Introduction
Every drawing consists of the views necessary to describe the shape of an object, and must provide its exact length, width, height, size, position of holes and any other details required for the manufacturing of the object. Providing this information on a drawing is called dimensioning.
Types of Dimensions
A drawing usually requires two types of dimensions:
(i) Size or functional dimension
(ii) Location or datum dimension
(i) Size or functional dimension: It gives the size of a piece or component as shown in Fig. 3.1. It is usually represented by the letter S.
(ii) Location or datum dimension: It fixes the relationship of the component parts (holes, slots, etc.) of a piece of structure. It may be from centre to centre, surface to surface or surface to centre, as shown in Fig. 3.1. Generally it is denoted by the letter L.
Elements of Dimensioning
The elements of dimensioning include the extension or projection line, dimension line, leader line, arrowhead and the dimension itself. The various elements of dimensioning are shown in Fig. 3.2.
Execution of Dimensions
The points given below have to be always followed while executing dimensions:
(i) Projection lines should be drawn from the visible features of the object and extend slighly beyond the dimension line, as shown in Fig. 3.3.
(ii) Mutual crossing of dimension and projection lines should be avoided. See Fig. 3.4.
(iii) Projection or extension line must be drawn perpendicular to the part to which it is to be dimensioned. See Fig. 3.5.
(iv) For arrowheads used at the ends of dimension and leader lines, the length may be taken as three times the width/depth as shown in Fig. 3.6. The size of the arrowhead should be proportionate to the size of the drawing.
Placing Dimensions
Dimensions may be placed according to either of the following two systems:
(i) Aligned system:In an aligned system, all the dimensions are placed above the dimension line such that they may be read either from the bottom or from the right hand side of the drawing. See Fig. 3.7.
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