10:1 Rule

 This is a requirement of the ISO/TS 16949 standard and applies to inspection.

When you inspect a part, you are required to use a gauge that has a resolution of 1/10^th of the total tolerance of the dimension being measured. 

Example: I am measuring a part which has a tolerance of +/- 0.3 mm, with a total tolerance of 0.6mm. I will need a gage that can discriminate to 0.06mm

4:1 Rule

 This is a requirement of the ANSI/NCSL-Z-540 and is a Calibration requirement.

If I am calibrating a gage I need to use a check standard that has an uncertainty of measurement that is less than or equal to 25% of the total gage tolerance.

Example: I am calibrating a caliper with a tolerance of +/- 0.03 mm, with a total tolerance of 0.6mm. I will need a Check Standard (master) that will have an uncertainty of measurement no greater than 0.015mm

 
10:1 Guideline in Calibration

Many people have used the 10:1 guideline for years in calibration. However it has  been used differently than explained above. Simply stated it has been used like this:

When calibrating a gage, use a check standard (master) that has a resolution equal to 10 times greater than the accuracy of the gage being calibrated.

Example: I am calibrating a gage with a tolerance of +/- 0.03mm, with a total tolerance of 0.6mm. I will need to use a check standard (master) that has a resolution of 0.06mm. 

This is a “loose guideline” that a person can use when deciding what type of check standard to use to calibrate a gage, but in practice the 4:1 rule is the proper method. If you do not follow the ANSI/NCSL-Z-540 Standard then you would not be required to meet the 4:1 rule and can default to the 10:1 Guideline. 

 

Article Courtesy of Carlis Stuber

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