DRAWING INSTRUMENTS AND THEIR USES

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Candace Barker
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1 Chapter - 1A DRAWING INSTRUMENTS AND THEIR USES Drawing Instruments are used to prepare neat and accurate Drawings. To a greater extent, the accuracy of the Drawings depend on the quality of instruments used to prepare them. The following is the list of Drawing Instruments and other materials required. a) Drawing Board b) T-square or Drafter (Drafting machine) c) Set Squares d) Protractor e) Drawing Instrument Box f) Drawing Sheet g) Drawing Pencils h) Drawing Pins/Clips a) Drawing Board: Drawing board is made from strips of well seasoned soft wood generally 25 mm thick. It is cleated at the back by two battens to prevent warping. One of the shorter edges of the rectangular board is provided with perfectly straight ebony edge which is used as working edge on which the T-square is moved while making Drawings. Battens Working edge (ebony) Fig. 1A.1 Note: When Minidrafter (Drafting machine) is used to prepare Drawings, the working edge is not used.

2 2 Computer Aided Engineering Drawing Drawing board size: Drawing boards are made in various sizes. The selection of Drawing board depends on the size of drawing paper used. The sizes of Drawing board recommended by Bureau of Indian Standards (B.I.S) is given below Standard size of Drawing boards Designation Size (mm) Designation Size (mm) B 15 B B1 7 B B2 7 5 b) T-square : T-squares are made from hard wood. A T-square consists of two parts namely the stock and the blade joined together at right angles to each other by means of screws and Pins as shown in figure 1A.2. The stock is made to slide along the working edge and the Blade moves on the Drawing board. Stock Working edge Screws Blade Fig. 1A.2 The working edge of T-square is used to draw parallel lines, vertical lines or inclined lines at 3, 6 o to the horizontal using set squares. Drafting machine (or Drafter): In a Drafting machine, the uses and advantages of T-square, set square, scales, protractors are combined. One end of the Drafter is clamped at the left top end of the Drawing board by a screw provided in the drafter. An adjustable head with a Protractor is fitted at the other end of the Drafter. Two blades made of transparent celluloid material are fitted to the adjustable head and are perfectly perpendicular to each other. These blades are used to draw parallel, horizontal, vertical and inclined lines. The blades always move parallel to the edges of the board. Use of Drafting machine helps in reducing the time required to prepare Drawing.

3 Drawing Instruments and their Uses 3 Parallel bar arm Scale Angle Parallel bar arm Scale Potractor Drawing board Drawing sheet Fig. 1A.3 c) Set Squares : Set squares are generally made from Plastic or celluloid material. They are triangular in shape with one corner, a right angle triangle. A pair of set squares (3 6 ) and 45 (45 set square are generally provided with Protractor) facilitate marking of angles as shown in figures 1A.4 and 1A.5. They are used to draw lines at 3, 6 and 45 to the vertical and horizontal. mm mm mm mm Fig. 1A Set Square Fig. 1A Set Square

4 mm mm 4 Computer Aided Engineering Drawing Fig. 1A Set Square with Protractor d) Protractor: Protractors are used to mark or measure angles between and 18. They are semicircular in shape (of diameter mm) and are made of Plastic or celluloid which has more life. Protractors with circular shape capable of marking and measuring to 36 are also available in the market Fig. 1A.7 e) Drawing Instrument Box : It consists of the following a) Large size compasses, b) Large size divider, c) Small size bow pen, bow divider, and d) Lengthening bar f) Drawing sheet: They are available in many varieties and good quality paper with smooth surface should be selected for Drawings which are to be preserved for longer time. Sizes of Drawing Sheets recommended by Bureau of Indian Standards (B.I.S) is given below,

5 Drawing Instruments and their Uses 5 Standard size of Drawing sheet Designation Size (mm) Designation Size (mm) A A A A A A g) Drawing Pencils: The accuracy and appearance of a Drawing depends on the quality of Pencil used to make Drawing. The grade of a Pencil lead is marked on the Pencil. HB denotes medium grade. Increase in hardness is shown by value put in front of H such as 2H, 3H etc., Softer pencils are marked as 2B, 3B, 4B etc. A Pencil marked 3B is softer than 2B and Pencil marked 4B is softer than 3B and so on. Beginning of a Drawing may be made with H or 2H. For lettering and dimensioning, H and HB Pencils are used. h) Drawing Pins and clips: These are used to fix the Drawing sheet on the Drawing board. Compass: Compass is used for drawing circles and arcs of circles. The compass has two legs hinged at one end. One of the legs has a pointed needle fitted at the lower end where as the other end has provision for inserting pencil lead. Circles upto 1mm diameters are drawn by keeping the legs of compass straight. For drawing circles more than 15 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). Refer fig. 1A.8. Fig. 1A.8 Compass

