Miscellaneous Drawing Instruments

1) French Curves

A French curve is a curved ruler used for drawing irregular curves that are neither circles nor circular arcs. It is made of wood, plastic or transparent celluloid. There are different forms and sizes of French curves. French curves are best suited to draw smooth curves/ arcs (which cannot be drawn by a compass) with ease. They are made in various shapes. Some set-squares also have these curves cut in their middle. French curve is free form template make of acrylic and is used to draw a smooth curve passing through a number of points. The outer profile of the French curve is adjusted such the smooth curve passes through more than three points and a curve passing through these lines are drawn. The next part of the curve is then drawn by using the next three points in addition to the last two points of the previous curve. A typical French curve is shown in Fig.1.

Fig.1. French Curve

To draw a smooth curve using French curve first set it by trial against a part of the line to be drawn, then shift it to the next portions. Each new portion should fit at least three points on the curve just drawn. It should be seen that the curve (radius) is increasing or decreasing smoothly and no corner should be formed on the curve.

2) Eraser

An eraser is made of rubber and is used to erase extra pencil work and/or wrongly drawn lines. Soft eraser is the most suitable one for erasing as it leaves a little or no impression on the drawing sheet and it should not spoil the surface of the paper. Frequent use of rubber should be avoided by careful planning.

Fig. 2 Eraser

3) Erasing shield

When, on a drawing, if a part of a line or some lines among many other lines need to be erased or modified, in normal way of erasing will damage the other nearby lines. Erasing shield is a thin metallic sheet having small openings of different sizes and shapes. A suitable opening is aligned to the line to be erased and the line is removed by the eraser.

4) Template

Templates are available for drawing circles, arcs, ellipses, triangles, squares and other polygons. Also, symbols used by various engineering faculties, such as architectural, mechanical, electrical etc. are used in the form of templates. A template is a thin, flat piece of plastic containing various cut-out shapes. It is designed to speed the work of the drafter and to make the finished drawing more accurate. Templates are available for drawing circles, ellipses, plumbing fixtures etc. Templates come in many sizes to fit the scale being used on the drawing.

5) Stencils

A stencil is a thin flat piece of celluloid used to write letters and numerals. This helps the draftsmen to write neatly and uniformly and at a faster rate.

6) Drawing Clips or Drawing Pins and Adhesive Tapes

Both drawing clips and drawing pins serve the same purpose. They are used to fix the drawing sheet firmly in position to the drawing board as one construct the drawing so that it does not shake during preparing drawing. The needle part of the pin is generally made of steel, while the head may be of plated mild steel or brass. Pins of about 15 mm to 20 mm diameter and about 1 mm thick flat heads made of brass are quite convenient, as they do not rust. Pins should be so inserted that the heads sit on the surface of the paper. Clips or adhesive tapes are often used instead of the pins.

Fig.3 Drawing Pins

Fig.4 Drawing Clips

7) Duster or Handkerchief

A Duster or a handkerchief is used for cleaning instruments. It is also used to sweep away the crumps or dusts formed after the use eraser on the drawing sheet. Duster should preferably be of towel cloth of convenient size. Before starting work, all the instruments and materials should be thoroughly cleaned with the duster. The underside of the T-square and the set-squares or the drafting machine which continuously rub against the paper should be frequently cleaned.

8) Drawing Instrument Box

Drawing instruments are generally sold in sets in boxes, but they are also available separately. An instrument box contains the following.

• Large compass
• Large divider
• Bow compasses, bow divider
• Lengthening bar
• Pen point for attachment
• Screwdriver
• Lead case
• Inking pen or liner or ruling pen

9) Roll - N – Draw

It consists of graduated roller, scale of 16 centimeter and protractor. It is ideal for drawing vertical lines, horizontal lines, parallel lines, angles and circles.

Fig. 5 Roll - N - Draw

10) Tracing Paper

It is a thin transparent paper. Figures below it can be seen easily and traced out in pencil ink.

11) Drawing Ink

It is used for making drawings in ink on tracing paper.

