10 Selection of Acceptable Parts There is no such thing as an "exact size".
11 Important Terms – Single Part Nominal Size – general size, usually expressed in mmsActual Size – measured size of the finished partLimits – 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.
17 The tolerance zone is under the zero (base) line
18 Possible positions of the tolerance zone in case of shafts ISO standard uses tolerance position letters with lowercase letters for the shafts.Fundamental Deviation: is the deviation closest to the basic size.
20 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 286USA ANSI B4.2Japan JIS B0401Germany DIN 7160//61France NF EUK BSI 4500Italy UNI 6388Australia AS 1654
21 Possible positions of the tolerance zone in the case of holes ISO standard uses tolerance position letters with capital letters for the holes.Fundamental Deviation: is the deviation closest to the basic size.
26 Example: A shaft of nominal diameter 25 mm is going to be manufactured Example: A shaft of nominal diameter 25 mm is going to be manufactured. IT grade is required to be IT7.Determine the tolerance on the shaft.
29 Fits Between Mating Parts Fit: degree of tightness between two parts.Fit types:Clearance Fit – tolerance of mating parts always leaves a spaceInterference Fit – tolerance of mating parts always results in interferenceTransition Fit – sometimes interferes, sometimes clearsNote these terms are used with multiple parts.
30 Clearance FitThe mating parts have such upper and lower limits that a clearance always results when the mating parts are assembled.
43 Basic Systems for Fit Specification In order to standardize dimensioning of fits, two basic systems are used:Basic Hole System: Minimum hole diameter is taken as the basis. Lower deviation for the hole is equal to zero. Dmax is prescribed according to the specified tolerance.Basic Shaft System: Maximum shaft diameter is taken as the basis. Upper deviation for the Shaft is equal to zero. dmin is prescribed according to the specified tolerance.
44 Basic Hole SystemToleranced dimensions are commonly determined using the basic hole system in which the minimum hole size is taken as the basic size.
49 Two ways of indicating tolerances on technical drawings Limits of a dimension or the tolerance values are specified directly with the dimension.Questions?
50 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?)
52 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.
53 ExampleFor a nominal diameter of 25 mm and for a fit specification of H7/j5 determine the following:Type of the tolerancing systemTolerance on the holeTolerance on the shaftUpper and lower limits of the hole (Dmax, Dmin)Upper and lower limits of the shaft (dmax, dmin)Type of the fit
54 Key + - a) H7/j5 Basic Hole System b) D = 25 mm, from the given table:+nominal sizeH7j5-c)