Actual Minimum Material Envelope

[Andrew T]

ASME Y14.5 - 2009, 1.3.26 Envelope, Actual Minimum Material - This envelope is within the material. A similar perfect feature(s) counterpart of largest size that can be expanded within an external feature(s) or smallest size that can be contracted about an internal feature(s) so that it coincides with the surface(s) at the lowest points. Two types of actual minimum material envelopes - unrelated and related - are described in 1.3.26.1 & 1.3.26.2.

ASME Y14.5 - 2009, 2.8.4 Effect of LMC - Where a geometric tolerance is applied on an LMC basis, perfect form at LMC is required. Perfect form at MMC is not required. This is the reciprocal of the MMC concept. 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. The total permissible variation in position is maximum when the feature is at MMC, unless a maximum is specified.

The 1994 standard references the actual mating size when describing the effect of LMC (1994 - 2.8.4). 1994 defines actual mating size as the dimensional value of the actual mating envelope. So, it appears that there is a change from the 1994 standard to the 2009 standard with respect to the effect of LMC.

Let's say that I have a plate with a hole in it. The hole has a position geometric tolerance at MMC. The inspector can use a pin gauge to find the value of the AME (related and/or unrelated). If the position call out is changed to LMC now the inspector cannot use a pin gauge since we need to know the AMME (related and/or unrelated). The AMME and the AME can (and most likely will) have different values. I had always thought of the unrelated AME as the size. If you asked me what size the hole was I would put the largest pin gauge in the hole that I could and then report that value to you. If this hole is specified at LMC, and we now care about the AMME, then is the size of the hole now different? It is the same hole, but depending upon being specified at MMC or LMC we are concerning ourselves with the high points of contact or the low points of contact (different envelope values/sizes).

So how will an inspector find the size of the hole specified at LMC? This seems difficult if not impossible to do in a non-destructive manner. Consider a small hole in a tall part. Can a CMM or a scan capture the entire surface of the hole to determine the AMME? If you cannot determine the AMME of the feature then how do you know how much bonus tolerance is allowed?

[Kelly_Bramble]

If the position call out is changed to LMC now the inspector cannot use a pin gauge since we need to know the AMME (related and/or unrelated).

No... You need to re-read the definitions you posted above from ASME Y14.5-2009.