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.

Read More

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.

Read More

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

Read More

Drafting Machine / Mini-Drafter

Mini-Drafter is a device used to draw parallel or inclined lines very effectively with ease. It is mounted on the top left corner of the drawing board by means of a clamping mechanism which is an integral part of the device. Mini-drafter is an instrument which combines together all the features of a T-square, set squares, scales and protractor and hence is a unified instrument capable of performing the functions of all these instruments put together. One end of the mini-drafter is clamped by means of a screw, to the distant longer edge of the drawing board. At its other end, an adjustable head having protractor markings is fitted. Two blades of transparent celluloid accurately set at right angles to each other are attached to the head.

The machine has a mechanism which keeps the two blades always parallel to their original position, irrespective of their position on the board. The blades have scales marked on them and are used as straight edges. The blades may be set at any desired angle with the help of the protractor markings. The L-shaped scale which is graduated in millimeters acts as the working edge of the mini-drafter. The L-Shaped scale also has a degree scale for angle measurement. The working edge can be moved to any desired location on the drawing board. In some machines, the blades are removable and hence a variety of scales can be used. Thus, by means of this machine, horizontal, vertical or inclined parallel lines of desired lengths can be drawn anywhere on the sheet with considerable ease and saving of time. Drafting machines are common among the college students and draughtsman.

Fig.1 Mini-Drafter

The drafting in the machine come in different sizes and a pattern called ‘Pantagraph’ type. The protractor head has a spring loaded clutch relieving handle, which rotates and locks at 150 intervals automatically. For setting any angle other than multiples of 150, the clutch spring is released and by rotating the centre knob, the zero line is set to the required angle and the friction clutch knob is tightened. It is capable of rotating 1800, thereby any angle can be set. They can be reversed with the help of dovetail slide fitting. There is a fine adjusting mechanism on the drafting head to set the scale parallel to the edge of the board. The scales also can be adjusted if there is any error in measuring between them. The angle of the drafting board can be adjusted by pedal operating system.

Procedure for Clamping Mini-Drafter

Set the protractor head with reference mark indexing zero degree, then fix the clamp of the mini-drafter at the top left corner either along the top horizontal edge of the board or along the left vertical edge of the board. With the drawing sheet placed underneath the scales of the mini-drafter, fix the drawing sheet to the drawing board with the scales of the mini drafter aligned either with the vertical or the horizontal borderlines of the drawing sheet.

Read More

Compass and Divider

Compass

Compass is used for drawing circles and arcs of required diameter. It consists of two metal legs hinged together at its upper end by means of joint known as knee joint. An adjustable or fixed needle is fitted on to the end of one of the legs whereas the other leg is provided with an attachment which can be fitted with a pencil lid or pencil depending upon the nature of attachment. There are different types of compass available in the market depending upon their sizes, such as

i) Large Size Compass

ii) Small Size Compass

Circles up to 120 mm diameters are drawn by keeping the legs of compass straight. For drawing circles more than 150 mm radius, a lengthening bar is used. It is advisable to keep the needle end about 1mm long compared to that of pencil end so that while drawing circles, when the needle end is pressed it goes inside the drawing sheet by a small distance (approximately 1mm). Curves drawn with the compass should be of the same darkness as that of the straight lines. It is difficult to exert the same amount of pressure on the lead in the compass as on a pencil. It is, therefore, desirable to use slightly softer variety of lead about one grade lower in the compass than the pencil used for drawing straight lines, to maintain uniform darkness in all the lines. As a rule, while drawing concentric circles, small circles should be drawn first before the centre hole gets worn.

Fig. 1 Compass

1) Large-Size Compass with Interchangeable Pencil and Pen Legs

It consists of two legs hinged together at its upper end. A pointed needle is fitted at the lower end of one leg, while a pencil lead is inserted at the end of the other leg. The lower part of the pencil leg is detachable and it can be interchanged with a similar piece containing an inking pen. Both the legs are provided with knee joints. Circles upto about 120 mm diameter can be drawn with the legs of the compass kept straight. For drawing larger circles, both the legs should be bent at the knee joints so that they are perpendicular to the surface of the paper.

