Important Terms – Single Part Nominal Size – general size, usually expressed in mms Actual Size – measured size of the finished part Limits – maximum and minimum sizes shown by tolerances (larger value is the upper limit and the smaller value is the lower limit) Tolerance – total allowable variance in dimensions (upper limit – lower limit) – object dimension could be as big as the upper limit or as small as the lower limit or anywhere in between.
The tolerance zone is under the zero (base) line
Possible positions of the tolerance zone in case of shafts Fundamental Deviation: is the deviation closest to the basic size. ISO standard uses tolerance position letters with lowercase letters for the shafts.
FYI: The ISO System of Limits and Fits (referred to as the ISO system) is covered in national standards throughout the world, as shown by the following list: Global ISO 286 USA ANSI B4.2 Japan JIS B0401 Germany DIN 7160//61 France NF E 02-100-122 UK BSI 4500 Italy UNI 6388 Australia AS 1654
Possible positions of the tolerance zone in the case of holes Fundamental Deviation: is the deviation closest to the basic size. ISO standard uses tolerance position letters with capital letters for the holes.
Fits Between Mating Parts Fit: degree of tightness between two parts. Fit types: –Clearance Fit – tolerance of mating parts always leaves a space –Interference Fit – tolerance of mating parts always results in interference –Transition Fit – sometimes interferes, sometimes clears
Clearance Fit The mating parts have such upper and lower limits that a clearance always results when the mating parts are assembled.
Basic Systems for Fit Specification In order to standardize dimensioning of fits, two basic systems are used: 1)Basic Hole System: Minimum hole diameter is taken as the basis. Lower deviation for the hole is equal to zero. D max is prescribed according to the specified tolerance. 2)Basic Shaft System: Maximum shaft diameter is taken as the basis. Upper deviation for the Shaft is equal to zero. d min is prescribed according to the specified tolerance.
Basic Hole System Toleranced dimensions are commonly determined using the basic hole system in which the minimum hole size is taken as the basic size.
Two ways of indicating tolerances on technical drawings Questions? Limits of a dimension or the tolerance values are specified directly with the dimension.
Indicating tolerances The dimension is given by: a shape symbol, nominal size, a letter indicating the position of the tolerance zone in relation to zero line, a number indicating the width of the tolerance zone. (quality of production?)
DIMENSIONING OF TOLERANCES - RULES The upper deviation should be written above the lower deviation value irrespective of whether it is a shaft or a hole. Both deviations are expressed to the same number of decimal places, except in the cases where the deviation in one direction is nil.
Example For a nominal diameter of 25 mm and for a fit specification of H7/j5 determine the following: Type of the tolerancing system Tolerance on the hole Tolerance on the shaft Upper and lower limits of the hole (D max, D min ) Upper and lower limits of the shaft (d max, d min ) Type of the fit
Key a) H7/j5 Basic Hole System b) D = 25 mm, from the given table: nominal size H7 j5 + - c)