It is important to understand the significance of the position of the object relative to the planes of projection. To get useful information about the object in the orthographic projections, the object may be imagined to be positioned properly because of the following facts :
- Any line on an object will show its true length, only when it is parallel to the plane of projection.
- Any surface of an object will appear in its true shape, only when it is parallel to the plane of projection.
In the light of the above, it is necessary that the object is imagined to be positioned such that its principal surfaces are parallel to the planes of projection.
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.
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. Representation of tangential curved surfaces figure below 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. Representation of corners and fillets figure below shows the application of the above principle.
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.