Why Engineering Drawings?Engineering drawing is a formal and precise way of communicating information about the shape, size, features and precision of physical objects.Drawing is the universal language of engineering.

Elements of Engineering Drawing

Engineering drawing are made up of graphics languagegraphics language

and word languageword language.

Graphicslanguage

Describe a shape(mainly).

Wordlanguage

Describe size, location andspecification of the object.

Basic Knowledge for Drafting

Graphicslanguage

Wordlanguage

Linetypes

Geometricconstruction Lettering

Projectionmethod

PROJECTION METHOD

Perspective

Oblique Orthographic

Axonometric Multiview

Parallel

PROJECTION THEORY

The projection theory is based on two variables:

1) Line of sight

2) Plane of projection (image plane or picture plane)

The projection theory is used to graphically represent

3-D objects on 2-D media (paper, computer screen).

Line of sightLine of sight is an imaginary ray of light between an

observer’s eye and an object.

Line of sight

Parallel projectionParallel projection

Line of sight

Perspective projectionPerspective projection

There are 2 types of LOS : parallel convergeand

Plane of projectionPlane of projection is an imaginary flat plane which

the image is created.

The image is produced by connecting the points where

the LOS pierce the projection plane.

Parallel projectionParallel projection Perspective projectionPerspective projection

Plane of projection Plane of projection

Disadvantage ofPerspective Projection

Perspective projection is notnot

used by engineer for manu-

facturing of parts, because

1) It is difficult to create.

2) It does not reveal exact

shape and size.Width is distorted

Orthographic Projection

Orthographic Projection

5

Orthographic projectionOrthographic projection is a parallel projection technique

in which the parallel lines of sight are perpendicular to the

projection plane

MEANING

Object views from top

Projection plane

1

2

3

4

51 2 3 4

Orthographic projection technique can produce either

1. 1. Multiview drawingMultiview drawing that each view show an object in two dimensions.

2. 2. Axonometric drawingAxonometric drawing that show all three dimensions of an object in one view.

Both drawing types are used in technical drawing for

communication.

ORTHOGRAPHIC VIEW

Axonometric (Isometric) Drawing

Easy to understand

Right angle becomes obtuse angle.

Circular hole becomes ellipse.

Distortions of shape and size in isometric drawing

AdvantageAdvantage

DisadvantageDisadvantage Shape and angle distortion

Example

Multiview Drawing

It represents accurate shape and size.AdvantageAdvantage

DisadvantageDisadvantage Require practice.

Multiviews drawing (2-view drawing)Example

Reading Isometric Sketches

Reading Isometric Sketches

Orthographic Projection

• An orthographic projection is a 2 dimensional representation of a 3 dimensional object.

Six Principle Views

• Any object can be viewed from six perpendicular views

The Glass Box

• One way to understand the standard arrangement of views on a sheet of paper is to envision the object in a glass box

• The outside observer would see six standard views of the object through the sides of this imaginary glass box

Glass Box Method

• Glass Box Method: – The object is placed in a glass box. – The image of the object is projected on the sides

of the box. – The box is unfolded.– The sides of the box are the principle views.

Glass Box Method

• The object is placed in a glass box.

• The side of the box represent the 6 principle planes.

Six Principle Views

OBSERVER MOVE AROUND

Front view Right side view

Top view

THE GLASS BOX CONCEPT

Bottom view

Left side view

Rear view

HeightWidth

De

pth

History

Example: Create the orthographic projections for the following object.

The height and depth of is equal to 3 grid squares.

Example: Create the orthographic projections for the following object.

Number of Required Views• When drawing an orthographic projection you

must include the appropriate number of views to fully describe the true shape of the part.

• You may use a fewer number of views if you can fully describe the part in the given views.– How many views are required to fully describe a

rectangular box?– How many views are required to fully describe a

sphere?

Number of Required Views

• You may use additional views, such as the left, back, or bottom views to describe an object if the object cannot be fully described in the three standard views.

– We will also learn about additional views this quarter such as section views, detail views and auxiliary views.

Each View Provides Two MeasurementsEach View Provides Two Measurements

• Top and Bottom views show DEPTH and WIDTH.

• Left and Right views show HEIGHT and DEPTH

• Front and Rear views show HEIGHT and WIDTH.

Measurement Transfer Between ViewsMeasurement Transfer Between Views

• The mitre line (drawn at a 45° angle) is used to transfer depth measurements between the top and right side (or left side) views.

Normal PlaneNormal Plane

• Normal Planes will appear as an edge in two views and a true sized plan in the remaining view when using three views such as a top, front and right side.

Inclined PlaneInclined Plane

• Inclined Planes will appear as an edge view in only one of the three views.

• The inclined plane will appear as a rectangular surface in the other two views.

Oblique PlanesOblique Planes• Oblique Planes will

not appear as an edge view in any of the six views since they are not parallel or perpendicular to the projection planes.

• They always appear as a “plane” and have the same number of corners in each of the six views.

Foreshortened Lines

Identify the foreshortened lines in the orthographic projection below.

Curved SurfacesCurved Surfaces

• A cylinder will appear as a circle in one view and a rectangular shape the other two views.

Axis and Center LinesAxis and Center Lines

• The axis appears where the cylinder appears rectangular.

• Center marks are used to identify the center of the cylinder where it appears circular.

Choosing the Front View

• Considerations when choosing the front view of an object.– Chose the view that shows the most features or

characteristics of the object– Choose the view that contains the least number of

hidden lines. – Choose the view so the part is oriented with its

longest length parallel to the bottom of the drawing.

First- and Third-Angle Projection

• There are two main systems used for projecting and unfolding the views:– Third-angle projection which is used in the United

States, Canada and some other countries– First-angle projection which is primarily used in

Europe and Asia

• You should understand both methods

Third-angle Projection

First-angle Projection

Object

We consider here the development of the orthographic views of a simple object.

Viewing

Direction

Picture Plane

For the Top view we view from the top!

Viewing

Direction

Point of intersection with picture plane

Projectors

Perpendicular to picture plane

Viewing

Direction

Intersections of allextreme points

Top View

Front View

Similarly, viewing from the front with parallel projectors

Top & Front Viewson opening up the page

Notice the interrelation

Similarly, the

Right Side View

Again notice the interrelation

Mitre

The third view can also be obtained by taking projections from the two views, using the mitre line, a line at 450

Two types of projections commonly used: I & III angle

In third angle, picture planes in between the viewer & object

In first angle, picture plane behind the object

Opening up of the box with the various views in III angle

The relationship on plane paper of the various views in III angle

TopView

FrontView

RightView

Left View

The relationship on plane paper of the various views in I angle

TopView

FrontView

Left View

RightView

Principles of Multi-View Drawings

1. The Front and the Top views are aligned vertically.

2. The Front and the Side views are aligned horizontally.

3. The front of the object in the top view faces the front view.

4. The front of the object in the side view faces the front view.

5. The depth of top view is the same as the depth of side view.

6. The width of top view is same as the width of front view.

7. The height of side view is same as height of front view.

Front View

Top View

Mitre

Front

Front View

Top View

Front

Drawing three views in III angle

Front View

Top ViewMitre

Front