ORTHOGRAPHIC
PROJECTION
CHAPTERFIVE
OBJECTIVES
1. Recognize and sketch the symbol for third-angle
projection.
2. List the six principal views of projection.
3. Sketch the top, front, and right-side views of an object
with normal, inclined, and oblique surfaces.
4. Understand which views show depth in a drawing
showing top, front, and right-side views.
5. Know the meaning of normal, inclined, and oblique
surfaces.
6. List the dimensions that transfer between top, front,
and right-side views.
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UNDERSTANDING PROJECTIONS
To make and interpret drawings you need to know how to create
projections and understand the standard arrangement of views.
You also need to be familiar with the geometry of solid objects and be able
to visualize a 3D object that is represented in a 2D sketch or drawing.
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Views of Objects
The system of views is called
multiview projection. Each view
provides certain definite
information. For example, a front
view shows the true shape and
size of surfaces that are parallel
to the front of the object.
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Multiview Projection
The system of views is called multiview projection. Each view provides
certain definite information.
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Parallel Projection
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The Six Standard Views
Any object can be viewed from six mutually perpendicular
directions,
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Revolving the Object to Produce Views
Revolving the Object to Produce Views. You can experience
different views by revolving an object.
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Principal Dimensions
The three principal dimensions of an object are width, height, and
depth.
The front view shows only the height
and width of the object and not the
depth. In fact, any principal view of a 3D
object shows only two of the three
principal dimensions; the third is found
in an adjacent view. Height is shown in
the rear, left-side, front, and right-side
views. Width is shown in the rear, top,
front, and bottom views. Depth is
shown in the left-side, top, right-side,
and bottom views.
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Projection Method
The outline on the plane of projection shows how the object appears to the observer.
In orthographic projection, rays (or projectors) from all points on the edges or contours
of the object extend parallel to each other and perpendicular to the plane of projection.
The word orthographic means “at right angles.”
Projection of an Object
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Horizontal and Profile Projection Planes
Specific names are given to the planes of projection. The front view is
projected to the frontal plane. The top view is projected to the horizontal
plane. The side view is projected to the profile plane.
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The Glass Box
One way to understand the standard arrangement of views on the sheet of
paper is to envision a glass box.
If planes of projection were placed parallel to each principal face of the object, they
would form a box.
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Unfolding the Glass Box
To organize the views of a 3D object on a flat sheet of paper, imagine the
six planes of the glass box being unfolded to lie flat.
Note the six standard views
(front, rear, top, bottom,
right side, left side).
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The Glass Box Unfolded
Lines extend around the glass box from one view to another on the planes of
projection. These are the projectors from a point in one view to the same point
in another view.
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Which Views to Present?

General Guidelines




Pick a Front View that is most descriptive of
object
Normally the longest dimension is chosen as
the width (or depth)
Most common combination of views is to use:
Front, Top, and Side View
Views other than the Principal Views are called
Auxiliary Views
15
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Conventional Orthographic Views
Width
Top View
Depth
Right
Side
View
Front View
Height
16
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Transferring Depth Dimensions
The depth dimensions in the top and side views must correspond pointfor-point. When using CAD or instruments, transfer these distances
accurately.
You can transfer dimensions between the top and
side views either with dividers or with a scale.
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You may find it convenient
to use a 45° miter line to
project dimensions
between top and side
views.
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Necessary Views
The top, front, and right-side views,
arranged together, are called the three
regular views because they are the
views most frequently used.
A sketch or drawing should contain only the views needed to clearly
and completely describe the object.
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One-View
Often, a single view supplemented by a note or by lettered symbols is
Enough.
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Choice of Front View
The view chosen for the front view in this case is the side, not the front, of the
automobile.
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Third-Angle Projection
To understand the two systems, think of
the vertical and horizontal planes of
projection, as indefinite in extent and
intersecting at 90° with each other; the
four angles produced are called the first,
second, third, and fourth angles (similar to
naming quadrants on a graph.) If the
object to be drawn is placed below the
horizontal plane and behind the vertical
plane, as in the glass box you saw earlier,
the object is said to be in the third angle.
In third-angle projection, the views are
produced as if the observer is outside,
looking in.
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First-Angle Projection
If the object is placed above the horizontal plane and in front of the
vertical plane, the object is in the first angle.
The biggest difference
between third-angle
projection and first-angle
projection is how the planes
of the glass box are
unfolded.
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Hidden Lines
Thick, dark lines represent features of the object that are directly visible.
Dashed lines represent features that would be hidden behind other surfaces.
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HIDDEN LINES
Hidden lines are used to represent surfaces that are
not directly visible in an orthographic view.
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Centerlines
The centerline pattern is used to:
• show the axis of symmetry for a feature or part
• indicate a path of motion
• show the location for bolt circles and other circular patterns
The centerline pattern is
composed of three
dashes: one long dash on
each end with a short
dash in the middle.
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PRECEDENCE OF LINES
A visible line always takes precedence over and
covers up a centerline or a hidden line when they
coincide in a view (A and B).
A hidden line takes
precedence over a
centerline (C).
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Centerlines continued…
Centerlines (symbol: ) are used to indicate symmetrical axes of objects
or features, bolt circles, and paths of motion.
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MODELS
One of the best aids to visualization is an actual model of the object. Models
don’t necessarily need to be made accurately or to scale. They may be made
of any convenient material, such as modeling clay, soap, wood, wire, or
Styrofoam, or any material that can easily be shaped, carved, or cut.
Try making a soap
or clay model from
projected views:
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SLANTED SURFACES
Slanted surfaces are surfaces that are not parallel to either the
horizontal or vertical axis.
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COMPOUND LINES
A compound line is formed when two slanted surfaces
intersect. The true length of a compound line is not shown in
the front, top, or side views.
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OBLIQUE SURFACES
Oblique surfaces are surfaces that do not appear
correctly shaped in the front, top, or side views
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PROJECTION BETWEEN VIEWS
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ROUNDED SURFACES
Rounded surfaces are surfaces that have constant radii,
such as arcs or circles. Surfaces that do not have constant
radii are classified as irregular surfaces
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