12) Clinograph (Adjustable Set Square)

Its two sides are fixed at 90⁰ and the third side can be adjusted at any angle.

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Pencil

The pencils are used for preparing the drawings on the drawing sheet, to draw different lines, shapes, symbols and to write texts in engineering drawing. The accuracy and the appearance of the drawing depend upon the quality of pencil used. Different grades of pencils are available depending upon the hardness of the lid. Pencils of various grades can be easily recognised by the letters marked on the body of the pencil. The pencils are generally graded as H, F, B and HB. H represents hardness; F represents Firm, B represents Softness and HB represents intermediate between hard and soft. The general designation of a pencil is associated with alpha-numeric symbols such as 2H, 3H, HB, B, 2B, 3B, etc. Fig.1 shows various grades of pencils. Drawing pencils are graded as 7B, 6B,…, HB, H, 2H, . . . in the increasing order of their hardness and decreasing order of their blackness. Generally, drawings are made with 2H pencils and finished with H or HB pencils.

Fig. 1: Pencil Grades

Pencils of Different Grades

Pencils are graded according to the hardness or softness of the lead. Selection of proper grade pencil or lead is important for quality drawing. One has to be careful in selecting a lead because very hard lead might penetrate the drawing, on the other hand, soft lead may smear. Quality and type of drawing paper is an important factor in selecting lead. One other importance consideration is the importance of line to be drawn. Inferior lines (like border lines, guide lines, construction lines and any other auxiliary lines needed to be erased later) are drawn using harder pencil. Comparatively softer grade pencil is used for drawing superior items (like object line, texts, symbols etc.).

Table 1 Pencil Usage Guideline for Different Line Types

Sl. No.	Task	Lead
1	Construction Lines	3H, 2H
2	Guide Lines	3H, 2H
3	Lettering	H, F, HB
4	Dimension Lines	2H, H
5	Leader Line	2H, H
6	Hidden Lines	2H, H
7	Cross Hatching Lines	2H, H
8	Centre Lines	2H, H
9	Phantom Lines	2H, H
10	Stitch Lines	2H, H
11	Long Break Lines	2H, H
12	Visible Lines	H, F, HB
13	Cutting Plane Lines	H, F, HB
14	Extension Lines	2H, H
15	Freehand Break Lines	H, F, HB

In drawing, standard pencils (lead encased in wood) and semi-automatic pencils are made use. Pencil leads are made of graphite with kaoline (clay) of varying proportion to get the desired grades. More the kaoline higher the hardness. The hardest pencil is 9H grade and softest pencil is 7B grade. Softer lead pencils are used to produce thicker and darker line work, but they wear out quickly. Medium grade of H, 2H are used for general line work as well as for lettering. Harder grade leads produce lighter and thinner lines. Most construction line work is done with 4H, 5H and 6H pencil leads, producing thin but also sufficiently dark by exerting pressure. Depending upon the individual's touch and the style of writing, the right pencil may be selected. In general H, HB and 2H are used.

General Selection

• HB- For freehand works, sketching.
• H- For outlines, visible lines, finishing, dimensioning, lettering, arrows etc.
• 2H - For drawing construction lines, dimensions lines, section lines, and center lines.
• 3H, 4H- For drawing minute details
• B- For shading

Great care should be taken in mending the pencil and sharpening the lead, as the uniformity in thickness of lines depends largely on this. To prepare the pencil lead for drawing work, the wood around the lead from the end, other than that on which the grade is marked, is removed with a pen-knife, leaving about 10 mm of lead projecting out. The conical point is used in sketch work and for lettering etc. The lead may be sharpened into different forms.

i) Conical point

ii) Chisel edge

iii) Bevel edge

iv) Cylindrical edge

With the chisel edge, long thin lines of uniform thickness can be easily drawn and hence, it is suitable for drawing work. The chisel edge is prepared by rubbing the lead on a sand-paper block, making it flat, first on one side and then on the other by turning the pencil through a half circle. For making the conical end the pencil should be rotated between the thumb and fingers, while rubbing the lead. The pencil lead should occasionally be rubbed on the sand-paper block (while doing the drawing work) to maintain the sharpness of the chisel edge or the pointed end. Instead of wooden pencils, Mechanical clutch pencils with a different lead size (lead dia 0.3, 0.5, 0.7 or 0.9 mm) and grade like H, 2H, HB etc., are also available. They are easy to handle as there is no reduction of holding length pencil leads can be replaced, as per required grade of hardness. They produce lines of uniform width without sharpening.

