Lettering

The verbal information, in writing, given in the drawing is known as lettering. Lettering plays an important role as far as logical comprehension of the drawing is concerned, particularly for those parts of the drawing which cannot be shown by lines. A well meaning drawing must be associated with clean dimensioning with good letter quality. Therefore, lettering should be lucid, legible and uniform in appearance and easy-to-write for rapid freehand writing. The art of writing letters such as alphabets and numbers etc. is known as lettering. Lettering forms an important part of drawing and is used to write letters, dimensions, notes and such other necessary information as may be required for complete execution of the drawing of an object.

Certain principles need to be followed for developing good writing skill for lettering. One must have knowledge of the following parameters in order to master the art of good lettering.

• Shape and proportion of each letter
• Order and direction of the stroke
• General composition of letters
• Rules for combining letters into words
• Skill of writing the letters in plain and simple style so that the lettering can be done freehand

Letter Styles

Letter styles are generally classified as Gothic, Roman, Italic and Text. They were all made with speedball pens and are therefore largely single-stroke letters. If the letters are drawn in outline and filled in, they are referred to as “filled- in” letters. The plainest and most legible style is the Gothic from which the single-stroke engineering letters are derived. The term roman refers to any letter having wide down ward strokes and thin connecting strokes. Roman letters include old roman and modern roman, and may be vertical or inclined. Inclined letters are also referred to as italic, regardless of the letter style; text letters are often referred to as old English.

Fig.1 Classification of Letter Style

Depending upon the spacing between words and thickness of strokes, letters may be classified into two types.

1) Extended and Condensed Letters

To meet design or space requirements, letters may be narrower and spaced closer together, in which case they are called “Compressed” or “Condensed” letters. If the letters are wider than normal, they are referred to as “Extended” letters.

2) Light Face and Bold Face Letters

Letters also vary as to the thickness of the stems or strokes. Letters having very thin stems are called Light Face Letters, while those having heavy stems are called Bold Face Letters. 

Single-Stroke Letters

The Bureau of Indian Standards (IS: 9609-1990) recommends single stroke lettering for use in engineering drawing. These are the simplest forms of letters and are usually employed in most of the engineering drawings. The word single stroke should not be misconstrued to mean that the letter should be made in one stroke without lifting the pencil. It actually means that the thickness of the line of letter should be such as it is obtained in one stroke of the pencil. The horizontal lines of letters should be drawn from left to right and vertical or inclined lines from top to bottom.

Single-stroke letters are of two types.

1) Vertical – Stem of vertical letters are perpendicular to the horizontal

2) Inclined - Inclined letters lean to the right, the slope being 75° with the horizontal.

According to the height of letters, they are classified as:

    i) Lettering ‘A’

    ii) Lettering ‘B’

In lettering ‘A’ type, the height of the capital letter is divided into 14 parts, while in lettering ‘B’ type, it is divided into 10 parts. The height of the letters and numerals for engineering drawing can be selected from 2.5, 3.5, 5, 7, 10, 14 and 20 mm according to the size of the drawing. The ratio of height to width varies but in case of most of the letters it is 6:5. The details of the characteristics of the types of lettering are given in Table 1 and Table 2.

Table 1 Characteristics of Type ‘A’ Lettering

Table 2 Characteristics of Type ‘B’ Lettering

Table 3 Different Sizes of Letters for Different Purposes

Fig. 2 Single-Stroke Vertical Capital Letters and Figures with Approximate Proportions

Fig. 3 Single-Stroke Inclined Capital Letters and Figures with Approximate Proportions

Fig. 4 Single-Stroke Vertical Lowercase Letters and Figures with Approximate Proportions

Fig. 5 Single-Stroke Inclined Lowercase Letters and Figures with Approximate Proportions

Single-stroke inclined capital letters and figures are shown in Fig. 3. The lower-case letters are usually used in architectural drawings. Vertical and inclined lower-case alphabets are shown in Fig. 4 and Fig. 5 respectively. The width of the majority of letters is equal to the height. All letters should be uniform in shape, slope, size, shade and spacing. The shape and slope of every letter should be uniform throughout a drawing. For maintaining uniformity in size, thin and light guide-lines may first be drawn and lettering may then be done between them. The shade of every letter must be the same as that of the outlines of drawings, i.e. intensely black. Therefore, H or HB grade of pencil is recommended for this purpose.

The spacing between two letters should not necessarily be equal. The letters should be so spaced that they do not appear too close together or too much apart. Judging by the eye, the background areas between the letters should be kept approximately equal. The distance between the words must be uniform and at least equal to the height of the letters. Lettering should be done as can be read from the front with the main title horizontal, i.e. when the drawing is viewed from the bottom edge. All sub-titles should be placed below but not too close to the respective views. Lettering, except the dimension figures, should be underlined to make them more prominent.

