16727_MEC 314 Fits and Tolerances TBO

7/28/2019 16727_MEC 314 Fits and Tolerances TBO

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Metrology and Measurement

MEC314

Sunil Sharma


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Fits and tolerances


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Important terms

Nominal size = a dimension

used to describe the general size

Basic size = the theoretical size

used as a starting point for the

application of tolerances

Actual size = the measured size

of the finished part after

machining

Limits = the max and min sizes

shown by the toleranced

dimension

Allowance = the min clearance

or maximum interferencebetween parts, or the tightest fit

between two mating parts


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Tolerancing why is it needed?

Tolerances are used to control the variation

existing on all manufactured parts.

Toleranced dimensions control the amount of

variation on each part of an assembly. Tolerances allow for interchangeable parts.

Parts made by different companies functionally

fit although they may not perfectly fit.

Tolerance = the total amount a dimension may vary and is the difference

between the maximum and minimum limits (e.g. 4.650 0.003)


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Important terms (cont)

Tolerance = the total allowable

variance in a dimension

Max material condition (MMC)

= the condition of a part when it

contains the greatest amount of

material

Least material condition (LMC)

= the condition of a part when itcontains the least amount of

material possible


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Tolerancing

Maximum Material Condition (MMC)- when it containsthe most material (Weighs the most)

Least Material Condition (LMC)-

when it contains the least material (Weighs the least)


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Tolerance Dimensioning

Tolerance is the total amount that a specific

dimension is permitted to vary;

It is the difference between the maximum and the

minimum limits for the dimension.

For Example a dimension given as 1.625 .002

means that the manufactured part may be 1.627

or 1.623, or anywhere between these limitdimensions.


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Tolerances

The Tolerance is 0.001 for the Hole as well as for the Shaft


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Tolerances

Plus-or-minus Dimensioning

Unilateral Tolerance

Bilateral Tolerance


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Fits


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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 th

Shaft is equal to zero. dmin is prescribed according to the specified

tolerance.


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Basic Systems for Fit Specification

tolerance zone

Fund. Dev.

Fund. Dev.

BHS BSS

basic size

basic size

hole

tolerance zone

shaft

hole

shaft

tolerance zone


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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

hole

2) 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 same

relative 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 numberscalled IT numbers.

These numbers range from IT0 to IT16. The smaller grade numbers specif

a smaller tolerance zone.


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Application,Process Tolerance (mm) IT GradeSlip blocks, reference gages 1-2 1High quality gages, plus gages 2-3 2Good quality gages, gap gages 3-5 3Fits produced by lapping 4-10 4Ball bearings, Diomand or fine boring, fine

grinding

5-12

5

Grinding, fine honing 6-20 6High quality turning, broaching 12-35 7Center lathe turning and boring, reaming 14-50 8Horizontal or vertical boring machine 30-80 9Milling, slotting, planing, metal roll ing or

extrusion

50-100

10

Drilling, rough turning and boring, precision

tubing 70-140 11Light press work, tube drawing 120-240 12Press work, tube rolling 150-500 13Die casting or molding, rubber moulding 250-1000 14Stamping 400-1400 15Sand casting, flame cutting 500-2000 16

International Tolerance Grade Numbers


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Fit Types

There are three types of fitsa) 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.

hole

Cmin

shaft

mm93.25

mm95.25

min

max

d

dmm02.0sT

mm00.26

mm03.26

min

max

D

Dmm03.0hT

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 fits

a) Clearence Fits b) Interference Fits c) Transition Fits

Interference Fits

The mating parts have such limits that the lowest shaft diameter is larger than the

largest hole diameter.

mm02.0sT

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

holeshaft

maxI

mm02.25

mm04.25

min

max

d

d

mm00.25mm01.25

min

max

DD mm01.0hT


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Fit Types

There are three types of fits

a) Clearence Fits b) Interference Fits c) Transition Fits

Transition 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 shaftmm02.0sT

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

maxI

mm02.25

mm04.25

min

max

d

d

mm00.25mm03.25

min

max

DD mm03.0hT


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Specification for the Fits

tolerance zone

Fund. Dev.

BHS basic size25 H7/g6

fund. dev.

for the hole

IT grade

for the hole

fund. dev.

for the shaft

IT grade

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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Fundamental Deviations (Letter Specification)

+

-

HOLE SHAFT

A

BC

DE F

GH

J KMN P

R S T U V XY Z

a

bc d

e fg h

jk

m np r s

tu v

xy z

dF

dF dF

dFbasic 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 atolerance 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/j5

determine the following:

a) Type of the tolerancing systemb) Tolerance on the hole

c) 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 fit


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Example

a) H7/j5 Basic Hole System

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

m0 m21H7 mm

m4- m5j5 mm

nominal sizeH7

j5

+

-

c)

m21mhT

m9 msT


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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 m shf TTT

or, m,5max mI m.25421max mC

m.30maxmax m CITf


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Tolerance is denoted by 2 symbols, a letter

symbol and a number symbol, called a grade

Letter symbols range from A to ZC for holes

and from a to zc for shafts

18 grade of tolerances are known as

fundamental tolerances


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Fundamental deviations


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Fundamental Deviations (Letter Specification)

+

-

HOLE SHAFT

A

BC

D

E F GH

J KMN P R

S T U V XY Z

a

b c d

e fg h

jk

m np r s

tu v

xy z

dF

dF dF

dFbasic 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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Runout of gear reflects total variation of specificelements of the tooth surface from axis ofrotation of the gear.

Runout is commonly measured in directionperpendicular to axis of rotation of gear, theradial runout

Also determined by measuring the variation of

specific gear surface elements in a directionparallel to axis of rotation, the axial runout

Runout is twice the eccentricity

16727_MEC 314 Fits and Tolerances TBO

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