tolerancing system

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Tolerancing System

In an assembly process the degree of "clearance" or "tightness" desiredbetween mating parts is important.

In a manufacture of a machine, quality is a primary consideration. Manufacturing precisiontaken into the product determines its quality, its cost and selling price.

Parts of a machine are designed in order to make a function. The working parts have a

definite relationship with each other: free rotation, free longitudinal movement, clamping

action, permanent fixed position. Precision is the degree of accuracy necessary to ensure thefunctioning of a part as intended.

There are two types of surfaces:1) Nonmating parts - left in their original rough-cast form

2) Mating parts - are machined to proper smoothness and must be at correct distance from

each other. So greater manufacturing precision is required for these surfaces. However

because of impossiblity to make a distance to an absolute size some variation must beallowed.

TOLERANCE is the allowable variation for any given size in order to achive a properfunction.

Basic size : 1.500 in

Tolerance on the shaft : 0.002 inTolerance on the slot : 0.001 in

Allowance : 0.004 (tightest

possible fit - maximum materiallimit)


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Cumulative tolerance effect : Dimensions will vary in position by the sum of the tolerances.This difficulty can be eliminated by dimensioning from one position (base-line dimensioning)


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Typical tolerances available with specific machining operations

A clearance fit (the shaft is always smaller than the hole)

Tolerance on shaft : 0.002

Tolerance on hole : 0.002minimum clearance : 1.500 - 1.495 = 0.005 in

maximum clearance : 1.502 - 1.493 = 0.009 in

clearance 0.005 in for the tightest possible fit


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An interference (force) fit (when the shaft is always larger in diameter than the hole - partsmust be assembled by pressure or heat expansion)

Tolerance on shaft : 0.001Tolerance on hole : 0.001

minimum clearance : 2.000 - 2.003 = -0.003 in (the tightest fit 0.003 in interference)

maximum clearance : 2.001 - 2.002 = -0.001 in (the loosest fit 0.001 ininterference)

Maximum clearance=Minimum interference

Minimum clearance=Maximum interference

A transition fit exist when the maximum clearance is positive and the minimum clearance is

negativeTolerance on shaft : 0.005

Tolerance on hole : 0.005


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minimum clearance : 1.000 - 1.007 = -0.007 in (the tightest fit 0.007 in interference)

maximum clearance : 1.005 - 1.002 = 0.003 in (the loosest fit 0.003 in clearance)

Transition fits are used only for locating a shaft relative to a hole, where accuracy is

important but either a clearance or an interference is permitted.

Standardized Tolerances

We use standardized tolerances for fits of parts.

Cylindrical Fits:

Decimal inch system, based on ANSI Standard (B4.1 - 1967(1979))

Millimeter system (ISO), based on ANSI Standard (B4.2 - 1978)

ANSI - American National Standards InstituteISO - International Organization for Standardization

Basic - Hole System : (Nominal) size is given to HoleBasic - Shaft System: (Nominal) size is given to shaft

Basic(or Normal) Hole System : Hole dia. constant, shaft dia. changing either smaller or

larger to satisfy the required fit (min hole dia. always nominal size)

Basic (Normal) Shaft System : Shaft dia. constant, hole dia. can be smaller or larger to satisfy

the required fit (max shaft always nominal size)Normal - Hole System is prefered because of availability of drills and reamers in machine

shop. But for a transmission system bearings must fit the shaft so shaft system is used.

Decimal inch system, based on ANSI Standard (B4.1 - 1967(1979))

The letter symbols are used in ANSI Standard

RC, running or sliding fit

LC, locational clearance fit


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LT, locationa transition fit

LN, locational interference fitFN, force or shrink fit

RC# (The letter symbols are used in conjunction with numbers)

the lower RC numbers are the tighter fits

the higher numbers are the looser fitsFN4 - Represents a complete fit (class 4 force fit)


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Appendix 8A : Running and Sliding Fits
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_a.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_a.htm


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Appendix 8B : Locational Clearance Fits

Appendix 8C : Locational Transitional Fits

Appendix 8D : Locational Interference Fits
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_b.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_c.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_d.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_b.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_c.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_d.htm


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Appendix 8E : Force Fits

EXAMPLE

RC6 fit

Nominal size : 1 in.

Nominal Size RangeLimits of Clearance

Standard Limits

Hole H9 Shaft e8

0.71 - 1.19

(Nominal Size : 1 in)

1.6 +2.0 -1.6

4.8 0 -2.8

1.0000 - 0.9984 = 0.0016 (clearance)

1.0020 - 0.9972 = 0.0048 (clearance)

Appendix 8A , page 661

FN4 fit

Nominal size : 2 in.

Nominal Size RangeLimits of Clearance

Standard Limits

Hole H9 Shaft e8

(Nominal Size : 2in)2.3 +1.2 +4.2

4.2 0 +3.5

2.0000 - 2.0042 = -0.0042 (interferance)2.0012 - 2.0035 = -0.0023 (interferance)

Appendix 8E , page 666
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_a.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_a.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htm


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http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htm


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ANSI Standart (b4.2 - 1978) related to ISO standarts

In ISO System letters are used to show different types of fits according to deviation from 0line.
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htm


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Hole tolerances are shown with capital letter A-Z

Shaft tolerances are shown with small letters a-z

Quality of tolerances is given with a # (H7, g6, m6, k8)
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htm


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Quality 1-5 (measuring instruments)

Quality 5-11 (mechanical parts)Quality 11-18 (coarse work)
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htm


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Appendix 9A : Basic Hole Clearance Fits
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app8_e.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_a.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_a.htm


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Appendix 9B : Basic Hole Transition and Interference

Fits

Appendix 9C : Basic Shaft Clearance Fits

Appendix 9D : Basic Shaft Transition and InterferenceFits

EXAMPLE

H7s6 fit

Basic dia. Tolerance pos & #

60H7

60.030

60.000

s660.072

60.053

Interference fit60.030 - 60.053 = -0.023 (max fit)

60.000 - 60.072 = -0.072 (min fit)

Appendix 9B , page 671

h9D9 fit

Basicdia.

Tolerancepos & #
http://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_b.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_b.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_c.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_d.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_d.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_b.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_b.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_b.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_c.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_d.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_d.htmhttp://www.me.metu.edu.tr/courses/me114/Lectures/tolerancing_files/app9_b.htm


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25

h925.000

24.948

D925.117

25.065

Clearance fit25.065 - 25.000 = 0.065 (max fit)

25.117 - 24.948 = 0.169 (min fit)

Appendix 9C , page 672

Copyright 2003 , Middle East Technical University
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