Figure 5-4.-Primary (principal) planes of projections.is normally placed between the point of sight andthe object. For the purpose of studying any typeof projection, it must be assumed that the planesof projection are in fixed positions. Once theobject is placed in a definite imagined position,it should never be changed. If a different view ofthe object is desired, the location of the point ofsight is changed.The PROJECTION LINES (or LINES OFSIGHT) are the imaginary lines from the eye ofthe viewer (point of sight) to points on the object(fig. 5-2). By the use of projection lines, pointson the object are projected on the image plane.These points are the points at which theprojection lines appear to pierce the image plane.By the projection of the prominent points, lines,and surfaces of an object, a complete view ofthat object can be projected on the plane ofprojection.The relationship between the point of sight(station point), the plane of projection (imageplane), the projection lines (lines of sight), andthe manner in which they are used for eachindividual type of projection will be discussed inthe following sections.ORTHOGRAPHIC PROJECTIONWhen you are called upon to draw athree-dimensional object or figure, it is customaryto represent the parts and forms on theflat plane of the drafting paper in such amanner that all features are shown in theirtrue dimensions and in their true relationshipwith other features on that part of the object.To do this, you must draw a number ofviews of the object from different angles.Projecting these essential views into a singleplane is known as ORTHOGRAPHIC PRO-JECTION. The term orthographic is derivedfrom the word orthos meaning perpendicular orright-angular.Multi-view ProjectionWhen an object is viewed through aplane of projection from a point at infinity,an accurate outline of the visible face of theobject is obtained (fig. 5-3). However, theprojection of one face usually will not providean overall description of the object; otherplanes of projection must be used. Establishingan object’s true height, width, and depth requiresfront, top, and side views, which are calledthe PRINCIPAL PLANES OF PROJECTION.Figure 5-4 shows the three principal (orprimary) planes of projection, known as theVERTICAL, HORIZONTAL, and PROFILEPLANES. The angles formed between thehorizontal and the vertical planes are calledthe FIRST, SECOND, THIRD, and FOURTHANGLES, as indicated in the figure. Cur-rently, however, for technical reasons, onlythe use of first- and third-angle projection ispractical.FIRST-ANGLE PROJECTION.— A fineexample of first-angle projection using a cube is5-3
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