Design (CAD) and other technologies. There’s not much room left in a 4-5 year program unless we dispense with, say, Chemistry. I don’t think that’s a viable option. So it seems most GD&T training happens on the job or at least within the realm of the employer. And employers are reluctant to spend ample dollars on training despite apparent costs savings – often because such cost savings aren’t adequately measured or the cost barrier of training everyone. The result is few engineers are trained. Many have exposure once to a 3-4 day class, and the result, without continuous training, is sometimes more disruptive than total ignorance. A designer wholly ignorant of GD&T is one thing, they are obviously easy to spot. A designer who has had some training, but may have a misunderstanding or a specific skill gap, will more likely result in undetected errors down the road. That hidden cost damages the overall value proposition, and becomes a barrier. So you see, complexity is a problem, regardless of your ideology. Hudson, come here.
But what about all those companies that aren’t using GD&T? Shouldn’t they be going out of business? Shouldn’t GD&T be a necessity for competing on quality products in this global economy? Many of the companies who skirted GD&T in the past tended to be low volume, vertically integrated, or reliant on a +5 sheet metal alchemist with a forge at the local renaissance faire. But many are observing the trend of companies giving secondary consideration to GD&T seems to be increasing, rather than decreasing. Is there another factor at work?
Let’s think about scrapping parts. One of the rather correct tenets of GD&T is that you can never manufacture a perfect part at nominal tolerances. Too much variation from that perfect part results in scrap. GD&T optimizes the management of those variances. But what if manufacturing in general advances to a point where variances are rather small for most applications regardless, with little or no added cost? Perhaps we’re starting to see this manifest itself. Case in point a quote from Lean Startup:
“One often hears commentators lament the loss of manufacturing jobs in the United States over the previous two decades, but one rarely hears about a corresponding loss of manufacturing capability. That’s because total manufacturing output in the United States is increasing (by 15 percent in the last decade) even as jobs continue to be lost. In effect, the huge productivity increases made possible by modern management and technology have created more productive capacity than firms know what to do with.”
Since we’re already talking Lean Startup, let’s take the entrepreneurial perspective to GD&T as an interesting exercise. GD&T is a product that (as far as you are concerned) is perfect in every way. Perhaps you can even prove conclusively how perfect it is, by how it measurably performs, improves yields, or saves money. Trouble is, too many companies don’t seem to be interested. Do you blame your flawless creation – or do you blame the consumer of that technology process? That’s the tricky part of entrepreneurship. Just being right is not enough. You have to be right, and make your solution compelling. And as compelling as GD&T is, in today’s continually improving manufacturing environment, perhaps the luster of an evolved 1965 standard has dulled somewhat.
Does that mean GD&T needs immediate evac? No. But it’s clear that something has to change, and MBE represents that opportunity. However, up to this point, MBE is merely lifting GD&T and the standard as is into 3D as appended annotations. Worse yet, the transition is losing further focus on drafting skill, merely because drawings are seen as an anachronism, regardless of the universal versatility of their presentation. MBE is an opportunity to leap forward in technology and provide true visualization for dimensioning and tolerancing in a way previously unimagined – and therefore increasing the accessibility of the principles that drive GD&T. The question remains: will anyone truly seize that opportunity? What do you want me to do fetch your slippers for you? Gee sir, I’d like that.