Engineering Graphics PDF

1 ENGINEERING GRAPHICS (CTBT-ESC-103)  DRAWING INSTRUMENTS AND ACCESSORIES  Drawing Sheets and Papers  Drawing sheets and papers are the ‘canvases’ on which drawings are composed by pencils or pens. Drawing sheets are available in standard sizes.  The standard drawin...

1 ENGINEERING GRAPHICS (CTBT-ESC-103)  DRAWING INSTRUMENTS AND ACCESSORIES  Drawing Sheets and Papers  Drawing sheets and papers are the ‘canvases’ on which drawings are composed by pencils or pens. Drawing sheets are available in standard sizes.  The standard drawing sheets and drawing boards recommended by the Bureau of Indian Standards (BIS) are shown in Fig. 1. Each higher-numbered sheet is half the size of the immediate lower-numbered sheet.  For drawing practice, an A2-size (popularly called a half-imperial size) (and sometimes, A1 size) sheet is recommended. Fig. 1. Drawing Sheet 2  Drawing Board  Drawing boards are used to support a drawing sheet or paper. They are made up of soft wooden platens fastened together by two cross plates (battens), Fig. 2. A shorter edge of the board carries a hard ebony strip fitted in a groove. Fig. 2. Drawing Board  For engineering students, the use of a D2 size (650 mm×470 mm) drawing board (commonly called a half-size drawing board) is recommended. 3  Drafter  A mini drafter is a portable device used to draw parallel, inclined, and perpendicular lines speedily, mounted on the top left corner of the drawing board.  A drafter consists of a scale, a scale screw, a scale plate, steel bars, a bar plate, and a clamping mechanism, Fig. 3. An L-shaped scale is placed along both the arms. Fig. 3. Drafter 4  T-square  A T-square is a T-shaped device used to draw straight horizontal lines. It consists of a stock and a blade joined together at right angles, Fig. 4.  The inner edge of the stock, called the mating edge, is made perfectly smooth and straight.  The upper edge of the blade, called the working edge, is perfectly straight and set exactly at 90° to the mating edge of the stock. Fig. 4. T-square 5  Set squares  Two set squares (a) 45° set-square and (b) 30°– 60° set-square, are the most common drawing instruments.  The set squares are made up of transparent acrylic. Two edges of each set-square are perfectly set at right angles. (a) (b) Fig. 5. Set squares (a) 45° (b) 30°– 60° 6  Protractor  A protractor is used to draw and measure the angles. It is available separately or as merged in a 45° set-square.  Compasses  Compasses are used to draw circles or arcs. Two sizes of compasses, (a) large compass and (b) small spring bow compass are in common use. (a) (b) Fig. 6. (a) large compass (b) bow compass 7  Pencils/Lead Pens  A pencil is a primary tool in drawing. Pencils are designated by their grades. The quality of drawing largely depends on the selection and use of the proper grade of pencil.  For technical drawing, three grades of pencils, namely, H, 2H and HB are recommended.  Pencil Sharpener  A pencil sharpener is a device used to mend the pencils. It conveniently removes the wooden shell covering the lead.  Eraser  An eraser, Fig. 1.13, is used to erase an unwanted part of the pencil drawing. A non-dusting good-quality eraser is recommended. 8  Circle Template  A transparent circle template made up of acrylic is used to draw circles of different radii quickly. The common circle templates have circles from 2 mm diameter to 30 mm diameter on them.  Drawing Clips, Pins, and Adhesive Tape Fig. 7. Circle template  Drawing clips, pins, and adhesive tape are used to fix drawing paper/sheets on the drawing board. Drawing clips are used to clamp drawing sheet corners along drawing board edges. To fix the corners of the sheet on the surface of the board, we use a drawing pin or adhesive Tape. Fig. 8. Drawing clips and pins 9  Sheet Container  A sheet container is used to store and carry drawing sheets. Made up of plastic, the container has a detachable lid and belt. Fig. 9. Circle template 10  PRACTICAL LESSONS  Clamping a Drawing Sheet on a Drawing Board and Setting the Drafter  Place a drawing board on a table top or any other suitable surface. A table with an adjustable inclined surface is preferred.  Place the drawing sheet on the drawing board. All the drawing sheets, except A4 size, are positioned horizontally.  Fix a drawing clip (Clip 1) at the bottom right corner of the board. See INSET 1 for the proper clip placement.  Loosen the clamping screw of the mini drafter. Carry the drafter gently over the board and place its clamping strap over the top left corner of the board such that two of the inner faces of the strap will mate with the corresponding faces of the top edge of the board, INSET 2. 11  Move the drafter scale to the center of the sheet. Loosen the scale screw and match the 0-degree mark on the degree scale with the mark on the scale plate, INSET 4.  