Regular vs Irregular Features of Size, Whether They May Be Defined with Basic Dimensions and Profile 3

[superptrucker]

In "The Ultimate GD&T Pocket Guide, 2nd Edition" by Alex Krulikowski, on page 13 there is a table that distinguishes Regular vs Irregular features of size. One distinguishing feature is "May Be Defined With Basic Dimensions and Profile" and it shows No for Regular FOS, and Yes for Irregular FOS. I am having a hard time understanding the meaning of this. Can anyone clarify? Thanks in advance.

 

[Burunduk]

The definition from ASME Y14.5-2009 should clarify this (sorry for not quoting the latest standard, but this is the one I am used to working with):

"1.3.32.1 Regular Feature of Size. regular feature of size: one cylindrical or spherical surface, a circular element, and a set of two opposed parallel elements or opposed parallel surfaces, each of which is associated with a directly toleranced dimension. See para. 2.2."

The portions of para. 2.2. that describe directly toleranced dimensions are:

"(a) Limit Dimensioning"
i.e:
50.2
49.8

And,

"(b)Plus and Minus Tolerancing"
i.e:
50+/-0.2

The third part of para. 2.2 is
"(c) Geometric Tolerances Directly Applied to Features."
Which does not describe directly toleranced dimensions and does have to do with basic dimensions and profile - which may be relevant to an irregular FOS.

 

[chez311]

supertrucker,

As alluded to above it lies in the wording of the standard's distinction between a directly toleranced dimension utilized for a rFOS (Regular FOS per Y14.5-2009 para 1.2.32.1) and a directly toleranced feature utilized for an iFOS (Irregular FOS per Y14.5-2009 para 1.3.32.2).

Just to provide a resource, the below thread was very valuable to me. Theres quite a bit of what Evan calls "unresolved opinion-based" disagreement around these definitions due to the way they are defined in the standard. The relevant portion starts around JP's reply on (24 Apr 20 14:34) and especially with pmarc's comment (28 Apr 20 05:06).

https://www.eng-tips.com/viewthread.cfm?qid=467965

 

[Tarator]

superptrucker,

Maybe this will help...

 

[superptrucker]

I always thought that a FOS had to do with the geometry, but I guess according to the standard, if a have a simple rectangular block, and I use basic dimensions and an all over profile tolerance, the opposing sides of the block are not considered features of size because they are not associated with a directly toleranced dimension. Is this correct?

 

[Burunduk]

Yes it is.

 

[Burunduk]

Another thing worthy of note in the same context:
The only occasion when it matters if a feature is classified as a regular feature of size or irregular feature of size is for the involvement of rule #1. This rule only applies to a regular feature of size. The basically dimensioned and profiled feature on the left side of Tarator's figure doesn't need rule #1, because the limits imposed by the profile tolerance zone require perfect form if the feature is produced entirely at MMC (or entirely at LMC, unlike in the directly toleranced case).