Lecture Autodesk inventor Orthographic projections

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.

Orthographic Projections

Orthographic Projections

Parallel projection technique

Projectors normal to projection plane

Projection Plane

Projectors

NOTE: CAD Systems typically use orthographic projection

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

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

Axonometric Projection Classes

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

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

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:

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

Isometric Sketching

In sketching an isometric view, principal axes are aligned as shown below:

Orthographic Projections