AME of a contoured or planar feature?

[Burunduk]

ASME Y14.5-2018,
According to 7.16.2 and 7.16.4,
"Where datum feature B departs from
MMB, relative movement (rotation) can occur between the true geometric counterpart for datum feature B and the related AME of datum feature B"

These sub-paragraphs refer to figures 7-31 and 7-33, in which datum feature B is a "contoured datum feature" (an arc of less than 180°) and a planar face respectively. In the 2009 versions of these texts, there was no mention of the related AME.
I understand MMB for (secondary or tertiary) datum features without size, but I'm not sure that I understand the mention of AME.
Does this imply that features without size have AME? I was under the impression that they don't. Does this indicate another case of a liberal use of the term "size", this time in the definition of "Envelope, Actual Mating"?

 

[axym]

Burunduk,

That's another mysterious change that I never noticed before.

The new statement says essentially the same thing as the statement that follows it, but adds additional details that are partly incorrect. The relative movement (rotation) occurs because of the clearance between true geometric counterpart and the surface of datum feature B (not its AME).

If an AME was defined for a less-than-180 arc surface or a planar surface, which it is not, it would not apply in this context. I'm not sure what they were getting at.

If we apply a liberal use of the term "size", the true geometric counterpart in Figure 7-30 could be described as a related AME of datum feature B. But I don't see the connection to the MMB case in 7-31 (and especially not 7-33) at all.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Everything is now a feature of size. If that were not the case much of this, as in this particular example, would be gibberish. Think of it like "feature of size on demand" or "We can't define a feature of size, but we know it when it suits the example."

 

[Burunduk]

The new statement says essentially the same thing as the statement that follows it, but adds additional details that are partly incorrect.

Exactly. The statement that follows is carried over from the 2009 edition, and it absolutely makes sense:
"Datum feature B may rotate within the confines created by its departure from MMB and might not remain in contact with the true geometric counterpart."
The only thing the new statement adds is confusion.

Everything is now a feature of size

Maybe they are going in the direction of being similar to ISO GPS. I don't know whether ISO has a definition for a feature of size, but it looks like they don't need one, because there is the independency principle, and because position can locate a planar surface. Regarding rule #1, even if it is not going away, it could simply be defined to apply to external and internal cylinders, slots and tabs. Then, it may not matter if everything is a feature of size, or nothing is a feature of size.

 

[CheckerHater]

Could we approach it from different direction?

Not everything is feature of size, but everything can have envelope?

After all, we got used to the idea of using profile on flat features where flatness or parallelism could work perfectly well.

So maybe we should extend our principles towards wider definition of envelope? (I am not saying I like it).

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future