Tolerances and Fits

Deviations from the exact dimensions are inavoidable due to the nature of manufacturing processes. These deviations must not be so large that the part is useless. Designer specifies upper and lower limits to the dimensions

ddmax

dmin

Tolerance is the difference between the maximum andminimum size limits of a part:

minmax ddd

Too tight tolerance High manufacturing costToo large tolerance Part may not function as expected

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Fits

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TolerancesApplication, 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

Grinding, fine honing 6-20

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 thehole is equal to zero. Dmax is prescribed according to the specified tolerance.

2) Basic Shaft System (BSS)Maximum shaft diameter is taken as the basis. Upper deviation for theShaft is equal to zero. dmin is prescribed according to the specifiedtolerance.

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Basic Systems for Fit Specificationtolerance zone

Fund. Dev.

Fund. Dev.

BHS BSS

basic sizebasic size

hole

tolerance zone

shaft

hole

shaft

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International Tolerance Grade Numbers

• In order to establish a preferred fit we need1) 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 oftolerances 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. Thetolerance zones are specified in international tolerance grade numberscalled 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

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

International Tolerance Grade Numbers

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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)

Basic size 18-30 (Consider the 1st appearance)

• Tables can be used for both shafts and the holes.• Tolerance on a shaft or a hole can be calculated by using the formulas provided by ISO.

mm 021.0minmax dddTs

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Tolerance Designation (ISO R286)

Tolerance on a shaft or a hole can also be calculated by using the formulasprovided by ISO.

iKT where,

T is the tolerance (in m)

DDi 001.045.0 3 (unit tolerance, in m)

21DDD (D1 and D2 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

mm 0.021m 45.21 iKTs

Same result is obtained using Table A-11

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Fit Types

There are three types of fitsa) Clearence Fits b) Interference Fits c) Transition Fits

Clearence FitsThe mating parts have such upper and lower limits that a clearence alwaysresults when the mating parts are assembled.

holeCmin

shaft

mm 93.25mm 95.25

min

max

dd

mm 02.0sT

mm 00.26mm 03.26

min

max

DD

mm 03.0hT

Allowance of the fit corresponds to maximum material condition (Cmin).Allowance can be calculated by considering tightest fit.

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Fit Types

There are three types of fitsa) Clearence Fits b) Interference Fits c) Transition Fits

Interference FitsThe mating parts have such limits that the lowest shaft diameter is larger than thelargest hole diameter.

mm 02.0sT

Allowance of the fit corresponds to maximum material condition (Imax).

holeshaft

maxI

mm 02.25mm 04.25

min

max

dd

mm 00.25mm 01.25

min

max

DD

mm 01.0hT

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Fit Types

There are three types of fitsa) Clearence Fits b) Interference Fits c) Transition Fits

Transition FitsEither a clearance or an interference may result depending on the exact valueof the dimensions of the machined shaft and hole within the specified tolerancezones

hole shaft mm 02.0sT

Allowance of the fit corresponds to maximum material condition (Imax).

maxI

mm 02.25mm 04.25

min

max

dd

mm 00.25mm 03.25

min

max

DD

mm 03.0hT

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Fundamental Deviation

tolerance zone

Fund. Dev.BHS

Tolerance zones for the hole and theshaft are prescribed by IT numbers. Note that we also need the fundamentaldeviation for the shaft to completely 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 size

25 H7/g6

fund. dev.for the hole

IT gradefor the hole

fund. dev.for the shaft

IT gradefor 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/c11Basic Size (Datum)+

-fund. dev.

H11

c11

Th

Ts

(BHS)

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. 12.0 c,for mm, 34 FD mm. 16.0 IT11, mm, 34 TD

mm. 34min Dmm. 16.34max D

mm. 88.3312.034max dmm. 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).

(A-Z)…/h…, Ex: B8/h8

+

- h

Datum Line

shaft

+

- h

Datum Line

shaft

HH…/h…, snug fit.

Note: Fund. dev. table for BSS is not given in the textbook.

B

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Fundamental Deviations (Letter Specification)

+

-

HOLE SHAFT

A

B CD E F GH

J KMN P R S TUV XY Z

a

bc d e f g h j k

mn p r s t u v x y z

F

F F

Fbasic size

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)

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Tolerance on the Fit

Tolerance on the fit is defined as the sum of the tolerance on the hole andtolerance on the shaft.

minmaxminmax ddDDTTT shf

Cmin CmaxImax

Imin

Cmax

Imax

minmax CCT f minmax IIT f

maxmax CIT f

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) Type of the tolerancing systemb) Tolerance on the holec) Tolerance on the shaftd) Upper and lower limits of the hole (Dmax, Dmin)e) Upper and lower limits of the shaft (dmax, dmin)f) Type of the fitg) Tolerance on the fith) Allowance

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Example

a) H7/j5 Basic Hole System

b) D = 25 mm, from the given table:

m 0m 21H7

m 4-m 5j5

nominal sizeH7

j5

+

-

c)

m 21hT

m 9 sT

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Example

d) System) Hole (Basic ,0min Dmm 021.25021.025max D

e) mm 996.24004.025min dmm 005.25005.025max d

f) Interference fit.

g) Tolerance on the fit:m 30921 shf TTT

or, m, 5max I m. 25421max Cm. 30maxmax CIT f

h) Allowance = Imax= 5 m.

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Fits, Interference and Clearence Values

In the second table, interference and clearence values for commonly usedfits are given.

For example, consider G7/h6, D = 20 mm.From the table we read:

m 7m 41

min

max

CC

clearence fit.

m 7m 28G7

m 13

m 06h

G7

h6Cmax

Cmin

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Fits, Interference and Clearence Values

Consider H7/m6, D = 20 mm.

From the table we read:

interference fit.

H7 m6

m 21m 13

max

max

IC

0m 21H7 m 8

m21 6m

nominal size

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