Measuring Sphere with different tip angles.

[[Ja...]]

I am trying to measure a large I.D. sphere that requires me to come in at different tip angles. I come in at A0B90, scan 90 degrees of arc, A0B-90, 90 degrees, A90B90, 90 degrees, etc...

I performed a test on the calibration sphere by coming in at A0B0 and scanning the sphere. I received (x, y, z, d) = (-.0012, .0005, -.0015, .9843) and a form error of .0001 with sigma = .0000.

Coming in at each of the four angles and scanning 90 degrees of the sphere with each, I received (x, y, z, d) = (-.0004, .0013, -.0011, .9837) and a form error of .0020 with sigma = .0003.

The absolute differences are (.0008, .0008, .0004, .0006).

Is this proof that the four tip angle measurement is not as accurate as the single measurement?

I need to probe a large I.D. sphere and hold concentricity of .001. This doesn't seem possible anymore. Perhaps a star is my only option?

[[Da...]]

Did you qualify each of the angles as separate probes? If not, try that first.
How was the sphere manufactured ?

[[Ja...]]

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The probe angles are separate styli under the same stylus system and they all get calibrated every evening.

I'm not entirely sure how the sphere was manufactured.

[[Ja...]]

I have attached two images. The first is the view from X+, the second from Z+. It is clear that there is an issue with tip angle correlation.

[[Da...]]

Do you recall all the points into one sphere ? Spheres are a real PITA ! If the sphere is generated in software for a milling operation, then they are usually pretty close to spherical. If they are turned in a lathe then there are several variables that will mess with the spherocity, making things very hard to do. In a turned form the actual radius programmed doesn't always coincide with the diameter. Form errors will show differently on different areas of the sphere.

[[Da...]]

Wow, doesn't look good.

[[Ja...]]

I don't recall the points, I simply make many "Circle Paths" using different tip angles. The images are at 100x magnification. That being said, I just ran another tip through the same procedure and the results were just fine. This is a problem with the individual tip and it's angles. Perhaps I need a full scanning calibration on it, or perhaps I should replace the tip.

I'll do a bit more testing after a full calibration and make a decision. Thanks Dave.

[[Er...]]

I have had very good success checking Spherical Radii using 4 circle paths as shown. We have to hold .003" profile and this method has worked very well. Maybe something to try.

[[Ri...]]

Measuring a single feature with multiple tips on an XXT, especially an RDS, is difficult.

I have done this before multiple times on an active head holding .001" Positions, but it is difficult. One thing that I've done to help me, and maybe it's just a placebo effect, but I will manually qualify the styli, and then let the software go back and automatically qualify the tips for me.

I also have (on super complex probes) setup a probe check measurement plan with the reference sphere, and manually tweaked the X,Y,Z values of the probe to get all of the error out. - This is kind of a last ditch effort for me.

[[Ja...]]

Thanks for the advice Richard.

My issue turned out to be that somebody had crashed the probe coming straight downward and the shaft had gotten pushed up into the body. I tried tightening and realized I couldn't fit a tool in there. I replaced the tip and now everything seems to be in agreement and repeating.

It's strange that our daily calibration didn't notify me that there was something wrong.