Lecture Notes On Drawing Instruments And Sheet Layout PDF

Chapter DRAWING 1 INSTRUMENTS AND SHEET LAYOUT 1.1 INTRODUCTION Drawing is an art of representing objects or forms on a flat surface or a canvas chiefly by means of lines, using any of a wide varie...

Chapter DRAWING 1 INSTRUMENTS AND SHEET LAYOUT 1.1 INTRODUCTION Drawing is an art of representing objects or forms on a flat surface or a canvas chiefly by means of lines, using any of a wide variety of tools and techniques. It generally involves making marks by moving graph- ite pencils, ink pen, wax colour pencils, crayons, charcoals, pastels, or markers on a plane surface such as paper, canvas, etc. Engineering drawing is a graphical way to convey an unambiguous and accurate description necessary for engineered items. It is made in accordance with the standard conventions for layout, nomenclature, interpretation, appearance, size, etc. The purpose of engineering drawing is to provide exact geometrical configuration for the construction or analysis of machines, structures, or systems. Today, the mechanics of the drawing task has been largely automated and greatly accelerated through a number of computer softwares. This chapter deals with an introduction and use of drawing instruments and accessories commonly required in preparing engineering drawings. 1.2 INTERNATIONAL AND NATIONAL CODES Engineering drawing follows certain codes of practice. International Organisation for Standardisation (ISO) recommended international standards for engineering drawing in 1982. At present, these are adopted by 164 countries out of 205 total countries in the world. The standards published by ISO are designated as ISO XXXX:YEAR, where XXXX represents a unique number allocated to the standard and the YEAR represents the year of publication. If a standard has been published before and is updated, the number remains the same as the previous number but the YEAR changes to the new year of publication. Each country has its own standard organisation. For example, in the United Kingdom (UK), it is the British Standards Institution (BSI), in the United States of America (USA), it is the American National Standards Institute (ANSI) and in Germany, it is the Deutsches Institut fur Normung (DIN). In India, the Bureau of Indian Standards (BIS) is engaged in the preparation and implementation of standards, operation of certification schemes both for products and systems, organisation and management of testing laboratories, creating consumer awareness and maintaining close liaison with international standards bodies. The standards published by BIS, irrespective of developed or adopted from ISO, are designated as IS YYYY:YEAR, where YYYY represents another unique number allocated to the standard and the YEAR represents the year of publication. In addition to this, the BIS also publishes some special bulletin which contains a copy of Indian Standards defining special area of interest.  1.2 Engineering Drawing SP 46:2003 Engineering Drawing Practices for School and Colleges is such a special bulletin of Bureau of Indian Standards, which provides standard codes to be used for engineering drawing practice by all the students and practicing engineers. 1.3 DRAWING INSTRUMENTS To be proficient in engineering drawing, it is essential to be familiar with the drawing instruments and the techniques of using them. Great care must be taken for the proper choice of drawing instruments to get the desired accuracy with ease. Following is a list of common drawing instruments and accessories: 1. Drawing board 9. French curves 2. Mini drafter 10. Set squares 3. Drawing sheet 11. Eraser or rubber 4. Drawing Pencil 12. Sheet fasteners 5. Compass (pivot joint type and spring bow type) 13. Template 6. Divider (pivot joint type and spring bow type) 14. Pencil cutter 7. Protractor 15. Sand paper pad 8. Ruler (scale) 16. Brush or towel cloth 1.4 DRAWING BOARD Figure 1.1(a) shows a conventional drawing board. It is made of softwood which provides a flat surface. Engineers and draftsmen use the drawing board for making and modifying drawings on paper with pencil or ink. The working surface must be smooth and free from cracks, bumps and holes so that pencils can easily draw lines. Usually, the surface is covered with a thin vinyl sheet to prevent damage while using the compasses and the dividers. The standard sizes of drawing boards prepared according to recommendations of Bureau of Indian Standards are given in Table 1.1. Drawing boards designated by D00 and D0 are used for drawing offices whereas D1 and D2 are used by engineering students. (a) (b) Fig. 1.1 Drawing board (a) Conventional (b) Modern Table 1.1 Designation and size of Drawing Board (All dimensions in are millimetres) Designation Length × Width Tolerance on Thickness Tolerance on Recommended for use Length/Width Thickness with sheet sizes D00 1525 × 1220 ±5 22 ±1 A0 D0 1270 × 920 ±5 22 ±1 A0, A1 D1 920 × 650 ±5 22 ±1 A1, A2 D2 650 × 470 ±5 22 ±1 A2, A3 D3 500 × 350 ±5 22 ±1 A3, A4  Drawing Instruments and Sheet Layout 1.3 Ancient drawing boards were provided with ebony strips on their left edge to guide the T-squares. As the T-squares are outdated, the modern boards do not require such strips. Presently drawing boards are sup- ported on steel frame as shown in Fig. 1.1(b). The steel frame provides mechanical linkages which help in controlling the height and the inclination of the drawing board to suit comfortable working in standing position. A tall drawing stool prepared according to IS 4209:1989 is generally used for sitting purpose. 