Suppression of envelope - Thickness defined independently of shape of flexible part? (ASME Y14.5)

[Th.Ro.]

Is there a way to indicate that the envelope principle should not apply when defining the thickness of a flexible part, for example a gasket made of a flexible rubber sheet?

This is how the part should be allowed to look like this when not constrained:

But on the drawing (and if installed) it should look somewhat like this

In ISO this would be an easy thing to define but with ASME Y14.5 invoking the envelope principle by default it seems to be impossible...

 

[greenimi]

2.7.2 Form Control Does Not Apply (Exceptions to
Rule #1)
The control of geometric form prescribed by limits of
size does not apply to the following:
(a) stock, such as bars, sheets, tubing, structural
shapes, and other items produced to established industry
or government standards that prescribe limits for
straightness, flatness, and other geometric characteristics.
Unless geometric tolerances are specified on the
drawing of a part made from these items, standards for
these items govern the surfaces that remain in the asfurnished
condition on the finished part.
(b) parts subject to free-state variation in the unrestrained
condition. See para. 5.5.

2009/ page 28


So, I think that the flexible part are exempted from envelope requirement.

 

[Th.Ro.]

After having another look at ASME Y14.5 (2009) I think I found the solution:

So I think the drawing indication should be something like this:

 

[greenimi]

I think you are correct.
The Independency symbol is used to get rid of the rule#1.

But since in your original post you mentioned flexible part/ gasket made of a flexible rubber sheet, I think rule#1 is not in effect in the first place.

 

[3DDave]

greenimi - the part that people skip is "produced to established industry or government standards that prescribe limits for straightness, flatness"

There has to be traceability on the drawing to those standards for them to apply, not just being pretty sure that there might be some that could. So if the material spec was to an ASTM requirement that included flatness, that would be fine, but if it's just bulk material properties for being rubber, then the drawing should specify the limits.

 

[Sem_D220]

"This is how the part should be allowed to look like this when not constrained" and the embedded image that follows imply Free State Variation. That means (b) in greenimi's quote is in force. It doesn't have to do with industry/government standards.

Edit: rephrased for clarity.

 

[3DDave]

Sem, I think I'm finally starting to understand your approach.

 

[greenimi]

Sem, 3DDave and all,

The question I have is: how do you know that we are dealing with a flexible part (and no rule#1 applicable)?

Is it the Independency symbol mandatory or you just have to find the material used and make the assumption from there? My questions are related with the scenario in which F symbol (free state variation) is not used, as shown in the OP original posting.

 

[Sem_D220]

greenimi,
The OP showed in the first image that the part can be bent when not restrained.
If this information is not provided and all you have is a drawing not showing the view with >= 1.005 limit in bent condition - how do you know a part is subjected to free state variation in the unrestrained condition? In the drawing, it will be shown as it is in the second image in the OP (flat). A drawing note should specify the restraint method. From there one should conclude that rule #1 doesn't apply.

 

[pylfrm]

There should always be a clear procedure to determine whether the envelope principle applies to a particular size tolerance, but ASME Y14.5-2009 para. 2.7 is poorly worded and does not provide that. The drawing creator must take responsibility for eliminating the ambiguity when needed. In this case, using the independency symbol is the proper solution.


pylfrm