Number of Decimal Places for tolerancing (Metric vs Inch)

[RonNicholls]

Trying to dimension a print properly and came across an intriguing portion of the ASME Y14.5-2009 standard.

I have always understood that if you use (for example) 3 decimal places in the "nominal" dimension, then the "tolerance" should have the same number of decimal places (example: 33.3" +/- 0.5). This would match what is stated in "INCH" dimensioning in the ASME14.5-2009 standard (section 2, pages 25/26).

It does not seem applicable to metric tolerancing if you read the ASME standard. It seems you can ues as many decimal points in the nominal and tolerance as you want....meaning they don't have to match. You could have 25.3mm +/- 0.123456789 as a callout.

How does this make sense & does anyone see it differently?

 

[Belanger]

Yes, the rules are a little different for metric.
It kind of makes sense, though, because when you're measuring something to verify pass/fail, you're not really verifying the nominal. You're verifying how close you are to the nominal. So that's really where the number of decimal places is important.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[KENAT]

Conventions for inch & metric have a number of differences in ASME.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[CheckerHater]

Ron,
You can look at it this way:
Your dimension and your tolerance have the same accuracy.
But then in metric you just drop the zeroes following your nominal (theoretically correct) value.
So the difference is purely cosmetic.

 

[dgallup]

Lots of differences between inch and metric drawings, ASME or not.

You never display trailing zeros in metric. This also means you can not use the number of decimal places to indicate default tolerances.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[drawoh]

...

It does not seem applicable to metric tolerancing if you read the ASME standard. It seems you can ues as many decimal points in the nominal and tolerance as you want....meaning they don't have to match. You could have 25.3mm +/- 0.123456789 as a callout.

...

The standard does not say you cannot do this. Obviously, it is a silly thing to do. The standard assumes some level of intelligence and professionalism on your part.

The standard does mean that you should explicitly attach tolerances and/or feature control frames to everything. This, actually, is a good idea, and not nearly as much work as you might think.

--
JHG