Datum and LMB

[[An...]]

So, I see that Pic. 4-17 seems to be the law.
Nevertheless I would like you to understand what LMB and MMB is.
it's not so difficult if you are ready think in an unconventional way.
From my side: end of the thread

[[Ja...]]

Andreas,

If you know the true unconventional meaning behind LMB and MMB, please feel free to share before ending the thread. All we have to go by are the standards. If you know of a better explanation, by all means speak your mind.

Side question: Have you considered involving yourself with the ISO or ASME standards committees?

[[An...]]

The "Wall-Thickness-Principle" allows further deviation in perpendicularity
and therefore a larger boundary-diameter.
Could be necessary when it comes to the
design of a "Checking-Gage" in order to substitute the CMM.
(attributive check)

Secret "virtual" lines within the material are not helpful
for the real people out there at the CMMs and design-departments.

Pic_6-13_a.pdf

[[Aa...]]

I'm not sure why you insist on pretending that LMB works the same way as MMB, only with different sized boundaries. That's not what the standard says. You've made it clear many times in this forum that you don't like the ASME standard. But your insistence that the standard is written wrong will not help those who need to react to how the standard is actually written.

[[Ja...]]

Andreas,

I think I am understanding what you are trying to say, but I do have an issue with your Pic_6-13_a.
Wouldn't the gage that you propose accept a part whose measured perpendicularity value was greater than .005 when produced at it's LMC? You could have a part measuring .010 Perpendicularity when produced at LMC size 15.966 but still fit in your gage. That part would be non-conforming, no?

I do not know much about ISO, so correct me if I am wrong, but something is telling me that you are treating this as if we had reciprocity?

Then again - If a part is toleranced as shown and is allowed more orientation error as it gets larger, there is no way that it can be a mating component. If it were a mating component, then your logic would hold true. The application of LMC must have some other purpose, like wall thickness preservation or something along those lines.

Also, it may help to keep the conversation on either LMC as applied to a feature or LMB as applied to a datum feature.

[[An...]]

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To formulate a little bit more precise.
Picture 4-17 combines two topics in one graphic constellation:


1.) Modifer (L)
2.) Datum B



1.) Ensuring a minimum of wall thickness
2.) Defining a secondary datum feature perpendicular to A
made for CMM or as an inner Ø of a gage.

[[An...]]

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Correction!

I don't like illustrations that I don't understand on the first and the second glance.
For me,this forum is the only means to provocate some brain storms.

[[Me...]]

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I couldn't disagree more. Virtual condition is invaluable in assuring parts always mate at their most extreme deviations.

[[Ja...]]

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Andreas,

You are correct. The problem here is that the standard defines the size of B(L), as secondary to A, to be the LMC size of B (24.5) MINUS the geometric tolerance on B with respect to A (0.1) so the LMB size is 24.5 - 0.1 = 24.4. This is the size that B is to be simulated at per Y14.5 and cannot be done with gage elements as they would interfere with the part material.

If you simulate datum feature B by adding the 0.1 to the 24.5 and then add the difference between MMC and LMC size (0.5) for a total of 25.1, you are actually simulating datum B if it were referenced at MMB, not LMB. Per Y14.5.

Think of it mathematically - Let LMC size be L, MMC size be M, and the geometric tolerance be T.
LMB = L - T
MMB = M + T
Your method proposes
LMB = L + T + (M - L) = L - L + M + T = M + T = MMB

But LMB != MMB.

[[An...]]

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Yep!
This question brings it on the point.

The sense of (L) on a perpendicularity:
Allowing more skewing of a cylinder in dependence on it's outer diameter (UAME)
Simultaneously more skewing means an enlargement of the envelope diameter (RAME)

This envelope diameter (MCC in CMM-terms) is constrained to the datum plane A
and is then the datum feature B.

Therefore
When talking about MMB or LMB on outer diameters:
The mode of functioning is always the same.

MMB: all actual RAME-diameters have to lie between 15.966 and 16.034
LMB: all actual RAME-diameters have to lie between 15.966 and 16.052

Diameters smaller than 15.966 are illegal. (also if we talk about a datum feature)

There is no technical reason for any weird "substraction"!

[[An...]]

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Example of a weird substraction.

[[An...]]

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The "LEAST" creates the higher limit.
The "Maximum" creates the lower limit.

[[Ma...]]

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This applies to outer dimensions ( aka shafts and so on ). For holes it is exactly opposite.