General Tolerances - Metric
General Tolerances - Metric
(OP)
Guys,
In a metric drawing you cannot use the number of decimal places to control the acuracy in the general tolerance block, because standards do not allow trailing zeros in metric dimensions.
ISO get around this with a specific general tolerance standard, ISO 22768, which bases the accuracy on the length of the dimension and whether you require fine, medium, or coarse tolerances.
How do people that use ASME create a general tolerance block for metric drawings?
Cheers
Dave
In a metric drawing you cannot use the number of decimal places to control the acuracy in the general tolerance block, because standards do not allow trailing zeros in metric dimensions.
ISO get around this with a specific general tolerance standard, ISO 22768, which bases the accuracy on the length of the dimension and whether you require fine, medium, or coarse tolerances.
How do people that use ASME create a general tolerance block for metric drawings?
Cheers
Dave





RE: General Tolerances - Metric
Which standards do not allow them?
RE: General Tolerances - Metric
The following shall be observed where specifying millimeter dimensions on drawings:
(a) Where the dimension is less than one millimeter, a zero precedes the decimal point.
(b) Where the dimension is a whole number, neither the decimal point nor a zero is shown.
(c) Where the dimension exceeds a whole number by a fraction of one millimeter, the last digit to the right of the decimal point is not followed by a zero.
At one former company, we created the tolerance block just like normal, except we stated that zero and one place decimal dimensions hada tolerance of 1.5mm. 2 place decimals had a tolerance of 0.8. When we revised things later, we went to the ISO method of dimensioning and used ISO 2768-mK.
"Wildfires are dangerous, hard to control, and economically catastrophic."
Ben Loosli
RE: General Tolerances - Metric
http://www.eng-tips.com/threadminder.cfm?pid=1103
"Wildfires are dangerous, hard to control, and economically catastrophic."
Ben Loosli
RE: General Tolerances - Metric
As far as I am aware, ISO standards don't specifically prohibit trailing zeros in the text, but in diagrams I have never seen trailing zeros shown.
RE: General Tolerances - Metric
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: General Tolerances - Metric
That alone would dictate that trailing zeros are not to be used for tolerance places.
"Wildfires are dangerous, hard to control, and economically catastrophic."
Ben Loosli
RE: General Tolerances - Metric
ISO 2768 has some serious potential issues, I would not call it up on a drawing otherwise to ASME stds. Though maybe I'm biased because I got so burnt by my first involvement with it.
I would not use the trailing zeros with mm drawings to ASME unless it's clearly explained to be a deviation from 14.5. Even then I'm hesitant.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: General Tolerances - Metric
I have brought a major problem to their attention - the replacement format in many cases significantly changes the general tolerances. The older formats had +/-0.25 for whole numbers, new formats have +/-1 for whole numbers. Of course, that is a major FFF change so each drawing has to be completely re-dimensioned to capture the original design intent.
Of course, being a contractor, there is a limit to what I can push for. The bottom line is, beware of using company standards. They are often not controlled as well as ISO or ASME.
RE: General Tolerances - Metric
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: General Tolerances - Metric
"Wildfires are dangerous, hard to control, and economically catastrophic."
Ben Loosli