FITS and TOLERANCES
Control Measurements of
Finished Parts
Indicating the Desired Dimension
Indication of the Real Dimension of Parts
Due to the
inevitable
inaccuracy of
manufacturing
methods, a part
cannot be made
precisely to a
given dimension.
Indication of the Real Dimension of Parts
Indication of the Real Dimension of Parts
Selection of Waste Parts
Selection of Waste Parts
Waste results when the manufacturing process cannot
maintain size within prescribed limits.
Selection of Acceptable Parts
Selection of Acceptable Parts
There is no such thing as an "exact size".
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.
Characteristics of Tolerances
Characteristics of Tolerances
Connection btw the characteristics:
US = N + UD
LS = N – LD
T = US – LS =
The Possible Positions of the
Tolerance Zone
• The tolerance zone is above the zero (base)
line
• The tolerance zone is on the zero (base)
line
• The tolerance zone is under the zero (base)
line
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.
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
ISO standard uses tolerance position letters with capital letters for the holes.
Fundamental Deviation: is the deviation closest to the basic size.
Schematic representation of the
placement of the tolerance interval
A tűrésnagyság szabványos értékei
IT: International Tolerance
• 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.
Fits Between Mating Parts
ILLUSTRATION OF DEFINITIONS
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.
Clearance Fit (e.g.: H7/f6)
Clearance Fit (pl. H7/f6)
Clearance Fit (pl. H7/f6)
Transition Fit
Either a clearance or an interference may result
depending on the exact value of the dimensions
of the machined shaft and hole within the
specified tolerance zones.
Transition Fit (e.g.: H7/j6)
Transition Fit (e.g.: H7/j6)
Transition Fit (e.g.: H7/j6)
Transition Fit (e.g.: H7/j6)
Interference Fit
The mating parts have such limits that the lowest
shaft diameter is larger than the largest hole
diameter..
Interference Fit (e.g. H7/p6)
Interference Fit (e.g. H7/p6)
Interference Fit (e.g. H7/p6)
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. Dmax 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. dmin 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.
Fits in Basic Hole System
Basic Shaft System
• In this system, the maximum shaft is taken
as the basic size and is used only in
specific circumstances.
Fits in Basic Shaft System
Indicating Tolerances
Two ways of indicating tolerances on
technical drawings
Limits of a dimension or the tolerance values are
specified directly with the dimension.
Questions
?
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?)
Specifying Fits in Technical
Drawings
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 (Dmax, Dmin)
• Upper and lower limits of the shaft (dmax, dmin)
• Type of the fit
Key
a) H7/j5  Basic Hole System
b) D = 25 mm, from the given table:
H7 
 21 m
0 m
+
-
j5 
 5 m
- 4 m
nominal size
H7
j5
Th  21 m
c) Ts  9 m