Limits, FITS AND TOLERANCES
 Due to the inevitable inaccuracy of manufacturing
methods, a part cannot be made precisely to a
given dimension. The permissible variation on the
size is called tolerance.
 The two extreme permissible sizes on the
actual size are called limits.
 When two parts are to be assembled, the relation
resulting from the difference between their sizes
before assembly is called a fit.
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FIT - Condition of looseness or tightness between
two mating parts being assembled together.
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HOLE
SHAFT
Max Hole size – Basic Size =
Upper Deviation
Min Hole size – Basic Size =
Lower Deviation
Max shaft size – Basic Size =
Upper Deviation
Min shaft size – Basic Size =
Lower Deviation
CLEARANCE FIT
Maximum shaft dimension < Minimum hole dimension
INTERFERANCE FIT
Maximum Hole size < Minimum Shaft size
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TRANSITION FIT
Obtained by overlapping of tolerance zones of
shaft and hole …… Does not guarantee neither
clearance nor interference fit.
To obtain different types of fits, it is practice to
vary tolerance zone of one of the mating parts
HOLE BASED SYSTEM
Size of hole is kept
constant, shaft size is
varied to get different fits.
SHAFT BASED SYSTEM
Size of shaft is kept
constant, hole size is
varied to get different fits.
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Representation of Fit
A fit is indicated by the basic size common to
both components, followed by symbol
corresponding to each component, the hole
being quoted first. E.g. 45 H8/g7
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Fundamental Deviations on Shaft Size
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The selection of
letter freezes one
limit of hole / shaft
Representation of
Tolerance
1) Letter Symbol
H : lower deviation
of hole is zero
h : upper deviation
of shaft is zero
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2) Number or Grade (ITG) = IT01, IT0, IT1,….IT16 gives
the tolerance value (T).
Units in μm
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i =
15
The selection of
Tolerance grade number
freezes the other limit of
hole / shaft
Representation of
Tolerance
1) Letter Symbol
H : lower deviation
of hole is zero
h : upper deviation
of shaft is zero
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Representation of Fit
Together (Letter & Grade) on
both mating components
decide quality of fit
0.021
INTERFERENCE
FIT
0.022
0.013
Φ30.021
Φ30.035
Φ30.022
Φ30.000
H7 : Tol Grade 7 mean 21μ variation
(H means upper deviation zero)
p6 : Tol Grade 6 means 13μ variation
(p means upper deviation is 22 μ)
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FITS APPLICATIONS
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Unilateral and Bilateral Tolerances
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Compound Tolerance
60 ± 0.02
40 ± ???
0.08
100 ± 0.06
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Accumulation of Tolerances
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Progressive Dimensioning
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Limit Gauges
 The term ‘limit gauges’ signifies the use of gauges
for checking the limits of the components.
 GO gauge checks Maximum Material Limit (MML)
GO and NOT GO limits of plug gauge
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 Limit gauges ensure that the components lie
within the permissible limits, but they do not
determine the actual size or dimensions.
GO and NOT GO limits of snap gauge
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Taylor’s Principle
 Taylor’s principle states that the GO gauge is
designed
to
check
maximum
metal
conditions, that is, LLH and HLS.
 GO gauge can be designed to check more
than one dimension at a time.
 NOT GO gauge is designed to check minimum
metal conditions, that is, HLH and LLS.
 NOT GO gauge should check only one
dimension at a time.
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 Gauge Tolerance: Normal practice is to take gauge
tolerance as 10% of the work tolerance.
 Component that is manufactured outside the limits
should not be accepted by gauges.
 Component rejected by the GO gauge can be
reworked to maintain the limits because GO gauge
checks maximum material limit.
 Component rejected by the NOT GO gauge is
permanently rejected because NOT GO gauge
checks minimum material limit. Hence close gauge
tolerances will be provided on NOT GO gauges.
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