Chapter 10
Tolerancing
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Tolerancing
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Tolerances are used to control the
variation in size that exists on all
manufactured parts.
The amount that a size is allowed to
vary depends on the function of the
part & its assembly.
– electric drill vs. jet engine


The more accuracy required in a part
(smaller tolerance) the greater the
cost.
Tolerances allow for interchangeable
parts, which permits the replacement
of individual parts in an assy. instead
of replacing the whole system if a
part goes bad or fails.
Tolerance
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Tolerance is the total amount a
dimension may vary. It is the
difference between the maximum
size and the minimum size.
Tolerances can be expressed on a
drawing in several ways:
– direct limits, or as tolerance
values applied directly to a
dimension. (Fig. 10.8)
– geometric tolerances (Fig. 10.25)
– notes referring to specific
conditions
– A general tolerance note in the
titleblock. (Fig. 10.7)
Dimensions
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Limit dimensions:

Plus/minus dimensions:
– unilateral
– bilateral
Tolerancing Terms
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Nominal size: the “name” or general
size. Often expressed as a fraction.
Basic size: the theoretical size used as a
starting point for the application of
tolerances. Nominal size in decimal
format.
Actual size: the measured size of the
finished part.
Limits: the maximum & minimum sizes
shown by the toleranced dimension.
Allowance: the min. clearance or max.
interference between 2 parts.
MMC: the condition of a part in which
it contains the most amount of material.
EX: biggest shaft or smallest hole.
LMC: the condition of a part in which it
contains the least amount of material.
Tolerance, MMC, LMC?
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Limit dimensions:
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Plus/minus dimensions:
– unilateral
– bilateral
Complete Worksheet
TOL-1

Remember x.xxxx
– I.e. 0.1234
– I.e. 2.1200
Fit
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Fit: the degree of tightness between
mating parts.
The three most common types of fit
found in industry are:
– Clearance fit - there is always a
space between the 2 mating parts.
(shaft is smaller than the hole)
– Interference fit - the 2 mating parts
always interfere with one another in
assembly. (shaft is bigger than the
hole)
– Transition fit - sometimes a
clearance fit & sometimes an
interference fit between the mating
parts.
Clearance Fit
Interference Fit
Do on own
page 335, Figure
10.1a
•Hole Tolerance?
•Shaft Tolerance?
•Minimum clearance?
•Maximum clearance?
•Allowance?
Complete Number 1
of Worksheet 66
ANSI Standard Fits
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A group of English usit tolerance
relationships called preferred
precision fits have been developed.
They are specified in ANSI B4.1.
The five classes are:
– Running & sliding fits (RC)
– Clearance locational fits (LC)
– Transition locational fits (LT)
– Interference locational fits (LN)
– Force or shrink fits (FN)
These tolerances are specified in
Appendix 5 through 9, pages a25-a32.
The values in these tables are given in
THOUSANDTHS of an inch.
Example: 1.2 = 0.0012
Class RC9:
Running & Clearance Fit
Basic DIA = 2.0000
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Hole
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Shaft
Shaft Tolerance =
 Hole Tolerance =
 Max. Clearance =
 Min. Clearance =
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Class FN2:
Medium Drive fit
Basic DIA = 0.5000
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Hole

Shaft
Shaft Tolerance =
 Hole Tolerance =
 Max Interference =
 Min Interference =
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Complete Worksheets
Tol-2, Tol-3 and 66
Metric Fit - Terms
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Basic size: size to which limits of
deviation are assigned. (Fig.10.1, p
343)
– Must be same for both parts
– Basic sizes selected from chart in Table
10.2
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Deviation: difference between the
actual size and the basic size. (Fig.
10.14)
Upper deviation: difference between
the max. size limit & the basic size.
Lower deviation: difference between
the min. size limit & the basic size.
Fundamental deviation: the
deviation closest to the basic size.
(Fig. 10.18)
– Capital H indicates Hole
– Lowercase letter, I.e. f , indicates shaft
Metric Fit – Terms cont’d
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International Tolerance Grade
(IT): a group of tolerances that
vary depending upon the basic
size, but have the same level of
accuracy within a given grade.
(Fig 10.18)
– There are 18 IT grades. The
smaller the grade number, the
smaller the tol. zone.
– See Appendix 10 for IT grades.
Preferred Metric Fits
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There are 2 systems used to indicate
preferred fits in the metric system.
– Hole basis: the system of fits where
the min. hole size is the basic size.
The fundamental deviation for a hole
basis system is indicated by a capital
“H”.
» EX.
50H8
(50 = DIA in mm) (Basic size)
(H = using Basic Hole system)
(fundamental deviation)
(8 = closest running fit) (IT
grade)
» For mating part 50H8/f7 (Appendix 11)
– Hole Limits?
– Shaft Limits?
– Fit?
Preferred Metric Fits
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The second system used to indicate
preferred fits in the metric system.
– Shaft basis: the system of fits
where the max. shaft size is the basic
size. The fundamental deviation for
a shaft basis system is indicated by a
lowercase letter.
» EX. 50h7
(50 = DIA in mm) (Basic size)
(h = using Basic Shaft system)
(fundamental deviation)
(7 = closest running fit)
(IT grade)
» For mating part 50F8/h7 (Appendix ?)
» Limits and Fits?
– Hole Limits?
– Shaft Limits?
– Fit?
Try the following problem
Basic DIA 41mm
 Use Hole Basis System
 Sliding
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Try the following problem
Basic DIA 41mm
 Use Shaft Basis System
 Sliding
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Try the following problem
Basic DIA 58mm
 Use Shaft Basis System
 Force Fit
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Look Up
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What type of fit is a 6 H7/s6?
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Hole or Shaft Basis System?
Complete Worksheets
Tol-4 and 67