I'm trying to clarify my understanding about the requirements [read: best practices] for measurement resolution and measurement confidence.
The "Rule of 10s" is a commonly tossed around guideline for measurement resolution that, to the best of my understanding and searching, suggests the following:
- That your measurement systems should be able to resolve to a level of 10% of your total tolerance.
However, regardless of whether you consider "tolerance" to refer to the product or process tolerance, there still seems to be some confusion among those I've talked with.
As an example, take the following dimension:
125mm +/- 10mm
In this case, the total tolerance range is 20mm. Ten percent of 20mm would be 2mm. Therefore your measurement system should be able to resolve down to a 2mm level which would get you roughly 10 distinct "categories" into which your results fall.
An alternate theory that I've come across however, is:
- That the measurement resolution should be "ten times greater than your specification/tolerance as written.
- This is the "move the decimal place" approach and is more a "significant figures" discussion.
Taking the same previous example where the tolerance is +/- 10mm, increasing the resolution 10X would imply that measurements should be obtained at the 1mm level.
I don't believe this latter approach is the intent of the "10 to 1" rule, but I'm looking for some general consensus.
To complicate this question slightly, consider a scenario with an engineering drawing which defines a length as follows:
468.2mm +/- 6.48mm
(THIS is where the team arguments begin.)
The total tolerance is 12.96mm (6.48mm x 2), of which 10% would be 1.296mm. Being a good inspector, and dropping the decimal places to utilize a 1mm resolution measurement method, this still seems to be contradictory to the intent of the dimension.
Where the dimension or tolerance is specified out to multiple decimal places, should the "move the decimal place" method be invoked? If a dimension is specified out two decimal places, it would only make sense to be able to resolve it to 3 decimals.
Appreciate anyone who takes the time to read through this and give your thoughts!