Modifiers in GD&T 2

[arasto]

When we give MMC or LMC modifier in any position tolerance in GD&T, what will be the applicable tolerance of the feature when part is not made on MMC

 

[greenimi]

Bonus in the amount of the departure from the specified condition. Ex feature departs 1mm from the MMC THEN position tol became 1mm bigger.

 

[arasto]

Yes, got it. Thanks

 

[powerhound]

Minor detail, but the departure must be from MMC towards LMC.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[greenimi]

To John:

Can be “otherwise”?

Departure from MMC to where if it is not “LMC”?

If the feature departs from the MMC toward “the other boundary” (lets stay in the Y14.5 terms and call it virtual boundary) then the feature is non-conforming already, by violating its size requirements.

Am I right?

You might say, “ Yeehh, but you don’t know that!!” You will find it later if you are measuring the size (and you should).

 

[drawoh]

Minor detail, but the departure must be from MMC towards LMC.

If you call your feature up at MMC, the least material condition is allowed to have a significant positional error. Is this a valid LMC condition?

I like to user zero positional error at MMC. This means that the MMC outline must not be violated. The measurement below MMC is your positional tolerance.

--
JHG

 

[greenimi]

Quote: "I like to user zero positional error at MMC. This means that the MMC outline must not be violated. The measurement below MMC is your positional tolerance"

I agree! But that would be valid if you can "afford" the size tolerance increase. (like if you assembly something with clearance).

 

[KENAT]

I agree! But that would be valid if you can "afford" the size tolerance increase. (like if you assembly something with clearance).

Which is probably the main application for Position with MMC so reasonable initial stance.

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[greenimi]

Quote: “Which is probably the main application for Position with MMC so reasonable initial stance.
Yes it is, but also could be used to maintain the minimum clearance. Don’t you agree?

 

[KENAT]

I'd use LMC for that normally unless I'm misunderstanding.

Main place I'll use LMC is in patterns of holes for mass reduction or air flow or similar.

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[pmarc]

I would just like to touch one aspect that has not been mentioned yet, which, in my opinion, is in many real life situations largely ignored...

When people try to explain logic behind position tolerance specified at MMC (or LMC) something like this can be very often heard/read: "When the size of the feature departs from MMC (or LMC), additonal tolerance (bonus) is available..." so on and so forth.

But what size exactly are we talking about? Would someone like to clarify?

 

[greenimi]

Size of UAME if primary and RAME if secondary or tertiary. Am I right?

 

[greenimi]

Kenat,

Imagine two external features that are adjacent to one another (two buttons on a keyboard) and the minimum edge distance between them must be maintained. MMC would be the correct modifier to use.
See page 229 (Bryan Fischer book)- Tolerance stackup

 

[pmarc]

What do you mean by: "... and RAME if secondary or tertiary"? Not sure I get it.

What about feature of size controlled by position tolerance at LMC? Size of what should be used to calculate bonus?

 

[greenimi]

For LMC:

It's not the local size and it is not the actual mating envelope, it is the unrelated actual minimum material envelope. When that envelope departs from the LMC size, you have more material to work with and therefore can physically and by design permit more tolerance.

 

[greenimi]

For MMC: I will go with the size of UAME. (to much speed got myself into trouble and misspoken about the datum shift)

 

[pmarc]

That is right, greenimi. Thank you.

 

[greenimi]

Thank YOU for keeping me honest!!

 

[KENAT]

Sure, for patterns of 'male' features MMC would be the way to do what you describe, but I'd bet it's a less common application.

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[powerhound]

I guess I should have checked back sooner. Para. 2.7.1 (b) uses the specific terminology that I used. It states "MMC towards LMC." It was obviously important enough for the committee to be that specific. I didn't think it would raise this kind of ruckus. All it takes is one guy to try and pass off a part saying something like "It doesn't say which direction the departure has to go." to get the standard chaged so that no one else is history will ever make the same assumption.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II