GD&T. Apply Flatness to the Planar Feature of Size

[srikanthsatyam]

The GD&T book is saying that, In form controls only straightness can be applied to the feature of size.
For example if we apply the straightness to the planar feature of size, it controls the median lines of the two opposite planes.
Similarly we can apply the flatness to the planar feature of size, so it can control the median plane of the two opposite planes.

Am I correct, if not please explain me, thanks in advance.

 

[pmarc]

Yes, you are correct. ASME Y14.5-2009 shows this in figs. 5-8 & 5-9. This is new concept in ASME not defined prior '09 edition, though considered as a logical extension of straightness applied to the axis of cylindrical FOS.

 

[SeasonLee]

When will apply the flatness (or straightness) to planar feature of size? anyone who can provide an application example?
Thanks

SeasonLee

 

[fsincox]

Any time you want to exempt a FOS from "the envelope principle" and want to still exert some control other than just "I".

 

[MechNorth]

Commonly used to control flatness of stock material. Recall that Rule #1 doesn't apply to stock material. As I tell my students, using flatness on a feature of size controls the derived median plane, which is a difficult thing to verify UNLESS you modify it at MMC; in that situation, you are dealing with the virtual condition, which is easily checked.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Jim ... a quick side note. It's not required to use flatness of a FOS on stock sizes, because although Rule #1 doesn't apply to stock sizes, it is understood that there is an industry or gov't standard that already controls the form (para. 2.7.2).
I realize that you are saying that stock is an example of where to use FOS flatness, but I'm just adding that it's not a must.

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

http://www.gdtseminars.com

 

[MechNorth]

Good point, J-P. Which raises the question, where do you get the industry specs? I think we dealt with that in a thread a while back. They are available (as far as I've had to search for them), though they can take some digging.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Ah, yes... That's like the ole' saying that "established tooling or gaging tolerances shall apply."

Shrug.

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

http://www.gdtseminars.com