Gothic Letters

Gothic letters are formed by thickening the stems of single-stroke letters. These are mostly used for main titles of ink-drawings. The outlines of the letters are first drawn with the aid of instruments and then filled-in with ink. The thickness of the stem may vary from 1/5th to 1/10th of the height of the letters. Fig.6 shows the alphabets and figures in gothic with thickness equal to 1/7th of the height.

Fig.6 Gothic Lettering 

Read More

Types of Lines

Line is one important aspect of technical drawing. In Engineering Graphics, the details of various objects are drawn by different types of lines. Each line has a definite meaning and sense to convey. Lines are always used to construct meaningful drawings. Various types of lines are used to construct drawing, each line used in some specific sense. Lines are drawn based on some standard conventions mentioned in BIS (SP 46:2003). A line may be curved, straight, continuous or segmented. It may be drawn as thin or thick. Engineering drawing consists of organized combination of different types of lines. Lettering plays an important role as far as logical comprehension of the drawing is concerned, particularly for those parts of the drawing which cannot be shown by lines. A well meaning drawing must be associated with clean dimensioning with good letter quality.

Line Thickness

The thicknesses of lines are varied according to the drawing and are finalized either by ink or by pencil. Thickness of the line should be chosen according to the size and type of the drawing from the following range. (IS:10714-1983) 0.18, 0.25, 0.35, 0.5, 0.7, 1, 1.4 & 2 mm.

Inked Drawings

The line-group is designated according to the thickness of the thickest line. For any particular drawing a line-group is selected according to its size and type. All lines should be sharp and dense so that good prints can be reproduced.

Pencil Drawings

For drawings finalized with pencil, the lines can be divided into two line-groups. It is important to note that in the finished drawing, all lines except construction lines should be dense, clean and uniform. Construction lines should be drawn very thin and faint and should be hardly visible in the finished drawing.

Types of Line

Various types of lines used in general engineering drawing are shown in Table 1 as described by SP 46:2003.

Table 1 Types of Line as per SP 46:2003

1 (a)

1 (b)

1 (c)

Fig. 1 (a) (b) (c) Representation of Different Types of Line

1) Outlines

Lines drawn to represent visible edges and surface boundaries of objects are known as outlines or object lines or principal lines. These are represented by continuous thick lines.

2) Dimension Lines

Continuous thin lines, used for giving dimensions of the drawing, are known as dimension lines. A dimension line is terminated at its outer end with an arrow head touching the outline, extension line or centre line.

3) Extension Lines or Projection Lines

These are continuous thin lines used for dimensioning an object. They extend by about 3 mm beyond the dimension lines.

4) Construction Lines

These are thin continuous lines used for construction of objects.

5) Section Lines or Hatching Lines

These are thin continuous lines used for showing the section evidently. They are uniformly spaced thin lines drawn at an angle of 45 degree to the main outline of the section. The spacing between the lines is generally 1 mm to 2 mm.

6) Leader or Pointer Lines

These are continuous thin lines and are drawn to connect a note with the specific feature in the drawing.

7) Short-Break Lines

These are continuous, thin and wavy freehand lines drawn to show the break of an object for a short length. These are also used to show irregular boundaries.

8) Long-Break Lines

These are thin ruled lines provided with short zigzags at suitable intervals. They are drawn to show long breaks.

9) Hidden or Dotted Lines

These are closely and evenly spaced dashes lines of equal lengths. They are of medium thickness and are used to show the invisible or hidden parts of the object on the drawing.

10) Centre Lines

These are thin, long, chain lines composed of alternatively long and short dashes spaced at an approximate distance of 1 mm. The proportion of long and short dashes is 6:1 to 8:1. The short dashes are about 1.5 mm long. These are used to indicate the axes of cylindrical, conical and spherical objects. These are also used to show the centres of circles and arcs. Centre lines should extend for a short distance beyond the outlines to which they refer. Locus lines, extreme positions of movable parts and pitch circles are also shown by these lines.

11) Cutting-Plane Lines

These are long, thin chain line with thick ends. These are used to show the location of cutting plane.

12) Chain Thick

These lines are used to indicate special treatment on the surface.

13) Chain Thick Double-Dashed

These lines are used to show outlines of adjacent parts, alternative and extreme positions of movable parts, centroidal lines and parts situated in front of the cutting plane.

14) Hatching Lines

Hatching lines are the inclined parallel lines. The minimum space between these lines should be more than twice the thickness of the heaviest line in the drawing. It is recommended that these spacing should never be less than 0.7 mm. For showing the limits of partial or interrupted views and sections continuous thin free hand lines or continuous thin straight lines with zig-zag are used.

Read More

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