Move the drafter scale near the bottom edge of the sheet. Match the edge with the horizontal scale of the drafter. The sheet may be moved up and down pivoting about the Clip 1. Once the bottom edge of the sheet is matched perfectly with the horizontal scale, place another clip (Clip 2) near the bottom left corner of the sheet.  Move the scale to the top edge of the sheet, sliding gently over the sheet, and place the Fig. 10. Setting of drafter third clip (Clip 3) near the top right corner of the sheet. Use a drawing pin, INSET 5, or adhesive tape, INSET 6, to fix the top left corner of the sheet. 12  Preparing the Pencil and Lead Sticks  A penknife may be used to remove the wooden shell from the unlettered end of the pencil. A sharpener is good alternative to a knife.  Preparing the Compass  Working with Set-squares  The set-squares, in combination with T- square, can be conveniently used to draw lines inclined at 15°, 30°, 45° and so on. Fig. 10. Setting of Set squares 13  DRAWING MARGINS AND TITLE BLOCK  It prevents the actual drawing from getting damaged due to spoilage at the sheet edges.  The title block is an important part of the drawing. It is located at the bottom right corner of the frame attached to the frame Fig. 11. Drawing margins lines  The title block, commonly adopted in engineering colleges is depicted in Fig. 12. Each row is of 9 mm width. The lettering of 7 mm height is done to write the necessary information. Fig. 12. Title box 14  TIPS FOR GOOD QUALITY DRAWING  Sharpen the tip of the pencil from time to time by using a penknife and sandpaper.  Use a proper grade of the pencil and/or lead, i.e., H, 2H, or HB as the case may be. Avoid frequent use of erasers.  Complete a line, circle, or arc in one stroke only. Avoid overdrawing. Maintain constant hand pressure while drawing a particular line, circle, or arc.  Check frequently the zero setting of the drafter scale.  While moving the drafter scale from one point to another, care should be taken so that it does not rub with the drawing sheet.  Use a bow compass to draw smaller circles or arcs. A circle template should only be used to draw circles or arcs having a diameter of less than 5 mm.  Your drawing sheet gets stained by dirt on the drawing instruments, drawing board, and your hands. Keep all these are always clean. 15  Lines and Lettering  Lines and lettering are two important aspects of technical drawing. Lines are used to construct a drawing. Lettering is used to provide some specific information on it.  Lines  Lines are like the alphabet of a drawing language. As the alphabet is used to form meaningful words, various types of lines are used to construct meaningful drawings.  Lines are categories under Thick Medium Thin  Width  Their shape or construction  Width  Line width means the thickness of a line. The proportions between THIN, MEDIUM, and THICK lines are 1: 2: 4. The line group 0.25 is the most preferred line group for pencil drawing. 16  As a general rule, harder grade pencils are preferred for THIN lines and softer grade pencils for THICK lines. An H grade pencil is advised for THICK and MEDIUM lines. THIN lines may be drawn by a 2H grade pencil.  Their shape or construction Continuous Dashed 17 Line type Construction Weight Applications Continuous thick Thick Visible outlines, Object line lines Continuous thick line 18 Line type Construction Weight Applications Continuous thin Thin Projection lines, line Dimension line, Extension lines, Leader lines Projection lines 19 Extension lines Dimension line Section line 20 𝜙 = 20 Leader lines  Ratio between the lines and the gaps 2:1 Line type Construction Weight Applications Dashed medium Medium Hidden lines line 21 Line type Construction Weight Applications Chain thin line Thin Center lines Chain thin line  Ratio of the long dashes to the short dashes 6:1 22 Line type Construction Weight Applications Chain thin, thick Thin Cutting plane line at ends Chain thin, thick at ends 23  Lettering  Lettering is the art of writing a text on a drawing by using alphabets, numerals, and symbols.  Lettering Rules  Draw letters as simple as possible. Artistic or cursive lettering should be strictly avoided.  Draw letters symmetrical about the vertical axis or horizontal axis. Asymmetric letters like F, R, Z, 4, etc., may be drawn as they are.  Round off the sharp corners wherever necessary, e.g., D, P, S, etc.  The height of all the letters in one line should be the same.  Use single stroke vertical CAPITAL letters as much as possible. 24  Height and Width of Letters  The letters may have different heights depending on their purposes. BIS has recommended the heights of letters as: 1.8 mm, 2.5 mm, 3.5 mm, 5 mm, 7 mm, 10 mm, 14 mm, and 20 mm.  The height of the letters bears a direct relationship with the size of the drawing, i.e., large- sized letters for larger drawings and small-sized letters for smaller drawings.  The readers are advised to use letters of 10 mm, 7 mm, and 5 mm height to write titles, subtitles, and notes/dimensions, respectively on the pencil drawing.  