Positioning hole with edge distance? 1

[Rich68]

I am no GD&T expert, but I can't believe this is correct (See attached). It seems to me that this allows for the hole to float? I've always been under the impression that the feature had to have an absolute position.
If this is not valid, any feedback helping to explain how it is invalid would be greatly appreciated.

Thanks,
Rich

 

[Sem_D220]

If this was defined, for example by changing directly toleranced +/- radius to basic dimension and applying profile tolerance wrt A to the surface of datum feature B, the simulator would be able to expand from LMB size towards MMB size.

This, however, still wouldn't solve the issue of unlimited expansion of the simulator of the datum feature B referenced RMB in the position callout

pmarc, isn't the condition underlined first is the same condition that there is in fig. 4-29(a)? And doesn't the second underlined portion describe what happens under this condition? Not nitpicking or anything, just trying to understand.
It seems that if the RMB callout is pointless when the simulator is allowed to be "unlimited" within the range of LMB-RMB, this is also true for fig. 4-29(a).

 

[pmarc]

greenimi,
I was thinking in terms of the simulator, thus the expansion from LMB towards MMB (opposite to what standard says in the RMB definition). I should have been more precise about that.

Sem_D220,
Like I mentioned, situation shown in fig. 4-29a is different from OP's case because in that figure the secondary contoured datum feature B has a location relationship to the higher order datum, whereas in OP's scenario it has not.

Imagine how the gaging process for the part from figure 4-29a looks like in case of verification of the position callout to A|B. The part is held tight by cylindrical simulator A first and then the simulator B starts its expansion. And the key point is that because the part is not able to move as a result of expansion of the simulator B (because it's constrained by A), it's the simulator B that will stop its expansion at some point. Once it stops its expansion, we can say datum feature B has been simulated RMB.

In OP's case the part can freely float when put on A and so the expansion of B has no physical blocker and so it is impossible to simulate true RMB of datum feature B.

 

[greenimi]

I was thinking in terms of the simulator, thus the expansion from LMB towards MMB (opposite to what standard says in the RMB definition). I should have been more precise about that.

Thank you for your direct answer. I heard before, on this forum, that there are two ways of thinking about these simulators: sitting on the part OR sitting on the gage so I was slightly confused. Thanks again.

 

[Sem_D220]

pmarc,
Thanks for the thorough explanation. The fixed location of datum axis B in fig. 4-29a relative to datum A indeed does the difference. Thank you for clarifying.

 

[greenimi]

Pmarc,
Let me ask you: since RMB for datum feature B modifier is not quite “a correct or appropriate” selection then your recommendation is to use MMB/ LMB or BSC. Correct?
I remember another discussion where MMB on a secondary was not the correct selection due to the fact the secondary did not have a location relationship to the primary (which is also valid in the OP’s case). And, to be specific, has been concluded that “ If a secondary or tertiary planar datum feature is referenced at MMB and it has no location relationship to the higher order datums, then this is invalid callout.”

https://www.eng-tips.com/viewthread.cfm?qid=447862.

January 8, 2019


Now, I know it is a difference between a planar datum feature (in the scenario in which has been saying that MMB for secondary is a NO-NO) and a curved datum feature (“B” in the OP case), but I would like to ask you: where you draw the line ? For me a planar datum feature is just a feature with an infinite radii.
So, in other words I am asking: what would be the biggest radii from where MMB would not make sense/ MMB (for secondary). callout would not be valid. I hope you understand what I am asking.

In OP case: radii basic and no location relationship to the primary A, and “B” could be MMB/LMB or BSC
In the thread referenced above: planar surface, secondary and no location relationship to primary, “B” at MMB secondary is invalid.

How to judge and where you draw the line?

Could be a point where none of the modifiers make sense at all? No RMB, no MMB, No LMB, No BSC.

 

[chez311]

pmarc,

I too am wondering about where the line is drawn between a purely planar feature and a feature which does not qualify as a FOS in your explanation (ie: why RMB is not okay due to unlimited expansion for the OP's case and why it is acceptable for a planar feature - or what the difference is). I seem to be stuck on the same point of confusion as greenimi, as it seems like a planar feature would have see the same issue.

Imagine how the gaging process for the part from figure 4-29a looks like in case of verification of the position callout to A|B. The part is held tight by cylindrical simulator A first and then the simulator B starts its expansion. And the key point is that because the part is not able to move as a result of expansion of the simulator B (because it's constrained by A), it's the simulator B that will stop its expansion at some point. Once it stops its expansion, we can say datum feature B has been simulated RMB.

