General Tolerances - Metric

[dakeb]

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

 

[looslib]

ASME Y14.5-1994 section 1.6.1 Milimeter Dimensioning

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

 

[looslib]

This may be a better thread for this discussion.

http://www.eng-tips.com/threadminder.cfm?pid=1103

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[dakeb]

ASME Y14.5-2009 para. 1.6.1 (b) and (c) prohibits trailing zeros

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.

 

[PeterStock]

I have used the number of decimal places to control tolerances in metric parts. While there was no specific company spec exempting them from the no trailing zeros requirement, when the title block gives a tolerances for different numbers of decimal places, I consider that that section of Y14.5 (whatever version is in use) to be overriden by company practice. I see no problem with doing something like this when it is spelled out or ammounts to common practice. To spell it out, I would suggest adding that exception to Y14.5 to a company spec that is referenced on the drawing. And questions like this are better asked in the Drafting Standards, GD&T & Tolerance Analysis forum.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[looslib]

Our drawing note in the unspecified tolerance block stated that the draswing is to be interpretted in accordance with ASME Y14.5-year.

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

 

[KENAT]

These days, I suggest just putting a single +- in the block, not dependent on size, number of decimals, number of sig figs or anything else.

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.

Posting guidelines faq731-376

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

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

 

[dakeb]

I'm really keen to push for this to adhere to a formal, not a company, standard. The company title block has changed a few times over the years and it is a company practise that whenever a drawing is revised, the format gets changed to reflect the current year and copyright notice.

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.

 

[PeterStock]

Company "standards" are good when they are formaly stated in a controlled document and not just "what we have always done".

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[looslib]

When I changed the default settings of the CAD system to produce dimensions that conformed to ASME Y14.5, no trailing zeros, I quickly got called into a meeting with the engineering and manufacturing people. They had been using trailing zeros and new drawings weren't so that changed the tolerance on a lot of parts that the desitgner didn't catch at first. I had to explain that the standard that the company said we produced drawing to did not allow trailing zeros and if engineering wanted the tighter control, then the dimensions needed to be explicitly toleranced. After some grumbling, the managers agreed that what I had done was the correct way forward and all designers were sent a memo to be sure to chek their drawin tolerances.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli