Find a feature PCM?

[[Cl...]]

I have a part with a .0405 diameter ID sphere on the OD of the part. This sphere needs to be w/in 2 deg of another feature. The part is loaded onto a fixture with the sphere facing the operator. The position of the sphere may be slightly different each time it's loaded on the fixture so there is no guarantee that the stylus won't miss it. What I did is create a 2d curve that runs through the center of the sphere, starting and ending far enough on either side of it to compensate for it being either side of center. Then I use minimum coordinate to get the minimum point (-Y) of the curve in. I use that point in a secondary alignment as XYZ zero. and offset .02025 in X (putting the coordinate system at center of sphere). Now I set the sphere measurement to that alignment and it is able to hit and measure it every time. But there is a problem with this method. If the part is rotated too far CW, or CCW the minimum point can change from X+ to X-, throwing of the secondary alignment, the stylus will then miss the sphere. Right now we run the program, if it misses the sphere we stop, adjust the nominal (+ or -), change the offset direction and re-run the program, all is well. This takes up too much time. Looking for an alternative w/o PCM. Does anyone have a better idea on how I can have the probe hit this sphere every time?

[[No...]]

I may have a wrong imagination of your part in my head, but if you seek the -Y minimum coordinate of the sphere, then why do you offset the radius in X? I think a simplified drawing of the situation would be helpful.
Anyway, I don't know how the minimum coordinate is calculated for a curve, but if you can use a circle instead, the minimum coordinate is always calculated in radial direction, so the angle shouldn't matter anymore as long as the sphere is still covered.

[[Cl...]]

The first attachment shows the minimum point (inside the red box) right where the sphere begins, tangent to the part OD. I use it for XYZ zero of my secondary alignment. Then offset it half (.02025) the sphere diameter. This puts XYZ zero in the middle of the sphere. The problem is, if the part is loaded too far CW, or CCW from center the minimum point will be on the opposite side of the sphere and the .02025 offset will be wrong, causing the probe to miss the sphere feature. Right now we run the program, if it misses the sphere we stop, adjust the nominal (+ or -), change the offset direction and re-run the program, all is well. This takes up too much time. Looking for an alternative w/o PCM.

[[Cl...]]

I just thought I'd bump this one up.

[[No...]]

Wouldn't it be better to go for the sphere center (the deepest point of the sphere) instead of the edge? Then you wouldn't need the offset. If your probe head is capable of self-centering and your stylus is big enough (slightly bigger than the sphere), you could use the point found as the coordinates for a self-centering point, which would give you the sphere center quite precisely. That's how I usually do it.

[[Cl...]]

When the part is loaded on to the fixture it is clocked by eye, so it will never be in the same location. This is why I'm using a long radial 2d curve to "find" the sphere. The minimum point always ends up being on either the X+, or X- corner of the sphere where it meets the part OD.

[[Cl...]]

Figured it out, alI had to do was mask the first and last 50 points on the curve and
change to Y maximum point. Now it get the maximum point in the center of the sphere.

The curve was too long to get the correct minimum point.