Chapter 1: Basic Elements of Drawing

1.1 Introduction

Drawings can be created quickly and precisely using drawing tools. The quality of the tools has a significant impact on how accurate the drawings are. It is simple to achieve desired accuracy using high-quality devices. Therefore, it is crucial to get instruments that are as high-quality as possible.

Below is the list of minimum drawing instruments and other drawing materials which every student must possess:

1. Drawing board

2. T-square

3. Set-squares - 45° and 30°- 60°

4. Scales

5. Protractor

6. French curves

7. Drawing papers

8. Drawing pencils

9. Eraser (Rubber)

10. Drawing pins, clips or adhesive tapes

11. Drafting machine

12. Roll-n-draw

We shall now describe each of the above in details with their uses:      

1). Drawing boards

Drawing boards are scale plastic boards designed specifically for technical drawing. They are used to draw parallel lines easily and precisely, e.g., for three-dimensional projections and much more. They feature a mechanical device with rail-mounted rulers that run horizontally and vertically at an exact 90° angle.

Figure 1.1

Drawing board is made in various sizes. Its selection depends upon the size of the drawing paper to be used. The sizes of drawing boards recommended by the Bureau of Indian Standards (IS:1444-1989) are tabulated in table 1-1. For use in schools and colleges, the last two sizes of the drawing boards are more convenient. Large-size boards are used in drawing offices of engineers and engineering firms.

The drawing board is placed on the table in front of the student, with its working edge on his left side. It is more convenient if the table-top is sloping downwards towards the student. If such a table is not available, the necessary slope can be obtained by placing a suitable block of wood under the distant longer edge of the board.

2). T-square

T-Square is basically a scale attached to a vertical head and as per definition, it is a tool used to draw horizontal lines on the Drawing Sheet, apart from this it is also used to support Set Square which can be used to produce lines in multiple of 15°.

The measuring portion is called Blade and is used to draw horizontal lines on Drawing Sheet. The edge of the scale is called Working Edge. The remaining black portion is called Head/Stock which is used to support the T-Square against the slotted edge of the Drawing Board so that it can facilitate drawing parallel horizonal lines as shown in figure 1.2

Figure 1.2

T-SQUARE is basically available in four different size for different size of sheet namely T0, T1, T2, T3.

3). Set-squares

A triangle-shaped tool that is often made of plastic or metal and is used to draw angles and straight lines. Set squares can also be used to draw perpendicular lines, angles, and complete shapes, such as squares, rectangles, and triangles.

They're particularly helpful for checking to see if a shape has a right angle - a set square will always contain a 90-degree angle, so you can use them to identify these angles in shapes.

Two different types of set squares are present in a geometry box, one with 90-45-45 degree angles and the other with 30-60-90 degree angles.

Figure 1.4

The set-squares are made of wood, tin, celluloid or plastic. Those made of transparent celluloid or plastic are commonly used as they retain their shape and accuracy for a longer time. Two forms of set-squares are in general use. A set-square is triangular in shape with one of the angle as right angle. The 30°-60° set-square of 250 mm length and 45° set-square of 200 mm length are convenient sizes for use in schools and colleges.

Figure1.5

4). Scales

The scales are the ratio of the linear dimension of an element of an object as represented in the original drawing to the actual linear dimension of the same element of the object itself.

The scales in engineering drawing are a set of levels or numbers which are used in a particular system as a measuring or comparing parameter. The engineering scales are made in a variety of graduation to meet the requirements of many different kinds of engineering works. The engineering scale adopted for drawing should have a bearing on the degree of accuracy required of scaled measurements on prepared drawings.

Full scale, Enlarging scale, and Reducing scales in Engineering drawing

Drawing an object at its true size is not always possible. A depiction of a particularly large object, for instance, cannot be plotted at full scale since it would require adjusting the size of the drawing on the drawing page. Objects that are very small cannot be drawn at full size because they would be difficult to read and draw. The following scale types are used in engineering drawings so that things can be accommodated and conveniently plotted and read:

Full scale: When an engineering drawing is prepared to the actual size of the object, the scale used is termed as full-size scale and the drawing is known as full-size drawing. 

A scale with a ratio of 1:1 is known as a full-size scale in engineering drawing.

Example: A 30 mm radius plain disc is represented on the drawing by a circle of 30 mm radius.

Enlarging scale: When a very small object such as components of a wristwatch, is enlarged in some regular proportion to accommodate its drawing. So, when the drawing is prepared larger than the actual size, the scale is said to be an enlarged scale, and the drawing is said to be an enlarged-sized drawing. A scale in which the ratio is larger than 1:1

Reducing scale: When the object is of large size, the actual dimensions of the object have to be reduced to accommodate the drawn object in the drawing sheet. So, when a drawing is prepared smaller than the actual size of the object, the scale used is termed as reducing scale and the drawing is known as a reduced-sized drawing.

A scale in which the ratio is smaller than 1:1

Figure 1.6

Designation of Scales in Engineering Drawing

The complete designation of an engineering scale shall consist of the word "SCALE' followed by the mention of its ratio, as follows:

SCALE 1:1 for full size

SCALE X:1 for enlargement scales

SCALE 1:X for reduction scales

Representative Fraction (RF)

The representative fraction is defined as the ratio of the dimension of an element of an object in the drawing to its actual linear dimension of the same element of the object itself.

The value of R.F. is always expressed in the form of x:y, in which both x and y are lengths converted into the same units. Hence, RF has no decimal and it is unitless.

RF for full-sized scale = 1

RF for reducing scale < 1

RF for enlarging scale > 1

It is also highlighted that RF is the ratio of linear dimensions and not the ratio of areas or volumes. In case, the quantities are given in units of areas, then take the square root to convert them into lengths units. Similarly, if the quantities are given in units of volume, then take the cube root to convert them into lengths unit.

BIS recommended Scales in engineering drawing

The standard values of RF for scales in engineering drawing as recommended by BIS are as follows:

Full Scales - 1:1

Reducing Scales - 1:2, 1:5, 1:10, 1:50, 1:100, 1:200, 1:1000, 1:2000, 1:5000

Enlarging Scales - 50:1, 20:1, 10:1, 5:1, 2:1

The scales in engineering drawing have to be chosen for depends upon various factors like the size of the object, functionality of the drawing.

The requirement for the construction of a scales in engineering drawing

The information required for the construction of plain, diagonal, or a vernier scales are as follows:

RF of the Scale

The maximum length to be measured by the scale

The minimum length to be measured by the scale (i.e., Least count)

The length of the scales in engineering drawing is determined by the formula

Length of the scale = representative fraction x Maximum length to be measured.

Types of scale in engineering drawing

The scales in engineering drawing are classified as the following:

1. Plain scale

2. Diagonal scale

3. Vernier scale

4. Comparative scale

5. Scale off chords

Plain scale

In a plain scale a line divided into a suitable number of equal parts or units, the first of which is subdivided into smaller parts.

The use of plain scale in engineering drawing is to measure up to two consecutive units, i.e., a unit and its immediate subdivision. For example - meter and decimetre, feet and inches, etc.

Diagonal scale

A diagonal scale in engineering drawing is used when the measurements are required in three consecutive units; i.e., a unit and its immediate two subdivisions. A diagonal scale's measurement is more accurate than a plain scale's measurement.

For example, meter, decimetre and...