What does a tolerance really mean?

[breyman]

Hi,

We have debate going on in our company right now as to the true defintion of a simple tolerance.

Here's the very simple question:

Say you had a rod with a diameter dimension and tolerance of 1.000" +/- 0.005", what is the exact upper bound that that part can be and still satisfy this tolerance?

One group says that the part can be no larger than exactly 1.005", meaning that if it was actually 1.00500000001" (assuming you could measure to this accuracy), it is out of spec.

The other group says that as long as the first 4 digits aren't more then 1.005", than it is okay, i.e. 1.00599999999" would be "in spec"

I'm personally on the side of the first group and this has been my understanding all along, but there's oddly enough support/debate that I'm confused now. What are your thoughts?

 

[looslib]

At some point a line has to be established because the more precise the measurement, the more it costs to obtain. If you have processes that say inspection procedures shall be 10X more accurate than the measurement being taken, that is the line and there is no further need to go finer. A borderline measurement still has to fit the design criteria.
If it doesn't fit at the next assembly level, then the design needs refined.

If I have 1.003 +/- .002 on a drawing I would think any value between 1.0010 and 1.0050 on the chosen measurement device would be good. 1.0009 or less and 1.0051 or higher is out of spec.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[TheTick]

Anyway, problem resolved, and it's always fun to be right.

It's even MORE fun to be ALWAYS RIGHT!

 

[flash3780]

The uncertainty of measurement must be built into the acceptability range. So, if you can only measure a +/-.01 inch dimension to an accuracy of +/-.001 inches the acceptability limit must be within +/-.009 of the nominal value. If you're borderline, you could always use a better measuring tool.

Of course, reasonable people can often make judgments if things are close by checking the actual values of the mating parts. This happens much more often in one-off or short-run production than in mass-production.