Mechanical Engineering Drawing MECH 211/M / Lecture #9 Chapters 11 - - PowerPoint PPT Presentation

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Mechanical Engineering Drawing MECH 211/M / Lecture #9 Chapters 11 and 12 p Dr. John Cheung Dimensioning Dimensions: Define the numerical values of a feature Size Location Location Surface texture (Manufacturing processes)

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SLIDE 1

Mechanical Engineering Drawing / MECH 211/M

Lecture #9 Chapters 11 and 12 p

  • Dr. John Cheung
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SLIDE 2

Dimensioning

  • Dimensions: Define the numerical values of

a feature

Size Location Location Surface texture (Manufacturing processes) Geometric characteristic (square round cone Geometric characteristic (square, round, cone,

etc)

2

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SLIDE 3

What Makes for Good Dimensioning

  • Proper line types, spacing and arrowheads
  • Placements is of the dimensions

Placements is of the dimensions

  • What to dimension and what not to

dimension clear and unambiguous dimension – clear and unambiguous

3

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SLIDE 4

Scale

  • Drawings are generally to scale
  • Dimensions that are not to scale (NTS)

( )

Draw a heavy straight line under the dimension

  • Standard scales should be used

Standard scales should be used

  • Large objects reduced in size
  • Small objects increased in size
  • Small objects increased in size
  • Dimensions are show in real (not scaled size)

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SLIDE 5

Nomenclature

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SLIDE 6

Nomenclature Nomenclature

  • Dimension: A numerical value which defines

Dimension: A numerical value which defines size or relative position.

  • Basic dimension: Theoretically exact size of
  • Basic dimension: Theoretically exact size of

the feature. R f di i Di i di l

  • Reference dimension: Dimension not directly

used, but indicated for clarity.

  • Dimension line: Thin dark solid line that shows

the extent and the direction of the feature.

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SLIDE 7

Nomenclature Nomenclature

  • Extension line: Line that shows which feature

Extension line: Line that shows which feature is associated with the size.

  • Visible gap: Gap between corners of the
  • Visible gap: Gap between corners of the

feature and extension lines. L d li E i li h h h i

  • Leader line: Extension line that shows the size
  • f a inaccessible feature.
  • Diameter/Radius symbols: φ/ R followed by

the size of the feature.

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SLIDE 8

Nomenclature Nomenclature

  • Arrowhead: Symbol at the end of dimension

Arrowhead: Symbol at the end of dimension line.

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SLIDE 9

Dimensioning Text

  • Text is usually 3mm or 0.125’’ high
  • Space between lines of text is 1 5 mm or

Space between lines of text is 1.5 mm or 0.0625’’

  • Text should be legible do not crowd
  • Text should be legible, do not crowd

dimensions D l bj li Li b

  • Do not letter on object lines. Lines may be

broken ‐ clarity

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SLIDE 10

Size and Position

  • Size dimensions

Size dimensions Position Position dimensions dimensions

  • 1. Horizontal
  • 1. Horizontal position

2 Vertical 2 Vertical position

  • 2. Vertical
  • 2. Vertical position
  • 3. Diameter
  • 3. Angle
  • 4. Radius

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SLIDE 11

Size and Position Size and Position

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SLIDE 12

Tabular Dimensioning

  • Series of objects with like features but varying dimensions
  • The variables are given in a tabular form.
  • Commonly seen in catalogues, handbooks etc

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SLIDE 13

Co‐ordinate Dimensioning

  • Coordinate system must be defined
  • Dimensions are given decimals form (no fractional

dimensioning)

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SLIDE 14

Dimensioning Rules

  • Each feature is dimensioned only once
  • Dimensions should be placed in the most descriptive

view view

  • Dimensions should specify only the size and position
  • The manufacturing method should only be shown if it is

a design requirement a design requirement

  • Angles shown on drawings as right angles are assumed

to be 90 degrees unless otherwise specified, and they need not be dimensioned need not be dimensioned

  • Dimensions should be located outside the boundaries
  • f the object whenever possible.

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SLIDE 15

Dimensioning Rules

  • Dimension lines should be aligned and grouped where

possible to promote clarity and uniform appearance.

  • Crossed dimension lines should be avoided whenever

Crossed dimension lines should be avoided whenever

  • possible. When dimension lines must cross, they should

be unbroken.

  • The space between the first dimension line and the

The space between the first dimension line and the

  • bject should be at least 3/8 inch (10mm).
  • The space between dimension lines should be at least ¼

inch (6mm). inch (6mm).

  • There should be a visible gap between the object and

the origin of an extension line. (1.5mm)

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SLIDE 16

Dimensioning Rules

  • Extension lines should extend 1/8” (3mm) beyond

the last dimension line.

  • Extension lines should be broken if they cross or are

Extension lines should be broken if they cross or are close to arrowheads.

