Irregular FOS type (b), ASME Y14.5-2018

[Burunduk]

Does anyone know why para. 3.35.1 (b) references Figure 7-40 as an example for a "feature or collection of features that may contain or be contained by an unrelated actual mating envelope other than a sphere, cylinder, or pair of parallel planes."?
Seems to me that the datum feature simulator for A is a pair of parallel planes, so why "other than"?

 

[chez311]

From conversations I have had this figure is both incorrectly referenced and labeled. It says "Irregular and Regular Features of Size as Datum Features" - there are only iFOS in this figure and no rFOS (no directly toleranced DIMENSIONS only directly toleranced FEATURES - see

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

The reference in 3.35.1(b) is also incorrect, all three datum features are pairs of parallel planes so they are all iFOSa, so the reference should be in 3.35.1(a).

 

[Burunduk]

chez311,
I think you are right. The title of the figure is misleading. Seems like the error was caused by carrying over the figure from the '09 standard (Fig. 4-33) with the title unchanged while changing the 80 dimension to basic and controlling the sides by profile. In the 2009 standard, I suppose it was meant as a directly toleranced dimension although the lack of tolerance seemed odd (In a real-world application I would look for a title block general tolerance but it isn't typical for figures in the standard). It's unfortunate because the committee was on the right track of clarifying the confusion between "directly toleranced feature" and a "directly toleranced dimension" which is related to Regular FOS, defining a "directly toleranced dimension" in a separate paragraph (3.27). However, the title of figure 7-40 may contribute to people keeping thinking that the basic 80 and 30 widths are regular FOS. The reference to this figure from the "type b" FOS definition makes things even worse.

 

[aniiben]

Chez311,
You seem to know this stuff. Could you, please clarify my doubts in the referenced discussion?