sie1010 lesson 5.2 - dimensioning and tolerancing (part 2)

8/9/2019 SIE1010 Lesson 5.2 - Dimensioning and Tolerancing (Part 2)

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SIE 1010 Engineering Design Graphics

Lesson 5.2 Dimensioning and Tolerancing

Dr Ivan Lee

[email protected]


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Lesson Outline

Dimensioning

Units of Measurement

Terminology Associated with Dimensions

Arrangement, Placement and Spacing of Dimensions

Using Dimensions to Specify Size and Location of Features

Dimensioning Rules and Guidelines

Finish Marks

Tolerancing

Definitions

Tolerancing Methods and Tolerance Accumulation

Geometric Tolerances

Tolerancing of Mated Parts

Preferred Metric Limits and Fits


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Variation in Part Sizes

In manufacturing, same process is typically employed to mass

produce a single part

Parts are then combined with other mass-produced parts to

create commercial products

Mass-produced parts must be interchangeable

However, parts produced by same manufacturing process are

not exactly the same slight variations in part size

Tolerancing a dimensioning technique used to ensure partinterchangeability by controlling variance in manufactured

parts


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Tolerancing

Tolerance range within which a dimension is allowed to vary

Controls amount of variation on each manufactured part

Amount of variation depends on function of part and assembly

As long as size and location of part features fall within tolerance zone,

the part should function properly within an assembly

Critical to success of manufacturing

Ensures interchangeability of parts

Directly influences cost and quality of manufactured parts

Parts made to high accuracy are expensive Depending on type of product, extremely accurate parts may not be

warranted

For example, parts for a plastic toy need not be as accurate as

automotive parts


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Tolerance Definitions

Tolerance specifies total permissible variation of a size

Difference between upper and lower limits of size

Example of specification: 3.25 0.03

Basic size: 3.25

Upper limit (max value): 3.28

Lower limit (min value): 3.22

Tolerance: 0.06 (Upper Lower limits)

Actual part size can range anywhere between 3.22 and 3.28, and still

function properly Basic size theoretical size from which tolerance is assigned

Actual size measured size of a finished part


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Tolerancing

General rule

Tolerances should be stated as generously as possible while still

ensuring the part will function properly

Allows a wider variety of processes to manufacture the part, thereby

keeping part costs low

Manufacturing quality is a function of part accuracy

High quality parts small variations in size and shape

Tight tolerance zones need to control part variability in

manufacturing


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Tolerance and Machining Process


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Tolerance Declaration

Tolerances may be expressed in different ways

Direct tolerancing methods

General tolerance notes

Geometric tolerances


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Direct Tolerancing Methods

Lower limit Upper limit

Upper limit

Lower limit

Limit dimensioning

Plus-and-minus dimensioning


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General Tolerance Notes

General tolerances are given in a note or in the title block

Example of a general note for Metric dimensions

ALL METRIC DIMENSIONS TO BE HELD TO 0.05

For basic size = 65.00, upper limit = 65.05, lower limit = 64.95

Example for English dimensions

ALL DECIMAL DIMENSIONS TO BE HELD TO 0.002

Example for angular dimensions

ALL ANGULAR TOLERANCES 1 DEGREE


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General Tolerance Notes

Tolerances may also be specified in terms of number of

decimal places found in the dimensions

Example of general note

UNLESS OTHERWISE SPECIFIED, TOLERANCES ARE AS FOLLOWS:

Millimetres:

X = 0.5

X.X = 0.25

X.XX = 0.12

A tolerance added to a dimension always supersedes the

general tolerance


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Tolerance Accumulation

When location of a feature depends on more than one

tolerance values tolerances will be cumulative

Chain dimensioning technique example (a)

Dimensions are specified in sequence relative to each other

Tolerance accumulation between surfaces X and Y is 0.03


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Base line dimensioning technique example (b)

All dimensions of a given type are specified from the same datum

Tolerance variation between surfaces X and Y is reduced to 0.02


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Direct dimensioning technique example (c)

Distance between features are directly dimensioned

Tendency for tolerance accumulation can be further controlled

Maximum variation between surfaces X and Y is 0.01


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Geometric dimensioning and tolerancing (GDT) is a methodof defining parts based on how they function, using standard

