GD&T: True or False? 3

[stevepmd]

The following questions are not intended as an indictment of GD&T, but rather to provoke some critical thought on current theory and application.

T or F: The only reason for applying GD&T specs on component drawings is to reduce mfg costs by allowing relaxation of linear tolerances.

T or F: The application of GD&T specs on component drawings are rarely accompanied by relaxation of linear tolerances.

T or F: If the sum total of costs associated with learning, interpreting and implementing GD&T in the workplace exceeds component mfg cost saved, then GD&T is not cost-effective and thus has no legitimate usage in the engineering workplace.

T or F: GD&T theory has been "sold" to engineers by guru "disciples" in an evangelical manner - a body of commandments that must be "believed in" rather than implemented based on any practical merit.

T or F: The fact that interpretation of GD&T callouts often evoke heated discussions, indicates that GD&T theory as currently defined is flawed and possibly fundamentally defective.

T or F: The fact that "none", "a little" or "a lot" of GD&T can be arbitrarily applied to a component drawing - and still produce identical components - points to the arbitrariness of GD&T as it is currently defined and applied.

T or F: In the absence of a note to the contrary, all GD&T specs on a drawing require mandatory inspection.

T or F: Many GD&T specs are uninspectable and are thus ignored by shop and inspection personnel.

Word Problem: A component drawing with only linear dimensions/tolerances and no GD&T specs is submitted to the shop for fab. A second drawing of the same component with GD&T specs applied is also submitted to the shop for fab. After fabrication, the components are inspected and found to be geometrically identical. Question: what value has GD&T contributed to the fabrication and/or "quality" of this component?

Multiple choice: GD&T specs on a drawing are instructions to a) the machinist; b) the inspection department.

T or F: GD&T specs do not inform or instruct the machinist because component defects as covered by GD&T are a function of fabricating machine precision and thus beyond the control of the machinist.

 

[KevinDeSmet]

It is my belief thatg GD&T is only really practical for very large, highly automated assembly runs that demand a 99.9999% success rate of parts mating together, etc...

Automotive, aerospace, consumer electronics.

But one of builds or low volume builds for machine design, to take an example, I believe using GD&T is looking way to deep into it. It slows everything down having to take into account every little nook and cranny and its pedantic engineering think.

Certified SolidWorks Professional

 

[drawoh]

KevinDeSmet,

I am running SolidWorks, and most of my drawings are metric. As per ASMY 14.5, I delete the trailing zeros. This means that I have to go through my drawings and apply tolerances to each and every dimension, somehow.

Applying tolerances and feature control frames on most fabrication drawings takes maybe five minutes extra on a typical drawing. If it takes significantly more than that, it is because I am trying to sort out a messy tolerance stack. Messy tolerance stacks are problems that usually affect assembly and function. These must be detected and corrected.

GD&T is a convenient and clear way to express tolerances. I do not see a down side to preparing good drawings, and I do not see a down side to GD&T.

JHG

 

[drstrole]

Using ASME Y14.5 is actually a dimensioning and tolerancing standard that defines dimensioning and tolerancing paractices. The standard (or other dimensioning and tolerancing standard being used) needs to be indicated on the drawing. If the dimensioning and tolerancing standard is not being specified on the drawing how do the users, or the drawing creator know or understand the meaning of what is put on the drawing. GDT is a large portion of the standard that helps correctly define the part requirements.

Drstrole
GDTP - Senior Level

 

[pmarc]

One may want to use coordinate dimensioning only and live without GD&T because in his/her opinion it's unnecessary, unrealistic, too complicated, expensive in training, time consuming in application on drawings, uninspectable, etc. However as long as the same person is not a designer, manufacturer and inspector of a part simultaneously, there is no guaranty that complete and undistorted information about the product (free of assumptions and misunderstandings) will be transefered from one step of development process to the other. There is no possibility that part's geometry will be defined in full, precise and unambiguous manner.

 

[ptruitt]

I hope my competitors are not using GD&T and my suppliers are using it. In fact, if my suppliers are not proficient in using GD&T... (...hmmmmmm... I'll finish my sentence with those suppliers on a case-by-case basis.)

Old-school dimensioning is/was 1-D, GD&T is 3-D. I don't design many 1-D parts these days, if I ever did.

Peter Truitt
Minnesota

 

[KENAT]

Actually, GD&T can be useful as part of a 'risk management' strategy even on low volume stuff, especially if that low volume stuff is individually a relatively high proportion of your business.

For a company that has say, $50000000 turnover in a quarter, not being able to ship an individual unit worth say 3000000 because some holes didn't line up on parts to be mated or similar is pretty significant.

Likewise if the items have to assemble/install at a customer site or similar.

On the other hand, if you are virtually hand building units, with on site machining capabilities etc, then the impact of ill fitting parts may be mitigated.

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