  • Leader lines used to dimension circles or arcs

h ld b di l should be radial.

  • Dimensions should be oriented to be read from the

bottom of the drawing.

  • Diameters are dimensioned with a numerical value

preceded by the diameter symbol.

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SLIDE 17

Dimensioning Rules

  • Concentric circles should be dimensioned in a

longitudinal view whenever possible. Fig.11.29 Wh di i i i t th t f

  • When a dimension is given to the center of an

arc or radius, a small cross is shown at the center.

  • The depth of a blind hole may be specified in a

note. C b d (CBORED) f d (SFACED)

  • Counterbored (CBORED), spotfaced (SFACED),
  • r countersunk (CSK) holes should be specified

in a note.

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SLIDE 18

Ali d d U idi ti l Aligned and Unidirectional

  • Aligned Dimensions

text placed parallel to the dimension line vertical dimensions read from the right side of the drawing

  • Unidirectional Dimensions

Read from bottom of page

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SLIDE 19

Dimension Outside the View

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Dimension Line Methods

  • Use the clearest methods

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SLIDE 21

Grouped Dimension Lines

  • Stagger grouped dimensions

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Breaking Extension Lines

  • Do not break extension lines for object lines
  • Break extension lines for arrow heads

Break extension lines for arrow heads

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SLIDE 23

Centre Lines as Extension Lines

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SLIDE 24

Radial and Diametric Dimensions

  • More than half a circle:

diameter L th h lf i l

  • Less than half a circle
  • r arc: radius
  • Leaders to point

Leaders to point towards centre of the circle or arc (Radial) Id i l h l

  • Identical holes can

have their dimensions grouped with an X g p

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Dimensioning Arcs Dimensioning Arcs

  • Arc in dimensioned where they are true shape

y p

  • Value is located inside the arc if it fits
  • If not numeral alone or including leader is moved out
  • Cross is indicated with or without dimensions for centre of

all arcs except small and unimportant radii

  • For long radius false center with jogged leader can be
  • For long radius, false center with jogged leader can be

used

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SLIDE 26

Dimensioning Chained Features

  • Smaller dimension should be placed closer to the object to avoid

unnecessary line crossing

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SLIDE 27

Detailed Explanations

  • Leader lines are used to detail manufacturing requirements

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SLIDE 28

Not to Scale Dimensioning

  • Not to scale (NTS) features are indicated with an underline

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SLIDE 29

Reference for an Extension Line

  • Dimensioning is always performed between crisp surfaces
  • When flat surfaces are not available

extension lines with reference marks are used

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SLIDE 30

Best View

  • Feature is dimensioned in the view where it is seen best

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SLIDE 31

Symbols

  • Do not draw a view

for a feature that could be indicated by a symbol. y y

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SLIDE 32

Symbols

Section view is needless as Counterbore Countersink Spotface symbols in the top view means this

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SLIDE 33

Keyseats and Keyways

  • Dimension key seats from the bottom of the key seat to
  • pposite end of the shaft

F k f f k b f h l

  • For keyways, from top of keyway to bottom of hole

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SLIDE 34

Centre Distances

  • By giving centre to

centre distances d dii f d and radii of ends

  • One radius

dimension is only dimension is only needed, but number of places number of places need to be mentioned

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Concentric Circles

  • Dimensioned in the longitudinal view

A

35

A SECTION A ‐ A

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SLIDE 36

Threads

  • Threads are

dimensioned with l l local notes

  • Internal or tapped

threads on the threads on the circular view

  • External threads on
  • External threads on

the longitudinal view

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SLIDE 37

Superfluous Dimensions (Redundant) Superfluous Dimensions (Redundant)

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SLIDE 38

Types of Tolerances (Chapter 12)

  • Dimensional tolerances (limits of the linear
  • r angular dimensions)

g )

  • Positional tolerances (limits of linear or

angular location of features within a part angular location of features within a part

  • Geometric tolerances (abatement form

shape or position of a specific feature) shape or position of a specific feature)

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SLIDE 39

Fundamentals

  • The not desired but permitted dimensional variation of a

i f d h i i certain feature due to the economic aspect in manufacturing (imperfect machining processes).

  • Tolerances are essential when two or more parts are

Tolerances are essential when two or more parts are assembled together ‐ clearance

  • The amount of the permitted variation is related to the

The amount of the permitted variation is related to the functions of the parts (quality).

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SLIDE 40

Fundamentals

MMC

MMC – Maximum Material Condition LMC – Least Material Condition

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What is Important?

