why are bigger probing spheres more accurate than smaller ones

[Paul Schuyler]

1) Can anyone explain in a sentence or two why bigger probing spheres are more accurate than smaller spheres?
And also:
2) Why is locating your artifact near the position where you will actually measure your part more accurate than locating the sphere at the far end of the table?
In both cases, I sort of understand why, but I can't clearly explain why, in which case I don't really understand it.
PaulS

[kingsld1]

On a perfectly flat plane with a machine that tracked perfect, there would not be any difference in the readings. I've never seen either one of those things. There's always a little bit of chatter; a little bit of a wave in the surface. The CMM always has a little bit of difference between where it's been told to go and where it actually ends up. A bigger stylii filters out more of the valleys that occur in a surface. A small styli reaches deeper into the valleys. Since the machine does not perfectly repeat its location, sometimes a small style will reach deep in the valley and sometimes it will hit the peaks. A large style physically cannot go as deep into the valley and that reduces the variation that you could see. It's not that the big stylii is more accurate, but that it has better repeatability.

[JEFMAN]

1/ in addition of what kingsld1 said, a small ball has also a small stem (shank ?), which is more deformable than a large one, the probing force is the same for both styli most of the time.
2/ the cmm has some geometrical defects, which are compensated by the "21 parameters matrix". This matrix is done during the manufacturing of the cmm, and it's modified during cmm calibrations, but some defects can appear, which are not compensated (temperature gradient, crashes, some stupid people who leans on the gantry...). If you measure where the calibration sphere is fixed, those defects are less important because they are taken into account in the stylus calibration. If you measure with a long stylus at A90B0 and A90B180, the pitch defect of the cmm (example : difference of temperature between the top and the undersize of the granite) can be important. If the sphere is where you measure, the stylus calibration "compensate" this defect.
Some more information at the #13 here... Hope this helps.
https://www.pcdmisforum.com/forum/pc...check-your-cmm

[Wes Cisco]

+1 to the previous comments.

For clarity I would reword kingsld1's last sentence to read: "It's not that the bigger ruby (or stylus tip) is more accurate, but that is acts as a passive filter for very small surface imperfections."


HTH & ymmv

[Cumofo]

+1 to the previous comments.

For clarity I would reword kingsld1's last sentence to read: "It's not that the bigger ruby (or stylus tip) is more accurate, but that is acts as a passive filter for very small surface imperfections."


HTH & ymmv

There is nothing passive about the Demon. It will destroy your probe files, your alignments, your clearplanes, your career, and leave you in a heap of rubble without warning.

[Paul Schuyler]

very good, thanks to all.
paulS