The body height of lowercase letters is taken as 0.7 times the height of capital letters. The tail and the stem of the lowercase letters are drawn 0.3 times the height of a capital letter. 25  The height-to-width ratio varies from letter to letter. Most of the letters follow the ratio 7 : 5 or 7 : 6.  The line width of a letter depends on its height. Large-sized letters have more line thickness than small-sized letters.  The inclined letters are sloped to the right at 75° from the horizontal. The inclinations of all the letters should be the same.  The distances between two letters in a word may not be necessarily the same. The spacing between words may be equal to the height of the letters.  While lettering a fraction, keep the height of the numerator and denominator equal to 3/4th of the height of a non-fractioned number.  The spacing between division bar and the numerator or denominator should be such that the total height of the fraction will be twice of that of a non-fractioned number 26  Dimensioning  A drawing without dimensions is meaningless. Dimensions are necessary to show the exact size of an object. Dimensioning refers to the act of giving dimensions, i.e., length, width, height, diameter, etc., of the object.  Dimensioning is often done by a set of elements, which includes extension lines, dimension lines, leader lines, arrowheads, and dimensions.  An extension line is a short line drawn perpendicular to an object line. These lines start immediately or a few millimeters from the ends of object lines and extend a few millimeters beyond a dimension line.  A dimension line is drawn between two extension lines parallel to the object line. As a rule, there must be one and only one dimension line between any two extension lines. 27  A leader line (or leader) is a line that connects a note or a dimension with the feature to which it applies. Leaders are drawn at suitable angles, preferably 30°, 45°, or 60°, and are never drawn horizontally or vertically.  An arrowhead is drawn at each end of a dimension line. The tip of an arrowhead touches the extension line.  The closed and filled arrowhead is most commonly adopted. It is in the form of an isosceles triangle, having a height three times its base. 28  System of dimensioning  Aligned System  Unidirectional System  Aligned System  Dimensions are placed perpendicular to dimension lines.  Horizontal and inclined dimensions can be read from the bottom of the drawing. Vertical dimensions can be read from the right-hand side of the drawing.  All dimensions are placed above the midpoint of dimension lines. 29  Unidirectional System  Dimensions are always placed vertically 5 irrespective of dimension lines.  All dimensions can be read from the bottom of 40 the drawing.  Horizontal dimensions are placed above the 60° midpoint of dimension lines. Vertical and inclined dimensions are placed in the middle of dimension lines by breaking them.  Aligned system is quite popular and mostly used in industries 30  Rules of dimensioning  Between any two extension lines, there must be one and only one dimension line bearing one dimension.  As far as possible, all the dimensions should be placed outside the views. Inside dimensions are preferred only if they are clearer and more easily readable.  All the dimensions on a drawing must be shown using either Aligned System or Unidirectional System. In no case should, the two systems be mixed on the same drawing.  The same unit of length should be used for all the dimensions on a drawing. The unit should not be written after each dimension, but a note mentioning the unit should be placed below the drawing. 31  Dimension lines should not cross each other. Dimension lines should also not cross any other lines of the object. However, extension lines may cross each other or outlines of the object.  All dimensions must be given. As far as possible, there should not be a need for calculation, assumption, or direct measurement for any dimension.  Each dimension should be given only once. No dimension should be redundant, i.e., no dimension should be repeated directly or indirectly.  When it is necessary to place a dimension within a sectioned area, leave a blank space for the dimension 32  Avoid dimensioning hidden lines. Keep dimension lines 6 – 8 mm away from the object line and also from each other.  If the space between two extension lines is too narrow to mark arrowheads and the dimension then one of the following ways  Draw arrowheads touching the outsides of the extension lines and pointing toward each other. Place the dimension above the dimension line.  Draw arrowheads above and place the dimension at one end of the dimension line outside of the extension line.  Draw arrowheads above and place the dimension at the end of the leader, which terminates on the dimension line.  For two consecutive dimensions, replace two intermediate arrowheads by a dot and place the dimensions above, depending on the space availability.