Poor Fr (Pitch runout) repeatability with RT Table

[[Ya...]]

I want to certify a master gear for noise inspection. The pitch runout control on that master gear is pretty tight, 6um. This is a brand new master gear with Gleason's certificate with it and never been used. On the original certification, the Fr is 3.3um with uncertainty 3um.

I try to measure this gear with Calypso plus Gear-Pro and RT table, the reading I got always jump.
I measure the gear right after probe re-qualification and RT-Axis qualification, I got runout 4um. Then, I did nothing, but just run the same program again and again, then, I got runout to be 8.7um, 9.2um, after few times (around 10 times), the reading becomes 13.6um. Then, if I re-do the RT-Axis and probe qualification, the Fr becomes 3.3um again.

1) Am I suppose to do the RT-Axis every time when I run a program with the RT-Axis (The Rotary table is fixed on the CMM table with bolts, normally, we do not touch the rotary table at all) ? The default limit is 250 times, right? Is that means if I use less than 250 times, I should not need to worry about the RT-Axis, am I right?


2) How is the repeatability for Fr on your machine? 10um uncertainty is too big for me. I used to work with a klingenberg gear analyzer, it can repeat within 1um if we calibrate it every 24 hrs.

3) Does anyone have any hit for me why my pitch runout is so unstable?

Thanks in advance.

[[He...]]

Since you get good results with a freshly defined RT-axis I think it safe to say that the RT-axis is causing the problem.
Even if the RT is fixed in the machine the RT will move relative the machine origin because of temperature fluctuations.
On most machines the scale have 0 temperature coefficient which means that the scales will not grow or shrink depending on the temperature. The machine will however change its size. The stone that most modern Zeiss' are built with have a coefficient of around 4.2 µm/meter and degree C.
The X- and Y-scales are normally fixed at the end of the scale where the zero point is.
That means if your temperature changes 1°C the rotary table will change its position with 4,2 µm for every meter distance it is to the machine zero point.

There are a few ways to reduce this problem. If you measure your datums using the rotary table this to will get the same error as the gear measurement and the will cancel each other out. You will still have problems with tooth thickness, pitch diameter and all that but those normally have bigger tolerances. How do you measure your datums on the master gear?

We always measure our rotary table axis on each part. It only takes a few minutes and that way we more or less know that the current RT-axis is a good one.


If I were you I would get myself a good temperature logger. Preferably one with several different sensors that can be set at different locations around the room. Zeiss sells a system called Tempar (expensive as the pope's virginity) but it works very well from what I have seen.

[[Ri...]]

Which method are you using for the RT-axis calibration?

I've always felt like the Cylinder method (on your part) is the best method. I would suggest doing it for every part that is ran, especially if you are chasing microns.

[[Ch...]]

Agreed. I've seen and experienced this at many places. Proper RT Qual on each part is required. (Pattern/Symmetry/filtering, etc)
Good luck!