Envelop requirement 1

[elinah34]

Hello,

In ISO Envelop requirement is by definition a perfect FORM at MMC condition. I couldn't figure out if it also limits the feature's form tolerance in any other case.
For example, if I have a hole with a size dimension of 100+/-1 with E modifier - the form should be perfect at MMC, i.e 99 mm diameter. But what is the allowed deviation of form if the size is otherwise, e.g at LMC, i.e 101 mm diameter?
If you can refer me to a good source that explains it, it will be great.
Thanks

 

[Belanger]

According to ISO 14405-1, paragraph 3.12 of the 2011 version that I have:

Envelope requirement
Simultaneous use of a combination of the two-point size as the specification operator applied for the least material limit of the size and either the minimum circumscribed size or the maximum inscribed size as the specification operator applied for the maximum material limit of the size."

In other words, as long as the feature passes the envelope (which was the MMC idea that you mentioned), and as long as it passes the two-point size spec (which ignores form because it's simply two opposing points), then the size is considered good. This of course assumes that there are no other specification modifiers being invoked.

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

 

[elinah34]

Thanks Belanger.
So the form tolerance isn't controlled if the size isn't at its MMC?

 

[greenimi]

So the form tolerance isn't controlled if the size isn't at its MMC?

I think it is (the form it is controlled) within the size tolerance.

 

[Belanger]

As mentioned, it is controlled by the limits of size. The two conditions in my last paragraph ("as long as..." and "as long as...") result in that form allowance.
To use your example of a hole given as Ø 100 ± 1, the maximum "ovality" it can have is 2 mm. That is unlikely to happen, but it would be the extreme, legal form error.

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

 

[elinah34]

Thanks!