Engineering Drawing Principles and Applications

Introduction

In engineering drawing, it is often required to make drawings that show the interior details of an object, which are not visible to the observer from outside. The method of showing the interior details by hidden lines makes the view confusing and difficult to understand, as shown in Fig. 4.1. To overcome this difficulty, complicated objects are assumed to be cut by an imaginary plane. The part of the object between the imaginary cutting plane and the observer is assumed to be removed, as shown in Fig. 4.2. The exposed or cut surfaces are identified by section lines or hatching lines. The view obtained is called the sectional view, as shown in Fig. 4.3. Hidden lines and details behind the section are usually omitted from the sectional view, unless they are required for clarity. The sectional view is very important in daily life, as it helps in manufacturing and explaining the construction of complicated machines and their parts.

Cutting Plane or Sectional Plane

The imaginary plane by which an object is assumed to be cut is called the cutting plane or the sectional plane. The cutting plane used in sectioning is indicated by a line in a view adjacent to the sectional view. This line is called the cutting plane line. It is a long chain that is thin, but is thick at the ends. Arrowheads, put at the ends, indicate the direction in which the cut away object is viewed, i.e. the direction of sight as shown in Fig. 4.4. At the end of the cutting plane line, capital letters such as X – X, Y – Y or A – A are often marked to identify the cutting plane line with corresponding section. When the cutting plane line coincides with a centre line, the cutting plane line takes precedence over a centre line. In case of symmetrical objects, the cutting plane line is omitted, unless it is needed for clarity.

Section Lines or Hatching Lines

The lines used to represent the material which has been cut by a cutting plane are called section lines. These are also called hatching lines. Section lines are thin, equally spaced, parallel and inclined lines drawn in the area of the view where a cutting plane cuts the material of the object.