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Positioning hole with edge distance?

Positioning hole with edge distance?

Positioning hole with edge distance?

(OP)
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

RE: Positioning hole with edge distance?

Quote:

...this allows for the hole to float?
The 4 holes are grouped together under a common callout, which actually gives two position requirements for the holes: the upper tolerance of .030 controls their location on the part (I think that's the answer to your question). We know this because that tolerance references all 3 datums.
The lower tolerance of .005 controls the hole-to-hole location, which is often more important to the function.

That said, I suppose some of the folks on here may wish to discuss datums B and C, but I'm just trying to keep it simple and answer your basic question for now.

RE: Positioning hole with edge distance?

Rich68,
You are correct that the position specification, as a whole, is incorect.

In radial direction the tolerance zones are not fully defined with basic dimensions relative to datum axis B. The basic hole-to-edge dimension should be replaced with for example basic "pitch" circle radius.

Datum feature selection, mentioned by J-P, is different part of the story.

RE: Positioning hole with edge distance?

I think that what the OP is concerened about is the 4X .XXX basic dimention from datum feature B surface to the surface of the edge of the hole.

RE: Positioning hole with edge distance?

(OP)
If the basic edge dimension went to the center of the holes instead of the edge and datum B was the surface of the radius (Not center axis), would it be ok?

RE: Positioning hole with edge distance?

Agreed with pmarc. As it stands now, regardless of the fact that the hole-to-edge distance is ambiguous (does it intersect with the center of the arc? its not clear) - its an incorrect specification of the hole true position. As it stands the hole axes is technically defined by the directly toleranced diameter, which means the axis true position changes with the size of the hole which is not allowed. As pmarc noted this should instead be fully defined with basic dimensions. This could be done in a variety of ways, including a basic pitch diameter/angular dimensions or linear x/y basic dimensions.

pmarc/JP - I assume in reference to datum selection you guys are referring to the lack of 180 degree opposed points on what seems to be referenced as a FOS datum B?

RE: Positioning hole with edge distance?

Rich68,

I'm hesitant to say that would be okay, with the caveat that datum feature B would have to be defined with a profile tolerance and its radius would then have to be basic as well. Right now datum feature B seems to be defined as FOS which it is not, since it lacks 180deg opposed points.

RE: Positioning hole with edge distance?

I would add to what chez311 said above, that if the hole location dimension is kept to the surface of the radius, datum feature B symbol, in that case, should not be aligned with the radius dimension line, but either attached beneath the profile FCF that will define the arc geometry or connected to the surface itself or to an extension curve from the surface, clearly separated from the radius dimension line.

Datum C is also defined in a questionable way. From the way it is aligned with the basic angle dimension line, it suggests that the datum is meant to be a center plane derived from two non-parallel planes contacting two non-parallel surfaces. Is it OK to relate the datum feature symbol to an angle dimension similarly to how it is done with width dimensions defining parallel-surfaces features of size? When we had a GD&T training course at work, the instructor suggested it to me as a valid scheme when I asked him how to define a center plane datum to be derived from a dovetail-type feature. But, I never found anything that supports it in the Y14.5 standard. It would be interesting to get additional opinions about this.

RE: Positioning hole with edge distance?

pmarc,

The tolerances are weird, and I cannot see what would make me do it like that. Are they wrong?

I assume the datum A is the bottom face, and that we are not seeing the whole drawing. The physical inside radius is the secondary datum feature, regardless of size. The two inside holes are centred about an axis defined by angle XX, datum feature C. The outside holes are located by their edges from the inside radius, on a radius defined by the inside holes. Inspection of this thing would be a pain. I am glad I am not designing an inspection fixture for it. The imaginary radius the holes are shown located on is not very meaningful beyond showing design intent.

If the drafter is trying to locate holes to clear screws, this is the wrong way. I am being pedantic here.

--
JHG

RE: Positioning hole with edge distance?

I think that the true problem in OP's case is proper understanding of how basic dimensions work and what the difference between datum feature B and datum B is.

In case of position tolerance, basic dimensions are used to locate position tolerance zones from datum(s)/datum reference frame, not from datum features. So if we imagine for a moment that datum feature B in OP's example is regular 360 degrees cylinder controlled with +/- toleranced diameter, I guess very few of us (if anyone) would locate the holes from the edge of the datum feature B. Instead of that, there would most likely be a basic diameter dimension applied to the "pitch" circle of the pattern of the holes and together with the implied 0 linear dimension between the center of the "pitch" circle and the datum axis B this would fully define true position of the 4 tolerance zones relative to datum B. I will risk and say that this would be the most natural way to define it.

OP's case is different mainly because datum feature B is not a feature of size, however what is important is the fact that a datum feature simulator B used to establish a datum B is a contoured surface from which a center/axis can be derived. And from that center/axis the basic "pitch" radius dimension should be defined by default.

