Tolerances Fits

7/29/2019 Tolerances Fits

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Chapter 11

Tolerancing

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Tolerancings Tolerances are used to control

the variation in size that existson all manufactured parts.

s

The amount that a size is allowedto vary depends on the functionof the part & its assembly.

electric drill vs. jet engine

s

The more accuracy required in apart (smaller tolerance) thegreater the cost.

s Tolerances allow forinterchangeable parts, whichpermits the replacement ofindividual parts in an assemblyinstead of replacing the wholesystem if a part goes bad or fails.


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s Tolerance is the total amount adimension may vary. It is thedifference between the maximum sizeand the minimum size.

s Tolerances can be expressed on adrawing in several ways:

a general tolerance note in thetitleblock.

direct limits, or as tolerance values

applied directly to a dimension. (Fig.11.8)

geometric tolerances (Fig. 11.31a)

notes referring to specificconditions (page 364)

Tolerance


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2.500 0.004 0.503 0 .003

0.5002.496+0.008

-0.000+0.006

-0.000

.500

.506

2.504

2.496

s Limit dimensions:

s Plus/minus dimensions:

unilateral

bilateral

Dimensions


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Tolerancing Termss Nominal size: the name or general

size. Often expressed as a fraction.

s Basic size: the theoretical size used as

a starting point for the application of

tolerances. Nominal size in decimalformat.

s Actual size: the measured size of the

finished part.

s

Limits: the maximum & minimum sizesshown by the toleranced dimension.

s MMC (maximum material condition): the

condition of a part in which it contains

the most amount of material.

EX: biggest shaft or smallest hole.

s LMC (least material condition): the

condition of a part in which it contains

the least amount of material.


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0.500

0.506

2.504

2.496

Tolerance, MMC, LMC?

s Limit dimensions:

s Plus/minus dimensions:

unilateral

bilateral2.500 0.004 0.503 0 .003

0.5002.496+0.008

-0.000+0.006

-0.000

Tolerance? Width

Hole

MMC?


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Complete Activity10 TOL-1

s

Remember x.xxxx I.e. 0.1234

I.e. 2.1200


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Fit

s Fit: the degree of tightnessbetween mating parts.

s The three most common types of fit

found in industry are: Clearance fit- there is always a

space between the 2 matingparts. (shaft is smaller than thehole)

Interference fit- the 2 matingparts always interfere with oneanother in assembly. (shaft isbigger than the hole)

Transition fit- sometimes aclearance fit & sometimes aninterference fit between themating parts.


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Tolerancing Terms

s Basic size: the theoretical sizeused as a starting point for theapplication of tolerances.

s Nominal size in decimal format.

s Allowance: the min. clearance ormax. interference between 2 parts.

Clearance

Interference

Clearance

Interference

Minimum

Maximum

Min or Max?


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Clearance Fit


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Interference Fit

ALLOWANCE =

0.001

0.001


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Page 362 - Hands On

11.1a & b in groups onthe board and c by

yourself

Hands-On (not Figure) 11.1 Are thefits clearance, transitional or

interference? Use categories below

(Mistakes in textbook)

Maximum clearance/

interference?

Minimum clearance/

interference?

Allowance?

Shaft Tolerance?

Hole Tolerance?

CBA


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Complete Activity 10TOL-3 #s 4, 5, & 6

Which is between

Largest Hole / Smallest Shaft?

Largest Hole / Largest Shaft?

Smallest Hole / Largest Shaft?

Smallest Hole / Smallest Shaft?

Shaft (Tab)

Information given?

Size of tab, clearance fit, allowance

0.005, and tolerance on slot is .002.

Minimum or maximum interference?


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Basic Systemss Basic Hole System

Standard reamers, etc., are used to

produce holes and standard gaugesused to check sizes. Shafts aremachined.

Determine Minimum hole size,apply allowance, then apply

tolerances.s Basic Shaft System

Not used often. However,advantageous when shafts cannot

be easily machined to size or whenseveral parts fit onto one shaft.

Determine Maximum shaft size,apply allowance, then applytolerances.


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ANSI Standard Fitss A group of English unit tolerance

relationships developed calledpreferred precision fits. They arespecified in ANSI B4.1.

s

The five classes are: Table 11.1pg 367 Running & sliding fits (RC) Clearance locational fits (LC) Transition locational fits (LT)

Interference locational fits (LN) Force or shrink fits (FN)

s These tolerances are specified inAppendixes 5 through 9, pagesa23-a30. The values in thesetables are given inTHOUSANDTHSof an inch.

Example: 1.2 = 0.0012


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Class RC9:

Basic DIA = 2.0000

s Hole limits =

s Shaft limits =

s Shaft Tolerance =s Hole Tolerance =

s Max. Clearance =

s

Min. Clearance =

RC means?

9 means?


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Class FN2:

Basic DIA = 0.5000

s Hole limits =

s Shaft limits =

s Shaft Tolerance =s Hole Tolerance =

s Max. Interference =

s

Min. Interference =

FN means?

2 means?


