Examining the 2009 Irregular feature of size definition

[3DDave]

1.3.32.2
Irregular Feature of Size.
irregular feature of size: the two types of irregular features of size are as follows:

(a) a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope
that is
a sphere, cylinder, or pair of parallel planes
(b) a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope
other than
a sphere, cylinder, or pair of parallel planes

______________​

In comparing (a) to (b) they cover the universe. Logically, (b) is NOT (a), so (a) AND (b) includes all features.

Frankly, this is sloppy and I have no idea how it came about. My guess is that it was to accept the special cases that are given as examples in the standard with no particular thought to what unwanted items might fit under that infinite umbrella.

I'd say that, given that everything is now a FOS of some type, that a lot of the standard could be simplified by just referring to everything as "feature" instead of classifying them. Make simple rules for special shapes and let users figure out the rest - the same way they have to do now.

Under this a flat surface with a locating dimension is a feature of size. That also means and single surface or any surface pair on the infamous Z-extrusion where the small end faces aren't in opposition is a feature of size of some type.

By creating this definition, the committee eliminated the meaning for "feature of size" because everything can fit this definition.

What the committee failed to do was to define what it meant to "contain or be contained". Because of this any reader can decide for themselves what it means, hence the argument about cones. It also opens the old arguments that a cylindrical notch of 1 degree can be considered FOS, or one of 181 degrees, or that maybe a 359 degree cylindrical notch might not "contain" because it's actually open.

For a pre-'2009 feature of size containment was not required. It was a matter of being able to identify the feature as a spherical, cylindrical, or an opposed pair of planar surfaces. A simple yes-no conformance proposition. Now the answer is always "yes." It's great that the simple bugs aren't worked out of the definitions.

The '2009 version also messed the FOS simple definition up with the inclusion of "a circular element." Why not pairs of linear elements? Same reason for excluding LMB datum targets when allowing MMB datum targets I suppose.

Thanks to dtmbiz for drawing attention to this flaw.

Is it still present in the '2018 version? I bet it is.

 

[chez311]

I agree with the sentiment that the definition introduced in 2009 opens the door to interpretation that just about any type of feature or shape can be considered a FOS. Personally I am not convinced of this fact and I think certain features are precluded from being a FOS based on the behavior of the applicable AME, however the definition of IFOSb is certainly open ended enough to allow multiple interpretations.

I think the only real improvement in the definition for 2018 is the specific mention of UAME instead of just AME however the associated definitions of AME/UAME/RAME are mostly unchanged. In relation to your question about linear elements the 2009 definition does mention "a set of two opposed parallel elements or opposed parallel surfaces" and in 2018 explicitly states "a set of two opposed parallel line elements or opposed parallel surfaces".

3.35 FEATURE OF SIZE
feature of size: a general term that is used in this Standard to refer to instances in which both a regular and an irregular feature of size apply.
3.35.1 Irregular Feature of Size irregular feature of size: there are two types of irregular features of size, as follows:
(a) a directly toleranced feature or collection of features that may contain or be contained by an unrelated AME that is a sphere, cylinder, or pair of parallel planes. See Figure 7-41.
(b) a directly toleranced feature or collection of features that may contain or be contained by an unrelated AME other than a sphere, cylinder, or pair of parallel planes. See Figures 7-40 and 11-29.
3.35.2 Regular Feature of Size
regular feature of size: one cylindrical surface, a spherical surface, a circular element, or a set of two opposed parallel line elements or opposed parallel surfaces associated with a single directly toleranced dimension. See subsection 5.2 and para. 5.8.1(e).

 

[CheckerHater]

I wonder if feature of size should still be limited to having only one size? And how to achieve it?
Say, head of hex bolt - six features, one size.

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

 

[3DDave]

Great - does it define what is meant by opposed? For me it means that there are colinear surface normal pairs. That seems to exclude "elements" as they aren't a surface and therefore cannot have surface normals.

 

[greenimi]

3DDave,

I got this attachment from my trips to linkedin where a similar question has been asked.
Somehow opposed term is "defined" --standardized or not.


If you are searching hard enough here on eng-tips you see that are "problems" with circularity definition, cylindircity definition, derived median line straightness definition, etc. so why not with IFOS definition too?

 

[3DDave]

Since "feature of size" implies a size, the committee should have used "volumetric feature" to differentiate those that envelope a volume from those that don't and left "feature of size" as a special case of "volumetric feature" to describe that special case.

Instead they decided that poisoning the whole thing was better so they did not have to make a detailed explanation of how they are evaluated differently in the other sections. No doubt it saved a lot of work in avoiding dealing with the rest of the section members.


How many legs does a dog have if you call his tail a leg? Four. Saying that a tail is a leg doesn't make it a leg.

Abraham Lincoln

And calling a cone a feature of size does not make it so. Thank Abraham Lincoln for setting the correct logic.