Drawing Instruments

DRAWING INSTRUMENTS

Following is the list of commonly used drawing instruments and accessories.

Accuracy and Speed in the preparation of drawings depends on the quality of these instruments and accessories.

1. Drawing Board 

2. Mini Drafter (or) T square 

3. Instrument box 

4. Set Squares (45° and 30-60°) 

5. Protractor

6. Engineer's scale 

7. French curves 

8. Drawing pins (or) clips (or) Adhesive tape 

9. Pencils

10. Eraser

11. Pencil sharpener 

12. Card board sheet (or Bottom sheet) 

13. Drawing sheet 

1. Drawing Board

Drawing boards having flat and smooth surface made up of seasoned softwood are generally used. It consists of 4 to 6 strips of about 18 mm thick. The wooden strips are cleated at the back by two battens by means of screws. One of the shorter sides of the board is provided with ebony edge (also known as working edge) with which a T-square can be used (If minidrafters are used instead of T-square, this ebony edge will not be used). Fig. 2.1. shows a drawing board.

The standard sizes of drawing boards that are commonly used in practice are given below in Table 2.1.

The drawing board is placed on a table with ebony edge at the left side, in a slightly inclined position, inclined towards the draughtsman at a convenient height.

2. Mini Drafter (or) T-square

T-square is made of hard quality wood (or plastic) which consists of a long strip called blade, rigidly fixed by screws at right angles to a shorter strip called head (or stock). The T-squares are used on drawing board in such a way that the stock of the T-square is kept perfectly touching on the ebony edge of the drawing board.

T-squares are mainly used, 

i) To draw horizontal lines and horizontal parallel lines 

ii) To act as a base for drawing inclined and vertical lines using set squares.

Fig. 2.2(a) shows a conventional T-square and Fig. 2.2(b) shows the position of a T-square on a drawing board.

Now a days T-square is replaced by Mini drafter. Mini drafter is a simplified version of the drafting machine which is normally used for professional drafting of engineering drawings. A mini drafter mainly consists of two scales at right angles which can do the function of T-square, Set square, Protractor and Scales. A typical mini drafter is shown in Fig. 2.3(a). One end of the mini drafter has a clamp to be fixed on the drawing board and the other end has an adjusting knob having a protractor mark attached with an L-shaped transparent scale accurately making right angles. In normal position one of the blades of the scale is horizontal and the other vertical. The scale may be set and clamped at any angle by means of the adjusting knob.

Compared to Mini drafter, T-square gives better accuracy in large size drawings, but the speed of drafting is low. For small size drawings and for study purpose the use of mini drafter is normally prefered in practice due to its several advantages and multi functionality of drawing horizontal, vertical and inclined lines, marking angles, parallel lines at any inclination etc.,

Fig. 2.3(b) shows a mini drafter fitted on a drawing board.

3. Instrument box

An instrument box consists of various drafting instruments like compass, divider etc., A standard three bow set instrument box contains the following

1. Large size compass (150 mm long with interchangeable pencil and pen legs) 

2. Large size divider (150 mm long) 

3. Small bow compass (95 mm long) 

4. Small bow divider (95 mm long) 

5. Small ink bow compass 6. Lengthening bar

Large size compass is used to draw large size circles and arcs. It consists of two legs hinged together at its upper end by `Knee Joint. An adjustable needle is fitted on one end of the legs and the other leg is provided with an attachment which can be fitted with pencil lead or an inking device. (Fig. 2.4 a)

Small bow compass is used to draw circles, arcs and circular curves of diameter less than 50 mm. It is usually of the centre adjustment type in which a knurled nut is placed at the centre to adjust the distance between the legs. (Fig. 2.4 c)

Large size dividers consists of two legs pivoted together at their upper end and are equipped with pointed needles at their lower ends. Large size dividers are used to divide curved lines or straight lines into desired number of equal parts. They are also used to transfer a length from drawing sheet to the scale (or) to transfer a length from the scale to the drawing sheet. (Fig.2.4 b)

Small bow divider is usually of centre adjustment type in which a knurled nut is placed at the centre to adjust the distance between the legs. Small bow divider is used as the large divider but for making minute divisions and large number of short distances. (Fig. 2.4 d).

Small ink bow compass is similar to small bow compass but an inking needle attached with one leg instead of pencil lead (Fig. 2.4. e). Small ink bow compass is used to draw small circles in ink, and for tracing of drawing with ink.

Lengthening bar is an attachment to be attached with large size compass to draw very large circles and arcs. Usually a circle or an arc of radius more than 180 mm is drawn with a large size compass attached with lengthening bar to the leg of the compass holding the lead. A lengthening bar is shown in Fig. 2.4 (f).

4. Set Squares

Set squares are made of transparent celluloid or transparent plastic material so that the work underneath the set square can be seen. Two types of set squares are normally used, 45° - 45° angles set square and 30° – 60° angles set square. 45° – 45° set squares are used to measure and draw inclined lines at 45° where as 30° – 60° set squares are used to measure and draw 30° and 60° inclined lines.

Set squares are normally used on the base of a T-square (Fig. 2.5 a) or on a mini-drafter (Fig. 2.5 b), so that the set square can be conveniently moved horizontally. 

Set squares are normally used 

i) To draw perpendicular lines (or) the lines at 30°, 45° and 60° to the horizontal. 

ii) To draw inclined lines at 15°, 75° and 105° by using both 45°-45° and 30°-60° set squares. 

iii) To draw parallel (or) perpendicular line to any given line keeping one side of set square moving on the T-square (or) Mini drafter.

