Non-Circular Features of Size tolerance zone

[claforet]

Hi,

On the drawings where I work we have a general tolerance called out for the posiition of non-circular features of size. Normally, this is a rectangular tolerance zone but the plant has asked me to make this tolerance zone cylindrical. Does anyone know if this is forbidden at all by the ASME Y14.5M-2009 standard?

Thanks

 

[Belanger]

If it says it's for non-features of size, and there is no diameter symbol, then it is to be interpreted as a "boundary" situation, similar to Fig. 7-34 in the 2009 standard. Notice that this is not a rectangular tolerance zone; it actually takes on the same shape as the true feature. And a cylindrical zone (by adding the diameter) probably doesn't help the situation of the plant.
I'd say that you and them should noodle Fig. 7-34 and see if that's OK. (The difference is that you might have one feature control frame trying to control the entire thing, whereas Fig. 7-34 breaks it into two feature control frames.)


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

 

[dingy2]

A diametrical tolerance zone applies to features of size that are round or spherical and can be applied in either the MMC or RFS mode.

Non-round features should not have a diametrical tolerance zone since one would lose orientation. Having a .005 positional tolerance on a non-round shape means .005 smaller on a hole or large on a pin than its MMC size or actual size, depending upon how it is shown in the feature control frame.

Dave D.

http://www.qmsi.ca

 

[pmarc]

claforet,
By saying non-circular FOS, do you mean something like features of rectangular/square shape or rather two opposite parallel planes (e.g. sloth width)? Based on the fact that you mention about rectangular tolerance zone, I assume the first option, but I would just like to have it clear.

Answering as briefly as possible to your question, ASME Y14.5-2009 does not specifically forbid defining cylindrical tolerance zone for non-circular features of size like rectangular holes, etc., but it does not show any example of such application too. Why? Probably because it is quite illogical and inpractical to apply cylindrical tolerance zone to features that are nominally not round/cylindrical.

Basic questions that I would ask to the plant are: Why do they need it? What is the function of a non-circular FOS that cylindrical tolerance zone is required?

Maybe the concept that J-P described could work? Are you able to show any sketch that presents the issue?

 

[ptruitt]

The question makes me wonder if your company is hoping for too much good to come out of default tolerances. I can see the need for situations similar to Figures 7-38 and 7-46 (Y14.5-2009) in many scenarios... Multiple single-segment feature controls of position and profile might be needed.

Peter Truitt
Minnesota

 

[fcsuper]

This request may also come from a misunderstanding of how the axis is determine using the FCF and surface of the feature. The closest method I can think of to get the interpretation for which they are asking is to use multiple symmetric FCFs. That would be ugly. Have you considered composite profile FCF?

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
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