Drawing tolerance vs. Measurement device

#1
Hello,

I'm struggling with what I understand to be a measurement standard and what our QC department is saying.

I have a drawing that dimensions a length to ±0.25mm. My question is (again, based on what I think is the correct approach), what instrument I use to measure this length has to be capable / repeatable to 1/10th of the tolerance (10:1 ratio). Is that 10:1 compared with the .25mm or the .01 decimal place?

Any help would be appreciated.

Thank you
 
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#2
The 10:1 "rule" is just a guideline of sorts, and other information is needed in order to determine the device to use. In your case, the tolerance range is 0.50mm (~.020"), so using the 10:1 ratio your gage should resolve to 0.05mm (.002"). Other factors to consider include the risk(s) of error near the tolerance limits, and the thing being measured (is it 20 feet long or 2 inches, e.g.).
 
#3
Ahh, OK, so it is based on the tolerance range, in this case 0.50mm. OK, I was thinking I needed to account for the hundredth decimal place when considering a measurement device.

That helps. Thank you for the clarification.
 
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