5. Protractor

Protractors are also made of transparent celluloid (or) transparent plastic material which are available in semi-circular (Fig. 2.6) and circular shapes.

Semi circular protractors are labelled from 0° to 180° whereas the circular protractors are labelled from 0° - 360°

Protractors are used to draw and measure angles and to divide circles or sectors into any number of equal parts.

6. Engineer's Scale and Ruler

A set of 8 scales made up of either cardboard or plastic are used to make full size, reduced size and enlarged size drawing conveniently. These scales are designated from M, to Mg. (Scales are explained in detail in Chapter 4)

A steel ruler (ordinary metric scale of 30 cm length) is also used by an engineer, to rule straight lines and to measure the distances in centimetres and millimetres.

7. French Curves

French curves are made of transparent celluloid or transparent plastic material having an edge composed of several different curves, available in a variety of shapes and sizes as shown in Fig. 2.7.

French curves are normally used to draw a perfectly smooth curve through predetermined points.

8. Drawing pins, Clips and Adhesive tape

Drawing pins (Fig. 2.8 a), Drawing clips (Fig. 2.8 b) and Adhesive tapes (Fig. 2.8c) are used for fixing the drawing sheet on the drawing board.

Drawing pins are normally fixed on all the four corners of the drawing sheet, though they offer a firm grip they damage the drawing board surface.

Drawing clips are normally used on left and right corner of the upper edge of drawing sheet with the drawing board. But for drawing sheets of smaller sizes, drawing clips cannot be used.

Both drawing clips and drawing pins may obstruct the free movement of minidrafters and set squares. Hence now a days transparent adhesive tapes are used, which are available in rolls with various thickness. Pieces of adhesive tapes are used on the edges of drawing sheet, both on horizontal and vertical edges.

9. Pencils

Pencils are used for preparing the drawings on the sheets. Pencils are graded according to the relative hardness of their graphite lead. They are graded as HB, H, 2H etc. H stands for hardness and B stands for blackness and softness. HB is the medium grade pencil. A hard pencil produces a thin line while a soft pencil produces a thick line.

The grades H, 2H, 3H etc. denote the degree of hardness in an increasing order. Similarly grades B, 2B, 3B etc., indicate the degree of softness in an increasing order.

H grade - used for making drawings, finishing lines, outlines, visible lines and hidden lines.

2H grade - used for construction lines, dimension lines, section lines and centre lines. 

HB grade - used for drawing borderlines, freehand sketching, title block, arrow heads, object lines, lettering and numbering. 

Wooden drawing pencils of hexagonal cross section are traditional types of pencils (shown in Fig. 2.9a) are used for engineering drawings. They are sharpened with either conical point (or) chisel point. Chisel pointed ends (Fig. 2.9b) are used for drawing straight lines while the conical pointed ends. (Fig. 2.9c) are used for general purpose including writing, dimensioning etc.,

Mechanical clutch pencils (Fig.2.10) of lead size 0.5 mm diameter are mostly used now-a-days because of uniform line width it can offer. However clutch pencils can be used only for drawing lines, not recommended for lettering.

10. Eraser

Eraser (or rubber) is used to remove unnecessary lines, writings and portions on the drawing. A soft natural rubber (or) nylon rubber is generally used as eraser. 

11. Pencil Sharpener

A mechanical sharpener equipped with cutter is generally used to sharpen the pencil. The sharpener removes the wood, leaving the lead exposed so that it may be pointed to any desired shape. A pencil sharpener will sharpen the pencil with conical head. Further if chisel end is prefered a sand paper may be used.

12. Card board sheet

A card board sheet thicker than the drawing sheet is available in any size in the market with various colours. The card board sheet can be used as a bottom sheet for fixing the drawing sheet. Size of card board sheet is slightly lesser than the drawing board and normally cardboard sheet is fixed on drawing board using drawing clip. Drawing sheet is then laid on cardboard sheet and fixed using adhesive tapes. A card board sheet used as a bottom sheet for drawing sheet will ensure the uniform line thickness and fine finishing of the drawing.

Table : 2.2. Standard sizes of drawing sheets as per BIS 

13. Drawing Sheet

Drawing sheets are white papers of good quality, used for the preparation of pencil drawings. One of the surfaces of the drawing sheet is normally smooth. This smooth surface is used for drawing. Bureau of Indian standards (BIS) recommends ‘A’ series of paper sizes A0, A1, A2, A3 and A4. Drawing sheets, sizes of which (untrimmed sizes) are given in Table 2.2.

It is interesting to note that the length of paper to width of paper in all the varieties is in the ratio of √2 : 1 rounded off to the nearest millimetre. The surface area of the drawing sheet designated by A0 is 1m2. Successive paper sizes in the series are obtained by halving the preceding paper size parallel to the shorter side. For example, A1 size drawing sheet is obtained by halving A0 size sheet.

Fig. 2.11. shows the relationship among various sizes.

Selection of the size of the drawing sheet depends on the size of the object to be drawn and the scale used.

In practice A0, A1, A2, A3 and A4 sheets are used. But Drawing sheet of A2, ie. size 420 × 594 mm, known as half imperial sheet is generally used by engineering students. 

Drawing sheets are generally used with their longer sides placed horizontally.