Letters 5/07/09

May 5, 2009
Most readers concur that Geometric Dimensioning and Tolerancing is a plus when it comes to turning out high-quality designs quickly

GD&T in the crosshairs
Most readers concur that Geometric Dimensioning and Tolerancing is a plus when it comes to turning out high-quality designs quickly. But there are some who feel it is too confusing and the real solution is for software vendors to simply add it to their CAD programs. What do you think?

GD&T: Good or a waste of time?
The main problem I see with GD&T is that even though the authors claim it to be faster once you’re skilled (“GD&T – Early Warning for Bad Designs,” March 5), when defining solid models via parametric or direct modeling, you don’t use GD&T. Thus, it actually does take more time from the designer ’s point of view, because drawings with automatic dimensions are part of every MCAD system, but GD&T is not part of defining the model.

I think proponents of GD&T should work with MCAD software vendors to embed GD&T into applications so it appears during the model-definition stage. In other words, based on constraints and perhaps some intuitive questions, the software would generate appropriate GD&T dimensions. This would take the complexity out of dimensioning and let designers focus more on products, not the symbolism.

Jim Watkins

Jim Watkins’ comments are typical of managers in companies that somehow remain in business. It amazes me that we’re engineering, manufacturing, and inspecting in the 21st century, yet we continue to hang on to dimensioning and tolerancing practices that can be traced back to the 17th century, despite the financial and stressful problems they cause. Think about it. When’s the last time you used any tool, or followed any procedure, as it was practiced in the 17th century?

Experience has proven time and time again that when quality goes up, costs come down, and projects are competed faster when companies and their suppliers are proficient in using GD&T. I have seen GD&T training create proficiency in the auto industry. I’ve seen it eliminate a two-year backlog (even as they received new orders) in under a 1.5 years. I’ve also seen companies that spent 5 years (and longer) in product development, improve after adopting GD&T to the point it only took two years to get a new product released. So the notion that applying GD&T takes longer is nonsense. Practice and training create proficiency, and proficiency improves the comfort and speed with which people work.

As for the comment about MCAD vendors automating the application of GD&T specifications, this is a valid gripe. Several leaders in GD&T training have worked with MCAD vendors to improve the how software creates GD&T specifications. However, like most businesses, MCAD vendors prioritize their work based on customer input. When customers start requesting automated GD&T applications, vendors will start to deliver.

Until that day, designers will just have to keep thinking in terms of geometric boundaries and documenting their thoughts, whether in 3D model annotation or on 2D drawings.

Michael Adcock

Although it does take time to design tolerance zones for mating parts and fasteners with GD&T, it takes a lot more time without it. Without it, you can be modeling product assemblies with worst case holes and shaft sizes and locating dimensions.

I recently arrived at my current company and found every assembler spending more time with a grinder opening up holes to fit than assembling products. The culprit: MCAD automatic dimensioning, standard print tolerances, and engineers who did not check worst-case scenarios in their models. Most engineers were quite surprised when shown worst-case scenarios that resulted from the “quick” and automatic — but poor — dimensioning and tolerancing. We are now putting GD&T into practice.

John Joyner

Be the first to identify this device from a past issue of MACHINE DESIGN and win a fabulous prize, along with the honor of seeing your name in an upcoming issue. E-mail entries to [email protected] and put “Gadget” in the subject line.

I used to work as a design engineer in a small manufacturing company making fairly simple products. We stopped using GD&T because production people were not experienced enough to understand it. We reverted to standard tolerancing. I am a big fan of GD&T, but in our case it was not worth the effort.

Mark Wales

I, too, was a design engineer in a small company and encountered a similar situation as yours (Mark’s). But instead of giving up on the “better system,” I worked wi th machine operator s and helped educate them on an asneeded basis. Initially, I did meet with some reluctance, but overall, most operators were eager to improve their skills and understanding. As a result, there was a significant interest among operators and inspectors, and we actually developed in-house training for basic GD&T training, as well as individual training for more in-depth understanding. It takes commitment from management and engineering to use the proper tools and raise the standard for all involved.

George (from the Web)

GD&T might have been one of the big reasons that I was let go in a company I worked for about five years ago. The work ers and management could not understand GD&T and found it confusing and intimidating. The workshop seemed to like making accurate fixtures for nonaccurate parts, and they did not believe in extra tolerance. So parts never fitted their fixtures. It was a sorry sight. GD&T is recognized as a cross-industry best practice

Anon. (from the Web)

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