Solids of revolution

when some of the plane figures are revolved about one of their sides – solids of revolution is generated some of the solids of revolution are:

1. Cylinder:  when a rectangle is revolved about one of its sides, the other parallel side generates a cylinder.   
2.  Cone: when a right triangle is revolved about one of its sides, the hypotenuse of the right triangle generates a cone.
3. Oblique cylinder: when a parallelogram is revolved about one of its sides, the other parallel side generates a cylinder.
4.  Sphere:  when a semi-circle is revolved about one of its diameter, a sphere is generated..
5. Truncated and frustums of solids – when prisms, pyramids, cylinders are cut by cutting planes, the lower portion of the solids (without their top portions) are called, either truncated or frustum of these solids. Some examples are shown in figure 7.

Figure 7.  Illustrates some examples of truncated / frustrum of solids.

Visibility
When drawing the orthographic views of an object, it will be required to show some of the hidden details as invisible.  To distinguish the invisible portions from the visible ones, the invisible edges of the object are shown on the orthographic views by dashed lines. However, in practice, these lines of dashes conveniently and colloquially, but wrongly called as dotted lines. To identify the invisible portions of the object, a careful imaginative thinking is essential. 

Rules of visibility
When viewing an object, the outline of the object is visible. Hence the outlines of all the views are shown by full lines. All the visible edges will be shown as solid lines as shown in figure 8. Figure shows the frustum of a pentagonal pyramid.

Figure 8. Front view of the object. The visible edges are shown as solid lines and the hidden edges are shown as dashed lines.