ASME/ANSI symbols

Table height with flatness


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Geometric D&T

Five categories of geometric controls

Form

Orientation

Location/Position Runout

Profile

Example of feature control frames Shaft with circularity control


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Examples of geometric dimensioning and tolerancing


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GDT symbols


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Mated Parts

Tolerance of a single stand-alone part is of little importance

When it is mated with other parts in an assembly, tolerancing

becomes critical

Mated parts must be toleranced as a system to fit within prescribed

degree of accuracy

In the pulley assembly below, shaft must turn freely within bushing,

while bushing is force-fit into pulley


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Pulley Support Assembly

Exploded view of pulley support assembly

Pulley support assembly

Parts in an assembly must betoleranced as a system to achieve

prescribed degree of accuracy


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Types of Fits

Fit refers to the degree of tightness or looseness between two

mating parts

Tolerances can be specified for mating parts to achieve desired fit

Different types of fits

Clearance fit

Interference fit

Transition fit

Line fit


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Clearance Fit

Internal member (e.g., shaft) is always smaller than external member

(e.g., hole of bushing)

Shaft is free to turn inside bushing


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Interference Fit

Internal member (e.g., bushing) is always larger than external member

(e.g., hole of pulley)

Requires two parts to be forced together, without using adhesive or

mechanical fasteners


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Transition Fit

Ranges between a pure clearance fit and a pure interference fit

Either internal shaft or external hole may be larger, so parts either slide or

are forced together

If an assembly calls for transition fit, the two sets (hole, shaft) of

components can be measured and sorted into groups according to size

(e.g., small, medium, large)

Components are then assembled components from one group being

mated with corresponding components from matching group

Selective assembly method relatively inexpensive way to manufacture

clearance or interference fits


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Line Fit

One of the limits on both

members (hole, shaft) are equal

This means shaft and hole may

have the same size


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Allowance

Allowance is the tightest possible fit between two mated

parts

Allowance = Smallest hole size Largest shaft size

Clearance fit

Allowance: Minimum clearance between two parts (positive)

Interference fit

Allowance: Maximum interference between two parts (negative)


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Mated Parts with Parallel Surfaces

Different types of fit

Typically refer to cylindrical features (shafts, holes)

May also apply to parts with parallel surfaces that fit inside one

another

Fit of mated parts with parallel surfaces


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Basic Hole Systems

A reference system, or method of calculation, to relate

tolerances and allowance to a basic size

Applies to a system of mated parts to achieve a particular

type of fit (clearance, interference, transition)

Tolerances and allowances are pre-determined for a basic size based

on the desired fit

Two reference systems

English units

Metric units


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ANSI B4.2 1978 (1994), Preferred Metric Limits and Fits

Standards and tables for tolerancing fitted parts using metric units

A tolerance is specified using a special designation (e.g., 40H7)

Basic definitions

Basic size Size to which limits or deviations are assigned (40)

Deviation Algebraic difference between a size and the

corresponding basic size

Upper deviation Algebraic difference between max limit of size and

the corresponding basic size

Lower deviation Algebraic difference between min limit of size and

the corresponding basic size

Fundamental deviation Upper or lower deviation that is closest to

the basic size (H)


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Illustration of Definitions

Illustration of definitions for Metric Limits and Fits

IT grade number1. Establishes the magnitude of

a tolerance zone

2. A smaller number indicates a

smaller tolerance zone

Fundamental deviation

1. Establishes the position

of a tolerance zone with

respect to the basic size

2. Expressed by tolerance

position letters

3. Upper-case (e.g., H) for

hole dimensions; lower-

case (e.g., h) for shaft

dimensions


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More definitions

Tolerance Difference between max and min size limits

Tolerance zone Represents the tolerance and its position in relation

to the basic size

International tolerance grade (IT) A group of tolerances that varydepending on the basic size, but provide same relative accuracy

within a given grade; designated by 7 in 40H7 (IT7). There are 18 IT

grades: IT0, IT1, and IT01 to IT16. The smaller the grade, the smaller

the tolerance zone.

