1. TOLERANCES - Introduction Nearly impossible to make the part to the exact dimension by any means of manufacturing approach tolerances of the dimension.
- Dimension 30 (mm) won’t be made exactly as 30 (mm)
- It may be made as (mm) or (mm).
- maximum may be (mm) 30
(a)
(b)
Fig. 1
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2. Introduction (a) 30.01 (shaft) (b) 30.005 (hole)
- situations for assembly of (a) and (b)?
(a) (shaft)
(b) (hole)
(a) and (b) are impossible to be assembled without any special treatment
(a) (shaft)
(b) (hole)
(a) and (b) are assembled with a possibility of poor Function of the system (see Figure 2)
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3. Introduction
Figure 2 . L’ L
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4. Introduction In summary, designers need to specify tolerances for
(a) Parts manufacturing interchangeable
(b) System function satisfactorily with low cost
Since greater accuracy costs more money, the designer will not specify the closest tolerance, but instead will specify as generous a tolerance as possible.
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5. Objectives of the lecture:
Introduction
Objectives of the lecture:
To learn principles behind those rules or standards for determining tolerances.
To learn procedure of using the standards for determining tolerances.
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6. Tolerance is always a positive number
Basic Concept
Definition of Tolerance:
Tolerance is the total amount a specific dimension is permitted to vary, which is the difference between the maximum and the minimum limits.
Tolerance is always a positive number
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7. (b) 1.250-1.251 hole Clearance fit (a) 1.2513-1.2519 shaft
Basic Concept
Three types of fits
(a) shaft
(b) hole Clearance fit
(a) shaft
(b) hole Interference fit
(a) shaft
(b) hole Transition fit
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8. Limits:
upper-limit and lower-limit
The maximum and the minimum sizes indicated by a tolerance dimension.
The limits for hole are 1.250” and 1.251”
The limits for shaft are 1.248” and 1.247”
Lower limit Upper limit
The tolerance can also be defined as upper limit – lower limit
on one same dimension
Hole tolerance = =0.001
Shaft tolerance = =0.001
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9. Allowances:
an international difference between the maximum material limits of mating parts. It is the minimum clearance (positive allowance) or maximum interference (Negative allowance) between parts.

10. Allowance = Min Hole – Max Shaft
Allowances:
Allowance = Min Hole – Max Shaft
For the previous example,
Clearance fit
Allowance = =0.002
Shaft limit
Hole limit
Allowance is associated with two dimensions of two parts that form a fit
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11. Basic concept Examples Figure 5 Allowance=1.250-1.248= 0.02
Shaft tolerance = =0.001
Hole tolerance= = 0.001
Allowance= = 0.02
Max clearances= = 0.04
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12. Tolerance representation
The unilateral form
The bilateral form
The limit form
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13. Tolerance representation
In general or
Positive First
Large Limit on Top
Small limit first
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14. Standard
Standard (ISO, etc.): limits a freedom of choices but promotes the exchange of parts manufactured with
different approaches
different equipment
different worker
in different cultural and societal situations
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15. Different countries and regions together to develop - Concepts - Rules
Standard
Different countries and regions together to develop
- Concepts
- Rules
- Systems
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16. Methodology for Determining Basic Size 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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17. Methodology for Determining Basic Size
Basic Shaft System
Reason: in some applications, the shaft could be better made as a reference
Different fits with the same shaft
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18. Basic Shaft System Methodology for Determining Basic Size
the maximal shaft size as the basic size
Reason: Cold-finished shaft.
- cold forging
- cold molding
- cold rolling
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19. Example Basic hole system Methodology for Determining Basic Size
Basic size =0.5
0.505
0.502
Basic shaft system
0.500
0.499
Basic hole system
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20. Example: Basic Hole System
Given: Tolerance for the hole = 0.002
Tolerance for the shaft = 0.03
Allowance = 0.02
Basic dimension =0.500
To determine: (a) the limit of the shaft
(b) the limit of the hole
Solution:
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21. - Because Basic hole system, Basic dimension=0.5,
Known:
- Allowance=0.02
- Tolerance for hole=0.002
- Tolerance for shaft=0.03
- Because Basic hole system, Basic dimension=0.5,
Min. Hole dimension = 0.5
Therefore:
Max. Hole dimension = Min. Hole + Hole tolerance
= = 0.502
- Max. Shaft dimension = Min. Hole – Allowance
= = 0.498
- Min. Shaft dimension = Max. Shaft + Shaft tolerance
= = 0.495
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22. Example Basic hole system 0.502 0.498 0.500 0.495
Basic hole system
The basic size = 0.500
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23. Example Known: The minimal hole size: 0.500+0.002=0.502
Allowance=0.002
Tolerance for hole= 0.003
Tolerance for shaft= 0.001
The minimal hole size:
=0.502
Basic shaft system
0.505
0.502
0.500
0.499
Basic shaft system
The maximal hole: =0.505
The minimal shaft size:
=0.499
The basic size=0.500
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