Machine Drawing

Orthographic Projections 47

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d:\N-Design\Des3-1.pm5 Seventh Print

No line Line

Fig. 3.9 Representation of tangential

curved surfaces

3.6.1 Hidden Lines

While obtaining the projection of an object on to any

principal plane of projection, certain features of the

object may not be visible. The invisible or hidden

features are represented by short dashes of medium

thickness. Figure 3.8 shows the application of hidden

lines in the projection of an object.

3.6.2 Curved Surfaces

Certain objects contain curved surfaces,

tangential to other curved surfaces. The

difficulty in representing the surfaces can be

overcome if the following rule is observed.

Wherever a tangential line drawn to the curved

surface becomes a projector, a line should be

drawn in the adjacent view. Figure 3.9 shows

the representation of certain curved surfaces,

tangential to other curved surfaces.

Certain objects manufactured by casting

technique, frequently contain corners filleted

and the edges rounded. When the radius of a rounded corner is greater than 3 mm and the

angle between the surfaces is more than 90°, no line is shown in the adjacent view. Figure 3.10

shows the application of the above principle.

(a)

Fillet

Corner

(b)

Fillet

Corner

Fig. 3.10 Representation of corners and fillets

If true projection is followed in drawing the view of an object containing fillets and

rounds; it will result in misleading impression. In conventional practice, fillets and rounds are

represented by lines called runouts. The runouts are terminated at the point of tangency

(Fig. 3.11).

3.7 Selection of Views

For describing any object completely through its orthographic projections, it is important to

select a number of views. The number of views required to describe any object will depend

Fig. 3.8 Application of hidden lines