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8/11/2019 3. (Tambahan) Tolerances_fits
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Chapter 1
Tolerances and Fits
Middle East Technical University
ME307 Machine Elements I
Fall 2006-2007
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Tolerances and Fits
Deviations from the exact dimensions are inavoidable due to the nature
of manufacturing processes. These deviations must not be so large thatthe part is useless. Designer specifies upper and lower limits to the
dimensions
d
dmax
dmin
o erance s e erence e ween e max mum an
minimum size limits of a part:
minmax ddd =
Too tight tolerance High manufacturing cost
Too large tolerancePart may not function as expected
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Fits
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Tolerances
Application, Process Tolerance (m)
Slip blocks, reference gages 1-2
High quality gages, plus gages 2-3
Good quality gages, gap gages 3-5
Fits produced by lapping 4-10
Ball bearings, Diamond or fine boring, fine grinding 5-12
r n ng, ne on ng -
High quality turning, broaching 12-35
Center lathe turning and boring, reaming 14-50
Horizontal or vertical boring machine 30-80
Milling, slotting, planing, metal rolling or extrusion 50-100
Drilling, rough turning and boring, precision tubing 70-140
Light press work, tube drawing 120-240
Press work, tube rolling 150-500
Die casting or molding, rubber moulding 250-1000
Stamping 400-1400
Sand casting, flame cutting 500-2000
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Basic Systems for Fit Specification
In order to standardize dimensioning of fits, two basic systems are used
1) Basic Hole System (BHS)
Minimum hole diameter is taken as the basis. Lower deviation for the
hole is equal to zero. Dmax is prescribed according to the specifiedtolerance.
2) Basic Shaft System (BSS)
Maximum shaft diameter is taken as the basis. Upper deviation for the
Shaft is equal to zero. dmin is prescribed according to the specifiedtolerance.
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Basic Systems for Fit Specification
tolerance zone
Fund. Dev.
BHS BSS
Fund. Dev.
basic size
basic size
hole
tolerance zone
shaft
hole
shaft
tolerance zoneDr. Serkan Da
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International Tolerance Grade Numbers
In order to establish a preferred fit we need
1) The magnitude of the tolerance zone for the shaft and the hole2) Fundamental deviation for the shaft (in BHS)
Fundamental deviation for the hole (in BSS)
International tolerance grade numbers (IT numbers) designate groups of
tolerances such that tolerances for a particular IT number have the samerelative level of accuracy but vary depending on the basic size.
The magnitude of the tolerance zone is the variation in part size. The
tolerance zones are specified in international tolerance grade numbers
called IT numbers.
These numbers range from IT0 to IT16. The smaller grade numbers specifya smaller tolerance zone.
IT numbers are given in Tables A-11 and A-13 in the textbook.
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Application,Process Tolerance (m) IT Grade
Slip blocks, reference gages 1-2 1
High quality gages, plus gages 2-3 2
Good quality gages, gap gages 3-5 3
Fits produced by lapping 4-10 4
Ball bearings, Diomand or fine boring, fine grinding 5-12 5
International Tolerance Grade Numbers
Grinding, fine honing 6-20 6
High quality turning, broaching 12-35 7
Center lathe turning and boring, reaming 14-50 8
Horizontal or vertical boring machine 30-80 9
Milling, slotting, planing, metal rolling or extrusion 50-100 10
Drilling, rough turning and boring, precision tubing 70-140 11
Light press work, tube drawing 120-240 12
Press work, tube rolling 150-500 13
Die casting or molding, rubber moulding 250-1000 14
Stamping 400-1400 15
Sand casting, flame cutting 500-2000 16
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International Tolerance Grade Numbers
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.
Use Table A-11 (tolerance grades, metric series)
as c s ze - ons er t e s appearance
Tables can be used for both shafts and the holes.
Tolerance on a shaft or a hole can be calculated by usingthe formulas provided by ISO.
mm021.0minmax === dddTs
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Tolerance Designation (ISO R286)
Tolerance on a shaft or a hole can also be calculated by using the formulas
provided by ISO.
iKT =where,
s t e to erance n m
DDi 001.045.0 3 += (unit tolerance, in m)
21DDD = (D1 andD2 are the nominal sizes marking the beginning and
the end of a range of sizes, in mm)
( )( )66.110 ITITnK =
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International Tolerance Grade Numbers
Example: Consider the previous example (D = 25 mm and IT grade of
IT7) and calculate the tolerance on the shaft using the formulas given in
ISO standards.
DDi 001.045.0 3 +=
Note: When the nominal sized marking the beginning and end of a range ofsizes are not available, nominal size can be directly used to calculate i.
i = 1.341 m
( )( ) 166.110 67 == K
mm0.021m45.21 === iKTs
Same result is obtained using Table A-11
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Fit Types
There are three types of fits
a) Clearence Fits b) Interference Fits c) Transition Fits
Clearence Fits
The mating parts have such upper and lower limits that a clearence always
results when the mating parts are assembled.
holeCmin
shaft
mm93.25
mm95.25
min
max
=
=
d
dmm02.0=sT
mm00.26
mm03.26
min
max
=
=
D
D
mm03.0=
hT
Allowance of the fit corresponds to maximum material condition (Cmin).
