Projection is defined as an image or drawing of an object made on a plane. All drawings used in the field of engineering are based on the principles of projection. That is why engineering drawings are capable to precisely convey the external as well as internal features of objects in terms of their shape and size. Projections can be classified on the basis of line of sight and the position of plane on which the drawing is made.
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Orthographic Projections
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Orthographic Projections
Parallel projection technique
Projectors normal to projection plane
Projection Plane
Projector s
NOTE: CAD Systems typically use orthographic projection
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Orthographic Projection Categories
Axonometric
Three object faces visible
No principal axes
parallel/perpendicular to projection plane
Multiview
Only one object face
visible
One object face (i.e., two
principal axes) parallel to
projection plane
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Axonometric Projections
Trimetric – no equal angles
All principal axes foreshortened unequally
Classed according to angles made by principal axes when projected onto projection plane
Dimetric – two angles are equal
Two principal axes are equally
foreshortened
Isometric – all angles equal (iso equal,
metric measure)
All principal axes foreshortened equally
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Axonometric Projection Classes
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Axonometric Projection Classes
Top View
Front View
Front View
Axes Only
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Isometric Scaling
Lines parallel to any
principal axis are
foreshortened equally
( scaled proportionally)
Measurements can be
made parallel to any of
the three principal axes
NOTE: In a trimetric projection, measurements along a given principal axis are proportional; however, measurements along different principal axes are not scalable
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Isometric Projections
All 3 principal axes equally foreshortened same scale
Can be visualized as looking down the
diagonal of a cube
Trimetric Projection Isometric Projection
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Isometric Drawings
Each axis is foreshortened
to approximately 82% of its
true length
To correct for this, an
isometric projection can be
plotted at a scale of 1/.82
(~1.22). This convenient
approximation of an
isometric projection is
called an isometric
drawing
It can be shown that, in an isometric projection:
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Isometric Viewing
About a vertical axis (45 ± 90n)º (n is an
integer), then
An isometric projection is obtained by rotating the view:
Out of the horizontal plane, ±35.26º
Front Front, Side Top, Front, Side
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Isometric Sketching
In sketching an isometric view, principal axes are aligned as shown below:
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Orthographic Projections