Dual Dimensioning Standards 1

[cygnas]

OK, here we go.....We are having a major battle here on how to go about a standard of dual dimensioning. Previously we have always dimensioned in english units only but we have decided to add dual (in/mm) dimensions to all of our drawings. Our biggest disagreement is with the conversion of the inches dimension to the "equivalent" millimeters dimension with accuracy. When we were using only inches we had a standard of the following: .xxx ± .005, .xx ± .020, and .x ± .030. Is there an "accepted" standard for dual dimensioning while not sacrificing accuracy in the conversion process? Should the metric tolerance hold the same amount of decimal places as the english? It seems like we are running into the "round up or round down" when it comes to the metric tolerance. If rounding is done it sacrifices accuracy. We need to keep the english dimension the "master" dimension and the metric would be the "slave". Thanks for any suggestions.
Cygnas

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[looslib]

I would use the default/primary dimensioning units for any GD&T. The dual/secondary values are reference only.


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[Heckler]

Another standard we use is ANSI/IEEE STD 268 American National Standard for Metric Practices. This standard has been adopted by the DOD for what it's worth. See appendix C Rules for conversion and rounding subset C6 Conversion of Linear Dimensions of Interchangeable Parts.

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[McLeod]

The reference that ctopher gave is the one you need. IEEE/ASTM SI 10 has very specific rules for conversion and rounding, which are only intuitive in hindsight. Genium's Drafting Manual describes these rules in section 6.9 - Dimensioning & Tolerancing (Inch/Millimeter Conversion).

There are two rounding methods described, A & B. Method A is simpler, but may result in converted limits which are outside the original limits. Method B ensures that the converted limits are always inside the original limits.

Both methods use rounding tables to determine the precision of the converted limits. The tables are based on the total tolerance spread of the original dimensions. e.g., if the inch tolerance is at least .004, but less than .04, the millimeter limits are rounded to 2 decimal places. Whether the last retained decimal place is rounded up or down depends on the method.

In method A, the rounding rules are: if the dropped digit is over 5, round up; under 5, round down; exactly 5, round to the nearest even number.

In method B, the last retained decimal place is rounded down for the upper limit, and rounded up for the lower limit, thereby ensuring that the original limits are maintained.

 

[ewh]

McLeod,

Thanks for specifying that for us. I knew that the standard existed, but wasn't able to find out the specifics. Most major CAD packages seem to follow method A.