1.5 MINI DRAFTER A mini drafter is used to draw horizontal, vertical or inclined parallel lines of desired lengths anywhere on the drawing sheet with considerable ease. It is basically an arm-type drafting machine which combines the functions of a T-square, set-square, protractor and scale. The size of the mini drafter is specified by the length of arms, usually 43 cm long. Figure 1.2(a) shows a mini drafter used by the students of school and colleges. It consists of linear and circular scales, an adjusting knob, steel bars, a bar plate and a clamping knob. The linear scale is in the form of a pair of blades fixed at right angles and graduated in millimetres. In normal position, one of the blades of the scale is horizontal and the other is vertical. This can be set and clamped at any angle with the help of the circular scale and the adjusting knob. The bigger version of the mini drafter is called a drafting machine. It is permanently fixed on a large drawing board and is used in industries. 1.5.1 Clamping the Mini Drafter The following procedure should be adopted for clamping the mini-drafter on a drawing board: 1. Set the circular scale of the drafter at zero degree and tighten the adjusting knob. 2. Take the clamping knob towards the top edge of the board and align the linear scales with the verti- cal and the horizontal boundary lines (or edges in case boundary lines are not drawn) of the drawing sheet. 3. Firmly grip the scales and tighten the clamping knob of the mini drafter, as shown in Fig. 1.2(b). The mini-drafter mechanism will keep the scale always parallel to the original position wherever it may slide over the board. Clamping knob Steel bars Bar plate Adjusting knob Linear scale Circular scale (a) (b) Fig. 1.2 (a) Mini-drafter (b) Clamping mini-drafter  1.4 Engineering Drawing 1.5.2 Rolling Ruler Figure 1.3 shows a rolling ruler. It consists of a rolling cylinder which enables it to roll over the drawing sheet. This facilitates to draw parallel straight lines. Sometimes a small protractor is also attached in the middle portion of Fig. 1.3 Rolling ruler it. The rolling ruler must be handled and rolled carefully otherwise it may lead to error. However, a mini drafter should be preferred as it serves the desired purpose with ease and accuracy. 1.5.3 T-Square Figure 1.4 shows a T-square. Its name comes from the gen- eral shape of the instrument where the head is supported on the edge of the drawing board and the long transparent Fig. 1.4 T-square plastic scale slides on the drawing sheet. This scale is used to draw parallel horizontal lines. The T-square serves as a guide for the set-squares for drawing parallel lines at commonly used angles 30º, 45º and 60º. Some T-squares are designed with adjustable heads to allow angular adjustments of the blade. A mini-drafter is used as an alternative of a T-square. 1.6 DRAWING SHEET A drawing sheet comprises a thick paper onto which the Table 1.2 Paper sizes for ISO-A series drawing is made. It is available in standard thickness Series Paper size (mm × mm) and size. The thickness is specified by weight in grams per square metre (gsm). A sheet with 150 to 250 gsm is A0 841 × 1189 suitable for drawing. Size of the sheet depends upon the A1 594 × 841 size of drawing. The Bureau of Indian Standards in its A2 420 × 594 bulletin is 10711:2001 recommends “ISO-A series” of A3 297 × 420 paper size for the drawing sheet as given in Table 1.2. A4 210 × 297 This series of paper always has a length-to-width ratio A5 148 × 210 A0 has an area of 1 square metre. Successive sizes in A6 105 × 148 the series are designated as A1, A2, A3, etc., which are A7 74 × 105 obtained by halving the preceding size. Figure 1.5 shows A8 52 × 74 the relationship among various sizes. In addition to the ISO-A series, there is a less common special elongated size and exceptional elongated size of drawing sheets. These sizes are obtained by extending the shorter sides of the ISO-A series to lengths that are multiple of the shorter side of the chosen basic format. Such elongated size sheets are used when greater length is required. Coloured sheet with thickness equivalent to drawing sheet is commonly called card sheet. It is spread on the drawing board before fixing the drawing sheet. The card sheet helps to prevent the drawing sheet from getting impressions of the flaws, holes or knots that may be present on the surface of the drawing board.  Drawing Instruments and Sheet Layout 1.5 74 148 297 594 1189 52 105 210 841 420 Fig. 1.5 Drawing sheet sizes of ISO-A series (in mm) 1.6.1 Selection of Sheets The drawing sheet should be white enough to produce better impression of the drawing than any dull white or light yellowish type. The card sheet of black colour is preferred as it gives less strain on eyes while working. Students generally use A1 size card sheet and A2 size drawing sheet. 1.6.2 Fixing the Sheets First, spread the card sheet on the drawing board and fix all its four corners with the help of drawing clips. The edges of the card sheet should be parallel to the edges of the drawing board. Now spread the drawing sheet over the card sheet aligning the edges and fix with adhesive tape. It is suggested to place the drawing sheet slightly towards the lower right corner of the card sheet to enable the drafter easily move over the whole working area of the drawing sheet. 1.6.3 Keeping the Drawing Sheet Valuable drawings need satisfactory handling and stor- age facilities in order to preserve them in good condition. Figure 1.6 shows a sheet holder which can be used to store sheets (both card and drawing sheets) and carry during travel. It is a cylinder shaped box made up of plastic. The inner diameter is 8 cm and the length can be varied from Fig. 1.6 Sheet holder 70 cm to 130 cm.  1.6 Engineering Drawing 1.7 DRAWING PENCIL (a) In engineering drawing, a pencil is used to create marks on the drawing sheet via physical abrasion. It contains a graphite lead with either wooden or mechanical type pro- (b) tective casing. A good quality pencil draws line of uniform shade and thickness. A wooden pencil with hexagonal Fig. 1.7 Pencil (a) Wooden (b) Mechanical cross-sectional shape is shown in Fig. 1.7(a). A mechanical pencil shown in Fig. 1.7(b) is basically a lead holder that requires a piece of lead to be manually inserted. It contains a mechanical system, either propeller or clutch type, to push lead through a hole at the end. Such a pencil is easier to use and always guarantees a sharp point. 