  • Understanding of tolerances
  • Selection and calculations

Selection and calculations

  • Prescription of tolerances
  • Tolerance of a size: the difference between

the maximum and the minimum allowed size of the specific dimension

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SLIDE 42

Size Nomenclature

  • Nominal Size

The general size (used for general identification of part) Example, Ø 15 shaft, M8 bolt

  • Basic Size

Theoretical size (size from which limits are worked out)

  • Actual Size

ctua S e

Measured size of the actual part

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Tolerance Nomenclature

  • Limits

the max and min sizes shown by tolerances

  • Allowance

for mating parts – min clearance or max interference

g p

  • Tolerance

total allowable variance total allowable variance

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SLIDE 44

Material Condition Nomenclature

  • Maximum material condition (MMC)

where part contains maximum amount of

material

  • Least material condition (LMC)
  • where part contains minimum amount of

material

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SLIDE 45

Fit Conditions

  • Clearance fit

space between mating parts

  • Interference fit

no space between mating parts

p g p

  • Transition fit

clearance or interference fit

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Dimension Types

  • Direct limits

(limit dimensioning)

  • Tolerance value

(plus or minus dim) (plus or minus dim)

  • Unilateral Tolerances

(only in one direction from basic size) (only in one direction from basic size)

  • Specific note

(The * dimensions φ2+0.001)

  • General note

(All diameters φ2+0.001)

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Clearance and Interference Fits

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Transition Fit

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How to determine fits? How to determine fits?

  • Evaluate the allowance

d h f and the interference

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Functional Dimensioning

  • Start by tolerancing the most important features
  • Functionality of the assembly has to be defined
  • Functionality of the assembly has to be defined
  • The assembly and manufacturing processes must

be defined

  • Tolerances should be as “coarse”

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Tolerance Stack‐up

  • Tolerances taken in the same direction from one point of

reference are additive – tolerances stack‐up or

p

accumulation of tolerance

  • Tolerance stack‐up can be eliminated by careful selection

and placement of dimensions p

  • If Z not given, it will be governed by both X and Y (.01

instead of intended tolerance of .005)

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Tolerance Stack up Tolerance Stack‐up

Tolerances stack‐up may cause assembly problems (Different datum used)

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Tolerance Stack‐up Tolerance Stack up

Dimensioning with respect to a common base would help

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Tolerance Stack‐up

Providing tolerances for the locating dimensions is a better solution

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Dimensioning methods

  • If limits are shown up and down, largest limit up
  • If shown side by side, smallest limit first
  • For angular dimensions, it can be in general note or it can be

mentioned similar to that of linear dimensions

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Metric Tolerancing (ISO)

International tolerance (IT) Grades

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Metric Hole Based Fits

Minimum hole size is the basic size

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Limit (Imperial) vs. Note (ISO/Metric) Tolerancing

Hole Tolerance = .025

(ISO/Metric) Tolerancing

Shaft Tolerance = .016 40 g6 Loosest fit 40.025‐39.975 = .050 g 40 H7 Tightest fit 40 000 – 39 991 = 009 40.000 39.991 = .009

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Basic hole and shaft system‐Imperial size

Hole Basis Fit Shaft Basis Fit

Basic Size .500 Largest shaft .500 Smallest hole .500

Interference fit Clearance fit Interference fit Clearance fit

  • Hole Basis fit: the basic size is the minimum DIA of the hole and fit is

calculated based on this

Interference fit Clearance fit Interference fit Clearance fit

calculated based on this.

  • Shaft Basis fit: the basic size is the maximum diameter of the shaft and

the fit is calculated base on this the fit is calculated base on this.

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SLIDE 60

Example – Run Fit

0.500 is the lower limit hole 0.496 is the upper limit shaft 0 004 is the ALLOWANCE

Example Run Fit

0.004 is the ALLOWANCE 0.496 is the upper limit shaft 0 003 is the shaft tolerance

.503 500 .496 493

0.003 is the shaft tolerance 0.493 is the LOWER LIMIT SHAFT 0 500 i th l li it h l

.500 .493

0.500 is the lower limit hole 0.003 is the hole tolerance 0.503 is the UPPER LIMIT HOLE 0.500 is the smallest hole 0.496 is the largest shaft 0.004 is the tightest fit 0.503 is the largest hole 0.493 is the smallest shaft 0.10 is the loosest fit g

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Geometric Tolerancing

  • Used to limit the abatement in the geometric or

Geometric Tolerancing

positional variation of features

Total flatness tolerance,

Flatness tolerance

, 0.05mm. This entire tolerance zone may move up and down within the size tolerance zone Total height tolerance 0.1 mm

indication in drawing

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Geometric

Example of Feature Control Frames

tolerance symbol (Parallelism) Geometric tolerance value Size dimension Reference Datum Geometric Geometric Geometric tolerance symbol (Roundness) Geometric tolerance value

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SLIDE 63

Dimensioning and tolerancing symbols

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Straightness of an Axis

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Roundness

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Examples of Geometric Tolerancing Examples of Geometric Tolerancing

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