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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. Compare using a 2D CAD program with sketching on a sheet
of paper.
7. List the dimensions that transfer between top, front, and
right-side views.
8. Transfer depth between the top and right-side views.
9. Label points where surfaces intersect.
Technical Drawing with Engineering Graphics, 14/e
Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman
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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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© 2012, 2009, 2003, Pearson Higher Education,
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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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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
Multiview Projection
The system of views is called multiview projection. Each view provides
certain definite information.
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Upper Saddle River, NJ 07458. • All Rights Reserved.
The Six Standard Views
Any object can be viewed from six mutually perpendicular
directions,
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Upper Saddle River, NJ 07458. • All Rights Reserved.
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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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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.
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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).
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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.
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
The Orthographic Projection
The front, top, and right-side views of the object shown now without
the folding lines.
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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.
Technical Drawing with Engineering Graphics, 14/e
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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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Upper Saddle River, NJ 07458. • All Rights Reserved.
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.
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
Two Views
Many objects need only two views to clearly describe their shape. If an object
requires only two views, and the left-side and right-side views show the object
equally well, use the right-side view.
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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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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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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Position of the Side View
Sometimes, drawing three views using the conventional arrangement
wastes space.
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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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© 2012, 2009, 2003, Pearson Higher Education,
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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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© 2012, 2009, 2003, Pearson Higher Education,
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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.
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
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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).
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
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Centerlines continued…
Centerlines (symbol: ) are used to indicate symmetrical axes of objects
or features, bolt circles, and paths of motion.
Technical Drawing with Engineering Graphics, 14/e
Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
VIEWS OF SURFACES
There are terms used for describing a surface’s orientation to the plane of
projection. The three orientations that a plane surface can have to the plane
of projection are normal, inclined, and oblique.
Note how a plane surface
that is perpendicular to a
plane of projection
appears on edge as a
straight line
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
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ANGLES
If an angle is in a normal plane (a plane parallel to a plane of projection) it will
show true size on the plane of projection to which it is parallel.
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
SIMILAR SHAPES OF SURFACES
If a flat surface is viewed from several different positions, each view will
show the same number of sides and a similar shape. This consistency of
shapes is useful in analyzing views.
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
INTERPRETING VIEWS
One method of interpreting sketches is to reverse the mental
process used in projecting them.
Technical Drawing with Engineering Graphics, 14/e
Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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:
Technical Drawing with Engineering Graphics, 14/e
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© 2012, 2009, 2003, Pearson Higher Education,
Upper Saddle River, NJ 07458. • All Rights Reserved.
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