Can I put Position control on a Radius

[tmackay]

I've never come across this before so I turn to Eng-Tips

I have a checker marking up my drawing to add position control to a radius.


My thought is No. Because I'm thinking you need a complete hole or shaft to measure two opposing sides to derive the center to make sure it falls within the positional tolerance.

We are working with ASME Y14.5-1994 and I couldn't find anything specific in Section 5. But there are no examples of radii with positional tolerance.

 

[JNieman]

That would be exceptionally difficult and may not actually represent the requirements of the geometry. We make some large rings with radii that are either clearance for mating components, or not-quite-precise locating surfaces for other components. Using profile-of-a-surface has yielded adequate production without incurring excessively challenging measurement. For these situations, the center-point can vary, but so long as it's within the min/max boundaries, I don't care about the size of the radius nor the center point location - thus profile made more sense to me.

Personally, I hate to apply anything but profile to a radius that under 90d or arc. Under 180d of arc I start to use caution, personally. Whether it's legal or not... I don't know absolutely, off the top of my head. I just want to add caution. It can be difficult to accurately measure/gauge such a requirement, in my experience.

My personal feelings may be guided by the heavy use of CMMs and laser scanners more than hard gauging, though.

 

[drawoh]

tmackay,

Ask your checker how he thinks someone will inspect the feature.

--
JHG

 

[SeasonLee]

tmackay

You are right, position can only apply to a FOS(feature of size) or a pattern of FOS. Attached is a tip from Tec-Ease Aug.1999.

Season

 

[mkcski]

Profile is the control of choice for a non-feature of size. Profile alone is pure form control. You can "add" datums to control orientation and location of the radius (from other part features).

Certified Sr. GD&T Professional

 

[powerhound]

5.2(a) in the 1994 standard is where it says that position is used to define the center, axis, or center plane of a feature of size. A radius is not a feature of size.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[pylfrm]

tmackay,

In support of SeasonLee's and powerhound's answers, the following paragraphs from ASME Y14.5M-1994 should convince you (if you are not already) that your radius is not a feature of size:
[ul]
[li]1.3.13 (feature axis definition)[/li]
[li]1.3.35 (true geometric counterpart definition)[/li]
[li]1.3.11 (actual mating envelope definition)[/li]
[/ul]



pylfrm

 

[mkcski]

tmackay:

This does NOT apply directly to your post about what are "regular" FOS. But from an "expanding your understanding" there are related terms to consider. For example: there are "irregular" features of size that are not "full" features but that can capture an actual mating envelope. Please see the attached.

Certified Sr. GD&T Professional

 

[powerhound]

Adding to mkcski's post, the concept of an irregular feature of size does not exist in the 1994 standard that you are using so don't bother looking for it there. It's only in the 2009.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[mkcski]

Powerhound

Thanks for adding the important distinction! I assume everyone has migrated to 2009 since it been almost 10 years since its release (and 1994 is almost 25 year old technology)

Certified Sr. GD&T Professional

 

[JNieman]

Some standards are defined in design program contracts, some of which may last several decades, leaving 'migration' out of reach.