TOLERANCES - Introduction Nearly impossible to make the part to the exact dimension by any means of manufacturing approach - tolerances of the dimension.

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TOLERANCES - IntroductionNearly impossible to
make the part to the exact dimension by any means
of manufacturing approach - tolerances of the
dimension.
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- situations for assembly of (a) and (b)?
Introduction

• (a) 30.01 (shaft)
• (b) 30.005 (hole)
• (a) and (b) are impossible to be assembled
  without any special treatment
• (a) 30.00 (shaft)
• (b) 30.20 (hole)
• (a) and (b) are assembled with a possibility of
  poor Function of the system (see Figure 2)

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Introduction

• .

Figure 2
L
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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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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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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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Basic Concept
Three types of fits

• (a) 1.247 - 1.248 shaft
  
• (b) 1.250-1.251 hole
  Clearance fit
• (a) 1.2513-1.2519 shaft
  
• (b) 1.2500-1.2506 hole
  Interference fit
• (a) 1.2503-1.2509 shaft
  
• (b) 1.2500-1.2506 hole
  Transition fit

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Limits

• 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

The tolerance can also be defined as upper limit
lower limit on one same dimension
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Allowances
an intentional difference between the maximum
material limits of mating parts. It is the
minimum clearance (positive allowance) or maximum
interference (Negative allowance) between parts.
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Allowances

• Allowance Min Hole Max Shaft

• For the previous example,
• Clearance fit
• Allowance 1.250-1.2480.002

Allowance is associated with two dimensions of
two parts that form a fit
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Examples
Figure 5
Basic concept

• Shaft tolerance 1.248 - 1.247 0.001
• Hole tolerance 1.251-1.250 0.001
• Allowance1.250-1.248 0.02
• Max clearances1.251-1.247 0.04

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Tolerance representation

• The unilateral form
• The bilateral form

• The limit form
• 2.245 - 2.250
• 0.495 - 0.500
• 2.247-2.253

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Tolerance representation

• In general
• or
• Positive First

• Large Limit on Top
• Small limit first

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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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Standard
Different countries and regions together to
develop - Concepts - Rules - Systems
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Basic Hole System
Methodology for Determining Basic Size

• 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
Methodology for Determining Basic Size

• Reason in some applications, the shaft could be
  better made as a reference

Different fits with the same shaft
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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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Methodology for Determining Basic Size
Basic size 0.5

• Example
• 0.502 0.498
• 0.500 0.495

0.500 0.499
0.505 0.502
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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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Known - Allowance0.02 - Tolerance for
hole0.002 - Tolerance for shaft0.03 - Because
Basic hole system, Basic dimension0.5, Min.
Hole dimension 0.5

• Therefore
• Max. Hole dimension Min. Hole Hole tolerance
• 0.5
  0.002 0.502
• - Max. Shaft dimension Min. Hole Allowance
• 0.5 -
  0.02 0.498
• - Min. Shaft dimension Max. Shaft Shaft
  tolerance
• 0.498 -
  0.03 0.495

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Example

• 0.502 0.498
• 0.500 0.495

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Example
Known Allowance0.002 Tolerance for hole
0.003 Tolerance for shaft 0.001 The minimal hole
size 0.5000.0020.502
Basic shaft system
0.505 0.502

• 0.500
• 0.499

The maximal hole 0.5020.0030.505

• Basic shaft system

The minimal shaft size 0.500-0.0010.499
The basic size0.500