6 6 Computer Aided Engineering Drawing Divider: A Divider has two legs of equal length with two steel needles. The legs are hinged at a common point. A Divider is used to divide curve or straight lines into required number of equal parts. It is used to transfer dimensions from one part of drawing to the other. Dividers are also used to set off given distances from the scale to the drawing. Refer fig. 1A.9. Fig. 1A.9 Divider

7 Chapter - 1B LINE TYPES, DIMENSIONS AND LETTERING Lines: Figure 1B.1 shows various lines used in Engineering Drawing. (i) A Thick line A Thick line (ii) B Medium thickness B Dotted line or Dashed line (iii) C Thin long chains C Centre lines (iv) D Thin line D Dimension line / Extension lines (v) E E Section Plane lines (vi) F F Long Break lines (vii) G G Continuous thin wavy lines Fig. 1B.1 Visible edges and surface boundaries (outlines) of objects are shown using continuous thick lines shown by line A-A. Line B-B shows dotted lines or dashed lines used to show hidden edges of objects. These lines are of medium thickness. Centre lines shown by C-C are thin and long chains used to indicate the axes of solids such as Prisms, cones and Pyramids, lines of symmetry, pitch circles etc. These lines extend for a short distance beyond the outlines. Line D-D shows thin and continuous lines called Dimension lines used to show the Dimension. These lines are terminated at the outer ends by pointed arrow heads touching the out lines. These lines are also used to show projection lines, leader lines, hatching lines etc. Section plane lines or cutting plane lines are thin and long chain lines. At the ends, the lines are marked thick. Refer line E-E. Long break lines are thin lines with short zig-zags. As the name indicates, they are used to show long breaks.

8 8 Computer Aided Engineering Drawing Continuous thin wavy lines are used to show, short break lines, irregular boundary lines etc. Dimensions of arrow head : Arrow heads are used at the ends of the Dimension lines as shown below. The arrow heads are pointed at the ends. The length L of arrow head is usually three times the width b. For clarity, arrow heads are filled freehand. b L L = 3b Fig. 1B.2 Dimensioning: Dimensioning is done to show the size of objects. There are two systems of Dimensioning. They are, i) Aligned system and ii) Unidirectional system. i) Aligned system of dimensioning φ Sq Sq Aligned system of Dimensioning Fig. 1B.3

9 Linetypes, Dimensions and Lettering 9 In this system of Dimensioning, the dimensions are placed above the Dimension lines so that they may be read either from the bottom or from the right hand side of Drawing as shown in figure 1B.3. ii) Unidirectional system of dimensioning As the name indicates, in this system of Dimensioning, all the Dimensions are placed in one direction only so that they may be read from the bottom of the Drawing only. This is shown in figure 1B.4. It is very important to note that the non-horizontal Dimension lines are interrupted in the middle to facilitate insertion of Dimensions. φ Sq Sq Unidirectional system of dimensioning Fig. 1B.4 Principles of Dimensioning: a) As far as possible, the Dimensions should be placed outside the Drawing. b) They should be placed at least 8 mm from the outlines and also in between dimension lines. c) Dimensions should not be shown between dotted lines. d) Dimension indicated in one view usually not repeated in other view/views excepting where it is a must for the purpose of identification, clarity or both.

10 Computer Aided Engineering Drawing e) Dimensions should be shown in one unit only such as mm. The unit need not be shown for every Dimension. It can be shown in a prominent place in the Title block of Drawing sheet as Dimensions in mm f) While Dimensioning, it is necessary to show the minimum necessary dimensions to describe the object. Conventions used to represent materials: While making Drawings involving materials such as steel, concrete, glass, etc., the Engineer or Draughtsman uses conventions as shown in figure below. Steel, Cast Iron, Copper and its alloys, Aluminium and its alloys, etc. Metals Tin, Lead, White-metal, Zinc, etc. Marble, Porcelain, Stoneware, Slate, etc. Glass Glass Packing and Insulating material Asbestos, Felt, Fibre, Synthetic resin products, Paper, Rubber, Cork, Linoleum, Leather, Wax, Insulating and Filling materials, etc. Wood Wood, Plywood, etc. Liquids Oil, Water, Petrol, Kerosene, etc. Concrete A mixture of Cement, Sand and Gravel

(As per New Revised Syllabus of Anna University) Department of Mechanical Engineering. SATHYABAMA UNIVERSITY Jeppiaar Nagar, Chennai

(1*,1((5,1* *5$3+,&6 (As per New Revised Syllabus of Anna University) Dr. S.RAMACHANDRAN, M.E., Ph.D. Professor & Head K. PANDIAN, M.E., E.V.V.RAMANAMURTHY, M.Tech., R. DEVARAJ, M.E., Associate Professors