Hi Euric,
The system you describe is used for inches-based drawings.
In metric drawings, the tolerance is based on the distance. The drawing will typically be on an A0, A1, or A2 sheet. The sheet will have frame and a title block, and the title block will list the default tolerances. Here's what I have in some of my drawings:
UNLESS OTHERWISE SPECIFIED DIMENSIONS ARE IN MILLIMETERS
LINEAR TOLERANCES:
0-6 �0.1 OVER 6-30 �0.2 OVER 30-120 �0.3 OVER 120-315 �0.5 OVER 315-1000 �0.8 OVER 1000 �1.2
Thus, small distances automatically get tight tolerances and large distances get looser tolerances, which is often what you want.
In inches drawings you will have "naked decimals" and you will have extra zeros to the right of the decimal to specify the tolerance. In metric drawings, you do NOT have naked decimals and you will NOT have any extra trailing zeros. Some people use commas in metric drawings instead of periods. The obvious advantage is that a comma is easier to see and reproduces better than a period, reducing the chance for errors. For example,
Inches drawing: .10 (if you don't see the period, you might think this is 10 inches)
Metric drawing: 0,1 (even if you don't see the comma, the zero reminds you that it's 100 �m)
There are plenty of standards documents for drawings and tolerancing, but this is a very involved subject. The modern trend is to use geometric tolerancing, which has big advantages over old-fashioned tolerancing but is a lot more complicated. I you want, I could send you some drafting standards documents off-list next week.
John
Euric wrote:
Does anyone know anything about drawing tolerances? I am interested in finding out if someone is aware or can point me to info on "self-tolerancing". I'm not sure if this is the correct term or not, as I can't find anything under this term on Google.
If I specify a dimension as 10 mm, without stating a tolerance, the self tolerance would default to �0.5 mm. For 10.0 mm, the self tolerance is � 0.05 mm, for 10.00, it would be � 0.005 mm, etc.
I believe the way it works, is that a number like 10 mm � 0.5 mm can be seen as 9.5 ~ 10.5 mm. When properly rounded upward, any number between 9.5~10.5 mm would round back to 10. For 10.00 mm � 0.005 mm, this relates to 9.995~10.005 mm. Again, any rounding of any number inbetween 9.995 and 10.005 would be 10.00.
Thus self tolerances allows for a range of numbers that would effectively be equal to the desired dimension if rounded. I'm not sure if this is the actual explanation used to describe this method, but it is one I understand.
So if anyone is aware of this method and has used it, please let me know. I tried to explain to a co-worker today that with metric dimensions if no tolerances are given on a drawing, the number of digits past the decimal determine the tolerance by implying self-tolerance as explained. She never heard of it and I couldn't find any referances to back up my explanation.
Euric
