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Posted: Thu Jul 02, 2020 10:23 am
by bighern
How often should the calibration sphere itself, be calibrated? We have a customer that wants us to show that this is something we do. And where would you send it to have this done? And while my sphere out for calibration, and i need to calibrate my probe,
what options do i have? any thoughts or info is welcome. thanks.


Posted: Thu Jul 02, 2020 10:35 am
by CMM Guru
The answer totally depends on your, and your customer's, requirements. You determine this statistically. In a perfect world, you would send you sphere out, say, once a week for a few weeks. After you see it is stable, you would then switch to maybe once a month. And after that is stable a few times, you would switch to quarterly, semi-annual, then once a year.


Start with once a year. If you get stable results back for a few years, go to every 2. And you would have documentation available to back up your reason since you can prove your sphere is stable over 2 years. We (another company) used to do every 3 years if I remember.

If your customer is asking this kind of question, it will need to be done by a certified lab with traceable equipment to NIST.

What to do in the meantime? Have 2 spheres. Unfortunately.


Posted: Thu Jul 02, 2020 10:43 am
by CrashN8
I cannot give advice on how often... we see users calibrate anywhere from everyday to once a week to once a month. Every once in a while we have people contact us with complaint that system is not accurate and can see that last tip calibration was 2 years ago :lol: :lol:

Anyway, you can choose a frequency that meets your needs, probably close to once per week and then write a small CMM-Manager program that includes tip calibration. Notice the Calibrate All button in Tip Manager... There is an option in the Calibrate All dialog to add to program, this adds a Program Step. Then save the program and run it anytime you need to re-calibrate tips.

Also notice we added an option to automatically save Tip Summary after Calibrating All tips. This option is added in 3.10 that releases in the next few weeks.



Posted: Thu Jul 02, 2020 10:44 am
by CrashN8
After reading CMM Guru's comments and re-reading your original post, I realize that I have answered your question in the wrong context. Next time I will read more carefully :lol:

Anyway, I hope my answer gave some extra insight either way.


Posted: Fri Jul 10, 2020 1:56 am
by chuck swanson
Boeing requires us to send ours out annually. We keep two of them in the system so we always have one in house.


Posted: Fri Jul 10, 2020 9:24 am
by CMM Guru
Here's what NIST says about this (essentially what I said in the first answer):

How often must I calibrate and instrument or artifact?

You need to calibrate often enough that the instrument or artifact is known well enough that the quality of your product is not adversely affected. You must know what uncertainty you require of the instrument or artifact and you need to know how the calibration is expected to change with time. Most systems for setting calibration intervals assume that the calibration slowly drifts over time, or that the change is at least describable by some reasonable statistical distribution. A good reference for these types of calculations is the National Conference of Standards Laboratories International (NCSLI) Recommended Practice Number 1 (RP-1).

You must remember that when an instrument comes in for calibration, if it is seriously out of calibration you must suspect the judgements made on the basis of data from this instrument. This may involve recalling parts or other instruments for rechecks. Thus, it is a bad practice, economically, to set recalibration intervals that are too long. On the other hand, if every instrument is in calibration when checked, the intervals may be too short, at least in the sense that you are paying for unneeded calibrations. The rules in RP-1 are an attempt to optimize the tradeoffs.

The referenced document is available from NCSLI at ... 5ef6325073


Posted: Tue Jul 14, 2020 8:46 am
by myork1967
If the cal sphere is not beat to heck, pitted ect..or ceramic no chips broken ect ect, seems to me you could check it on your own cmm.

When calibrating the CMM, all the equipment is calibrated so you do have traceability. I would probably have one touch sensor and probe put back just for this.....just my 2 cents...


Posted: Tue Jul 14, 2020 1:46 pm
by CMM Guru
Except. You would have to calibrate you tool on a certified artifact to start with. Which means you can't calibrated your probe on the sphere and then use that probe to check the sphere...