Regarding drawings with millimeter dimensions, Y14.5 – 1994 and -2009 both state in para. 1.6.1(b) ‘Where the dimension is a whole number, neither the decimal point nor a zero is shown.’
Our standard title block instructs tolerances on dimensions according to the number of decimal places expressed by that dimension. So, according to our company standard, a 20.00 dimension provides limits of 20.25 / 19.75 but according to para 1.6.1 (b) that is illegal – we should only be saying 20 with no decimal point etc. and stating each tolerance alongside each dimension – something that we would find tiresome and time wasting.
We are very reluctant to have to show the tolerances against every whole number. It seems ironic that for imperial units additional trailing zeros can be used ‘as necessary’ (para 1.6.2(b)).
Can you suggest a legal way around it?
Look forward to your reply / suggestions; thanks for a very useful website.
You’re correct. ASME Y14.5 1994 and 2009 both prohibit the use of trailing zeros on metric dimensions and tolerances. So, using title block tolerances that indicate something like this where the tolerance value is based on the number of decimal places is completely inappropriate:
.X ± .5
.XX ± .25
This practice for general tolerances originates with the expression of imperial units (inches), but in too many cases has not updated as the company changed from imperial to metric units. The correct method for displaying general tolerances (if you use them at all) for metric units is based on the dimensional value. These can use any interval (range of linear dimensions) imaginable, but one example is shown below. These tolerances are usually listed as a schedule or table near the title block.
0 > 20 ±0.1
20 > 80 ±0.2
80 > 120 ±0.4
120 > 200 ±0.6
I do tend to caution against an over reliance on general tolerances, simply because as general tolerances they are not tied to the functional requirements (failure modes) of each dimension. Considering the maximum allowable tolerance for each dimension may be more work for the designer, but manufacturing and purchasing will appreciate the difference in cost and lead times.
ASME GDTP Senior Level
Dimensional Engineering Mentor