Position - Sphere to Sphere

[JacobCheverie]

I am in a situation where I have two spherical datums -A- and -B-. The print calls out the position of -A- with respect to datum -B-. There is a diametric tolerance zone indicated on the print.

Of the three principal axes, along which two are measurements taken and then used to calculate Position? How does one determine which axes are relevant to the situation at hand?
I would assume sphere to sphere should be measured in regular 3D space (i.e. spherical tolerance zone). Why am I incorrect?

[Mike Ruff]

If only one spherical datum controls the location of another sphere, then the 3D distance between the two is the only thing controlling it. A sphere by itself cannot control orientation, so the axes are irrelevant.

[KIRBSTER269]

I am not sure if I'm following exactly what your asking, but your Sphere's are controlled by a common center point, So technically all rotations are free. Unless you have Max Material, Then there should be no shifts in your X, Y, and Z. Depending on how you align the part, can you see which direction your off.

[JacobCheverie]

Mike Ruff That is exactly correct to my understanding. Here is another piece of info. I am aligning my coordinate system Y axis to the line formed between centers. This leaves the Position being reported in Y and Z (dx = 0). The issue is that when I report it on the CMM as a Position, it is giving me the 3D distance as the length of the vector from sphere center to sphere center and reporting the individual projected lengths along X and Z (such that sqrt(dy^2 + dz^2) = distance between centers). Engineering argues that because these spheres will eventually contain balls with a cylindrical I.D., the deviation in Z does not matter. I argue that it does as it is representative of an improper mating surface. Thoughts?

KIRBSTER269 I briefly explained my alignment in my reply to Mike. Level Z, rotate Y to sphere center line, translate X, Y, and Z to datum sphere. We have MMB but my question is on the theoretical side (i.e. proper reporting/GD&T)

At the end of the day, a sphere is a 3-Dimensional entity; it's exact location is defined in 3-D space. Sphere to Sphere position should be measured by projection along three principal axes, regardless of the tolerance zone indicated on the print. Otherwise, there is room for "axis favoritism".

[Matthew D. Hoedeman]

3D-DISTANCE, use +/- one half of the position tolerance.