1.7.1 Grading of Pencils Pencils are graded according to the proportion of graphite to clay mixture in the pencil lead. A set of pencils ranges from hardest to softest as follows: 9H 8H 7H 6H 5H 4H 3H 2H H F HB B 2B 3B 4B 5B 6B 7B Medium A pencil that is considered the medium grade is designated by the letter HB. The grade becomes harder shown by the value of the figure preceding the letter H, viz. 2H, 3H, 4H, etc. Similarly, the grade becomes softer shown by the figure preceding the letter B, viz. 2B, 3B, 4B, etc. A hard pencil produces thin, grey line while a soft pencil produces thick line. 1.7.2 Selection of Pencil Engineers prefer harder pencils which allow for a greater control in the shape of the lead and line intensity. Usually, three line qualities are needed in engineering drawing. Thick black lines are used to represent visible and outlines, thin grey lines are used for construction work and medium thick lines are used for dimensioning. Pencil manufacturers have not established uniformity in grades. A pencil with grade H may vary in hardness from one brand to other brand. Moreover, amount of pressure exerted on the pencil also varies with user. Hence, with experience and preference one should select the trade name and grade of pencil that suits the needs. Humidity affects the graphite core of lead pencils. On dry days, the pencil leaves more dust or residue than on days of high humidity. On damp days, pencil lines appear more black or dense. When continuing the drawing on a day of high humidity, use a pencil with one grade harder to produce drawing quality similar to that on a dry day. Different line qualities may also be obtained by varying the amount of pressure exerted on the pencil, but this should not be attempted without experience. 1.7.3 Working End of Pencil The working end of a pencil may have a number of different shapes, namely; conical, chiselled or bevelled as shown in Fig. 1.8. These ends are carefully prepared by blade-type Fig. 1.8 Working ends of pencil (a) Conical pencil cutters and sand papers. Conical point is used for (b) Chiseled (c) Beveled  Drawing Instruments and Sheet Layout 1.7 general purpose including writing, dimensioning and making arrowheads. Chisel edge is suitable for draw- ing straight lines while bevelled is preferred for drawing circles and arcs. Always maintain uniform sharpness of the pencil lead to produce a uniform thick line. A pencil with too sharp point breaks easily and a too dull point produces fuzzy lines. Hold the pencil comfortably and naturally. Keep the pencil aligned with the drafting instrument and tilt at an angle of approximately 45° in the direction of pulling. Lines may increase or decrease in thickness when direction of the stroke is changed. Maintain even pressure to produce the line of uniform thickness. Hold the compas 1.8 COMPASS here during use Self-centreing A compass is used to draw circles, arcs device and curves. In engineering drawing, Set screw use of a pivot joint compass and a spring bow compass are recommended. Knee joints 1.8.1 Pivot Joint Compass Needle A pivot joint compass shown in Fig. (b) 1.9(a) is used to draw circles, arcs and Extension arm circular curves of diameter greater than 20 mm. It consists of two legs pivoted together at its upper end which pro- vides enough friction to hold the legs of the compass in a set position. One of the legs is equipped with pointed (a) (c) needle at the lower end while the other Fig. 1.9 (a) Pivot joint compass (b) Legs bended at knee joints leg is equipped with a setscrew for (c)Extension arm inserted to draw large circles mounting either a pen or a pencil at- tachment on the compass. Both legs of the compass are provided with a knee joint so that when bigger circles are drawn, the needle point and the pencil lead point may be kept perpendicular to the drawing sheet (see Fig. 1.9(b)). An extension bar can be inserted in the leg equipped with marking leg to increase the radius of the circle (see Fig. 1.9(c)). 1.8.2 Spring Bow Compass A spring bow compass shown in Fig. 1.10 is used to draw circles, arcs and circular curves of diameter less than 50 mm. They are usually of the centre adjustment type in which a knurled nut is placed at the centre to adjust the distance between the legs. Side adjustment type bow-compasses are also available in which a knurled nut is placed at the side to adjust the distance between the legs. Both legs of the compass are provided with a knee joint to keep the needle point and the pencil lead point perpendicular to the drawing sheet. 1.8.3 Working with Compass To draw a circle with a compass, adjust the opening of the legs of the compass to the required radius. Hold the compass and place the needle point lightly on Fig. 1.10 Spring bow the centre. Slightly press the needle point into the drawing sheet and rotate the compass  1.8 Engineering Drawing Sandpaper pad (a) (b) Fig. 1.11 (a) Setting length of needle point (b) Preparing pencil lead marking leg around it. Always rotate the compass clockwise. While drawing circles and arcs with compass, consider the following: 1. As the needle is required to be inserted slightly inside the paper, it is kept slightly longer than the lead point (see Fig. 1.11(a)). 2. The lead of the compass should be sharpened with a single elliptical face (see Fig. 1.11(b)). 3. An even pressure should be applied during rotating the compass to have uniform thickness of the line. 4. It is important that the circles and arcs produced with the compass are of the same quality as corresponding pencil lines. Since much pressure on the compass cannot be exerted as with pencils, a lead of the compass that is one grade softer than the lead of the pencil should be used for corresponding line work. 5. When many circles are drawn using the same centre, the needle of the compass may tend to bore an oversized hole in the drawing sheet. To prevent these holes, a device called a horn centre or centre disk may be is placed over the centre point. 