Theory

Whether working on projects related to civil engineering, mechanical designs or electronic schematics, the ability to interpret and apply scales is a foundational skill. Scales ensure that drawings accurately represent real-world dimensions, providing a universal language for engineers to communicate design specifications effectively. It is possible to produce drawings of small items that are the same size as the objects they depict. A drawing of 100 mm in length could represent a pen that is 100 mm long. Full-scale drawings are those that are drawn to the exact size of the things. For such drawings, standard full-size scales are employed.

Scale

A scale can be described as the proportion between the real dimensions of an object and its linear dimensions as shown in a drawing of the same object element. The scales generally used for general engineering drawings are shown in table below-

Scale Reference Table

Some situations might not allow for the preparation of full-size drawings. As a result, they are depicted proportionately bigger or smaller. A decreasing scale (1: 5) is used when drawings are made smaller than the actual size of the items (such as enormous equipment, buildings, bridges, etc.). Small machine parts, mathematical apparatus, timepieces, and other items are portrayed in drawings that are larger than they actually are. It is claimed that these are drawn with an expanding scale (5: 1).

The scales can be expressed in the following three ways:

1. Engineer's Scale: In this instance, the relationship between the object's actual dimension and its dimension on the design is expressed numerically in the manner, for example, 10 mm = 5 m.
2. Graphical scale: Graphical scale: The drawing itself has the scale drawn on it. The engineer's scale may decrease and results may become inaccurate as the drawing ages. With graphical scale, this is not the case since the scale will likewise reduce if the artwork does. Because of this, survey maps frequently employ the graphical scale.
3. Representative fraction: The Representative Fraction (abbreviated R.F.) is the ratio of the object's length as drawn to its actual length on the drawing. R.F.= (Length of the drawing)/(Actual length of object) 3.1. When a 1 cm long line in a drawing represents 1 metre length of the object, the R.F. is equal to 1cm/ 1 m = 1 / (1 x 100 cm)= 1/100 and the scale of the drawing will be 1 : 100 or 1/100 full size. The R.F. of a drawing is greater than unity when it is drawn on an enlarging scale. 3.2. For example, when a 2 mm long edge of an object is shown in drawing by a line 1cm long the R.F. is 1cm/2mm = 10mm/2mm = 5. Such a drawing is said to drawn on scale 5 : 1 or five times full-size.

Types of Scales

The scales used in practice are classified as under:

1. Plain scales
2. Diagonal scales
3. Comparative scales
4. Vernier scales
5. Scale of chords

Plane Scale

A line that has been divided into an appropriate number of equal parts, or units, with the first unit being further divided into smaller parts, makes up a plain scale. A unit and its sub-division, or two units, are represented by plain scales.

In every scale,

1. The zero should be placed at the end of the first main division, i.e. between the unit and its sub-divisions.
2. From the zero mark, the units should be numbered to the right and its sub-divisions to the left.
3. The names of the units and the sub-divisions should be stated clearly below or at the respective ends.
4. The name of the scale (e.g. scale, 1 : 10) or its R.F. should be mentioned below the scale.