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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:
Fundamental Deviation =
Fund. Dev. Value =
IT =
IT Value =
Limits =
Tolerance =
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Try the following
problem in groups onthe board
Given: Basic DIA 41mm, useShaft Basis System, Sliding Fit
Write Specification:
Maximum Clearance / Minimum Interference =
Minimum Clearance / Maximum Interference =
Allowance =
Hole:
Fundamental Deviation =
Fund. Dev. Value =
IT =
IT Value =
Limits =
Tolerance =
Shaft:
Fundamental Deviation =
Fund. Dev. Value =
IT =
IT Value =
Limits =
Tolerance =
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Try the following
problem in groups onthe board
Given: Basic DIA 58mm, useShaft Basis System, Force Fit
Write Specification:
Maximum Clearance / Minimum Interference =
Minimum Clearance / Maximum Interference =
Allowance =
Hole:
Fundamental Deviation =
Fund. Dev. Value =
IT =
IT Value =
Limits =
Tolerance =
Shaft:
Fundamental Deviation =
Fund. Dev. Value =
IT =
IT Value =
Limits =
Tolerance =
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Complete Activity 11Pages Tol-4 & 5
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