True Position and datum and MM

[[An...]]

Any thoughts?
See attached.

Contribution_11_05_2020_1.pdf

[[Ma...]]

Moin Andreas,

Ja "B" ist als Profilform zu "A" angegeben. Und in deiner Pos. 0,3 zu AB hat "B" Material-Bedingung. Wie Du das in Calypso für "B"
anwendest weiß ich leider auch nicht. 😕

[[An...]]

Meine Frage zielte auf die Sinnhaftigkeit von (M) ab. (in diesem speziellen Kontext)

[[SH...]]

It is a wrong call out, the datum must be feature of size..


Humble request, please use English language.

IMG_20200511_143314.jpgIMG_20200511_143300.jpg

[[Aa...]]

So, what, Andreas, is this a poll to see how many people who are not familiar enough to recognize an example lifted directly from the standard will give answers different from the explanation of this example in the standard?

The MMB of that datum feature is the limit set by the bilateral profile tolerance of 0.2, 0.1 outside the basic geometry, and 5.1 away from the axis of A.

I do agree that this part of the standard is somewhat confusing. Typically, MMB introduces DRF mobility. In this case, it provides a fixed simulator that the datum feature must contact, but it sets the position of that simulator at a position that is different from the basic position. And no, the standard's interpretation is not immediately intuitive to most who have coordinate measurement for their bread-and-butter.

[[An...]]

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Well,maybe I am looking for someone who practices the same unconventional thinking than me.
And I am still not willing to give up.

See attached.

Contribution_11_05_2020_2.pdf

[[Aa...]]

Aha! I see what you did there. You've altered it to make it essentially equivalent to the example in 4-30(b).

And yes, this is a glaring hole in the definitions of the standard. It fails to define the difference between cases when these interpretations are to be applied (when is contact between the datum feature and the simulator required, and when is it optional?). I've held for a few years that this is something that needs to be addressed.

Oh, shoot! I may have just outed myself as an unconventional thinker...

[[Me...]]

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Doesn't the standard already make a clear distinction that Fig. 4-31 applies to a planar datum feature that is offset relative to a datum axis? Rotation would be left uncontrolled in one direction if the datum feature was not required to be in a least one point of contact.

[[To...]]

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Check out fig 4-31

[[SH...]]

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Ok, got it.

IMG_20200511_213233.jpg

[[An...]]

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Welcome to the club!

[[Aa...]]

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Yes, 4.16.7 points our that in 4-31(c), B is offset relative to A by 5 mm. Of course, that is not in contrast to 4-30(b), where B is offset from A by 15 mm.

I think you took "offset" to mean "not centered on the preceding datum axis". But how far off center would warrant requiring contact?

[[Me...]]

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Yes, I feel that "offset" as used in 4.16.7, means not being centered about the datum axis.

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I'll have to hang my hat on the last sentence of 4.16.7

Where the datum feature simulator
and the higher precedence datum axis do not limit
rotation in both directions about the datum axis, the
datum feature must always contact the datum feature
simulator.

[[Aa...]]

Indeed. I had overlooked that sentence. Good catch.

[[An...]]

Maximum Material Condition.
Sometimes hard to understand.
See attached.

Contribution_13_05_2020_4.pdf

[[SH...]]

😮 😮 😱 😱

[[Me...]]

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I agree that it can be confusing at times…

It should be noted that both the (M) modifier and [BSC] indication will behave in the same manner since both are doing nothing more than defining a specific fixed location of the datum feature simulator.

The (M) modifier is only defining the simulator location at 5.1, just as the term [BSC] is defining a 5.0 simulator location. Datum feature shift / displacement is not allowed in either case because the datum feature is not being constrained or limited by the simulator. Section 4.11.9 addresses this specific issue and goes on to state the following;

“If the datum feature simulator geometry is
such that it does not fully limit or constrain the feature
such as rotating away from the datum feature simulator
beyond the established boundary limits, as shown in
Fig. 4-31, illustration (c), then the feature must remain
in contact with the datum feature simulator, and datum
shift or displacement is not allowed.“

I do realize that the part will physically rotate differently based on the as-produced datum feature geometry, in addition to the datum feature simulator location. But this does make the MMB modifier any less applicable than the [BSC] indication.

[[An...]]

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So what is the real meaning of the (M),assigned to datum B? (Pic. 4-31c)

See attached.

Contribution_14_05_2020_1.pdf

[[Me...]]

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It's indicating that the datum feature simulator is to be established at the MMB of 5.1. This will produce the "Click Phenomenon" and does not allow datum shift or displacement.

The "Click-Clack Phenomenon" is described in paragraph 4.16.4 of the standard.

[[Wi...]]

Id go and punch the engineer that made the drawing

[[Wi...]]

Has to be a feature of size.....

[[Aa...]]

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Not if you follow the standard. Andreas' example was copied directly from Y14.5-2009 (He even included the figure number for us.)

[[An...]]

My point is that I can't see a significant difference between "[BSC]" and "(M)"
when it comes to a kind of "Datum-Benefit of B".

The "datum B-benefit" is immanent to Pic. 4-31b.
The datum B(M) is in my opinion superfluous.

Whether 5.0000 or 5.10000 seems to be marginal.

Correction:
Whether 5.0000 or 5.10000 does not seem to be marginal.

[[An...]]

See attached.

Contribution_16_05_2020_1.pdf

[[Me...]]

Andreas,

The tolerance zones are not allowed to rotate as they are to be basically oriented / located relative to the applicable datum feature simulators. Aside from that, I believe that you've shown that the (M) modifier will bias the part rotation in one direction. If this is something that is not desired, then the designer would need to consider another option.