Basic size

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How to determine the tolerance
for a given dimension?
Input or pre-condition is:
1. design (dimension)
2. manufacturing technology (e.g., casting)
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Standard
Standard (ISO, etc.): limits a freedom of choices but promotes the exchange
of parts manufactured with
- different approaches
Unified approach
to determine the
tolerance
- different equipment
- different workers
- in different cultural and societal situations
To assist in the determination of tolerance
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2
Standard
Different countries and regions
together to develop
 Concepts
 Rules
 Systems
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Standard
Example: Tolerance range is 0.0.5 with the nominal
dimension of 30. Restrict to 30 to 30+0.05 only.
Basic idea:
To give restriction on the possibility of the dimension and then its tolerance
 Nominal dimension is positive
integer with the last letter 0 or 5.
(a)
(b)
30
30
 Limit dimension is the same as the
nominal dimension. For instance, the
low limit dimension for a hole is 30
which is the nominal dimension.
 Tolerance can only be a certain
groups.
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Basic hole versus basic shaft concept:
making the nominal dimension as one of the dimension
limit
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Basic Hole System
Purpose: take a hole as a reference to determine the
shaft limit given allowance and tolerances.
The minimal hole size as the basic size.
Reason: in some applications, the hole can be made
more precise (Reamers, Broachers, Gages), while the
machining of the shaft varies.
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Basic Shaft System
Reason: in some applications, the shaft could
be better made as a reference
Different fits with the same shaft
The maximal shaft size as the basic size
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Example
Basic size =0.5
0.502
0.500
0.498
0.495
0.505
0.502
Basic hole system
0.500
0.499
Basic shaft system
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Dimensions are initially determined by designers
Basic size (preferred)
from a point of view of function.

 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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To1 determine the tolerance
Figure
D  DT
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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.
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
10-18
18-30
IT 1
IT 5
0.0012
0.008
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)
 A Different location is given a name (letter)
50.005
50.030
G
H
k
J
h
50
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 (basic hole system)
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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 (basic shaft
system)
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Basic shaft system with the indication of
three types of fits


Basic hole system with the indication of
three types of fits
Basic shaft: the maximum diameter is the basic or nominal diameter.
Basic hole: the minimum diameter is the basic or nominal diameter.
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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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 The tolerance for a part (A) is also
constrained by the fit of the part with the
other part (B).
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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
 Fit
Design and manufacturing
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Procedure for determining the tolerance
1.
2.
3.
4.
Basic size selection
Determine the preferred fit
International Grade
Determine tolerances
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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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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

60H7


(
60.030
60.000
)
60.021
60.002
60k6
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(
60.021
60.002
)
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Summary (expected to know):
1.
Tolerance is about one part but it has effect on the fit of two parts.
2.
It is the fit that makes sense for the quality of machine running.
3.
Tolerance is determined by the constraints of (a) machine running
condition, (2) manufacturing technology, (c) design to meet the function.
4.
Standard is to assist in determination of the tolerance fast and to facilitate
the part exchange.
5.
Standard consists of several tables and charts.
6.
Procedure: (a) decide basic hole or basic shat, (b) decide the basic
dimension, (c) decide the fit (which contains the IT grade), (d) find the
tolerance with the basic basic size and tolerance zone code (e.g., H7).
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