Posted by John-Paul Belanger (126.96.36.199) on November 01, 2002 at 22:12:21:
In Reply to: Double Plus or Minus Tolerance posted by David E. Wagher on October 28, 2002 at 11:56:02:
I agree with Mr. Marz-- a double plus/plus (or minus/minus) is definitely awkward, but is probably not illegal. It is most likely derived from the ISO system of limits and fits (if you have ever seen a dimension such as 16f7 -- nominally 16 mm with a tolerance class of f7). In your case, both a hole and a pin could nominally be called .250, but the hole might have a plus/plus and the pin may have a minus/minus, to ensure that there is always clearance. The real authority on this is the Dimensioning & Tolerancing standard (ASME Y14.5M-1994) and it does not mention double unilateral tolerances, but it doesn't prohibit it either (paragraph 2.3.1.a).
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