Directly toleranced radius with a "located" center, ASME Y14.5-2018 4

[Burunduk]

One of the changes from the 2009 to the 2018 version of Y14.5, is the interpretation of the tolerance zone of a radius dimension with a direct tolerance.
According to 5.16.1 (page 39), the problematic crescent shaped tolerance zone from the previous versions is still intact, but only when the center of the radius "is not located". The new thing is - "When the center of the radius is located via dimension(s), the arcs are concentric". No example is shown for the new definition.

Does anyone here know or understand the reasoning behind this change and how the new scheme with the located/dimensioned center is to be implemented correctly? Is the idea here to establish some sort of true center on which the tolerance zone arcs are based? If so, is this another case for basic dimensions unrelated to a geometric tolerance? Does this become sort of a profile tolerance that controls both the form and the location of the actual radius feature, only without explicit datum references? Alternatively, does it not control location and only change the shape of the tolerance zone? If so, would reference dimensions for center location be appropriate?

 

[powerhound]

3DDave and Burunduk. EVERY. SINGLE. THREAD.

John Acosta, GDTP Senior Level

 

[greenimi]

3DDave and Burunduk. EVERY. SINGLE. THREAD.

And? What is wrong with that? It is quite entertaining to be honest.....and it is also educational.
I like it. Give me more ammunition for "my battles"

 

[3DDave]

I can't call it what I like - you have made it clear that the vocabulary in the standard is inviolate.

Better finish that write up for the committee.

 

[3DDave]

" you should know that two concentric arcs as a tolerance zone that limits the feature's variation, can contain radii both larger and smaller than the limiting arcs. This would violate the dimensional limits of the radius. "

What a gem. No, it won't violate the limits because those are an additional limitation. One constraint cannot override the limitations of any other constraint. The feature has to meet all the limitations.

 

[Burunduk]

If your toleranced dimension is R2.4+/-0.3, and you use a tolerance zone overlay of two concentric arcs: R2.1 and R2.7 to inspect that, then the maximum radius that can fit in the tolerance zone exceeds 2.7 and the minimum radius that will pass is less than 2.1.
If you mean by "additional limitation" that the other constraint is the location of the center of the actual radius at the exact point you will fixture the tolerance overlay to, how is this controlled? I tried to lead you to this understanding by one of the questions you evaded - if the radius center is specified by basic dimensions, and you constrain the overlay to the basic location, what makes you think the actual (as produced) radius will also align to that theoretical center?

 

[3DDave]

The other constraint is the actual value of the radius. It has to meet the curvature limitation as well as the boundary. You cannot say "I can have a larger radius fit the boundary and miss the curvature" and say that proves anything - the part fails that inspection.

 

[Burunduk]

The curvature is the only thing being measured and reported for the 'R'+/- requirement, therefore the crescent shaped tolerance zone. That tolerance zone IS the radius value limitation. Naturally, the theoretical Max and Min (curvature) tangent arcs limit the variation to the acceptable actual radius range, and per the standard they also limit the form irregularity of the surface to stay within that zone. This is why concentric limits are not used in order to control the toleranced 'R' requirement. They are able to limit irregularities (differently), but they are useless at limiting the radius value to the limits set by the +/- toleranced dimension (or limit dimensioning).

There is no common practice to limit irregularities by concentric limits in addition to measuring the curvature, and never was. Typically, a best-fit radius or a radius value calculation based on sample points is the only thing generated at the evaluation.

 

[3DDave]

The concentric radii applies to located radii, not fillets. There's no crescent in a located radius tolerance zone.

 

[Burunduk]

Bring an example, please.

 

[Burunduk]

For what it's worth, figure 4-21 in the 2018 standard which is the new version of figure 1-23 of Y14.5-2009, shows all dimensions as basic, including the radius value. This indicates that a radius with a "located center" should be controlled by profile, which makes sense. If anyone has an example of how a located center is useful or matters in conjunction with a directly toleranced radius dimension, please comment. I haven't been able to make sense of its mention along with the concentric tolerance zone in sub-para. 5.16.1 so far.