CMM Sphere Qualification

[UchidaDS]

Do you perform sphere qualification everyday or once a week or....?
I have told that instead of performing sphere qualification-->stylus qualification, they would perform size check, such as using a gauge ring and perform a "daily check".
Not too understand the theory behind this.

 

[MikeHalloran]

It sounds like something the CMM operator does in order to avoid being assigned to other work when (s)he's not real busy.



Mike Halloran
Pembroke Pines, FL, USA

 

[UchidaDS]

Nothing to do with that. It is about the need.

 

[MikeHalloran]

What part of a CMM drifts enough to need daily qualification?


Mike Halloran
Pembroke Pines, FL, USA

 

[UchidaDS]

There are a lot of parts need to measure. Drift? I can't tell if the CMM drifts. But how can you tell the CMM drifts or not drifts?

 

[MikeHalloran]

I can see a daily check against a length standard, maybe one axis per day, just to see if the air conditioning has screwed up, but checking a sphere is just make-work.



Mike Halloran
Pembroke Pines, FL, USA

 

[marktheyeti]

The level of qualification and/or "checking" required will depend upon usage and accuracy requirements. If you're taking measurements requiring accuracy near the limits of your machine (or at least bordering on the gage-accuracy-10x-required rule of thumb) then frequent qualification doesn't seem unnecessary. But, if you're machine accuracy is much better than the typical tolerances you work with, frequent qualification is not likely required.
Daily "checks" can be useful, especially if the CMM is accessible by many different people, not just one or two specific CMM operators. For instance, when I worked as a QA tech, we had QA personnel present for first shift, but not for second. The first task each morning was to clean the CMM table and run a short probe check for each probe (there were only 3 commonly used configurations.) This check was short (5-10 minutes) and made apparent any significant change to the machine and/or probe conditions. In my experience this was infrequent, but when it occurred it was usually from a part being fixtured incorrectly (leading to auto-drive into the part) or an operator accidentally driving a probe into a part. The morning check was a quick way to see if there were any big changes in machine performance without consuming too much time. It certainly was not make-work.
But for someone else's operation, it may be totally unnecessary. Your specific quality requirements and experience with your CMM's capability, reliability, and workload will determine the qualification / checking frequency.

 

[UchidaDS]

1. Let say if the part is calling out for .002" flatness. The machine has form measurement error of 5um (metric). Then the limit of the machine is fall in the at 10X rule, which means .002" flatness is its limit.
2. For Daily check do you use ring gage (setup with three different axis) to check different axis?
3. Is this daily check tends to check the probe or the machine or both?
4. I think one of the reason to perform sphere qualification is because wear, due to measure too many parts. 24hrs run.
5. Of course, if there is a crash due to malfunction or operation error, sphere qualification has to be perform.
6. I am hoping that you could share more information how you perform the daily check/criteria.

 

[marktheyeti]

Our CMM is checked daily against a two-sphere wishbone artifact. The check consists of measuring the spheres with each tip on the stylus in several different index positions of the head. Not EVERY position of course, just a few of the common ones in order to check for gross error. If the daily check shows an error, the tips are all visually checked for damage and the calibration program for that arrangement is performed. This checks and compensates every tip and index position in the arrangement.
I haven't ever seen us use a ring gage check.
Wear has never been an issue for us, as we don't typically use the scanning function and the machine isn't subject to 24/7 usage.
An outside calibration of the CMM itself, including laser alignment and some other stuff more complicated than I ever got into, is performed at a set interval (one or more years apart.)

Remember though, that your checking and calibration requirements will be different than mine. We're typically inspecting components for use in industrial products that rarely, if ever, are in life-critical applications. Further, any government-related work with more stringent quality requirements prompts a more thorough CMM inspection and qualification prior to part inspection. If your product has very tight requirements or is used in more critical applications, then the functional requirements of your CMM are different and require a different inspection/qualification routine. You will have to, through experience, determine the minimum level of machine inspection/maintenance required to assure manufacture of parts that consistently meet your approval criteria. You may find that your procedure needs to be modified after some time.

I hope this was helpful.