Tolerances
Introduction
• tolerancing
– technique of dimensioning parts within a required
range of variation to ensure interchangeability
• A tolerance should be as large as possible
without interfering with the function of the part to
minimize production costs.
EML 2023
Computer Aided Design
2
Tolerance Dimensions
• unilateral tolerance
2.250
+.000
-.008
• bilateral tolerance
2.250 ± .004
• limit tolerance
2.255
2.245
EML 2023
Computer Aided Design
3
Mating Parts
EML 2023
Computer Aided Design
4
Cylindrical Fits
EML 2023
Computer Aided Design
5
Cylindrical Fits – English Units
EML 2023
Computer Aided Design
6
Cylindrical Fits – English Units
EML 2023
Computer Aided Design
7
Cylindrical Fits – English Units
EML 2023
Computer Aided Design
8
Cylindrical Fits – English Units
EML 2023
Computer Aided Design
9
Cylindrical Fits – English Units
tolerance - difference between the
limits of size for a single feature ;
shaft .0025 ; hole .0040
limits of tolerance – the max and
min sizes of a single feature
allowance – the tightest fit between
two mating parts ; .0050
nominal size – the general size of
a shaft or hole ; 1.5
basic size – the size to which the
plus-and minus is applied
actual size – the measured size of
the finished part
fit - the degree of tightness or
looseness between two
assembled parts
EML 2023
Computer Aided Design
10
Cylindrical Fits – English Units
• clearance fit – gives a
clearance between
two assembled mated
parts ;
• min clearance
.005 ;
• max clearance
.0115
• transition fit
• interference fit
• line fit
EML 2023
Computer Aided Design
11
Cylindrical Fits – English Units
• clearance fit
• interference fit – a binding fit
that requires the parts to be
forced together, much as if
they were welded
• transition fit – may range from
a interference fit to a clearance
fit between the mated parts
• line fit – results in surface
contact or clearance when
limits are reached
EML 2023
Computer Aided Design
12
Cylindrical Fits – English Units
• The ANSI B4.1 standard defines a series of fits
between cylindrical features in inches for the
basic hole system.
• The types of fits covered in the standard are:
–
–
–
–
–
RC: running or sliding clearance fits
LC: clearance locational fits
LT: transition locational fits
LN: interference locational fits
FN: force and shrink fits
EML 2023
Computer Aided Design
13
Cylindrical Fits – English Units
• RC: running or sliding clearance fits
– RC1 to RC9
• LC: clearance locational fits
• LC1 to LC11
• LT: transition locational fits
• LT1 to LT6
• LN: interference locational fits
• LN1 to LN3
• FN: force and shrink fits
• FN1 to FN5
EML 2023
Computer Aided Design
14
Example – RC9 Fit
• hole and shaft have a basic diameter of 2.5”
• From Tables in Appendix 29
thousands of an inch
EML 2023
Computer Aided Design
15
Example – LT3 Fit
• hole and shaft have a basic diameter of 2.5”
• From Tables in Appendix 31
2.5012
2.5000
2.5008
2.5001
thousands of an inch
EML 2023
Computer Aided Design
16
Example – LN2 Fit
• hole and shaft have a basic diameter of 2.5”
• From Tables in Appendix 32
2.5012
2.5000
2.5021
2.5014
thousands of an inch
EML 2023
Computer Aided Design
17
Example Problem
18
Cylindrical Fits – Metric Units
• ANSI B4.2 standard
• basic size – the diameter
from which limits are
calculated
• upper and lower deviation –
the difference between the
hole or shaft size and the
basic size
• tolerance - the difference
between the maximum and
minimum sizes
EML 2023
Computer Aided Design
19
Cylindrical Fits – Metric Units
EML 2023
Computer Aided Design
20
Cylindrical Fits – Metric Units
• fundamental deviation – a
letter grade that describes
the deviation closest to the
basic size
• International Tolerance (IT)
grade – a series of
tolerances that vary with the
basic size to provide a
uniform level of accuracy
within a given grade
• there are 18 IT grades:
IT01, IT0, IT1, …, IT16
EML 2023
Computer Aided Design
21
Cylindrical Fits – Metric Units
• Hole basis
– a system of fits based on the minimum hole size as the basic
diameter
– the fundamental deviation for a hole-basis system is “H”
– Appendices 35 and 36 give hole-basis data for tolerances
• Shaft basis
– a system of fits based on the maximum shaft size as the basic
diameter
– the fundamental deviation for a hole-basis system is “h”
– Appendices 37 and 38 give shaft-basis data for tolerances
EML 2023
Computer Aided Design
22
23
Cylindrical Fits – Metric Units
EML 2023
Computer Aided Design
24
Cylindrical Fits – Metric Units
EML 2023
Computer Aided Design
25
Example 1
• determine the shaft and hole limits for:
– hole-basis system
– a close running fit
– a basic diameter of 49 mm
EML 2023
Computer Aided Design
26
27
Example 1
• use a preferred basic
diameter of 50 mm
• use a fit of H8/f7
EML 2023
Computer Aided Design
28
Example 2
• determine the shaft and hole limits for:
– hole-basis system
– a location transition fit
– a basic diameter of 57 mm
EML 2023
Computer Aided Design
29
EML 2023
Computer Aided Design
30
Example 2
• use a preferred basic
diameter of 60 mm
• use a fit of H7/k6
EML 2023
Computer Aided Design
31
Example 3
• determine the shaft and hole limits for:
– hole-basis system
– a medium drive fit
– a basic diameter of 96 mm
EML 2023
Computer Aided Design
32
33
Example 3
• use a preferred basic
diameter of 100 mm
• use a fit of H7/s6
EML 2023
Computer Aided Design
34
Nonstandard Fits, Nonpreferred Sizes
• determine the shaft and hole limits for:
– hole-basis system
– a close running fit
– a basic diameter of 45 mm
(do not change to a preferred size)
EML 2023
Computer Aided Design
35
36
Nonstandard Fits, Nonpreferred Sizes
• use a fit of H8/f7
EML 2023
Computer Aided Design
37