A ball and GD&T 2

[Diametrix]

I have a ball from a manufacturer with the following specs: Diameter - 0.500+/-.005" and "sphericity" - .005" Since the term sphericity is somewhat vague I wanted to get away from the notes describing the dimensions and show it with GD&T, however it seems that GD&T fails me here. The way I understand it, I can't use a composite profile tolerance here because there is no locating dimensions. It is almost like I need a basic dimension with tolerances (don't throw rotten tomatoes at me here) and to have a profile tolerance... which naturally is not possible. Any ideas how to show something like that on a drawing without a verbal description?

 

[Diametrix]

You just need to interpret it per its definition in the standard

Is there a definition of the term "sphericity" in the standard?

The thickness or normal distance between the two spherical surfaces is roundness or sphericity.

You are right that definition of circularity (i.e. roundness) is quite simple. However, how it relates to the profile tolerance of the ball surface (that ambiguous term sphericity) is not so much. For example, you seem to equate them. To me it was news that profile tolerance of a ball will be twice it's circularity tolerance. That, after all, was my initial reason to try to avoid the term "sphericity" on the drawing - people seem to assign different meaning to it and there is no clear definition of the term in the standards.

 

[axym]

Hi All,

It seems that the intent here for "sphericity" is to control the surface with a tolerance zone comprised of two concentric spheres. I would say that Circularity applied to a sphere would not be equivalent to this. I agree with Garland23 that the cross sections in each cutting plane could be different sizes. So all of the cross sections could be circular within 0.25 but the surface would not necessarily be spherical within 0.25. I haven't tried to sketch this out to prove it though.

In any case, Dynamic Profile would be a better tool to use. This does not need to be specified in a composite FCF, despite the fact that most of the examples in Y14.5-2018 show it that way. Dynamic Profile applied to a spherical surface with a basic diameter would create a tolerance zone comprised of two concentric spheres. The zone would be allowed to "progress", which in this case would be equivalent to allowing the diameter of the zone to float. This is exactly what is needed for sphericity.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Circularity, for a sphere:

(b) for a sphere, all points of the surface intersected
by any plane passing through a common center are
equidistant from that center

A circularity tolerance specifies a tolerance zone
bounded by two concentric circles within which each
circular element of the surface must lie, and applies
independently at any plane described in subparas. (a)
and (b) above.

So, a single point is a common center for a sphere. If one section was small and perfectly circular and another perpendicular to it was large and perfectly circular, then a third section perpendicular to them both would govern the amount of variation allowed between them.

 

[Belanger]

Garland23, your thinking is somewhat correct, so don't feel bad for pushing the point. However, I think the catch is that if each of those circular tracings are different sizes (within the size tolerance of 0.8), there will be some circular rings at a different orientation that feel the high/low mismatch in a different direction, and that would exceed the circularity tolerance at that new sweep.

The definition in Y14.5 given by 3DDave doesn't clearly say that -- it says "planes," implying that there are many individual rings swept around there -- but if we check every possible sweep in every orientation, the form error won't be able to reach the 0.8 mm provided by the size tolerance.

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