Engineering Drawing Principles and Applications

Introduction

It is always preferable to make the linear dimensions on a drawing the same size as the corresponding real dimensions of the object drawn. However, the drawing of a very big object, like a diesel engine, should be made considerably smaller than the object, whereas details of small precision instruments, watches, etc., are generally made larger than their real size. Objects large and small have to be drawn in such a way that the drawing can be read and handled conveniently. Thus if the linear dimensions of an object have to be enlarged or reduced, for drawing purposes, we need to use scales which enable us to enlarge (enlarging scale) or reduce (reducing scale) linear dimensions with uniformity. If the actual linear measurements of an object are shown in a drawing, the scale is called a full scale. The term ‘scale’ is defined as the ratio of linear dimension of an element of an object as represented in the original drawing to the real linear dimension of the same element of the object itself. All drawings should be drawn to scale and the scale used has to be stated on the drawing.

The complete designation of a scale shall consist of the word SCALE, followed by the indication of its ratio, as follows

SCALE 1 : 1 For Full Size

SCALE X : 1 For Enlarging Scales, X > 1

SCALE 1 : X For Reducing Scales, X > 1

As discussed above, different scales are used for convenience and specific purposes. The recommended scales for use on technical drawings are specified in the Table 6.1 [SP : 46 (1988)].

Representative Fraction or Scale Factor

The ratio of the drawing to the object is called the representative fraction, abbreviated as RF. In detailed words, representative fraction (RF) or scale factor (SF) is the ratio of length of a line in the drawing to actual length of the object represented.

The dimensions in both numerator and denominator of the fraction must be in the same units.