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    Chapter 1

    Tolerances and Fits

    Middle East Technical University

    ME307 Machine Elements I

    Fall 2006-2007

    Dr. Serkan Da

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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

    Dr. Serkan Da

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    Fits

    Dr. Serkan Da

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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

    Dr. Serkan Da

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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.

    Dr. Serkan Da

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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.

    Dr. Serkan Da

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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

    Dr. Serkan Da

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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

    Dr. Serkan Da

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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 =

    Dr. Serkan Da

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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

    Dr. Serkan Da

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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

    Dr. Serkan Da

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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

    Dr. Serkan Da

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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

    Dr. Serkan Da

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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.

    Dr. Serkan Da

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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.72.3316.01