Rounding of Tolerances 1

[randy64]

This is a question about rounding tolerances. It is a product of dual dimensioning. I know the evils of dual dimensioning, but our customer does it, and so it is what it is.

I searched the archives but have not seen this particular question as it applies to tolerances.

Question: When converting the inch tolerance in the FCF of .014 it gets us 0.3556. Our rounding standards state that a tolerance of .014 will get rounded to 2 places for the metric. I am arguing that the metric number in the FCF should be 0.35, due to the principle of never adding tolerance (could allow a too-big part that won't fit/go together). However, we have another general standard that defines how to round numbers. It's the usual stuff about 0-4 rounding down, 6-9 rounding up and rounding 5 depending on even/odd.

Bottom line, I am meeting resistance by folks that are blindly following the rounding standard i.e. .199 rounded to .20, versus my belief that a tolerance should never be rounded up i.e. .199 rounded to .19.

Is there something in AMSE that addresses this that will back up my position (assuming I'm correct. If I'm not I'm sure you all will let me know)?

Thanks.

 

[randy64]

DeSim,
We don't make primary units inches and then tolerance in metric.

What I meant to say is that on the drawing, metric is the primary unit, with inches in brackets (secondary). BUT, we design in inches and also base the metric on a conversion of the inches. It would seem to make more sense to always derive the secondary from the primary regardless of whether the primary was mm or inches, but that is not what the customer wants.

Re: Tolerance Block, the tolerance block on these drawings give both mm and inch tolerances based on number of decimal places and they are roughly equivalent.