To draw a circle, adjust the opening of the legs of the compass to the required radius. Hold the compass with the thumb and the first two fingers of the right hand and place the needle point lightly on the centre, with the help of the left hand. Bring the pencil point down on the paper and swing the compass about the needle-leg with a twist of the thumb and the two fingers, in clockwise direction, until the circle is completed. The compass should be kept slightly inclined in the direction of its rotation. While drawing concentric circles, beginning should be made with the smallest circle.

Fig. 2 Large-Size Compass with Interchangeable Pencil and Pen Legs

2) Lengthening Bar

Circles of more than 150 mm radius are drawn with the aid of the lengthening bar. The lower part of the pencil leg is detached and the lengthening bar is inserted in its place. The detached part is then fitted at the end of the lengthening bar, thus increasing the length of the pencil leg. It is often necessary to guide the pencil leg with the other hand, while drawing large circles.

Fig. 3 Lengthening Bar

3) Small Bow Compass

For drawing small circles and arcs of less than 25 mm radius and particularly, when a large number of small circles of the same diameter are to be drawn, small bow compass is used.

Fig. 4 Small Bow Compass

Divider

Dividers are used to transfer lengths to the drawings either from scales or from the drawing itself. Dividers are similar to the compass and are made in square, flat and round forms. However, unlike compass, a divider is provided with two needles on both the legs. Similar to the compasses, two sizes of dividers are used in technical drawings. One large divider and the other small spring bow divider. They are very convenient for setting-off points at equal distances around a given point or along a given line. The dividers are used for the following purpose.

1. To divide curved or straight lines into desired number of equal parts,
2. To transfer dimensions from one part of the drawing to another part
3. To set-off given distances from the scale to the drawing.

Fig. 5 Divider

1) Large-Size Divider

It is used to transfer dimensions and dividing lines into a number of equal parts. The divider has two legs hinged at the upper end and is provided with steel pins at both the lower ends, but it does not have the knee joints. In most of the instrument boxes, a needle attachment is also provided which can be interchanged with the pencil part of the compass, thus converting it into a divider.

2) Small Bow Divider

The small bow divider is adjusted by a nut and is very convenient for marking minute divisions and large number of short equal distances.

Bow Instruments

Bow pencil and bow pen compass are used for drawing circles of approximately 25 mm radius. Bow divider is used for marking or dividing smaller spaces. There are two types

(i) Integral legs with spring action

(ii) Two legs held with a curved spring on top with handle on it.

Bow instruments may have adjusting wheel and nut. To draw circles, it is better to mark the required distance separately and set the instruments and check. Then only the circles or arcs should be drawn on the drawing. Adjustment should be made with the thumb and middle finger. The instrument is manipulated by twisting the knurled head between the thumb and finger.

Fig. 6 Bow Instruments

Drop Spring Bow Pencil and Pen

Drop spring bow pencil and pen are designed for drawing multiple identical small circles. Example: Rivet holes, Drilled/reamed holes etc. The central pin is made to move freely up and down through the tube attached to the pen or pencil unit. It is used by holding the knurled head of the tube between thumb and middle finger while the index finger is placed on the top of the pin. The pin point is placed on the centre point of the circle to be drawn and pencil or pen is lowered until it touches paper. The instrument is turned clockwise and the circle is drawn.

Fig. 6 Drop Spring Bow Instruments

Read More

Set Squares

The set squares are triangular in shape with one of the angle as right angle and made of wood, tin, celluloid or plastic. Set squares made of transparent celluloid are the most satisfactory ones as the lines underneath them can be seen quite easily. Set squares of different sizes are available in the market. Set squares are used for drawing all straight lines except the horizontal lines which are drawn with T-square or mini drafter. Generally, two types of set squares are in use. They are:

(i) Thirty-Sixty degree (30⁰-60⁰) set square and

(ii) Forty-five degrees (45⁰) set square

The 30⁰-60⁰ set square of 250 mm length has three angles with the measures of 30⁰, 60⁰ and 90⁰ respectively. Similarly, the 45⁰ set square of 200 mm length has three angles with the measures of 45⁰ and 90⁰ respectively. Those made of transparent celluloid or plastic are commonly used as they retain their shape and accuracy for a longer time. Set squares sometimes lose their accuracy due to internal strains. So they should be tested periodically. Sometimes set squares have French curves.

