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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 Projector s 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 Classed according to angles made by principal axes when projected onto projection plane Trimetric – no equal angles All principal axes foreshortened unequally 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 Axonometric Projection Classes Top View Front View Front View Axes Only 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 It can be shown that, in an isometric projection: 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 Isometric Viewing An isometric projection is obtained by rotating the view: About a vertical axis (45 ± 90n)º (n is an integer), then 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 .. .Orthographic Projections Parallel projection technique Projectors normal to projection plane Projection Plane Projector s NOTE: CAD Systems typically use orthographic projection Orthographic Projection Categories... Isometric Sketching In sketching an isometric view, principal axes are aligned as shown below: Orthographic Projections ... 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