Originally Posted by
Kelly Bramble
I think I know what you're asking... let's see what happens.
Expressing size as a nominal with +/- tolerances is an option for expressing tolerances but not required. There are many acceptable methods to express size and tolerances with respect to ANSI or ASME dimensioning and tolerancing standards.
ANSI B4.1 specifies a required end item fit based on functional requirements. An RC6 is a Running Clearance fit based on a range for a given nominal size. The fits provided are intended for plain (non threaded) cylindrical parts and B4.1 makes recommendations on preferred sizes, allowances, tolerances, and fits for use wherever they are applicable.
I believe the fits are based on Experimental work long ago.
For a given size, e.g. RC6, Dia. 1.25 nominal size the desired MMC and LMC are provided for the hole and mating shaft feature. Also note that for the size given for the shaft feature is a double negative tolerance.
Tolerances may be expressed using a variety of methods, including double negatives however I don't recommend that.
In the default example given by the calculator, the hole could expressed as Dia. 1.2500 + .0025 / - .0000 and the shaft feature could be expressed as Dia. 1.2500 -.0020 / -.0036 (two negative tolerances with respect to the nominal).
In practice we ~ might actually use:
Hole: Dia. 1.2500 + .0025 / - .0000 (H9 Tolerance)
Shaft: Dia 1.2480 + .0000 / - .0016 (e8 Tolerance)
Clearance between the shaft and hole range between .00610 and .0020 depending on as-builts.