1.9 DIVIDER Fig. 1.12 Pivot A divider is used to divide lines or curves into a number of equal parts (using trail joint method), to transfer measurement from one part of the drawing to another part and divider to step-off a series of equal distances on the drawing. In engineering drawing, use of a pivot joint divider and a spring bow divider are recommended. 1.9.1 Pivot Joint Divider A pivot joint divider shown in Fig. 1.12 consists of two legs pivoted together at its upper end which provides enough friction to hold the legs of the divider in a set posi- tion. At the lower end both legs are equipped with pointed needle, but it does not have knee joint. In most of the instrument boxes, a needle attachment is provided which has to be mounted on the setscrew of the compass for converting it into the divider. 1.9.2 Spring Bow Divider A spring bow divider shown in Fig. 1.13 is used for marking minute divisions and Fig. 1.13 Spring large number of short distances. They are usually of centre adjustment type in which bow a knurled nut is placed at the centre to adjust the distance between the legs. divider  Drawing Instruments and Sheet Layout 1.9 1.9.3 Working with Divider To divide either a line or a curve into a given number of equal parts by trial, open the dividers to a rough approximation of the first division and step-off the distance lightly, holding the dividers by the handle and pivoting the instrument on alternate sides of the line at each step. If the dividers fall short of the end of the line, hold the back leg in place and advance the forward leg, by guess, one division of the remaining distance. Repeat the procedure until the last step falls at the end of the line. During this process, do not punch holes in the paper, but just barely mark the surface for future reference. To transfer measurements from one part of the drawing to another part, set the dividers to the correct distance then transfer the measurements to the drawing by pricking the drawing surface very lightly with the points of the dividers. To measure off a series of equal distances on the line, set the dividers to the given distance. Then step-off this distance as many times as desired by swinging the dividers from one leg to the other along the line, first swinging clockwise 180 degrees, then anticlockwise 180 degrees, and so on. 1.9.4 Equal Space Divider An equal space divider shown in Fig. 1.14 has usually 11 legs. They use multiple metal strips to form ten equal spaces that may be expanded or compressed to set at desired lengths. They can also be used to divide a line into equal number of parts with ease. However, their use in engineering drawing is limited as it may lead to slight error in the division. 1.10 PROTRACTOR Fig. 1.14 Equal space A protractor is used to draw and measure angles, and to divide circles or sectors divider into desired number of equal parts. They are available in semi-circular and circular shapes. A semi-circular protractor used by engineers, as shown in Fig. 1.15(a), is generally labelled from 0º to 180º in both direc- tions and graduated in increments of 1/2º. The line joining 0º-180º is called the baseline of the protractor and centre of the baseline is called origin of the protractor. Circular protractors as shown in Fig. 1.15(b) may be labelled from 0º to 360º (both clockwise and counter clockwise), or they may be labelled from 0º to 90º in four quadrants. (a) (b) (a) (b) Fig. 1.15 Protractor (a) Semi-circular (b) Circular  1.10 Engineering Drawing 1.10.1 Use of the Protractor To measure the given angle, first align the origin of the protractor over the vertex of the angle to be mea- sured. Then align one of the edges of the angle along the baseline of the protractor. The other edge of the angle is read from one of the two scales of the protractor, whichever is more convenient. Similarly to draw an angle, first align the origin of the protractor over the origin of the given line and align the line along the baseline of the protractor. Mark the point at required angle and join it with the origin of the line. 1.11 RULER (SCALE) A ruler is used to measure distances and to draw straight lines in centimetres and millimetres. A flat ruler with bevel edges is shown in Fig. 1.16(a). It is available in 15 cm or 30 cm length. One edge is calibrated in millimetres while the other is in half millimetres. Another variant of metric ruler has triangular cross section as shown in Fig. 1.16(b). It has three sides providing six scales, with each side showing two scales. They are easy to pick up. A clip is attached to identify the scale in use. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 CENTIMETRE INCHES 12 11 10 9 8 7 6 5 4 3 2 1 0 (a) Clip 10 1 2 11 12 0 60 58 56 54 52 50 48 46 4 2 0 80 (b) Fig. 1.16 Ruler (a) Flat with bevel edge (b) Triangular 1.11.1 Working with a Ruler Keep the edge of the ruler on the line on which the measurement is to be marked, looking from exactly above the required division. Mark the desired dimension with a fine pencil point. 1.11.2 Engineer’s Scale The word ‘scale’ usually employs for an instrument used for drawing or measuring the length of a straight line. It is also used to represent the proportion in which the drawing is made with respect to the object. It is used to make full size, reduced size or enlarged size drawing conveniently depending upon the size of the object and that of the drawing sheet. Usually, the engineers scale is made up of cardboard and as recom- mended by Bureau of Indian Standards are available in a set of eight scales. These are designated from M1 to M8 as shown in Table 1.3.  