General Tolerances on Title Block Advantage/Disadvantage?

[memoi567]

I am curious to your opinions on General tolerances on title blocks. I know of two ways to do it basically give a range or by deciding the amount of decimals.

For example:

0 to 1 +/- 0.01
0 to 6 +/- 0.03
6 to 24 +/- 0.06

or

x.x +/- 0.1
x.xx +/-0.01
x.xx +/-0.005

there is also a third way which is a combination of the two which would look like this:

	x.x	x.xx
0 to 1	+/- 0.01	+/- 0.004
0 to 6	+/- 0.03	+/- 0.01
6 to 24	+/- 0.06	+/- 0.03

So what do you use in your title block and what do you think are the advantages and disadvantages of each type?

[Kelly_Bramble]

Both way are often used in the USA. The tolerances should be based on the intended typical manufacturing process capabilities.

In the rest of the world, they use ISO 286 part I and II OR ISO 2768 as well as other manufacturing specific industry standards. On the engineering drawing there might be a note that reads:

UNLESS OTHERWISE SPECIFIED, TOLERANCES ARE PER. ISO 2768, Class M

https://www.engineersedge.com/mechan...tolerance_.htm

https://www.engineersedge.com/calcul...tolerances.htm

https://www.engineersedge.com/calcul...tolerances.htm

In my view, what we do in the USA is inferior and prone the miss-specification relative to function and manufacturing capabilities to what is done in the EU and Asia etc..

[memoi567]

In my view, what we do in the USA is inferior and prone the miss-specification relative to function and manufacturing capabilities to what is done in the EU and Asia etc..

What kind of miss-specification have you seen occur?

[Kelly_Bramble]

What kind of miss-specification have you seen occur?

I'm an engineering consultant and trainer in GD&T as well as DFM/DFMA.

Usually a miss-match between the target manufacturing process and the limits of tolerance specified. Many organizations have a default engineering drawing template and the default tolerance is never reviewed or considered.

So, imagine a machining tolerance block used on a weldment and so on.

I could go on for hours...

[memoi567]

As far as I can tell the ISO tolerance table is easier for the designer, but the table repeats a lot of the same tolerances just in different ranges and for different position. Thus, the decimal way ensures that the designer is more deliberate on the tolerances and dictating each dimension with the correct tolerance (by determining the amount of decimals displayed).

If any tolerance is outside the general tolerance range you have to dictate the tolerance on the dimension itself this is true for both standards, and really is not a factor.

These are the only advantages and disadvantages I can determine. I want to be able to go to my boss and say this standard is better because of this; and really I don't have a strong argument for either style. If my only argument is this is ISO standard or this is ASME standard then its not a good argument in my opinion.

[Kelly_Bramble]

the decimal way ensures that the designer is more deliberate on the tolerances and dictating each dimension with the correct tolerance (by determining the amount of decimals displayed).

Do consider that the imperial (inches) system requires matching significant figures for both limit, basic and expressed tolerance without leading zeros. For SI units (metric) there are no training zeros but there are leading zeros and significant figures are not to be maintained consistent..

This rules applies for all engineering drawing standards. ANSI, ASME and ISO.

Significant figures should be carried out as far as required to express the nominal dimension properly and prevent rounding error. Simply put this is why there are preferred whole numbers over infinite significant figure sizes and distances. Moreover each tolerance of variation on the part or end item should be considered relative to fit, function to ensure the part works and then the remaining features that are present and accounted for can be facilitated by the principal manufacturing process employed.