gd&t help provide better design

[bxbzq]

All gd&t textbooks state the benefits of GD&T are:
Improves Communication
Provides Better Product Designs
Increases Production Tolerance
I got first and third one, but I don't see good examples showing "provides better product design". From these statements GD&T is more than just a drafting tool, but a design tool. I'm in a position to convince people in department to use more gd&t but have limited knowledge and experience on this topic. So anyone minds sharing some good examples?

 

[CheckerHater]

I will say that GD&T together with its close relatives Direct tolerancing and Surface texture help you to evaluate your design.
Some very simple example: I saw the drawing of a part, simple rod with diameter being held to +/-0.0005 in. The said diameter didn’t actually fit to anything. It was fully exposed. It was almost like designer didn’t know about surface texture symbology and it was his attempt to say that surface of the part should be nice and smooth.
So as you are trying to apply your symbology properly, you also evaluate the part for its function and unnecessary cost.

 

[Belanger]

One reason why that second benefit is important is that GD&T tailors the tolerances to the function of the part or assembly. Think about a stepped shaft or spindle, where a smaller diameter is connected in line with a larger diameter. Traditional tolerancing would consider the diameters, the lengths, and perhaps leave the coaxiality to some implied tolerance.
But knowing the function may drive a more specific and accurate tolerancing scheme. For instance: if the part is a motor shaft that will be rotating at high RPMs, then we should be concerned about wobble and vibration -- in GD&T this would lead us to focus on a runout tolerance. However, suppose the part doesn't rotate; maybe it is just a plug that will be wedged into a hole and glued or welded in place. Then runout isn't needed, but perhaps a simple position tolerance.

So in general, GD&T allows better product designs because it provides more tools in our toolbox. Without these tools, we may have a clunky design simply because the limited language of coordinate tolerancing didn't allow us to specify a requirement that better suits the function.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[pmarc]

Another example taken from P.Drake's "Dimensioning and Tolerancing Handbook".

http://files.engineering.com/getfile.aspx?folder=4c616488-463d-4c51-a640-ce03d87464f6&file=gdt.pdf

 

[dingy2]

Pmarc's example is quite good and I also agree with J-P EXCEPT GD&T is sometimes applied when there is no real function and relationship between the features. That is unfortunate. Default GD&T is an example where everything is brushed with the same stroke although maybe only 1 feature has a function and relationship application.

Dave D.

http://www.qmsi.ca

 

[KENAT]

To some extent 2 is related to the 1 & 3, because you can better functionally tolerance your part/make use of available error budget and communicate your intent you can actually have a better part.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

bxbzq,

High quality fabrication drawings enhance design quality. If the fabrication drawings are prepared sloppily, the designer has not thought through tolerances. The parts can be hard to manufacture. The drawings can be bad enough that fabricators do not take tolerances seriously. There could be all sorts of assembly and functionality problems not observed by the designer.

A competent designer has a basic understanding of the fabrication processes, the assembly processes and the final end use of the product. GD&T is a language which allows the designer to specify exactly what is required from the fabricators. Since the language is based on standards, there is a mutual understanding of what the drawing means.

GD&T is not everything the designer needs to know, but metaphorically, it is a link in a chain.

--
JHG

 

[MechNorth]

Effective application of GD&T requires a real understanding of component functionality, not the superficial beliefs that we inherit from tribal knowledge. Rationalizing the controls we use and economizing on their application makes for a better design when the designer's hands aren't tied by corporate convention.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[bxbzq]

All good comments and examples. Thanks.