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Complete Activity 10

Tol-2 and Finish Tol-3

(#s 1, 2, & 3)


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Metric Fit - Terms

s Basic size: size to which limits of deviation areassigned. (Fig.11.9, p 365) Must be same for both parts Basic sizes selected from chart in Table 11.2 (p

371)s Deviation: difference between the actual size and the

basic size. (Fig 11.14, p. 369) (Tolerance)s Upper deviation: difference between the max. size

limit & the basic size. Hole system: Upper deviation (tolerance) is the

difference between the largest shaft size and thebasic size (minimum clearance or maximuminterference, also known as allowance).

Shaft system: Upper deviation (tolerance) is the

difference between the largest hole and the basicsize (maximum clearance or minimuminterference).

s Lower deviation: difference between the min. sizelimit & the basic size. Hole system: Lower deviation (tolerance) is the

difference between the smallest shaft size and the

basic size (maximum clearance or minimuminterference). Shaft system: Lower deviation (tolerance) is the

difference between the smallest hole and the basicsize (minimum clearance or maximum interference,also known as allowance).


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Metric Fit - Terms

s Fundamental deviation: the deviationclosest to the basic size. (Fig 11.18, p370)

s Preferred Fits: There are 2 systemsused to indicate preferred fits in themetric system. (Table 11.3, page 372) Hole basis

Capital H indicates Hole Basis System.

Shaft basisAny other capital letter, I.e. S, indicates

Shaft Basis System


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Basic Shaft System

Lower deviation

(Maximum clearance)

Upper deviation

(Minimum clearance /

allowance)

Fundamental deviation

for the hole equals the

allowance

Basic size applied to shaft the

type of fit will determine whether

the basic size will be the largest

shaft or the smallest shaft.Fundamental deviationfor the shaft equals the

basic size


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Basic Hole System

Basic size applied to hole the

type of fit will determine whether

the basic size will be the largest

hole or the smallest hole.

Upper deviation

(Minimum clearance /

allowance)

Fundamental

deviation for the

shaft equals the

allowance

Lower deviation(Maximum clearance)

Fundamental

deviation for thehole equals the

basic size


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Metric Fit Terms contd

s International Tolerance Grade(IT): a group of tolerances that varydepending upon the basic size, buthave the same level of accuracywithin a given grade. (Page 370, Fig11.17 & 11.16)

There are 16 IT grades. The smallerthe grade number, the smaller thetolerance zone.

Tolerance Value. See Appendix 10,page a31, for IT grades.


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Preferred Metric Fitss Hole basis: the system of fits where the minimum

hole size is the basic size. The fundamentaldeviation for a hole basis system is indicated by acapital H.

EX. 50H8

Basicsize is 50 which is the DIA in mm Tolerance Grade:

Hole Symbol: H8 Fundamentaldeviation is H - capital Hrefers to the Basic Hole system

Fundamental deviation value is thenominal size:

ITgrade is 8: Close Running fit

Tolerance value for8 is (Append. 10)

Mating part is f7 (Append. 11-12) Shaft Symbol: f7 Fundamental deviation is f

Fundamental deviation value is theallowance

Hole Limits? (Append. 11-12)

Shaft Limits? (Append. 11-12)

ISO Symbol: = 50H8/f7


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Preferred Metric Fitss

Shaft basis: the system of fits where the max.shaft size is the basic size. The fundamentaldeviation for a shaft basis system is indicated by alowercase letter.

EX. 50h7 Basic size is 50 which is the DIA in mm) Tolerance Grade:

Shaft Symbol: h7 Fundamentaldeviation is h - lower case

h refers to the Basic Shaft system Fundamental deviation value is the

nominal size: IT grade is 7: Close Running fit

Tolerance value for7 is (Append. 10) Mating part is F8 (Append. 13-14)

Hole Symbol: F8 Fundamental deviation value is the

allowance (Append. 13-14)

Hole Limits? (Append. 13-14)

Shaft Limits? (Append. 13-14)

ISO Symbol: 50h7/F8


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Look Up

s What type of fit is a 6H7/s6? (text hasa typo find H7/a6 and fix your text. It should be s instead ofa)

Hole or Shaft Basis System? Type of Fit?

s 6S7/h6?

Hole or Shaft Basis System?

Type of Fit?

s

Shaft Basis System, DIA 32,Locational Interference Write the specification (ISO Symbol)

for the above information.


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Try the following

problem in groups onthe board

Given: Basic DIA 41mm, useHole Basis System, Sliding Fit

Write Specification:

Hole:

Fundamental Deviation =

Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Maximum Clearance / Minimum Interference? =

Minimum Clearance / Maximum Interference? =

Allowance =

Shaft:


Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =


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Try the following


Given: Basic DIA 41mm, useShaft Basis System, Sliding Fit


Maximum Clearance / Minimum Interference =

Minimum Clearance / Maximum Interference =

Allowance =

Hole:


Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Shaft:


Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =


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Try the following


Given: Basic DIA 58mm, useShaft Basis System, Force Fit


Maximum Clearance / Minimum Interference =

Minimum Clearance / Maximum Interference =

Allowance =

Hole:


Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =

Shaft:


Fund. Dev. Value =

IT =

IT Value =

Limits =

Tolerance =


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Complete Activity 11Pages Tol-4 & 5

Documents

Tolerances Fits