Free State with Average Diameter

[powerhound]

Since all geometric tolerances apply in free state condition by default what is the purpose of the circle F in Fig. 6-53? I have always been under the impression that a circle F modifier is only used when there is a restraint note on the print and the specified feature is to be check unrestrained. This is why I'm okay with Fig. 6-54. There is a restraint note. Fig. 6-53 has no such note though.

My guess is that the circle F frees it from the rule that says a form tolerance must be a refinement of size tolerance but can someone confirm or deny that? In the case of 6-53 it's not a stretch for me to imagine that the high and low limit of an average is not a feature of size.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[drawoh]

powerhound,

I take it you are using ASME Y14.5M-1994. I cannot find the figure in ASME Y14.5-2009.

The figure looks okay to me. It controls free state circularity, and it controls size, independent of circularity.

--
JHG

 

[Belanger]

PH -- check out paragraph 1.4 (l) of 1994 or paragraph 1.4(m) in 2009. Apparently the free-state-as-default rule doesn't apply to non-rigid parts (1994 was actually more clear about it, if you ask me).

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

http://www.gdtseminars.com

 

[powerhound]

Yes, it's in the 94 standard. Thanks for your response.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

JP,

So could one safely infer that the opposite applies to non-rigid parts - that dimensions and tolerances apply in the restrained state? I never thought of it that way but I guess it does make sense.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Belanger]

Hmmm... I wouldn't say that. It just means that there is no implied condition. The 1994 standard directs us to the very end of the book, where "non-rigid parts subject to free state variation" may be controlled in 3 ways. These are paragraphs 6.8.1, 6.8.2, and 6.8.3.

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

http://www.gdtseminars.com

 

[fsincox]

Free state is the default condition UOS, in those otherwise specified conditions where restraint is required you need a modifier to differentiate which tolerances apply, now, in the restrained condition and which apply in the free state.

 

[powerhound]

Frank,
I'm specifically referring to figure 6-53 to which there is no restraint called out but the free state modifier is in the FCF.
The reason the question was asked was because I had a print come across my desk with the AVG diameter with a circularity callout in the free state. The problem was that it was a rigid part so I thought maybe there was something I was missing. It turns out the guy was trying to allow more circularity error than the size tolerance. I wound up telling him to add the note "PERFECT FORM AT MMC NOT REQUIRED" to that feature.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

Powerhound,
Keep in mind that "PERFECT FORM AT MMC NOT REQUIRED" unties form of the feature (pin) along its axis too. So you it will not only loosen up circularity but also axis straightness (unless there is additional callout on the print limiting the straightness).

 

[powerhound]

The part itself is about 1/2" long with a diameter of about 3/4" so hopefully releasing the straightness tolerance from the size tolerance won't do much damage. I'm never surprised by just how much a really bad machine shop can try to get away with though. While I had thought of the consequences of overriding rule #1 and it's effect on the straightness, I always appreciate someone checking me. Thanks pmarc.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II