Drawing Instruments and Sheet Layout 1.11 Table 1.3 Designation and description of Engineer’s scale Designation Description Scales Designation Description Scales M1 Full size 1:1 M5 5 mm to a metre 1:200 50 cm to a metre 1:2 2 mm to a metre 1:500 M2 40 cm to a metre 1:2.5 M6 3.3 mm to a metre 1:300 20 cm to a metre 1:5 1.66 mm to a metre 1:600 M3 10 cm to a metre 1:10 M7 2.5 mm to a metre 1:400 5 cm to a metre 1:20 1.25 mm to a metre 1:800 M4 2 cm to a metre 1:50 M8 1 mm to a metre 1:1000 1 cm to a metre 1:100 0.5 mm to a metre 1:2000 1.12 FRENCH CURVES French curves are used to draw smooth curves of al- most any desired curvature in mechanical drawings. They are made of transparent plastic having an edge composed of several different curves. They are avail- able in a variety of shapes and sizes. A typical set of three French curves available in Indian market are illustrated in Fig. 1.17. Fig. 1.17 French curves 1.12.1 Use of the French Curves French curves are used to draw a perfectly smooth curve through predetermined points in short steps. First plot the points by a light pencil to connect the points freehand resulting into a smooth flowing curve. Se- lect a suitable French curve and match a part of its edge with the freehand curve already drawn. Move the dark pencil along the part of the curve matching with the French curve edge. Now move the French curve forward to match the next segment of the freehand curve and darken it in the same manner. This will result into a series of plotted points. Figure 1.18 shows how a smooth line is drawn through a series of plotted points. The French curve in view A matches points 1, 2, 3, and 4. Draw a line from 1 to 3 only (not to 4). At B, the curve matches points 3 to beyond 6. Draw a line from 3 to 6. At C, it matches a point short of 6 to beyond 7. Draw a line from 6 to 7. At D, it matches a point short of 7 to beyond 9. Draw a line from 7 to 9. At E, it matches a point short of 9 to beyond 11. Draw a line from 9 to 11. While plotting the desired curve, consider the following: Fig. 1.18 Use of French curve in drawing predetermined curve  1.12 Engineering Drawing 1. French curve should be so placed that it intersects at least two points of the curve. If the curve is sharp enough, you may consider some more points on the curve. 2. Avoid abrupt changes in curvature by placing the short radius of the French curve toward the short radius portion of the curve. 3. Avoid working on the underside of the French curve. You may need to change your position around the drawing board, when necessary, so that you can work on the side of the French curve that is away from you. 1.12.2 Flexible Cord Recently a flexible cord shown in Fig. 1.19 is also used in place of French curves for drawing smooth curves with relatively great ease. It consists of a lead bar embedded in rubber covering. The flexibility of the material allows it to bend to any contour. Fig. 1.19 Flexible cord 1.13 SET SQUARES 23 22 A pair of right angled triangle is called set squares. A set 21 20 square has either 45º-45º angle or 30º–60º angle. The 45º 19 18 set square shown in Fig. 1.20(a) is a right-angled triangle 17 30° 16 16 15 15 in which acute angles measure 45º. The 30º–60º set-square 14 14 13 13 45° shown in Fig. 1.20(b) is a right-angled triangle in which 12 12 11 11 acute angles measure 30º and 60º. Set squares are usually 10 10 9 9 made of transparent plastic to see the work underneath. 8 8 7 7 6 6 They are used to draw lines inclined at 30º, 45º and 60º 5 5 4 4 with the horizontal. By using two set squares, lines inclined ° 3 3 45° 60 2 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 4 5 6 7 8 9 10 11 12 13 at 15º and 75º can also be drawn. 1 1 (a) (b) Fig. 1.20 Seta square (a) 45° (b) 30°–60° 1.14 ERASER An eraser, also known as a rubber, is used for removing pencil markings. An eraser that is popular is the art gum eraser, made of soft pliable gum. It is especially suited to removing large areas, and does not dam- age the paper. It leaves much residue which should be whisked away with a draftsman’s brush or cloth. A kneaded eraser is usually made of a grey or light blue material that resembles putty or gum. It erases by absorbing graphite particles and can be used for precision erasing. Generally, it leaves no residue. If this eraser becomes overly warm, the substance may break down leaving a stain on the drawing surface. A soft vinyl eraser has a plastic-like texture and is commonly white in colour. When large areas or dark marks are erased, the eraser causes smearing. Therefore, it is generally used to erase light marks and for precision erasing. Engineers favour this type of eraser for work on technical drawings due to their gentleness on paper. A harder eraser is designed for erasing lines in ink. Figure 1.21 shows an electric eraser with refill box. It has a knob in a short thin rod attached to a motor. The eraser knob turns at a uniform speed achieving a smooth erasure with a minimum of paper trauma. Electric erasers work quickly and completely. Holding the electric eraser steady in one spot Fig. 1.21 Electric eraser with refill may easily wear a hole or damage the surface of the material being erased.  Drawing Instruments and Sheet Layout 1.13 When there are many lines close together and only one of them is to be removed or changed, the desired lines may be protected by an erasing shield shown in Fig. 1.22. 1.15 SHEET FASTENERS Sheet fasteners are used to fix the card sheet and the draw- ing sheets on the drawing board. Figures 1.23(a-c) show drawing pins, drawing clips and adhesive tapes that can be used as sheet fasteners. 1. Drawing pins or thumb tacks They are easy to use and Fig. 1.22 Eraser shield remove. They offer a firm grip on the drawing sheet. Their use should be avoided as it damages the drawing board surface. Moreover the heads of the pin may obstruct the free movement of the mini-drafter. 2. Drawing clips They are made of steel or plastics and have a spring action. They can be used for fixing the card sheet and drawing sheets at all the four corners when their sizes are compatible with the drawing board. In case the drawing sheet is much smaller than the drawing board, it is not possible to use such clips. 