English to Metric Tolerance Issues 1

[TRRMan]

I'm wondering how people are dealing with dual dimensioning and the conversion from english units to metric. I have a tolerance that looks something like this;
INCH = .XX ± 0.01
.XXX ± 0.006
MM = .X ± 0.30
.XX ± 0.15
The problem is that things may pass QC in mm from the supplier but not pass QC interally in english. I know that sounds messy but until we convert over to all metric thats what I'm dealing with. Should I have both english values held to 3 places? I was just wondering what others have done.

Thanks

 

[looslib]

Tighten/loosen one or the other so they do pass.

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[MadMango]

Shouldn't one or the other be the controlling dimension that inspection inspects, with the other listed as reference?

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[TRRMan]

Yes in a perfect world it should be reference but in the real world I have China building my parts in mm and doing QC in mm because thats the tools they have and I have those parts being checked by my companys QC in English because those are the tools we have. I didn't make the mess I'm just trying to work with in it. We are checking to see if our QC can check everything in metric and at that point the issue goes away. I was just wondering if anyone else has had to deal with this and came up with a clean solution. I could loosen the tolerance but that could open up a whole host of other issues. I could make all the english dims 3 digits but then I would need to specify the tolerance everytime to make sure QC used the correct one and to do that on every drawing would become a problem. Thanks for any help.

 

[metalonis]

The problem appears to be in the conversion of the tolerances from inches to mm. Looks like the mm values were rounded instead of truncated. It's necessary to truncate the converted value so they end up being smaller than the original inch tolerance. That wy the millimeter dimensions will always be within the orignal inch dimensions.

For example, a 2.00 in dimension would have a size range of 1.99 to 2.01. The way you've converted the tolerance (0.01 in = 0.254 mm rounded to 0.3 mm) the equivalent range is 50.5 to 51.1 mm = 1.98 to 2.01 in. By truncating the converted tolerance to 0.2 mm you end up with 50.6 to 51.0 mm = 1.99 to 2.01 in. Everybody's happy.

 

[rconner]

While I don't know what if any standards specifically address this issue, I think in principal I like metalonis's approach of converting the original tolerance to in effect get effectively the same allowable "range" of actual dimensions (in either unit).

 

[rconner]

I have noticed ASTM E380, Standard for Metric Practice may in fact discuss "Rounding Tolerances Inches to Millimetres" (you may wish to obtain a copy of this standard). I think this may be allowed per this standard by a couple different explicitly defined methods, one of which "rounds to values nearest each limit" (meaning that even in the most "unfavorable cases" the two original limits will be changed by no more than a small percentage), and another that assures that the converted tolerances are never larger than the original tolerances (the latter of course necessary if "original gages" are used for inspection).