Allowance can be calculated by considering tightest fit.Dr. Serkan Da
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Fit Types
There are three types of fits
a) Clearence Fits b) Interference Fits c) Transition FitsInterference Fits
The mating parts have such limits that the lowest shaft diameter is larger than the
largest hole diameter.
mm02.0=sT
Allowance of the fit corresponds to maximum material condition (Imax).
holeshaft
maxI
mm02.25
mm04.25
min
max
=
=
d
d
mm00.25
mm01.25
min
max
=
=
D
Dmm01.0=hT
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Fit Types
There are three types of fits
a) Clearence Fits b) Interference Fits c) Transition FitsTransition Fits
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
hole shaftmm02.0=sT
Allowance of the fit corresponds to maximum material condition (Imax).
maxI
mm02.25
mm04.25
min
max
=
=
d
d
mm00.25
mm03.25
min
max
=
=
D
Dmm03.0=hT
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Fundamental Deviation
tolerance zone
Fund. Dev.
BHS
Tolerance zones for the hole and the
shaft are prescribed byIT numbers. Note
that we also need thefundamental
deviation for the shaft to com letel
specify the fit. ISO standard uses toleranceposition letters with capital letters for the
holes and lowercase letters for the shafts.
Letters are combined with tolerance grades to establish a preferred fit. Specification for the holes: H7, D8, J6
Specification for the shafts: h6, b8, k7
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Specification for the Fits
tolerance zone
Fund. Dev.
BHS basic size25 H7/g6
IT grade
un . ev.
for the hole
IT grade
for the hole
fund. dev.
for the shaft
H/(a-z) BHS (fund. dev. for the hole is zero) (A-Z)/h BSS (fund. dev. for the shaft is zero)
Letters I, L, O, Q, W, i, l, o, q, w are not used.
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Specification for the Fits, Example.
34 H11/c11
Basic Size (Datum)+
-
fund. dev.
H11Th
Ts(BHS)
c
What are the max. and min. limits for the hole and the shaft for the given fit spec.?
Table A-12 (Fund. devs. for shafts (BHS)).
mm.12.0c,formm,34 == FD
mm.16.0IT11,mm,34 == TD
mm.34min =D
mm.16.34max =D
mm.88.3312.034max ==d
mm.72.3316.012.034min ==d
(Table A-11)
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Specification for the Fits
In Basic Shaft System (BSS), fund. dev. of the shaft is given by h and
it is equal to zero. (Upper limit of the shaft is on datum line).
-
+Datum Line
B
,
- h shaft
+
- h
Datum Line
shaft
HH/h, snug fit.
Note: Fund. dev. table for BSS is not given in the textbook.Dr. Serkan Da
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Fundamental Deviations (Letter Specification)
+
A
B CD E
F rs
tuv
xyz
BHS: H/(a-g) (negative)
H/(j-k) (depends on size)H/(m-z) (positive)
BSS: (A-G)/h (positive)
(J-K)/h (depends on size)(M-Z)/h (negative)
-
HOLE SHAFT
H
J KMN P R S TUV X
Y Z
a
bcd
e fgh
jk
m
F
F F
Fbasic size
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Tolerance on the Fit
Tolerance on the fit is defined as the sum of the tolerance on the hole and
tolerance on the shaft.
( ) ( )minmaxminmax ddDDTTT shf +=+=
Cmin CmaxImax
Imin
Cmax
Imax
minmax CCTf = minmax IITf =
maxmax CITf +=
clearence interference
transition
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Example
For a nominal diameter of 25 mm and for a fit specification of H7/j5determine the following:
a) T e of the tolerancin s stem
b) Tolerance on the holec) Tolerance on the shaft
d) Upper and lower limits of the hole (Dmax,Dmin)
e) Upper and lower limits of the shaft (dmax, dmin)
f) Type of the fit
g) Tolerance on the fith) Allowance
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Example
a) H7/j5 Basic Hole System
b) D = 25 mm, from the given table:
m21H7
+
m5j5
+
-
nominal sizeH7
j5
+
-
c)
m21=hT
m9=sT
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Example
d) System)Hole(Basic,0min =D
mm021.25021.025max =+=D
e) mm996.24004.025min ==d
mm005.25005.025max =+=d
f) Interference fit.
g) Tolerance on the fit:
m30921 =+=+= shf TTT
or, m,5max =I m.25421max =+=C
m.30maxmax =+= CITf
h) Allowance =Imax= 5 m.
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Fits, Interference and Clearence Values
In the second table, interference and clearence values for commonly used
fits are given.
For example, consider G7/h6,D = 20 mm.
From the table we read:
m7
m41
min
max+=
+=
C
C
clearence fit.
m7
m28G7
++
m13
m06h
G7
h6
Cmax
Cmin
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Fits, Interference and Clearence Values
Consider H7/m6,D = 20 mm.
From the table we read:
interference fit.
m13max
=
+=C
H7 m6
max
0
m21H7
+
m8
m216m
++
nominal size
Dr. Serkan Da