Hole basis System of fits where min hole size is equal to the basicsize; fundamental deviation for a hole basis system is H

Shaft basis System of fits where max shaft size is equal to the basic

size; fundamental deviation for a shaft basis is h


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Tolerance Specification

40 H 7

Basic size

Fundamental deviation (position letter)

International tolerance grade (IT number)

Tolerance zone symbol

40 g 6

Basic size

Fundamental deviation (position letter)

International tolerance grade (IT number)

Tolerance zone symbol

Internal Dimensions (Holes)

External Dimensions (Shafts)


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Fit Between Mated Parts

A fit between mated parts is designated as follows

Basic size common to both components

Tolerance symbol for internal part (hole)

Tolerance symbol for external part (shaft)

Metric-unit fit designation

Shaft tolerance

Hole tolerance


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Preferred Basic Sizes

Whenever possible:

Standard, or preferred,

sizes of round metal parts

should be used

Basic size of mating partsshould be chosen from

the first choice sizes


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Preferred Hole Basis Fits

Hole basis is

preferred in most

cases Fits have a

fundamental

deviation of H

on the hole


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Preferred Shaft Basis Fits

Shaft basis should be used when acommon shaft mates with different holes

Fits have a fundamental deviation of h

on the shaft


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Description of Preferred Fits


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Tolerance Calculation Example 1

Given:

Basic size: 50 mm

Fit type: Free running, H9/d9

Calculation method: Hole basis

Find:

Tolerance limits on hole

Tolerance limits on shaft

Allowance


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ANSI Preferred Hole Basis

Metric Clearance Fits (ANSI B4.2-1978, R1984)


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From ANSI Preferred Hole Basis Clearance Fits:


Upper limit = 50.062

Lower limit = 50.000 (= basic size)

Tolerance limits on shaft Upper limit = 49.920

Lower limit = 49.858

Allowance

Allowance = Hole min Shaft max = 50.000 49.920 = +0.080


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Given:

Basic size: 30

Fit type: Medium drive, S7/h6

Calculation method: Shaft basis

Find:


Tolerance limits on shaft

Allowance


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ANSI Preferred Shaft Basis

Metric Transition and Interference Fits (ANSI B4.2-1978, R1984)


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From ANSI Preferred Shaft Basis Interference Fits:


Upper limit = 29.973


Tolerance limits on shaft Upper limit = 30.000 (= basic size)


Allowance

Allowance = Hole min Shaft max = 29.952 30.000 = -0.048


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Tutorial

Work on Tutorial 5 Questions


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More on Geometric D&T

Five categories of geometric controls

Form

Orientation

Location/Position Runout

Profile


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Form Controls

Straightness of axis

Circularity


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Form Controls

Flatness

Cylindricity


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Orientation Controls

Parallelism


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Orientation Controls

Perpendicularity Angularity


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Location Controls

Concentricity


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Position Controls

Position: hole location from surfaces

Position: hole location from hole

l


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Runout Controls

Circular runout

f l l


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Profile Controls

Profile of a line

Profile of a surface

i i


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Key Learning Points

Engineering drawings must include all the information needed

to build a part, assembly or system

Dimensions and general notes are used to describe the size and

location of part features, and details related to construction or

manufacture of the part A numerical value is associated with a dimension line

Defines the size, location, geometric characteristic, or surface texture

of a part or feature

Drawings are typically dimensioned using millimetres (Metric

system) or decimal inches (English system)

Terminology and guidelines for dimensioning should be

adopted for optimum readability

K L i P i


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Key Learning Points

Tolerancing is a dimensioning technique used to ensure part

interchangeability

Controls the variance that exists in manufactured parts

Specifies a range within which a dimension is allowed to vary

If the size and location of part features fall within the tolerance zone,the part should function properly (as designed) in an assembly

Tolerances may be expressed in different ways

Direct tolerancing, general tolerance notes, geometric tolerances

Dimensioning technique can affect accumulated tolerances Mated parts must be toleranced as a system to achieve desired

accuracy and function

ANSI B4.2 1978 (1994) Prefered Metric Limits and Fits

Standards for tolerancing fitted parts using Hole basis or Shaft basis

R f


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References

Fundamentals of Graphics Communication, Sixth Ed

Gary R. Bertoline, Eric N. Wiebe, Nathan W. Hartman,

William A. Ross, McGraw-Hill Co., Inc, (2011)

Engineering Design Graphics, 2nd

Edition James M. Leake, Jacob L. Borgerson, John Wiley & Sons,

Inc., (2013)

Engineering Design Process

Yousef Haik, Brooks/Cole Thomson Learning, Inc., (2003)

sie1010 lesson 5.2 - dimensioning and tolerancing (part 2)

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