In OP's case the part can freely float when put on A and so the expansion of B has no physical blocker and so it is impossible to simulate true RMB of datum feature B.

 

[pmarc]

greenimi,
The line is drawn at R = 10^6 mm .... Just kidding

My answer to your question is this:
- If the nominal datum feature (that has no location relationship to a higher order datum) is of any curvature - even a very large one - the MMB, LMB or BSC concept can be used (assuming that MMB, LMB or BSC size is determinable);
- If the nominal datum feature (that has no location relationship to a higher order datum) is planar, the MMB, LMB or BSC concept is invalid.

chez311,
I am not sure I fully understand your question, so let me put it this way:

If in OP's case (assuming datum feature A is the top or bottom surface) the inner radius was changed to perfectly flat horizontal surface, that surface could be referenced secondary RMB in the position callout and it would actually be the only meaningful choice out of RMB, MMB, LMB, BSC. This datum feature configuration would then be no different than many other common configurations of two mutually perpendicular planar datum features. The two simulators would have to be perpendicular to each other and because the two simulators would have no curvature, their perpendicularity would be the only thing to assure from gage design standpoint.

However, with secondary datum feature as an arc, the size of the curvature of the secondary simulator must be added to the equation, and because (like I have been trying to explain) the expansion of the secondary simulator within LMB-MMB bandwidth is unlimited in this case, there is no way to say what size of the curvature represents the RMB of datum feature B condition.

 

[chez311]

The two simulators would have to be perpendicular to each other and because the two simulators would have no curvature, their perpendicularity would be the only thing to assure from gage design standpoint.

However, with secondary datum feature as an arc, the size of the curvature of the secondary simulator must be added to the equation, and because (like I have been trying to explain) the expansion of the secondary simulator within LMB-MMB bandwidth is unlimited in this case, there is no way to say what size of the curvature represents the RMB of datum feature B condition.

pmarc,

Thank you for working through it with me - I did not think through my question very well, but you were able to read between the lines anyway, this helped me understand what you were trying to explain about unlimited expansion. For some reason it just wasn't clicking.

 

[pylfrm]

If in OP's case (assuming datum feature A is the top or bottom surface) the inner radius was changed to perfectly flat horizontal surface, that surface could be referenced secondary RMB in the position callout and it would actually be the only meaningful choice out of RMB, MMB, LMB, BSC. This datum feature configuration would then be no different than many other common configurations of two mutually perpendicular planar datum features.

I'd argue that the lack of a modifier symbol would imply a default of BSC in this case, and that RMB would not be meaningful.

thread1103-428497 had a related discussion, with particularly relevant posts by axym.


pylfrm

 

[pmarc]

pylfrm,

If you are saying that lack of modifier in this example implies BSC for secondary datum feature, then I would say you are technically correct, but because the datum feature is planar and has no location relationship to datum A, BSC is really no different than RMB.

Additionally, according to Rule #2 no modifier for a datum feature reference means RMB (despite that BSC technically makes more sense for planar datum features that have no location relationship to higher order datum(s)), that is why I favored RMB over BSC in the description of my example.

Is this the point of your comment?

 

[pylfrm]

pmarc,

From various statements in ASME Y14.5-2009 section 4, one might conclude that RMB is (in many cases) not a valid option for planar datum features. "Rule #2" in para. 2.8 does indeed imply otherwise though. I had forgotten about that.

My (slightly revised) point is that BSC better represents datum feature behavior in many cases that Y14.5-2009 would call RMB.

The standard's treatment of datum features is a complicated mess. I somehow manage to forget this every few months.


pylfrm

 

[pmarc]

My (slightly revised) point is that BSC better represents datum feature behavior in many cases that Y14.5-2009 would call RMB.

I fully agree with this statement.

The standard's treatment of datum features is a complicated mess.

And with this too.

 

[pylfrm]

The standard's treatment of datum features is a complicated mess.

I fully agree with this statement.

How would you say ISO compares in this regard?

(I notice OP never specified in this thread, although perhaps there are some clues.)


pylfrm

 

[pmarc]

pylfrm,
My apologies, but I simply didn't notice your question.

I would say the answer to it is not that simple. In ISO they put much more emphasis on theory and as a result they are probably more consistent in what they do than ASME is. But on the other hand their datum theory is really complex and, honestly speaking, difficult to comprehend. They have another portion of extra modifiers designed for datums only, which from my experience this don't make like easier (even though I see why they have been developed).