Actual Minimum Material Envelope

[AndrewTT]

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?

 

[pylfrm]

AndrewTT,

I'll try to answer based on the 2009 standard here because it seemed to be your main focus, and because I am more familiar with it.

There are multiple notions of size for a hole. They are not all relevant at all times though. When the envelope principle is in effect (perfect form required at MMC but not LMC), then a size tolerance effectively provides a maximum actual local size, and a minimum unrelated actual mating envelope size. When the envelope principle is overridden with a position tolerance at LMC (perfect form required at LMC but not MMC), then a size tolerance effectively provides a minimum actual local size, and a maximum unrelated actual minimum material envelope size.

Side note: For position tolerances at LMC, the surface interpretation takes precedence over the axis interpretation and associated bonus tolerance. The virtual condition boundary must exist entirely within the material of the part.

Scanning the surface, with a CMM or otherwise, should be a reasonable inspection method. Collect as many data points as needed to provide the desired confidence. This can indeed be problematic with long narrow holes, but no good alternatives come to mind.


pylfrm

 

[mkcski]

AndrewTT:

I agree with Pylfrms' response. However, if this is an actual production part and not just a case study (as a learning exercise), I have another consideration. LMC has very limited use - typically structural considerations like wall thickness and the ligament thickness between holes (like in a heat exchanger tube sheet) and may be misapplied. Please confirm LMC is appropriate for the design criteria.

If it is correctly applied, then excessively scan the surface of the hole for size and form condition - perfect form at LMC per 2.8.4. Since the UAMME is "in" the metal hard gauging is not possible. Then by a best-fit practice or some other analysis process, determine the size of the smallest circumscribed cylinder (that is in the metal). This is the size to use to determine bonus tolerance. The axis of the envelope must fall inside the positional cylindrical tolerance zone plus any bonus tolerance due a hole smaller than LMC size.

 

[AndrewTT]

Thank you for the responses pylfrm & mkcski. Yes, this is just a learning exercise. I am preparing for the 2009 certification test and noticed the wording of AMME when reading. I had been studying the 1994 standard so this caught my eye.

 

[mkcski]

AndrewTT

I studied for about 1 year - basically I had the 1994 Std memorized - get a life comes to mind. I took the Senior Level exam in 1999 in Baltimore MD - I live near York,PA. It lasted 5 hrs. Grueling at best, but I passed. As I remember it, there were mostly multiple choice questions and they were statements that either supported or contradicted the standard - theoretical more than application content. There were a few that required some calculations - bonus tol, etc. I found out later they avoided application questions due to the huge diversity of engineered parts that one may, or may not, be exposed too - makes sense and if fair but does not test "real" understanding of the subject.

Did you schedule a place and time?

I wouldn't mind hearing a synopsis of your exam experience.

Post more questions as you study.

Good luck with your exam.

 

[AndrewTT]

I will be taking the technician level test. I just began using/studying GD&T in January. I'm probably over preparing, but I don't want to have to take it twice.