Open tolerance for drawing

In the title box of drawing sheet we mention open tolerance, can any one help me in understanding this. Because I have seen some times some company gives very lose tolerance in title box and specify required tolerane with dimension but some places i have seen that we give tight tolerance in title box (which is valid for all drawings) and control tolerance through decimal places. So is there any standard for this and what is the right method ? Most of the components with which i deal is plastic components and unit which we use is metric.

G'Day dhanishd
as one on the recieving end of such things...I get to make it to the drawing... the tighter the tolerance (the more zeros you put in) the more expensive it becomes.

Many of my drawings are drawn using BS8888 (British Standard 8888) which superceeds BS308, which is in line with the ISO standard (world standard)
These standards cover much more than just the drawing layout and tolerances.
[url]www.g-tol.co.uk/in1.htm[/url]
[url]www.iso.org/iso/products/standards/catalogue_ics_browse.htm?ICS1=01&ICS2=100&ICS3=20&[/url]

this subject could get awfully deep real quick if we're not carefull
Bye Bye now
Andy

I don't think there is any rule at all about this. The title block tolerance should be the generic one that the greatest number of dimensions are to be held to. When a specific tolerance is placed on a specific dimension, it simply overrules the title block tolerance; it could be greater than or less than the title block tolerance.

If you are using the ISO system for your drawings, there is ISO 2768 (part 1 is default tolerances for linear dimensions, part 2 is for geometric tolerances). I must admit that I don't use this a whole lot since I'm based in the US where that standard is rarely invoked.