Tolerances on a drawing

[bbrejcha]

Sometimes explaining my problem on this forum helps me state the obvious and resolve the problem myself.  Talking thru it if you will.  I have a drawing that requires both inches and millimeters in that order.     If I adjust the tolerance or decimal places for the inch.... the millimeters adjusts what would seam appropriately.       I suppose a tolerance is a tolerance and should not matter if the millimeter to rounds up or down.    Im trying to match another drawing is a challenge to get the millimeters to fall in suit to match other drawings.  Is the trick to adjust the nominal slightly to get the mm to follow suit?

Bart Brejcha

Design-engine.com

[rreifsnyder]

Bart,

An example of what you seem to be talking about is in the case of a .005" tolerance which translates to .127 mm. You might think that it should round up to .13 when you reduce the metric to 2 decimal places but the problem is that the secondary units are then adding to the tolerance because translating that back to English units gives a tolerance of .005118". I know that would round out as well but the fact that it is actually larger is the issue. Secondary tolerances must never allow for parts being accepted that do not meet the primary tolerance. Because of this the metric tolerance rightly is rounded down.

[TomU]

Dual dimensions are a royal pain.  If I had my choice, they wouldn't be allowed.  Today we do everything from fudge the primary number (beyond the displayed decimal place) to manually recreating dimensions as dumb text.  For many numbers, there is simply no good way to show dual dimensions without losing some tolerance on the secondary values.  Our current internal 'policy' is to show the dual dimension tolerances at 8 decimal places when they don't match our customer's drawings.  🙄