3. Adhesive tape Generally, adhesive tapes having width of 10 mm to 15 mm are used to fix the drawing sheet on the card sheet. A length of around 40 mm is cut and fixed across the corners of the drawing sheet. Lighter coating of adhesive helps in removing the tape without tearing or marring the drawing sheet. (a) (b) (c) Fig. 1.23 Sheet fasteners (a) Drawing pins (b) Drawing clip (c) Adhesive tape 1.16 TEMPLATES Drawing templates or stencils are time saving devices that are used for drawing various shapes and standard symbols. They are especially useful when shapes and symbols repeatedly appear on the drawing. They are available in a wide variety of shapes including lettering, circles, ellipses, isometric circles, polygons and arrowheads. Figures 1.24(a) and (b) show a few of them that can be used to draw circles, polygons, arrow- heads, isometric circles and lettering. A template should be hold firmly while using to keep it from slipping out of position. They should be used only when accuracy can be sacrificed for speed. (a) (b) Fig. 1.24 Drawing templates (a) General purpose (b) Lettering  1.14 Engineering Drawing 1.17 PENCIL CUTTERS A pencil cutter or sharpener is used to prepare the working end of a pencil. A pencil with softer lead requires sharpening more often than with hard lead. Figure 1.25(a) shows a conventional sharpener which can produce only short length conical point. Figures 1.25(b) and (c) show blade-type cutters and mechanical sharpener. They are suitable for removing the wood from pencil. Thereafter, the desired working end can be prepared by rubbing over the sandpaper pad. (a) (a) (b) Fig. 1.26 (a) Sandpaper Pad (b) Conical point is produced (b) (c) Fig. 1.25 Sharpeners (a) Conventional (b) Blade cutter (c) Mechanical Fig. 1.27 Brush 1.18 SAND PAPER PAD A sand paper pad or block is used to sharpen the pencil lead. It should be kept within reach of the user as it is frequently required to prepare the pencil edge. Figure 1.26(a) shows a sand paper pad. Figure 1.26(b) shows a conical or needlepoint being produced by rubbing pencil lead on the sandpaper pad. 1.19 BRUSH OR TOWEL Figure 1.27 shows a brush. The brush, duster or towel cloth is used to keep the drawing surface clean by removing crump (formed after the use of eraser), graphite particles or accumulated dirt before they spread over the drawing sheet. Crump and graphite particles should not be brushed off with hands as they may spoil the drawing instead of cleaning it. 1.20 GENERAL PREPARATION FOR DRAWING Arrange the drawing board and stool so that work could be done comfortably without fatigue or eye strain. The working area should be well lighted. Natural light is the best, if available and ample. The drawing  Drawing Instruments and Sheet Layout 1.15 board should be arranged such that the light may come from the front-left (from the front right in case of a left-handed person). This minimises shadows cast by drawing instruments and hands. Every possible care must be taken to eliminate eye strain. Clean all the drawing instruments and accessories so that their surface may not spoil the sheet. Arrange them in a systematic manner, which is essential for saving time. Place the drawing instruments and reference publications on a small worktable adjacent to the drawing board. Clamp the mini-drafter on the drawing board and fasten the drawing sheet such that the mini-drafter can slide over the entire working area of the sheet. Switch to a harder pencil lead to draw fine or precise details. 1.21 PLANNING AND LAYOUT OF SHEET A proper planning and layout of drawing sheet facilitates the easy reading of drawings and interchange of information. A standard arrangement should ensure that all necessary information for understanding the content of drawing is included and sufficient extra margin is left to facilitate easy filing and binding wher- ever necessary. The Bureau of Indian Standards in its bulletin IS 10711:2001 specifies the size and layout of the standard drawing sheets. It is recommended that standard formats should be followed to improve readability, handling, filing and reproduction. Individual companies may use a slightly different layout for the sake of their own convenience but all necessary information is located at approximately the same place on most engineering drawings. Companies generally use pre-printed title block, borders and frames on drawing sheet to reduce drafting time and cost. 1.22 FRAMES AND BORDERS The drawing sheets of sizes greater than that of the ISO-A series sizes are called untrimmed sheet. The sheet cut from the untrimmed sheet are called trimmed sheet. The frame limits the drawing space. It is recom- mended that the frame must be provided on the drawing sheets of all sizes and should be executed with continuous lines of 0.7 mm width. Table 1.4 provide the sizes of the untrimmed sheet, trimmed sheet and drawing space. Figure 1.28 shows relation between untrimmed sheet and trimmed sheet of A3 size along with other information. Table 1.4 Preferred sizes of untrimmed sheet, trimmed sheets and drawing space Designation Untrimmed sheet Trimmed sheet Drawing space Number of grid (in mm) (in mm) (in mm) reference fields A0 880 3 1230 841 3 1189 821 3 1159 16 3 24 A1 625 3 880 594 3 841 574 3 811 12 3 16 A2 450 3 625 420 3 594 400 3 564 8 3 12 A3 330 3 450 297 3 420 277 3 390 638 A4 240 3 330 210 3 297 180 3 277 436 The space between the edges of the trimmed sheet and the frame is called a border. The width of the border is 20 mm on the left edge and 10 mm on the other edges. The larger border on the left edge helps in filing without damaging the drawing space. The border contains the following items: 1. Trimming marks Trimming marks are used as an aid to trimming the untrimmed sheet. The marks are provided in the borders at the four corners of the sheet. The marks are either in the form of two overlapping  1.16 Engineering Drawing untrimmed trimmed drawing space 10 5 1 2 3 4 5 6 7 8 50 A A Trimming Mark Grid Reference Number 3.5 mm high Centring Mark Grid Reference Letter 3.5 mm high Grid Reference 50 B B Fields Metric Reference Graduation (0.35 mm wide continuous line) Grid Reference Border (0.35 mm wide continuous line) drawing space C C untrimmed Orientation Mark trimmed Drawing Space Centring Mark (0.7 mm wide Grid Reference Field Line D D continuous line) (0.35 mm wide continuous line) 5 5 20 170 10 E E Drawing Frame (0.7 mm wide continuous line) Orientation Mark Title Block 65 F F 5 10 1 2 3 4 5 6 7 8 Fig. 1.28 Frames and borders Frame Frame Trimmed Trimmed Trimming mark Trimming mark Fig. 1.29 Trimming marks filled rectangles having 10 mm × 5 mm size or right angled isosceles triangles having 10 mm long sides as shown in Fig. 1.29. The sheets are trimmed to the outer edges of these marks and therefore remain on the sheet after it has been trimmed. 