There is, of course, another problem with datum feature B in OP's sketch, as mentioned by chez311. Since it is not a feature of size AND since it has been referenced RMB in the upper segment of the position feature control frame AND since it doesn't have a location relationship to a higher order datum A (I too assume that datum feature A is the bottom or top face), there is really no way to find a point at which the datum feature simulator B would stop its expansion. This is what makes selection of this directly toleranced arc as a datum feature at least questionable, in my opinion.

RE: Positioning hole with edge distance?

Quote (pmarc)

datum feature simulator B used to establish a datum B is a contoured surface from which a center/axis can be derived

Quote (pmarc)

there is really no way to find a point at which the datum feature simulator B would stop its expansion.

If the datum feature simulator can expand without limit, how can a distinct axis be derived from it? As the simulator grows in radius, the axis will dislocate further and further away from datum feature B, won't it? Please explain how these two statements go hand in hand.

RE: Positioning hole with edge distance?

Regardless of how big in diameter the simulator is, its axis technically doesn't change and is equally determinable.

The fact that as the simulator grows its axis will dislocate further and further away from the datum feature B exactly proves that this feature referenced RMB is a bad choice (because it also means that the position tolerance zones dislocate together with the axis).

RE: Positioning hole with edge distance?

pmarc,

I obviously clearly agree that datum feature B is not a FOS, and I mostly understand the practical implications and issues you describe with trying to simulate such a feature at RMB being that its not a FOS - however I'm not sure I follow why the simulator could expand "without limit". Could you explain a little why this is? If as you say "its axis technically does not change and is equally determinable" - even though "size" has limited meaning if it is not a FOS wouldn't this axis still be technically fixed to the simulator itself would only be able to expand (ie: the linear distance between said axis and the surface) within the limits specified?

RE: Positioning hole with edge distance?

Then, the basic pitch radius dimension scheme can only be valid under the condition that an MMB modifier is used, allowing to use a fixed size datum feature simulator. Correct? If this was already mentioned and I missed it, I apologize.

RE: Positioning hole with edge distance?

The simulator could expand without limit because the drawing doesn't define size of MMB and LMB for datum feature B relative to higher order datum A.

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. - but this time the problem would be reduced to the LMB-MMB band.

That is why in this case the only feasible solution (assuming the entire inner arc surface needs to be used as datum feature) is to define datum feature simulator B at a fixed size. This can be MMB but also LMB or even BSC.

RE: Positioning hole with edge distance?

Quote (pmarc)

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. - but this time the problem would be reduced to the LMB-MMB band.

I'm sorry but I lost you here once again. Will the expansion of the simulatir allowed to be unlimited, or will the size variation of the simulator be only between LMB and MMB?

RE: Positioning hole with edge distance?

Unlimited within LMB-MMB band making RMB pointless.

RE: Positioning hole with edge distance?

In fig. 4-29 (edit: illustration a), Y14.5 2009 datum B is called out RMB. Is the scheme shown there pointless?

RE: Positioning hole with edge distance?

Quote (pmarc)

Since it is not a feature of size AND since it has been referenced RMB in the upper segment of the position feature control frame AND since it doesn't have a location relationship to a higher order datum A (I too assume that datum feature A is the bottom or top face), there is really no way to find a point at which the datum feature simulator B would stop its expansion.

The underlined portion doesn't apply to fig. 4-29a, therefore what is shown in fig. 4-29a is not pointless.

RE: Positioning hole with edge distance?

Quote (pmarc)

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.

Wow, quite of learning from this discussion.

Just a quick question: Does the datum feature simulator geometry originates from LMB? Not saying it is not, but looks like it is the opposite from what expected.

RE: Positioning hole with edge distance?

Quote (pmarc)

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).

RE: Positioning hole with edge distance?

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.

RE: Positioning hole with edge distance?

Quote (pmarc)

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.

RE: Positioning hole with edge distance?

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.

RE: Positioning hole with edge distance?

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.

RE: Positioning hole with edge distance?

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.

Quote (pmarc 24 Feb 19 12:26)

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.

RE: Positioning hole with edge distance?

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

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.

RE: Positioning hole with edge distance?

Quote (pmarc)

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.

RE: Positioning hole with edge distance?

Quote (pmarc, 25 Feb 19 21:11)

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: Conical datum features/ Linear Extruded Shape/ Complex Shape had a related discussion, with particularly relevant posts by axym.

pylfrm

RE: Positioning hole with edge distance?

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?

RE: Positioning hole with edge distance?

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

RE: Positioning hole with edge distance?

Quote (pylfrm)

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.

Quote (pylfrm)

The standard's treatment of datum features is a complicated mess.
And with this too.

RE: Positioning hole with edge distance?

Quote (pmarc, 27 Feb 19 04:50)

Quote (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

RE: Positioning hole with edge distance?

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).

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