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Chapter 1
INTRODUCTION TO ENGINEERING DRAWING
The role of engineers is to design & develop the products. In their business, engineers have to prepare drawings to convey their ideas. The graphical language used by engineers is called as Engineering Drawing. Just as a picture speaks thousands of words, a complete technical drawing tells everything about the geometry of the product.
To draw accurate drawings, various instruments & accessories are used. These are explained on next slide.
1.3 DRAWING INSTRUMENTS AND ACCESSORIES
1.3.1 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. Indian Standards (IS) for drawing sheets and drawing boards as recommended by the Bureau of Indian Standards (BIS) are shown in Table 1.1.
1.3.3 Mini Drafter
A mini drafter is a portable device used to draw parallel, inclined and perpendicular lines speedily. It is mounted on a drawing board at the top left corner. A drafter consists of a scale, a scale screw, a scale plate, steel bars, a bar plate and a clamping mechanism, Fig. 1.3.
1.3.4 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. 1.4.
Roller Scale
A roller scale is a handy device used to draw parallel and inclined lines. It is a speedy device and may be used for practice in classrooms.
Compasses
Compasses are used to draw circles or arcs. Two sizes of compasses — (i) large compass and (ii) small spring bow compass are in common use. A large compass consists of a needle leg and a pencil leg hinged together at upper ends, Fig. 1.7(a). The two legs carry, respectively, a needle point and a pencil point at their lower ends. The pencil point can be interchanged with a pen point, Fig. 1.7(b). Lengthening bar, Fig. 1.7(c), is used to draw circles of diameter greater than 150 mm. Small spring bow compasses are of two types: bow pencil compass, Fig. 1.7(e) and Bow pen compass, Fig. 1.7(f).
Pencils/Lead Pens
The quality of drawing largely depends on the selection and use of proper grade of pencil. The grade of a pencil is printed near its blocked end. For technical drawing, three grades of pencils, namely, H, 2H and HB are recommended. A lead pen, Fig. 1.10, is an alternative to the pencil.
Lead Sticks
Lead sticks, Fig. 1.11, are used with compasses. HB and H grades are frequently needed for technical drafting. The end of lead sticks must be sharpened properly using sandpaper.
French Curve
A French curve is a template of freeform curves made up of acrylic or celluloid, Fig. 1.14. It helps to draw a smooth curve passing through a number of non-collinear points.
Circle Template
A transparent circle template made up of acrylic is used to draw circles of different radii quickly. The circle templates should only be used to draw circles of diameters smaller than 5 mm.
Lettering Template
Lettering template is a plastic plate on which letters are carved, Fig. 1.18. It may be used for double stroke Gothic lettering (Section 2.3.4).
Drawing Clips, Pins and Adhesive Tape
Drawing clips, pins and adhesive tape are used to fix drawing paper/sheet on the drawing board. Their use is explained in Fig. 1.21.
PRACTICAL LESSONS
Before the start of drawing work, the drafting table and other drawing instruments should be cleaned properly. The user should also clean his or her hands. This helps to keep the drawing work clean.
Clamping a Drawing Sheet on Drawing Board and Setting the Drafter
Refer Fig. 1.21.
- Place a drawing board on a table top or any other suitable surface. A specially designed drafting table (with a drawing board as a table top) may be used. The ebony edge of the board should be on your left-hand side.
- Place the drawing sheet on the drawing board. The bottom and right edges of the sheet should be approximately 1 cm each from the corresponding edges of the board.
- Fix a drawing clip (Clip 1) at bottom right corner of the board. See INSET 1 for proper clip placement.
- Loosen the clamping screw of the mini drafter. Carry the drafter gently over the board and place its claming 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. The distance of the clamp from the left edge of the board may be 5 mm to 10 mm, INSET 3. Tighten the clamping screw gently till the strap takes a firm grip on the board.
- Move the drafter scale to the centre 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. You must look directly from above the 0 degree mark to avoid the parallax error. Tighten the scale screw gently.
- 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. (If the sheet has a printed drawing frame, then the bottom horizontal line of the frame should be matched with the horizontal scale.) Now, move the scale to the top edge of the sheet, sliding gently over the sheet, and place the 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. The pin should be inserted at a point approximately 1 cm each from top and left edge of the sheet. In case of a sheet with a printed drawing frame, the pin should be placed outside the frame.
