new question on figure 7-45

[AndrewTT]

The continuous feature symbol is used in figure 7-45. It is connected to datum surface A.

2.7.5 Limits of Size & Continuous Features of Size - "...CF is used to identify a group of 2 or more FOS where there is a requirement that they be treated geometrically as a single FOS." That is pretty clear wording imo. CF cannot be applied to a surface but only to a FOS.

1.1.14 Figures - "...In some instances, figures are incomplete by intent." Has 1.1.14 been invoked in figure 7-45? Did the authors of the standard use the CF symbol incorrectly in figure 7-45 simply to convey that both surfaces make up datum A without having to show the dimension of that FOS, so as to not distract from the point of the figure (three segment composite tolerance)? To me this is not an incomplete figure so much as an incorrect figure. Leaving off the CF symbol would have been an acceptable incomplete figure. Or is this acceptable use of the CF symbol? Figures 2-8 thru 2-10 show the CF symbol associated with the size dimension. I would think that if there were alternate ways to use the CF symbol then figures 2-8 thru 2-10 would have been the place to show it, not buried in a figure on composite tolerances in chapter 7.

So, legit use of CF or not??????

 

[Belanger]

A good observation, and one that I believe has cropped up here in the past. I agree that CF shouldn't be used in that manner. Sure, they want the two surfaces to be treated as a continuous feature, but what's the tolerance on that coplanarity? It's not defined.

I supposed they could point to that statement that the drawings are incomplete, and if a flatness tolerance were added then it would be OK. But the standard prefers profile of a surface to be used to control conplanarity.

So my vote is that it was a goof, or at least a weak example because of this confusion. I don't have the draft of the proposed revision in front of me, but I think it is corrected there.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[3DDave]

It's to tell that datum A is both surfaces and not just on half. Any tolerance could be handled elsewhere, but isn't the topic of the figure.

I think there is too much read into the FOS explanation, which is about interpreting a CF when applied to a FOS; the key is the tail "... where there is a requirement that they be treated geometrically as a single feature of size."

The base definition is:

3.3.23 Continuous Feature Symbol
This symbol indicates a group of two or more interrupted features as a single feature. See Figs. 2-8 through 2-10 and 3-11 and para. 2.7.5.

 

[AndrewTT]

Based upon the wording of 3.3.23 I would have to say that it is an acceptable use of CF. I wish there was a note on 2.7.5 indicating that there is more explanation of the use of CF in 3.3.23 and vice versa. The index does not make finding all mentions of CF very easy.

 

[mkcski]

AndrewTT:

I have had issues with CF's too. Realize that the Figures in the Std are examples and the Std does not attempt limit the interpretation to the depicted example AND it does not present examples of all possible applications and/or interpretations.

FYI: I have a copy of the latest Draft of the next release of the Std. There are now two definitions for CF : "Continuous Feature" and "Continuous Feature of Size". And the definition of "Interruption" has been added. See attached

Certified Sr. GD&T Professional