CMM calibration using a perfect sphere that is supplied

J

jdm2008

#1
At our company we have a CMM(Faro), before use you are to calibrate it using a perfect sphere that is supplied( inputting several readings around the ball, the ball is attached to the arm).
Is this sufficient to calibrate the arm? I hesitate to the say this because I know what kind of reaction it ill entail, but our only purpose for calibrating the arm is to pass the ISO audit, in reality the arm is never used(long story and separate issue).
We have never been cited for non-compliance for this issue, but I think it's possible(even likely) that this is just dumb luck. What are your opinions?
 
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R

rsimano

#2
Re: CMM calibration

The ball itself should be sufficient as a calibration standard as long as the ball itself is validated/calibrated as a master on a regular basis to NIST.
 

Ninja

Looking for Reality
Staff member
Super Moderator
#3
Re: CMM calibration

... but our only purpose for calibrating the arm is to pass the ISO audit, in reality the arm is never used(long story and separate issue).
We have never been cited for non-compliance for this issue, but I think it's possible(even likely) that this is just dumb luck. What are your opinions?
Why not simply tag the arm with a "calibration not required - Do Not use" tag...it's way less work.

If you don't use it, why calibrate it? The audit doesn't need it calibrated. It only needs to be identified as to whether or not it requires calibration...and it doesn't since you don't use it. Take 14 seconds to tag it and move on to something more important.
 
R

rsimano

#4
Re: CMM calibration

Ninja has a good point. It would be more cost effective in the long run if its just to calibrate the arm only.

Is the arm needed for the sphere to calibrate the arm itself or the whole CMM before use?
 

Kingsld1

Involved In Discussions
#5
I'm not an arm user but a CMM user.

In the CMM world the calibration sphere is used only to determine the effective diameter of the stylus tip. (Effective diameter = true tip diameter - any deadband area in the probe switch)

Calibrating the CMM often involves a ball bar test and a linear accuracy test.

A better name for the calibration sphere is qualification sphere, which is the term used by our OEM.
 
E

erinpristel

#6
What you are doing is not a "calibration". It is a qualification of the probes/arm. The actual calibration is going to have to be done by an outside service. The CMM must also be "calibrated" and reference standards traceable to NIST. I would recommend taking that arm off of your calibration list as long as it has no effect on the rest of the CMM function. We have an annual calibration schedule for the entire CMM, separate from our probe qualification log that is filled out for each probe prior to use every day (which is what it sounds like you are doing).

Side note: My company was nearly cited in our last audit for lack of ability to find the calibration record for the CMM software. (We found it just in time) Some auditors may request it so make sure you have it ready with your calibration records.


Hope this helps!:D
 

Ninja

Looking for Reality
Staff member
Super Moderator
#7
What you are doing is not a "calibration". It is a qualification of the probes/arm. The actual calibration is going to have to be done by an outside service. (snip)

Side note: My company was nearly cited in our last audit for lack of ability to find the calibration record for the CMM software. (We found it just in time) Some auditors may request it so make sure you have it ready with your calibration records.


Hope this helps!:D
Hi Erin, Thank you for posting. I don't want to pick on you, but I do want to comment...

It is not mandatory that the calibration be done by an outside service, as long as the company has the means to do so in house. I calibrate my own CMM using a NIST traceable, calibrated grid (my CMM is optical). While an outside service may be deemed more desireable, it is not mandatory.

I also note that the calibration and validation of the software for most CMM's is part of the calibration procedure since the CMM is software controlled in the first place.
With respect to the actual programs for measuring specific features, I agree that each program for use needs to be validated...but that is primarily a paperwork excercise. Passing GRR using the equipment (especially if the GRR has a known control sample in it) is typically adequate for validating the program too.
 
E

erinpristel

#8
You are right. As long as it is NIST traceable you may calibrate the CMM in-house, i stated that improperly, i apologize all - thank you for replying; (we (our company) have an outside service complete our calibrations). We certify our probes on a daily basis when needed for production inspection using the NIST traceable spheres mentioned in the thread. Just wanted to add a reminder to have software validation evidence available, as you will undoubtedly be asked for it during an audit. I should have taken more time to reply.
 
S

sept68

#9
It is not mandatory that the calibration be done by an outside service, as long as the company has the means to do so in house. I calibrate my own CMM using a NIST traceable, calibrated grid (my CMM is optical). While an outside service may be deemed more desireable, it is not mandatory.
Does not the NIST standard have to be ~10x more accurate than the CMM?
 
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