According to ASME Y14.5 2009, the purpose of geometric dimensioning and tolerancing (GD&T) is to describe the engineering intent of parts and assemblies. GD&T specifications, when done correctly, provide only one way to encode and decode those standards and is not subject to interpretation. To have a beneficial effect, these specifications must be created with consideration of every piece of the manufacturing process from materials through final assembly.
In simple terms, GD&T offers a way to help clearly express design intent so that the manufacturer can choose the best process to meet the design requirements in the most cost effective manner. Good GD&T will also define how the part should be inspected. This is critical in setting the expectations that internal or external suppliers must meet.
Unfortunately, a lot of time and money is wasted interpreting GD&T, especially between user and supplier. Usually this comes into play at assembly when it is found that parts do not fit. GD&T is supposed to help avoid this, but failure to correctly understand design intent on the one hand and failure to understand manufacturability on the other ends up wasting time and adding cost to a process and ultimately to the parts produced.
If an experienced engineer made a list of what not to do and why, about 90% of mistakes made in manufacturing regarding GD&T could be eliminated.
Although the rise in popularity of GD&T has provoked the rise of consultants that aim to help companies implement GD&T, most do not represent the concept accurately. That is why it is important to go back to the source of the standards and teach a common way to use and implement GD&T, which would result in a more competitive manufacturing industry.
So, we as the manufacturing industries, have an opportunity: if we properly understood that GD&T starts in design, through manufacturing, to purchasing and assembly, and that it is a rule-based system leaving no room for interpretation, we could avoid cost related to misunderstanding.
Consider these common mistakes that disrupt effective use of GD&T:
Failure to locate datums on significant manufacturing locations. The net surfaces and control locations are all too commonly located in places that are not significant to manufacturing or the application of the part being defined.
Failure to consider orientation and the effects of gravity. Yes, a crankshaft can deflect due to gravity.
Using tolerances that are at or beyond the limit of the desired manufacturing processes. Maintaining tolerances at process limits is expensive and requires frequent maintenance. It is possible to design a part that cannot be manufactured. This is often the result of:
Failure to validate process and capabilities. To do GD&T well, one must understand manufacturing process capabilities in all three dimensions. GD&T can positively affect manufacturing quality based on the designer’s understanding of the capabilities of the manufacturing processes. Products must be designed with an understanding of tolerances, stack-ups, and the limitations of the processes that produce the elements of the final assembled product.
Failure to follow product/process changes through to GD&T standards. If GD&T updates lag product revisions, they cannot be helpful to producing a quality product at minimum cost.
Initially, GD&T is expensive. One would need to review each step in the manufacturing process as well as properly train the individuals involved. It is an upfront investment that many reject largely because the eventual cost of not understanding GD&T is not known. There could be, instead, a very significant return on investment if it is implemented.
The standards have matured over time and have become highly competent at conveying information accurately. Also, it is the best method available and still continually improving. The principals involved should learn GD&T and understand what it is, what it is not, and how it can assure that design intent is successfully and profitably manufactured.
An engineer with a wide view of manufacturing and thorough understanding of processes can be an invaluable benefit to company success if he or she can implement GD&T throughout the operation—every part, every process. ME
This article was first published in the August 2013 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 8/1/2013