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# Rounding of Dual Dimension Tols?

## Rounding of Dual Dimension Tols?

(OP)
If my primary dimensions are in MM and my secondary dimensions are INCHES in dual dimensioning, if I have a tolerance of 0.05mm it shows as 0.001" instead of rounding up to 0.002".  Is there a way to fix this?

### RE: Rounding of Dual Dimension Tols?

Pro/E always rounds secondary units so the tolerance range is smaller than the primary units.  .002" > .05 mm

You can either increase the primary tolerance to .051 mm which will round to .002" or increase the number of decimals on the secondary units to 4 in which case .05 mm will round to .0019".

### RE: Rounding of Dual Dimension Tols?

I have the same problem. mm is primary, inch is secondary. When I have tolerance -0,38 in primary, I have -0,014 in secondary. Why? If make right round-up it must be -0,015 (0,38/25,4=0,01496...). I can't write -0,381 becouse it, first of all, not right. Second - it mean that measuring tool must be 0,0001 class. It is expensive. Anyway, I think that this is mistake.

### RE: Rounding of Dual Dimension Tols?

No, it is intended functionality.  If the -,38 mm primary were rounded to -0.015" secondary you could accept parts measured in the secondary units that would be rejected in the primary units.

Dual dimensioning is almost always a mistake IMHO.

### RE: Rounding of Dual Dimension Tols?

Since the secondary units are not judged, I don't see why that would be an issue.  I do not believe secondary unit represent a second set of allowable specifications.  If the tols on the drawing are in mm's, then that is what applied...especially if using standard ISO tol charts.

Secondly, I should point out that dual dimensioning is no longer supported by ASME, prolly for reasons like the ones mentioned in this thread.

Personally, I would pick one set of units and stick to it.

Matt Lorono
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

### RE: Rounding of Dual Dimension Tols?

(OP)
heh, I knew the "Don't Dual Dimension" statements would come to this post and almost asked them not to be said.  Sadly, some engineers don't get to make the decisions themselves whether to dual dim or not... no matter how hard they try to convince their superiors.

Thanks for the summary of what's wrong with the secondary, that totally makes sense why they don't round up as you wouldn't want the parts to be accepted at a higher tolerance than you intended.

### RE: Rounding of Dual Dimension Tols?

Dual dimensions are just a reference people.  If you want metric, design in metric and vice versa.  Don't use the converted dimensions to manufacture to.  You'll end up with scrap.  Stop using the crutch and stick to one.

--
Fighter Pilot
Manufacturing Engineer

### RE: Rounding of Dual Dimension Tols?

Dual dimensions are not reference dimensions.  Unfortunately, you do sometimes get the problem where a part is designed with  one set of units & must fit other parts designed in the same units but the part is manufactured & inspected in a foreign country using another unit system.  If you are in that situation it is better to use Pro/E's dual dimension functionality than depend on someone you have never met, in a foreign country, performing the calculations manually.

I agree you can't always avoid them but they are still a problem waiting to happen.  I find increasing the number of decimal places is the best thing to avoid roundoff error.  And always use explicit tolerances rather than default tolerances as they can really bite you.

### RE: Rounding of Dual Dimension Tols?

(OP)
@fighterpilot:

The reason my company uses them is the projects and parts are always time critical, they never want a part to come in late only because a machine shop tried to convert them from metric to english and someone made a calculation error... sure it's their fault but my company will still have to pay in the end due to the project being delayed...

now you'll say "find a company that can do metric" and I say, go run your own business.

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