Conducting a Bias Study for a CMM with no Traceable Master or Reference value

G

guruprasadb

Dear Friends,

Have a nice day,

recently we have planned to Conduct MSA Bias study on CMM

as LC of CMM is 0.0001 mm

there was no traceable master or reference value .

So we have measured the one part for 10 times on the same CMM and calcuated average and considered as reference value.

and performed study

Is this process OK?

Thanks in advance
 
D

dickgent

I'm not sure I understand what you want to accomplish. Is this for a customer requirement, auditor, A2LA etc? OR Is this something you wrote into your procedures?
BIAS - def. The predicted difference on average between the measurement and the true value. Bias is also known as accuracy.

Your CMM calibration provider should provide you with bias reporting with masters (typically ball sets) traceable to NIST. He should be able to provide you with a detailed map of the extents of the CMM and the accuracy of each area. Typpically the CMM wil be less accurate at its furthest extents. You can perform Gage Reliability using your fixtures (parts or temporary tear down blocks), However not Gage R&R.
 
G

guruprasadb

Thank you . This is not for any audit purpose or not for any customer requirement , we want to study our process and we want to establish and get benfit, actually our calibration service provider only done calibration and said bais study is not in his scope, so as we are not having any master or reference value we have decided a procedure for establishing that reference value as mentioned above.

Thank you
 

Miner

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Admin
recently we have planned to Conduct MSA Bias study on CMM

as LC of CMM is 0.0001 mm

there was no traceable master or reference value .

So we have measured the one part for 10 times on the same CMM and calcuated average and considered as reference value.

and performed study

Is this process OK?

No. Bias is always in regards to a standard or a reference value obtain from a measurement taken by a more accurate and precise measurement device.

Let's take an example. You are measuring the diameter of a slip fit hole. If your CMM is set to return the best bit diameter of 8 points, it will be biased to the high side. The application demands the maximum inscribed diameter to function. You could take a thousand measurements, average them and obtain a calculated bias of zero. In reality, you are still biased to the high side.
 
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