understanding minimum material envelope and mating envelope on practice exam questions for GDTP preperation

[DM888]

I'm having an issue understanding the answers to the following questions. They're from a practice exam for the GD&T senior certification.

1) A tolerance of position or orientation is totally dependent on the size of unrelated actual minimum material envelope of the considered feature when the tolerance is
a) basic, b) zero at lmc, c) next to the size dimension, d) zero at mmc

2) A tolerance of position or orientation is totally dependent on the size of unrelated actual mating envelope of the considered feature when the tolerance is
a) basic, b) zero at lmc, c) next to the size dimension, d) zero at mmc.

The part that is confusing me is the use of minimum material envelope and mating envelope. I may not have a good understanding of their definition and how they apply to zero at mmc/lmc cases.

Thanks,

Dan

 

[3DDave]

It's the difference between how much material is in the part (minimum material) and how much room one needs to leave for clearance (mating envelope)

The first, minimum material envelope, is about how large a hole you could drill into an external feature and just barely hit air; in an internal feature it is how much you would remove to just clean up so that the feature is straight/perfect.

The second, unrelated mating envelope is how large the pin or small the hole in the mating can be and still fit, if the other feature is allowed to be rattled on to fit.

In the case of the related mating envelope then it tells how large the pin or small the hole in the mating part can be when the mating features as determined by the datum features are aligned.

I have used holes and pins as examples for the same reason the standard does - simplicity, not exclusivity.

 

[DM888]

It's the difference between how much material is in the part (minimum material) and how much room one needs to leave for clearance (mating envelope)

The first, minimum material envelope, is about how large a hole you could drill into an external feature and just barely hit air; in an internal feature it is how much you would remove to just clean up so that the feature is straight/perfect.

The second, unrelated mating envelope is how large the pin or small the hole in the mating can be and still fit, if the other feature is allowed to be rattled on to fit.

In the case of the related mating envelope then it tells how large the pin or small the hole in the mating part can be when the mating features as determined by the datum features are aligned.

I have used holes and pins as examples for the same reason the standard does - simplicity, not exclusivity.

Thank you for the explanation! Do you know why the answer to 1) is b and not d? And the same for 2), why d and not b.

 

[greenimi]

Thank you for the explanation! Do you know why the answer to 1) is b and not d? And the same for 2), why d and not b.

I guess you have to understand the position callout first:
What should lie within the tolerance zone (for position callout) ?
Answer this question and then we can go to the next one (we have to be able to walk before we can run)

 

[DM888]

Zero at lmc for the first question

 

[greenimi]

Zero at lmc for the first question

What about Zero at LMC ?

 

[3DDave]

Thank you for the explanation! Do you know why the answer to 1) is b and not d? And the same for 2), why d and not b.

Concern for:
Minimum material = Least material.
Mating material = Maximum material.

 

[Garland23]

I cannot find the term "minimum material envelope" in the Y14.5 standard.

 

[Burunduk]

I cannot find the term "minimum material envelope" in the Y14.5 standard.

2018 or 2009?

 

[3DDave]

I cannot find the term "minimum material envelope" in the Y14.5 standard.

It's part of ASME Y14.5-2018 and ASME Y14.5-2009.

 

[greenimi]

2018

3.30 ENVELOPE, ACTUAL MINIMUM MATERIAL
envelope, actual minimum material: a similar perfect
feature(s) counterpart of largest size that can be expanded
within an external feature(s) or of smallest size that can be
contracted about an internal feature(s) so that it coincides
with the surface(s) at the lowest points. This envelope is
on or within the material. There are two types of actual
minimum material envelopes, as described below.
(a) related actual minimum material envelope: a similar
perfect feature(s) counterpart contracted about an
internal feature(s) or expanded within an external
feature(s) while constrained in orientation, in location,
or in both orientation and location to the applicable
datum(s). See Figure 3-2.
(b) unrelated actual minimum material envelope: a
similar perfect feature(s) counterpart contracted about
an internal feature(s) or expanded within an external
feature(s), and not constrained to any datum reference
frame. See Figure 3-2.

 

[Garland23]

Thanks. I had done a word search in the PDF and having quotation marks around that phrase yielded nothing. Without the quotes I now easily find it!

 

[DM888]

What about Zero at LMC ?

You asked what should lie within the tolerance zone and I said zero at lmc. That is answer b (the correct answer to the exam question)
What am I confusing here?

 

[DM888]

Concern for:
Minimum material = Least material.
Mating material = Maximum material.

Doesn't the positional tolerance depend on the size whether it's zero at MMC or zero at LMC? In both cases there's ONLY a (bonus)positional tolerance when the feature departs from LMC or MMC.

 

[3DDave]

Doesn't the positional tolerance depend on the size whether it's zero at MMC or zero at LMC?

In both cases there's ONLY a (bonus)positional tolerance when the feature departs from LMC or MMC.

There are two components that determine the acceptable variation of a real feature. One is the explicit numerical value; the other is due to the departure from the relevant boundary for LMC and MMC.

If it is "totally dependent on the size of ..." then the explicit numerical value must be zero. Otherwise it would be dependent on both contributors and not totally dependent on one of them.

 

[greenimi]

You asked what should lie within the tolerance zone and I said zero at lmc. That is answer b (the correct answer to the exam question)
What am I confusing here?

I asked what should lie in the tolerance zone.
Do you know what a tolerance zone is?
If yes, then what can lie into the tolerance zone? What options you have ?

 

[DM888]

I asked what should lie in the tolerance zone.
Do you know what a tolerance zone is?
If yes, then what can lie into the tolerance zone? What options you have ?

for a hole, the axis of the hole.

 

[greenimi]

for a hole, the axis of the hole.

Ok. Axis of what? If the hole is not perfect, then axis of what ...you consider to be the axis of the hole?

 

[DM888]

I understand. I found the text in Y14.5 2009 below.
The LMC callout is based on the actual minimum material envelope because this represents the LMC of the hole,
and the MMC callout is based on the actual mating envelope because this represents the MMC of the hole.
Sorry if this was already explained!
I was too focused on the fact that both 0 at MMC and 0 at LMC are dependent on the size of the hole for positional tolerance.



2.8.2 Effect of MMC: Where a geometric tolerance is applied on an MMC basis, the allowed tolerance is dependent on the size of the unrelated actual mating envelope of the considered feature when considering effects based on the axis interpretation. The tolerance is limited to the specified value if the feature is produced at its MMC limit of size. Where the size of the unrelated actual mating envelope of the feature has departed from MMC, an increase in the tolerance equal to the amount of such departure is allowed.
2.8.4. Effect of LMC:
Where a geometric tolerance is applied on an LMC basis, the allowed tolerance is dependent on the unrelated actual minimum material envelope of the considered feature. The tolerance is limited to the specified value if the feature is produced at its LMC limit of size. Where the unrelated actual minimum material envelope of the feature has departed from LMC, an increase in the tolerance equal to the amount of such departure is allowed.

 

[Wuzhee]

The key in the text is the "location of the envelope".
For MME it's INSIDE, for AME it's OUTSIDE.

If the feature is at LMC then the LMC IS the UMME.
If it's at MMC then the MMC=UAME.