Geometric Tolerancing Can Increase Part Cost and Delay Schedules

Two parts in an assembly were not always mating properly. Occasionally, the mating parts evidenced interference when assembled. The drawing was updated to correct the occasional mismatching of assembly mates due to a tolerance stack-up issue. Immediately, the update drawing requirements were deemed beyond the capability of the factory to produce. The factory's outside, old-school, go-to shops decline making the parts based on the new requirements. Inspection could not measure the parts. Delay's, higher part costs and non-conforming parts were blamed on the addition of GD&T.

GD&T is suppose to be better! A Google search for GD&T advantages will lead to a well established mantra; GD&T will save any company money, reduce time to market, improve quality and lead to better designs. This mantra is not always true.

The change on the drawing you ask? Three concentric cylinders had an implied tolerance of ±.0025 [Editor's note: Original article was ± .005 inch in error] inch from a common center with the previous coordinate dimensioning scheme. The new GD&T dimensioning scheme made one cylinder a datum. The second critical cylinder feature was give a positional tolerance of Ø .007 inch in relation to the new datum cylinder. The irony is that GD&T made the part easier to make. A further irony is that most of the previous production parts made to the plus/minus coordinate dimensions would pass the new requirement.

It is in these ± dinosaur dominate ecosystems where GD&T is a liability and should be used with caution – or not used at all.

GD&T has created in many companies chaos in the organization, quarrels between stakeholders, led to quality escapes, created poorly fitting parts unable to function, produced more scrape, fashioned parts unable to be measured and customers degrading quality ratings of suppliers. No amount of wishing for a happy, well-ordered GD&T ecosystem is going to change the ugly, gritty reality that coordinate dimensions with plus/minus tolerances are easier to use. Therefore ± tolerances are used the most. That which is easy and common is used; that which is harder, but better, is avoided. Many hope for a better world to come as these plus/minus dinosaurs measurements die off to yield the ecosystem to GD&T systems capable of higher precision and greater efficiencies.

Why are GD&T benefits so hard for many companies to realize? I will suggest the most obvious reason why and when GD&T doesn't work for a company. (I will also explain 5 reasons a company must use GD&T in a later article.) In this article I will state clearly why GD&T is a bad idea in some companies. The problem is illustrated by the quote below:

Изучение GD&T подобно освоению нового языка. В начале вы знакомитесь с некоторыми ключевыми словами, далее постепенно выстраиваете словарный запас. . . .Понимание ключевых терминов, используемых на языке GD&T, является важной частью интерпретации рисунков.”

Основы геометрического измерения и толерантности.” Delmar, 2012. A. Krulikowski

Anyone struggling to understand the meaning of the above quote is facing the reason why many don't embrace GD&T.

To translate the quote (and make the point):

Learning GD&T is like learning a new language. First you learn some key words, then you build your vocabulary over time. . . .Understanding the key terms used in the GD&T language is an important part of interpreting drawings. . . . ”

Krulikowski, Alex. Fundamentals of Geometric Dimensioning and Tolerancing. Delmar, 2012.

If people inside and outside of an organization do not understand the same language then efficiencies and understanding are lost. We think of this as a matter of international trade but it is also true of technical processes. Coordinate dimension are common, easy to grasp and an easy means to communicate design intent. (There is a caveat to this last statement but that is discussed in the next article). Language is not easy to learn. GD&T has it's own meaning, symbology and nuances that are not part of the every day work life for many. If people you work with cannot understand what you are saying, then you are communicating nothing. Like a language, there must be a common understanding of meaning from the language used for the language to be useful.

Like a language, knowing a little bit of the language can create more trouble than not speaking the language at all. Ignorance of a new language motives people to the familiar old language. A dinosaur in this case maybe old but in this ecosystem, the dinosaur is familiar.

A few reasons for not learning the GD&T lingo:

  • A company will not pay for talent that can create meaningful GD&T. Skills cost extra and we don't use it.
  • Some bluff their way through their ignorance; hence others think a subject matter expert is present. Why spend money when we have an expert?
  • A company will not invest in training of talent to know GD&T. Training costs money.
  • A company invested in poor GD&T training for its talent. Despair over little return on the investment proving GD&T is too hard to learn.
  • Companies find ways around GD&T therefore avoiding the frustration of learning something new. I have enough problems in life without this new challenge frustrating me.
  • The “new” measuring system slows the company processes down because no-one knows how to read the GD&T properly. “Get 'er done now!”
  • No external force is driving the company to take up a serious implementation of GD&T. Customer requirements and demands drive our business. If the customer doesn't demand it, we don't do it.
  • Current outside suppliers do not understand GD&T. Finding new suppliers is a headache.
  • Vocational training and university engineering curriculum do not prepare graduates to use GD&T. Why train students to use GD&T when it does not help them secure jobs on graduation? (See the first statement in this list.)

GD&T works well where everyone who needs to measure knows GD&T. This body of common language include everyone in the supply chain, design area, inspection area and manufacturing area. Any person in the production chain that does not understand GD&T is a failure point certain to frustrate on-time delivery schedules.

The fact is, this “± dinosaur” maybe ancient but it is a massive and sometimes efficient creature. This dinosaur is well studied and understood in the ecosystem it dominates. This dinosaur will be present in companies for a very long time to come because of its familiarity and ease of use.

Many large companies are changing the ecosystem (i.e. the supply chain) the ± dinosaurs live in. These new ecosystems do increase profitability and reduce schedule time. These new ecosystems are places where the benefits of GD&T are necessary to thrive and the GD&T language is understood. However, outside of these GD&T ecosystems, the ± dinosaurs continue to rule. It is in these ± dinosaur dominate ecosystems where GD&T is a liability and should be used with caution – or not used at all.