2. Grid references A grid reference system, also called alpha-numeric referencing, provides an easy reference method to locate a specific area on the drawing for additions, modifications, revisions, etc. For execution, a grid reference border is drawn all around the outside of the frame at a distance of 5 mm. Starting from the centre of each sides, short lines are drawn at every 50 mm to form a reference field of size 5 mm × 50 mm. The corner reference fields may be longer or shorter than 50 mm to account for the remainders resulting from the divisions. Table 1.4 provides the number of reference fields on short × long sides of the standard size drawing sheets.  Drawing Instruments and Sheet Layout 1.17 The letters and numerals of nearly 3.5 mm height are written in vertical characters within the grid refer- ence field. Usually, letters are placed in chronological order from the top to the downwards on both left and right side reference fields (except for the A4 size sheet where they are placed in the right side area only). Letters I and O are not used. The numbers are placed in chronological order from the left to the right side on both top and bottom reference fields (except for the A4 size sheet where they are placed in the top side area only). Figures 1.30(a) and (b) illustrates the grid reference system for A1 and A2 size drawing sheets. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 4 5 6 7 8 9 10 11 12 A A A A B B C C B B D D C C E E F F D D G G E E H H F F J J K K G G L L Title Block H H Title Block M M 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 4 5 6 7 8 9 10 11 12 (a) (b) Fig. 1.30 Planning and layout of drawing sheet for (a) A1 size (b) A2 size 3. Centring mark Four centring marks are provided to facilitate positioning of the drawing sheet for photocopying, scanning, etc. The marks are placed at the centre of each of the four sides. The centring marks are 10 mm long starting from the grid reference border enters in the area of drawing space as shown in Fig. 1.28. The marks should be executed with a continuous line of 0.7 mm width. 4. Orientation mark Two orientation marks are provided to indicate the orientation of the drawing sheet on the drawing board. The marks are in the form of arrows as shown in Fig. 1.28. The marks are placed across the frame, one at a shorter side and one at a longer side, coinciding with the centring marks on those sides. One of the orientation marks always points to the draftsman. 5. Metric reference graduation A metric reference graduation shows usefulness in knowing the scale factor of the drawing which has been scanned or photocopied in a scale different than that of the original. The metric reference graduation starts from the left side frame and extends into the border for nearly 3 mm width. The graduation is 100 mm long, divided into 10 mm intervals and disposed symmetrically about a centring mark as shown in Fig. 1.28. 1.23 TITLE BLOCK A title block provides information for identification, administration and interpretation of the whole drawing. It is placed in the bottom right-hand corner of the drawing frame, where it is readily seen when the prints are folded in the prescribed manner. The size of the title block recommended is 170 mm × 65 mm for all sizes of drawing sheets. Figures 1.31 and 1.32 show the sample title block used by draftsmen in industries and engineering students in colleges respectively. A title block should contain the following information: 1. Name of the legal owner of the drawing (company, firm, organisation or enterprises) 2. Title of the drawing  1.18 Engineering Drawing Fig. 1.31 Title block used by draughtsmen in industries Fig. 1.32 Title block used by students in engineering colleges 3. Drawing sheet number 4. The scale 5. Symbol indicating the angle of projection used 6. The signature or initials of the staff designing, drawing, checking, approving officer and issuing of- ficer, along with dates. 7. Other information, as required Titles of drawings should be as concise as possible, consistent with adequate description. Multiple sheet drawings with the same identification number should be indicated as ‘N of P’, where N is the sheet number and P is the total number of sheet. The scale is the ratio of the linear dimension of an element of an object as represented in the drawing to the real linear dimension of the same element of the object itself. All drawings should be drawn to the scale for which, the selected scale should be large enough to permit easy and clear interpretation of the information depicted. The scale should be noted in the title block. When more than one scale is used, they should be shown close to the views to which they refer, and the title block should read ‘scales as shown’. If a drawing uses predominantly one scale, it should be noted in the title block together with the wording ‘or as shown’. All orthographic drawings are made either according to first angle or third angle projection. These are depicted in the title block by their corresponding symbols. Title block should also contain statement “All dimensions are in millimetres unless otherwise specified”. This means that all the features or dimension on drawing have a relationship or specifications given in the title block unless a specific note or dimensional tolerances is provided at a particular location in the drawing.  