+0/-.001 Tolerance question - Feature size is 1.249 +0/-.001 - Actually measures 1.2493 (.0003 OHL

Jim Wynne

With the example data given by both the OP and myself, you are absolutely right since rounding and truncation end up the same place...but if the value was instead 1.2487, it still would have been written as 1.249. And if it was 1.2495, it would have been written as 1.250 and would be deemed out of spec. Pretty sure that's rounding, not truncation.
There was nothing about rounding (specifically) in the OP's question, and the clear implication (to me) was that the measurement result should be truncated, not rounded. The clue is in the engineers reference to the "precision" shown on the print.


Looking for Reality
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Fair enough since we're both guessing here.
I suggest that rounding 1.2943 to three decimals and truncating 1.2943 to three decimals yields the same result...and we each made equally valid assumptions that resulted in the same thing...assumptions none-the-less. There was nothing about truncating (specifically) in the OP's question either, and the clear implication (to me) was rounding. Shows where implications get us on an internet forum.

We're off in the weeds (a tiny bit) here...it still ends up with either a defense of actions based on industry guidance which generally nets to "don't do that", or getting customer approval, or backing away and letting the engineer in question run the show. All this rest is side details.


Starting to get Involved
I went through posts describing "different schools"/"rounding"/"truncating" - nonsense for this thread.

@avodroctrebor is there a standard defined on the drawing? Probably yes (hopefully!). ASME and ISO are very clear how to do it:

you have a dimension 1.249 +0/-.001, so the upper limit is 1.2490000 (zeros to infinity)

and the lower limit is 1.2480000 (zeros to infinity).
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