For any drawing on drawing paper or tracing paper, lines should be black, particularly drawings which are to be reproduced. For this purpose, the pencil chosen must be soft enough to produce jet black lines as well as hard enough not to smudge easily. The point should not crumble under normal working pressure. The pencils should not be hard and cut grooves on the paper while drawing with normal pressure, Pencils H, 2H or 3H depending upon the paper (quality) and weather conditions are selected. In summer the pencil leads become softer due to rise in temperature, so slightly harder pencils can be made use of softer grade pencils are used on smooth surfaces for lettering and arrow head. During rainy season or when humidity is more, the drawing paper expands and wrinkles form, pencil leads become harder. So softer pencils are to be used. For better line work, i.e., dense black lines, prefer paper which is not having too much teeth (roughness).

Pencils used for drawing are always hexagonal in cross sections as they do not roll easily even when they are placed on slope surfaces. Its cross section helps in rotating the pencil, while drawing lines, to give uniform line thickness.

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Measurement Scale and Protractor

Measurement Scale

Scales are used for measurement of lengths and distances on the drawing. Scales are made of wood, steel, ivory, celluloid, plastic or card board. Stainless-steel scales are more durable. Scales may be flat or of triangular cross-section. 15 cm long and 2 cm wide or 30 cm long and 3 cm wide flat scales are in common use. They are usually about 1 mm thick. Scales of greater thickness have their longer edges bevelled. This helps in marking measurements from the scale to the drawing paper accurately. Both the longer edges of the scales are marked with divisions of centimetres, which are sub-divided into millimetres. Least count of such scale is generally 1 mm. The scale is used to transfer the true or relative dimensions of an object to the drawing. It is placed with its edge on the line on which measurements are to be marked and looking from exactly above the required division, the marking is done with a fine pencil point. The scale should never be used as a straight-edge for drawing lines. Parallax error will be nil or least while using thin/tapered edge scales.

A number of kinds of scales are available for various types of engineering design. Scale is a measuring stick, graduated on both its edges with different divisions to represent the corresponding actual distances of ground according to some fixed proportions. Such scales are known as RF scales or representative fraction scales. Such scales facilitate rapid marking off distances on drawing. Scales of various representative fractions (RFs) are available in the market e.g., 1:1, 1:2, 1:5, 1:10, 1:20, 1:25, 1:50, 1:100, 1:200, 1:500, 1:1000, etc. the proportion 1:50 means that 50 units on the ground is represented by 1 unit in the drawing. The card-board scales are available in a set of eight scales. They are designated from M7 to M8 as shown in Table 1.

Table 1 Scale Set Designation

Designation	Scale
M1	1:1
	1:2
M2	1:2.5
	1:5
M3	1:10
	1:20
M4	1:50
	1:100
M5	1:200
	1:500
M6	1:300
	1:600
M7	1:400
	1:800
M8	1:1000
	1:2000

Fig. 1 Scale

Protractor

Protractors are used for constructing and measuring angles. Protractors are generally semi-circular in shape. The base diameter of the semicircle serves as the straight edge. A semi-circular protractor can measure and construct angles of any measure between 0⁰ to 180⁰. Least count of the protractor is generally 1⁰. Protractor is made of wood, tin or celluloid. Protractors of transparent celluloid are in common use. The protractor is used to draw or measure such angles that cannot be drawn with set-squares. A circle can be divided into any number of equal parts by means of the protractor.

Fig. 2 Protractor

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