Does GD+T make your job harder or easier?

Recently at an interview I was asked this question and after a bit of thinking, I said easier. I said because it gives a machinist a good idea of what's important, how to tackle the part, and what needs to come first.
Apparently I am wrong, and just flat out told so. Coming from an environment where a hole is often not just a hole, but hole needing a TP of .003 and a 24 or better finish - but not marked on the print and if made to print gets returned on trumped up charges - perhaps my perspective has been skewed.
How would you answer this question, and why?

It's a strange question since the purpose of GD&T is not to make anyone's job easier or harder, it's to ensure that the manufactured part matches the design intent.

Given the way the question was posed, I would have answered the same way as you. Seems like your interviewer had an agenda for some reason.

Did they say why you were wrong?

If the designer knows what he needs, easier. If he doesn't, it makes it harder and more expensive. I get very involved with my customers design and processes. I would say about half the time, things are unnecessarily toleranced way too tight, but that is just a generalization. Bearing fits, punch and punch holder fits, dowel holes fine. I've seen 1/2 inch screw holes with .06 clearance given a .002 true position. If they want an inspection report that has to conform to the tolerances and want to pay for it, fine. Most of the time they want it less expensive, so I make them revise the drawing. I don't think it is a fair question without knowing the context.

Hardened --

I can't honestly say if GD&T makes my job easier or harder, but I do think GD&T embodies some of of the more poorly understood concepts in the industrialized world.

If I had a nickel for every time I've heard " . . a True Position tolerance of plus-or-minus so-many thousandths in all three axes . . . ", I could afford to fly first-class. Adding another nickel for every time I've heard ". . . a True Position tolerance of fourteen is the same as plus-or-minus five in all three axes . . .", I could stay at the Ritz.

I'm tired of explaining to others how come a hole center that's mis-located by 0.006 inch violates a 0.010 inch True Position tolerance, why hole that's mis-located enough to violate its tolerance can SOMEtimes become compliant if the hole is enlarged, or why holes in the same part having the same True Position errors aren't necessarily perfectly spaced.

John

I believe the usefulness of GD&T depends very much on the level of expertise of the designer invoking it AND the maker trying to interpret it; as soon as one or the other (or both) is deficient in understanding it turns into a catastrophe.
I've seen a lot of crap GD&T which makes my life harder.
I've seen a little bit of GD&T that was very useful in defining a critical aspect of a design and made my life easier.

In the prototyping scene that I mostly work in, it's typically been a problem rather than a help.
Too many engineers have no clue what they want to make, how they could realistically go about it or what tolerances they can get away with and still achieve their goal.
Also, the language of GD&T is arcane, and subject to lots of interpretation as the tolerancing gets more stringent and gets more icons in more boxes (even though it's supposed to be just the opposite!!).
When I'm confronted with a chicken scratch drawing with more GD&T on it than Heinz has got pickles, my best recourse is almost always to just have a conversation with the engineer and ask him what he wants to achieve.

Once the prototype has been verified to be functional, a more intelligent conversation can be had about GD&T for production drawings and the tool can more properly be applied to finding the cheapest way to make good parts and defining the boundary between a workable feature and one that will fail.
Almost nobody in my customer base goes to anything even remotely like the discipline necessary to do this well; tolerancing is mostly an afterthought and the GD&T I see most often reflects that.

Crap GD&T is no better than crap conventional dimensioning...it's pretty pointless if it hasn't been thought through.
It mostly isn't worth the effort to do the full-on interpretation. because it typically raises a lot of time consuming problems on projects that don't have the scope or budget to sort this out properly.
Typically, if there's doubt, it's cheaper for me to just bore and ream the damn bumper bracket bolt hole as accurately as my machine will conveniently permit, than it is for me to get in a tolerancing bunfight with the engineer so I can take proper advantage of the tolerance window.

Of course, this only applies to prototyping and limited production.
All rules change and the investment of effort becomes worthwhile as soon as you have lots to make and have to optimize your processes to make any money at it.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
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