CMM Qualifying Sphere Calibration

K

KosmoQA

#1
OK guys,
Can anyone tell me whether or not a CMM calibration sphere needs to be calibrated on a yearly basis? This is a question that has arisen a few times here at my location. We even asked a customer's auditor and he wasn't sure himself. The CMM is calibrated yearly, but does the sphere on which its styli are qualified need calibration as well?
 
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#2
Kosmo
From my experience, minimal wear takes place during use. With my CMM, I had the sphere calibrated once when new, and put a 5 year calibration period, with an annual inspection to check for damage/rust/misuse.
Paul
 
P

prabhudvp

#3
cmm sphere

Hi

The cmm sphere is solid built and does not wer out
One can schedule calibration based on cost and use

it shold be calibrated if some body drops it or hit while loading the parts & fixtures

also take care by holding good cloth,paper to avoid scratches

if you record your calibration reports, study the variation and
convince the auditor for revised calibration peroide

regards

Prabhu
 
S

sarasheed

#4
The Sphere is used to check the actual diameter of Stylii and inter relation between other stylii.The accuracy/calibration of the CMM as a whole is done with the help of Slip Gauge Test.If there is any variation in the diameter of the Sphere,it will be revieled in the Slip Gauge Test.This Slip Gauge Test,if carried out periodically,will automatically reviele the inaccuracy in the CMM incuding the inaccuracy due to Sphere.
If the accuracy in Slip Gauge Test is in acceptable limits,one need not worry about the calibration of the sphere.

We have procured Zeiss CMM WMM-850 in 1986 and so far we have not calibrated the Spheare but we are calibrating the complete CMM with Slip Gauge Test annually and the accuracy of the machine is with in specified limits.

sarasheed
 
K

KosmoQA

#5
Would a "Slip Gauge Test" be performed during the annual calibration, or is it another type of test?
 

CalRich

Involved In Discussions
#6
KosmoQA said:
Would a "Slip Gauge Test" be performed during the annual calibration, or is it another type of test?
Seems like the "slip gauge test" would be a verification activity, not part of the calibration.

Our lab regularly calibrates CMM spheres for a number of companies. Seems they have this done separate of the CMM calibration.
 

gard2372

Quite Involved in Discussions
#7
Kosmo,

With our CMM sphere, we had the initial cal cert form the OEM. If your CMM is calibrated annually, and you're daily probe cal's are done off of your sphere then you will want to track any deviations from the original circuferential profile of the sphere periodically. Depending on what your QMS manual states, often an organization can determine the intervals or even the allowable tolerance limits from original manufacture of the cal spheres.

I suggest you try www.cmmtalk.com they have a lot of info for programmers and operators of CMM and CNC machining shops like yours and registration is free. I pasted your question into their calibration forum, and these are the responses I received....






A CMM calibration sphere needs to be calibrated on a yearly basis? This is a question that has arisen a few times here at my location. We even asked a customer's auditor and he wasn't sure himself. The CMM is calibrated yearly, but does the sphere on which its styli are qualified need calibration as well?

gard2372

Posted - 3/16/2006 7:00:28 AM
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Plowboy

Posted - 3/16/2006 7:54:27 AM

Typically - NO - but it all depends on what your quality manual states and how big a pain your auditor wnats to be.
Most spheres are not calibrated after they are made.If they are damaged they are replaced and life goes on.
cmmguy

Posted - 3/16/2006 9:28:05 AM
--------------------------------------------------------------------------------
Master balls should be recal'd, but not necessarily on an annual basis. As plowboy pointed out, if they are damaged, then they should be chucked in the can.

Master sphere size in important to the measurement of features of size but not location. The Spheres can grow over time, depending on how they are made. And the new low wear stylii will wear the ball as you touch the same locations evertime you qualify your tips. And if you do Scanning probe qualification, you really are rubbing that sphere alot.

Calibrate it every two years and no one will complain. (You would calibrate your setting rings, wouldn't you??)
 
T

thenettrainer

#8
Um, er . . . . Absolutely! :)

I suppose there are instances where a shop does no "close" tolerance work, and has a CMM becasue a customer requires it, that kinda thang. However . . .

A sphere not only "grows" over time, it also loses it's "perfect" form. It's not unusual for a calib sphere's Form to change by several 10ths over the course of a year or two.

But isn't the issue really . . . . "Is my sphere any good??" Not "Will I be questioned or rejected in an audit?"

The cost of having a sphere inspected and certified is about $50. Hmmmm.
You're running a machine that cost the company around $100K. You expect reliable data from the machine. Right?

Spend the $50. :yes:

In my experience, this is a "possible" proceedured you might consider.

Spend a few bucks, and buy a 2nd calib sphere. (Assuming you've had one already for let's say a year or more) Put the new sphere into service. By ser. no., etc. So it's in your quality system.

Send the old one to a lab, get it recertified. Then put it away for a year. (Dip it in the protective "goop" is you need to, and put it in a well controlled temp environment till next year.)

Next year, send the newer one to be recertified. Put the old one back into the system.

Alternate each year. It'll only cost ya about $50 a year! :mg:

One other point to consider . . . What is the "climate" in your CMM area? If the temp/humidity is closely maintained, you'll have less of an issue. Why?

Because that sphere expands and contracts with the slightest temp change. And being a sphere, the molecules don't always "go back" to exactly where they were when at optemp.

FYI: I know if i were to ask 500 "savvy" people what the temp variation of a CMM room should be, I'd get over 99% telling me it had to be within 4 degrees. Well . . .

That might be true for "inspection rooms", etc. That's the allowable, acceptible temp range for measuring parts. And the tem can vary that 4 degrees over the course of any day. But . . .

For a CMM to function properly . . . . repeat perfectly . . . . Stay "in tune" . . . all of it's "parts" working correctly, the total amount of acceptible temp variation is . . . .

0.7 (7 10ths) of a degree over any 8 hour period!

If you machine/room, etc. varies more than that, best guess is . . . . if you do close tolerance work . . . . Spend the $50! :whip:
 
S

sarasheed

#9
CMM calibration sphere

It is better to ask the manufacturer of the CMM weather to calibrate the Sphere or not.The actual size of sphere is measured before sending the machine to the customers and same actual value of the sphere is fed in the software used for measurement and we ,as users,can not correct these valve in the sofrware.If we find some variation in the diameter of the spheare during calibration ,we can not take any decision.

sarasheed
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#10
I would say yes, the sphere should be calibrated. However, if used only for the qualification, then depending on frequency and how it is stored, you may be able to go to a 2 year schedule.

If you send it for calibration, I recommend talking to Oak Ridge National Lab (the actual cal lab is managed by Y12). They have the ability to calibrate it, or may make an alternative suggestion if their schedule is full.

Hope this helps.

Hershal
 
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