Drawing Instruments and Sheet Layout 1.19 1.24 SPACE FOR TEXT The space for text on a drawing sheet should provide all information necessary for the understanding the contents of the drawing. The space for text should be provided at the right-hand frame of the drawing space as shown in Fig. 1.33(a). The width of the space shall be equal to that of the title block, i.e., maximum 170 mm or at least 100 mm. If a figure takes up the whole width of the drawing sheet then the space for the text shall be provided at the bottom edge of the drawing sheet as shown in Fig. 1.33(b). The height of the space for text shall be chosen as required. 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 A A A A Explanation Instruction SPACE FOR TEXT B B B B References Location figures C C C C Revision table D Item references D D D Minimum 100 mm Maximum 170 mm Explanation Instruction References Item references E E E E SPACE FOR TEXT TITLE BLOCK TITLE BLOCK 65 F F F F Revision table 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 (a) (b) Fig. 1.33 Space for text (a) Right-hand edge (b) Bottom edge The space for text should provide the following information: 1. Explanations Here the explanation of special symbols, designation, abbreviations and units of dimen- sions should be given which are needed to read the drawing. 2. Instructions Here the instruction related to material, realisation, surface treatment, assembly placing, number of units and combined dimensions should be given. 3. References Here the reference should be made to supplementary drawings and other documents. 4. Location figures Location figures are used in architectural and building drawings. A location figure may comprise the following: (a) Schematic site plan with area, arrow indicating the north, building, part of building, etc. (b) Schematic plan of building with area, part, etc. (c) Schematic section through building with floor plan direction of view, etc. 5. Revision table Revision tables are used to record all document modifications, alterations or revisions which are made time to time to the drawing. In addition, any other factor which might influence the valid- ity of the drawing shall be located in the revision table. The method of recording may vary in detail, but commonly the necessary information is entered in a table made of thin or thick continuous lines. The word ‘ditto’ or its equivalent abbreviations should not be used. To facilitate extension of revision panel, entries for revision should begin from bottom upwards if the revision panel is a part of the title block as shown in Fig. 1.34 and from top downwards when revision panel is at the top right hand corner on drawing. It may contain the following information:  1.20 Engineering Drawing 5 mm DESIGNATION DETAILS OF REVISION DATE SIGNATURE Fig. 1.34 Revision table (a) Designation The identification of a change on a drawing may be a symbol, number or letter enclosed within a circle, square or triangle. The designation column should show the reference to this identification mark or appropriate grid reference. (b) Detail of revision The detail of revision column should show brief record of the changes in the drawing. (c) Date The date column should show dated initials of the person who carried out the revision. (d) Signature The signature column should show dated initials of the approving authority. (e) Other applicable information To accompany other information necessary for clarity regarding revision in the drawing, more columns can be added. 1.25 ITEM REFERENCES ON DRAWING AND ITEM LISTS If the drawing contains a number of items, or if it is an assembly drawing, a tabulated list of items is at- tached to the bottom right of the drawing frame, just above the title block. The list may be in conjunction with the title block. The item list included in the drawing should have its sequence from bottom to top, with headings of the column immediately underneath as shown in Fig. 1.35. It should be such as to be read in the viewing direction of the drawing. It is recommended that the item list be arranged in columns to allow information to be entered under the following headings: 1 2 4. 1 HEXAGONAL NUT 3. 1 WASHER 2 2. 1 SQUARE HEADED BOLT 5 mm. 1. 2 PLATES M.S. 3 S. NO. QUANTITY DESCRIPTION REFERENCE MATERIAL 4 Fig. 1.35 Item list (bill of materials) 1. Quantity The quantity column should show the total number of that particular item necessary for one complete assembly. 2. Description The description column should show the designation of the item. If the item concerns a standard part such as bolt, nut, stud, etc., its standard designation may be used. 3. Item reference The reference column should show the reference to the relevant item reference number. It is generally composed of Arabic numerals. To distinguish them from other indications the numerals used have either (a) height twice as used for dimensioning and similar indications, and/or (b) encircling. The item reference must assign in a sequential order to each component part shown in assembly and each detailed item on the drawing. The identical parts in an assembly should have the same item reference. 4. Material The material column should show the type and quality of the material to be used. If this is a standard material, its standard designation should be mentioned.  Drawing Instruments and Sheet Layout 1.21 5. Other applicable information To accompany other information necessary for finish products such as stock number, unit mass, state of delivery, etc., more columns can be added. 1.26 FOLDING OF DRAWING SHEETS The drawing sheet after completion of the drawing should be folded properly according to IS 11664:1986 recommended by Bureau of Indian Standards. Figure 1.36(a) shows the method of folding the drawing sheet Fig. 1.36(a) Folding of drawing sheets for filing or binding, all dimensions are in millimetres  1.22 Engineering Drawing intended for filing or binding while Fig. 1.36(b) shows the method of folding the drawing sheet intended to keep individually in filing cabinet. It can be seen that the title block of all the folded prints appears in topmost position. Depending upon the folding method adopted, suitable folding marks are to be introduced in the tracing sheets as a guide. Fig. 1.36(b) Folding of drawing sheets for storing in filing cabinet, all dimensions are in millimetres