ISO Method to Determine Tolerance
A practical problem statement:
 Basic
hole system
 Running of accurate machines
 Basic diameter, say 39.00
Question: to determine the tolerance?
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Basic size (preferred)
• Dimensions are initially determined by designers
from a point of view of functions.
• From the view point of function, the length of a
bar may be like 39.6
• From a point of view of manufacturing, 39.6 is not
a convenient figure, and therefore needs to be
rounded up (say, 40) (see figure 1)
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Figure 1
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Design a system (ISO standard) which is a set of tables
or charts to determine the tolerance.
Input to the system: basic size (decided by the designer),
machine running condition, manufacturing facility
available (option).
Output from the system: fit and tolerance.
Constraint to the system: machine running condition,
assembly requirement, manufacturing facility available
(option)
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Reference line for the shaft
(a)
30
Fundamental deviation
block. Its location is
measured with reference to
the basic size
(b)
30
Tolerance: composed of location of the fundamental
deviation block and thickness of the block
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Fundamental deviation (FD)
Deviation closest to the basic size or the location of the
FD block
International Tolerance Grade (IT)
FD can vary in an infinite number of possible numbers. To restrict FD
a finite number of possibilities, we group FDs into 16 as follows (IT1,
IT2, …, IT16, the number means grade:
Small Deviation
large Deviation
IT0, IT1, IT2, ....
IT16
large tolerance is given to large grade
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Figure 2
IT grades are further associated with manufacturing
processes
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Basic size group
1. With the same idea of IT grade, we group basic sizes
into groups. The tolerance is the same to any
dimension in the groups.
2. Large basic size gets large tolerance.
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Tolerance with respect to size group and IT group
Basic size
IT 1
IT 5
10-18
0.0012
0.008
18-30
0.0015
0.009
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 FD (block in the following diagram) is located with
respect to basic size (in total there are 27 FDs)
 Different location is given a name (letter)
 Different location of FD finally comes up with
tolerance
50.005
50.030
G
H
J
h
50
k
49.05
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H: a special location of FD, and this
location makes the minimum diameter
of the hole is the basic size of the
hole, so the tolerance of the hole
implies the basic hole system in the
mind of designer
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Shaft
h: a special location of FD, and this
location makes the maximum
diameter of the shaft is the basic size
of the shaft, so the tolerance of the
shaft implies the basic shaft system in
the mind of designer
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Basic shaft system with
indication of three types of fits
Basic hole system with
indication of three types of fits
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Figure 3
Complete set of
FDs for hole and
shaft
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A combination of location of FD and IT grade
Basic size
40 H7
Tolerance zone
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ISO Method to Determine Tolerance
Hole
- minimum hole size as basic diameter
- denoted by Capital letter (say, H)
Basic size
Fundamental Deviation
40 H8
IT grade
Tolerance zone
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Shaft
- maximum shaft size as basic diameter
- denoted by small letters (say, h)
Basic size
Fundamental Deviation
40 h7
IT grade
Tolerance zone
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Figure 4. Preferred fit
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ISO Method to Determine Tolerance
Preferred fit:
Product Function determines Fit. For instance,
two parts need to have relative motion, so we
require therefore clearance fit.
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Tolerance
Thickness of the FD block
Location of the FD block
 Basic size
 IT grade (required accuracy)
 Basic hole and basic shaft
 Quality of machine running
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ISO Method to Determine Tolerance
Procedure
1.
2.
3.
4.
Basic size selection
Determine the preferred fit
International Grade
Determine tolerances
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Procedure to determine tolerance
Re-visit the example:
 Basic hole system
 Running of accurate machines
 Basic diameter, say 39
Step 1: go to Figure 1, the closed size to 39 is 40.
Step 2: Go to Figure 4,
H8/f7
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Step 3: Go to Table 1a, we will find that under the size
40, and column H8
H8
f7
Fit
40.039
39.975
0.089 (max clearance)
40.000
39.950
0.025 (min clearance)
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Summary of the procedure
The following figures and table are used:

Figure 1: Get a preferred size as well as IT grade

Figure 4: Get a preferred fit

Table 1a: Get tolerance
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More Examples
Given:
basic hole system
locational transitional fit
basic diameter =57 mm
Figure 1 -> 60
Figure 4 -> H7/k6
Table 1: Hole
Shaft
60.030
60.021
60.000
60.002
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Shaft
60.021
60.002
-0.021
0.028
Max interference
Tolerance
Tolerance
Hole
60.030
60.000
Max clearance
0.019
0.030
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Representation on the drawing

60.030
60H7
60.000
(

60k6 ( 60.021
60.002 )
60.021
60.002
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)