Dual Dimensioning

Can anybody help me out with dual dimensioning? I have an original print
that is dimensioned in millimeters, but our QA department measures in
decimal inches. So if I want to make a new print with dual dimensioning,
how do I handle decimal places? Everything I've seen so far has the inch
dimensions at one more decimal place than the millimeter dimensions. But
how do I handle some tolerances then? For example, if the millimeter
dimension is 1 +0/-0.1, how do I convert this to inches? One decimal place
greater than the millimeter unit would be 0.0 +/-0.0. Or would it be one
decimal place greater than the tolerance decimal places? This would give
you 0.04 +/-0.00. But in order for it to make sense, I'd have to bring it
out to 3 decimal places, so the inch dimension would be 0.039 +0.000/-.004.

So, how many decimal places are correct?

 

By far the easiest thing to do is measure the part in the unit it's
made/designed in. Consider asking the QA guy to push the little inch/mm
button on his measuring device. If he doesn't have such a button, seriously
consider buying him some new tools--it'll be cheaper than doing the
engineering to dual dimension everything.

Unit conversion is fraught with issues, but at least in general terms,
you're correct--the "converted unit" (inches in your case) needs extra
decimal places to deal with round off error. I've been down the road you're
starting out on, and it's not a pretty process--good luck.

 

Yeah, I agree that would be the easiest thing to do, but getting QA to agree
to that may be a challenge.

If I can't do that, I'd still like to hear other people's input on the
issue. Is there a "correct" number of decimal places to use?

 

Jason, pardon me if I'm not understanding correctly -- but why get hung
up on number of decimal places? The number of decimal places in BOTH
English/Imperial and Metric units are only relevant relative to default
tolerances (in your title block or in the original print's title
block). It's generally true that if you use more decimal places than
three or four (in English/Imperial units) it MAY cause QA to have some
difficulty in verifying out to the extremes of the tolerances . . . but
only if the parts to be measured vary toward close to the extremes. But
if your company's QA department is so pig-headed that they refuse to
take their English measurements and convert ("hey guys, can you multiply
by 25.4 or do I have to do it for you?") then they rather deserve what
they get in difficulties. Even considering my intolerance for their
stupidities, if I were in your place I would round off tolerances to
three decimal places most times . . . or four if I felt that that the
tolerance was critical AND was already so tight that giving a slight bit
more via more decimal places would help prevent rejecting GOOD parts.
In your example (.039 +.000/-.004) I would just your conversion to be
appropriate.

Don't confuse the forest for the trees. Keep in mind the purpose of
tolerances and the purpose of QA (not so much to reject parts, but to
accept parts which are acceptable) and you'll keep your priorities
straight.

'Spork'

 

Great, thanks for your comments.