Fig. 1 45⁰ Set Square

Fig. 2 30°-60° Set Square

Uses

i) Set-squares are used for drawing all straight lines except the horizontal lines which are usually drawn with the T-square. Vertical lines can be drawn with the T-square and the set-square.

ii) In combination with the T-square, lines at 30° or 60° angle with vertical or horizontal lines can be drawn with 30°- 60° set-square and 45° angle with 45° set-square. The two set-squares used simultaneously along with the T-square will produce lines making angles of 15°, 75°, 105° etc. as shown in Fig. 3 and 4. In general, set-squares are used to draw angles of 15°, 30°, 45°, 60°, 75°, 90°, i.e, any multiples of 15°.

Fig. 3 Drawing lines at 30°, 60° and 90°

Fig. 4 Drawing lines at 15°, 75° and 105°

iii) Parallel straight lines in any position, not very far apart, as well as lines perpendicular to any line from any given point within or outside it, can also be drawn with the two set-squares.

iv) A circle can be divided in six, eight, twelve and twenty-four equal parts by using set-squares and T-square.

Read More

T- square

 A T-square is made up of hard quality wood such as teak or mahogany, etc. It consists of two parts - the stock and the blade - joined together at right angles to each other by means of screws and pins. T- square is used with its head against the ebony edge of the drawing board to draw horizontal lines, parallel lines and to guide/hold the setsquares, stencils etc. The stock is placed adjoining the working edge of the board and is made to slide on it as and when required. The blade is fitted with an ebony or plastic piece to form working edge of T-square. The blade lies on the surface of the board. Its distant edge which is generally beveled, is used as the working edge and hence, it should be perfectly straight. The nearer edge of the blade is never used. The length of the blade is selected so as to suit the size of the drawing board. Now-a-days T- square is also available of celluloid or plastic with engraved scale.

Fig. 1 T- square

Uses

1) The T-square is used for drawing horizontal lines. The stock of the T-square is held firmly with the left hand against the working edge of the board and the line is drawn from left to right as shown in Fig. 2. The pencil should be held slightly inclined in the direction of the line (i.e. to the right) while the pencil point should be as close as possible to the working edge of the blade. Horizontal parallel fines are drawn by sliding the stock to the desired positions. 

Fig. 2 Drawing using T-square

2) The working edge of the T-square is also used as a base for set-squares to draw vertical, inclined or mutually parallel lines. A pencil must be rotated while drawing lines for uniform wear of lead. The T-square should never be used on edge other than the working edge of the board. It should always be kept on the board even when not in use.

Testing the straightness of the working edge of the T-square

Mark any two points A and B (Fig. 3) spaced wide apart and through them, carefully draw a line with the working edge. Turn the T-square upside down as shown by dashed lines and with the same edge, draw another line passing through the same two points. If the edge is defective the lines will not coincide. The error should be rectified by planning or sand papering the defective edge. 

Fig. 3 Testing the straightness of the working edge of the T-square

T-square should never be used as a hammer or as guide for trimming papers. T-Square is used as a base for drawing various angles with the help of set squares. The standard ‘T’ square are designated as follows with dimensions shown in mm as per IS:1360-1989.

Table 1 Designation of T-square

Sl. No.	Designation of T-square	Blade length (mm)	To be used with Drawing Board
1	T0	1500 ± 10 mm	D0
2	T1	1000 ± 10 mm	D1
3	T2	700 ± 5 mm	D2
4	T3	500 ± 5 mm	D3

Read More