Gage Tolerance vs. Process Parameter Tolerance - Process qualification

R

rstaz

#1
Hello! This is my first post, I hope I am doing this right :)

I am running a process qualification. We will be looking at the best and worst case processing parameters. So, let's say I will be testing temperature at 180 and 190. One team member said that if the calibration tolerance of the controller is +/- 2, then after qualification, I must run production at 182 - 188. That's to ensure we are in the qualified window. Has anyone else taken this approach? I would be concerned about this if the calibration tolerance were large.

I am thinking about suggesting that we try to keep the calibration tolerance 30% or less of the process parameter tolerance--similar to a GR&R.

Any suggestions??

Thank you,

Renea
 
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G

Graeme

#2
Accounting for measurement uncertainty

Renea,

Welcome to the Elsmar Cove! Dive in and enjoy!

I'm making an assumption that what you are calling the "calibration tolerance" of the controller is its performance or acuracy specification - what I would use to determine if it passed or failed the calibration performance test. If that is correct, then ...

If the controller indicates 180, accounting for the accuracy means that all you can sure of is that the temperature is somewhere between 178 and 182. Likewise, a reading of 190 means that you are sure the temperature is somewhere betweeb 188 and 192. That would be the reason for keeping the controller somewhere between 182 and 188. If it is within that range you can be sure the temperature is within your limits of 180 to 190 even if the controller is off by + or -2.

This type of thing is actually fairly common. The 2 degrees you are eliminating on either end is called a guardband. In the calibration business we deal with that and other areas of measurement uncertainty all the time. For example, that controller would need to be calibrated using a temperature simulator with an accuracy of +/- 0.5 degree or better, and the test limits for us to say it is good would actually be +/- 1.5 degrees.

As for improving the performance of the controller - it is most likely that the temperature sensor attached to the controller is a thermocouple. If that is correct, then the thermocouple is probably the source of about 75% of that +/- 2 uncertainty, and there is very little you can do about that. That's just the way thermocouples are. Replacing the controller with a "better" one will only increase the cost of getting it calibrated.
 
R

rstaz

#3
Thanks, Graeme. I guess my problem was the fact that by accounting for the calibration uncertainty by narrowing the process window in production, you are increasing the ratio of calibration error to process tolerance. It makes me think it is similar to increasing the error in a "Gage R&R". I certainly see the logic, however. Thanks for your reply!

Renea
 

Miner

Forum Moderator
Staff member
Admin
#4
rstaz said:
Thanks, Graeme. I guess my problem was the fact that by accounting for the calibration uncertainty by narrowing the process window in production, you are increasing the ratio of calibration error to process tolerance. It makes me think it is similar to increasing the error in a "Gage R&R". I certainly see the logic, however. Thanks for your reply!

Renea
You will not change your ratios because you do not change your tolerance.

If you have access to the AIAG MSA manual, it has a discussion on gage performance curves. These curves have three zones: 1) A good part will ALWAYS be called good; 2) A bad part will ALWAYS be called bad; and, 3) where good may be called bad and bad may be called good.

Graeme's guard banding is established around the first zone. This does not change the tolerance, but will ensure that you do not ship bad product due to measurement error (maybe for other reasons, but not this one).
 
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