Dual Dimensioning Standards?

[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

 

[ctopher]

We always have metric dims be one decimal smaller than english units. Example: .xxx in .xx mm
I try not to round dims, I just take them out to another deciaml place. If you round up/down a metric dim, it can make a big difference in the inches.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP1.1 / PDMWorks 05
ctopher's home site

 

[CorBlimeyLimey]

Try this question in the Drafting Standards & GD&T forum1103


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[drawoh]

cygnas,

My recommendation is not to. When you round off inch and metric dimensions, they are not equal. A part can conform to one but not the other. You need to think of your drawings as being part of a contract that precisely defines the part you will accept and pay for. Ambiguous dimensioning is bad.

Pick which units are official, and make it clear that the other units are for reference, only. Better yet, do not do dual dimensioning. Avoid the confusion and the people with the wrong measuring instruments.

JHG

 

[MadMango]

Our tolerances are the same as above, being 3-place inch, it will be 2-place metric... 2-place inch, it will be 1-place metric.

The tolerances should use the same conversion factor, rounded up. Always remember that in dual dimensioning, the secondary dimensions are "for reference", that's why they are in brackets.

We use (sheet metal):
.X= [±].1 [X= [±]3]
.XX= [±].03 [X= [±]0.8]
.XXX= [±].010 [X= [±]0.25]

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[ewh]

It is not common to always round up. The standard we use is:

If the numbers after the required precision (number of decimal places) is less than 5, then there is no change in preceding digits (round down). For example, 2.4634 rounded to three decimal places would be 2.463.

If the numbers after the required precision is greater than 5, the preceding digit is increased by 1 (round up). For example, 4.37652 rounded to three decimal places would be 4.377.

If the number after the required precision is exactly 5, round off to the nearest even number. For example, 8.36500 becomes 8.36 when rounded to two decimal places. 8.35500 also becomes 8.36 when rounded to two decimal places.