dimensions from Datums

[designeng82]

This is so basic I feel shameful but there is a big debate at work right now.

Incase the attachment doesnt work, the picture shows a blob with a Profile tolerance of .5 all around it constrained to a parrallel datum A only. one of the surfaces of the blob is Datum B with a 2.5" basic dimension comming off to show the overall height of the blob.

Group 1, thinks the MMC of the 2.5 is 3.0 LMC is 2.0
Group 2, thinks the MMC is 2.75 and the LMC is 2.25 (because you get not profile tolerance on if off the datum surface
Group 3, thinks the 2.5 is unrelated to the .5 profile because it goes to datum B and therefore falls with the overall profile tolerance of the drawing.

Any help? Sorry this is so easy but when we looked though all the Tec-Ease Books almost all the examples go to holes and don't really deal with MMC of a linear dimension off a datum.

 

[TWJR]

For starters, I don't understand what datum B is. Normally, a datum is a plane, a diameter (of a revolved feature), a hole (or pattern), a slot, or some other feature. This appears to be just an imaginary line tangent to (I suppose) a radius. Could be following some rule I'm not aware of, but I've never seen this application of a datum. Someone enlighten me if I'm wrong.

 

[designeng82]

our parts are all soft silicon with no real flat surfaces, datum B is a tangent edge that has a flat horizontal area about an inch long. I agree, not ideal but it gives the measurement group a place to orient

 

[TWJR]

Sounds like something that should be inspected on an optical comparator with a transparency as the master, not a drawing with GD&T.

 

[waqasmalik]

i have not seen anything like this neither in AMEY14.5 2009 nor in 1994.

 

[pmarc]

It does not matter what the datum feature B is. Profile .5 wrt A is applied all around the contour shown and is not related to B, because the all around callout overrides the general profile tolerance. That being said, my vote is that GROUP 1 is correct here.

 

[drawoh]

pmarc,

I would agree with group[ ]1 for a width dimension. One end of the height is datum[ ]B, which is correct by definition. The maximum error all is at the top. I go with group[ ]2.

--
JHG

 

[SeasonLee]

Y14.5-2009 1.4 Fundamental Rules (m) states: Unless otherwise specified, all dimensions and tolerances apply in a free-state condition.

Parts made from soft silicon is a nonrigit parts, I think you need to add a restraint note on the drawing to describe the clamp conditions during inspection.

Season

 

[Belanger]

Group 1 is correct. The height of 2.5 is impacted by the profile tolerance of 0.5 (0.25 inward, all around, and 0.25 outward, all around).

Two key factors, as pmarc pointed out:
-- the profile tolerance of 0.5 does NOT reference datum B, so that bottom datum is irrelevant to the question.
-- the profile tolerance of 0.5 is all around, so the 0.25 general profile tolerance is irrelevant to the question.

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

 

[powerhound]

I'm with group 1 as well.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
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