GD&T question 5

[lwh723]

Hi,

So hoping some experts can help me put a question to bed. I work in the electronics industry, so really don't need to use GD&T at all. However, our data sheet drawing has this call out, and it's been the source of much debate with some customers. I've attached a picture of how I understand it, but some customers are taking it to mean that the 0.1 profile zone encompasses the B & C edges of the part. But those are datums, so shouldn't be included, right? In general, I think it's just a poorly done, but that's another issue.

 

[axym]

pmarc,

Yes, good catch. I think I got my material direction turned around because it's the bottom surface. The number after the (U) is the amount in the direction that adds material, and the datum plane sits on the high points of the surface. So you are given a tolerance for how much material is higher than the high points ;^). Odd, isn't it?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[lwh723]

So I've been following along with interest. Going back to my original question. Since datums are referenced in the profile tolerance, the outer profile zone isn't applicable on the datum edges, correct? So doesn't this effectively result in a +/-0.05 tolerance for my part?

 

[Belanger]

No, the profile is applicable on those edges. (The discussion showed that the datums as given are actually the center planes which are derived from the width and height.)

With that in mind, I would say that the middle paragraph within your original graphic gives the wrong numbers. Just based on that given GD&T, the dimensions of the part can be translated to 4.9 ± 0.1 by 8.2 ± 0.1. (You lose control of perpendicularity on the corners by translating it back to coordinate tolerances, but the individual dims would work out that way.)

Now, if that was not the intent, then the GD&T should be altered. But those are the results based on wrapping the profile all around.

The other side discussion was about a case where the datums are taken from those two surfaces only. (To designate that you'd have to move the datum symbols to be offset from the width and height dims.) If that were the case then you'd get only half of the 0.1 on those two surfaces, but the full 0.1 on the other two sides.

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

 

[3DDave]

axym -

This is why I suggested to the committee they use something like +.010/-.005 to indicate material added and removed about the nominal profile, but someone had already committed to (U) and that decision was -done- and could never be revisited.

 

[dtmbiz]

axym,

Your reply to greenimi...


Why would you say that the full tolerance is useable on all identified profile controlled surfaces with the datum center planes, when those dattum center planes are established by contact on "both" surfaces for each datum center plane ?

Wouldnt all identified profile controlled surfaces be contacted to establish the B|C datum center planes which would give half useable tolerance zones for all 4 surfaces?

 

[axym]

dtmbiz,

It's true that all of the profile-controlled surfaces would be contacted to establish the B|C centerplanes. But the "halving" effect only occurs when one individual plane is used to establish a datum. See the figures from my Mar 4 and Mar 6 posts.

The halving is not any kind of rule or condition that's being imposed - it's a consequence the "self-referencing" feature control frame. If the toleranced feature also establishes a datum plane, and the tolerance zone must be centered on the datum plane, then half of the tolerance zone is below the feature's high points (and therefore usable) and the other half is above the high points (and therefore not usable). In other words, the high points of the surface are perfect (coincident with the true profile). This doesn't occur when the datum feature is two opposed planar surfaces, because the high points of the two surfaces are not necessarily coincident with the true profile. This is very hard to explain in words - the pictures do a much better job.

The moral of the story is that in Y14.5 datum features are not all treated equally, with the same amount of tolerance. There are the halves and the halve-nots ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca