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@ McGraw-Hill Education

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Engineering Drawing with an Introduction to AutoCAD”, Dhananjay A Jolhe, Tata McGraw Hill, Ed. 2008

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.


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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.


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1.3.2 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. 1.2. The working surface of the board is planned perfectly. A shorter edge of the board carries a hard ebony strip fitted in a groove. This straight ebony edge, perfectly lined up with the edge of the drawing board, provides the guide for the T-square.


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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.


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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.


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Set-squares

Two set squares—(i) 45° set-square and (ii) 30°– 60° set-square, are the most common drawing instruments. A protractor is usually included in a 45° set-square, Fig. 1.5(a). The 30°– 60° set-square may include French curves in it, Fig. 1.5(b).

Protractor

Protractor is used to draw and measure the angles. It is available separately or as merged in 45° set-square.


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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.


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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).


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Dividers

Dividers are used to transfer lengths from one place to other. They are also used to set-off desired distance from the scale on the paper.


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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.


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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.


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Pencil Sharpener

A pencil sharpener is a device used to mend the pencils. It conveniently removes the wooden shell covering the lead. A common hand-held sharpener, Fig. 1.12(a), is recommended.

Eraser

A non-dusting good quality eraser is recommended for erasing unwanted part of the pencil drawing.


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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.


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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.


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Lettering Set-squares

Small sized transparent \set-squares without any graduations on their edges, Fig. 1.17, may be used for lettering purposes.


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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).


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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.


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Sheet Container

A sheet container, Fig. 1.20, may be used to store and carry drawing sheets.

Sandpaper

Sandpaper (or sandpaper block) is used to sharpen the pencil lead and lead sticks. Fine grade wood sandpaper, should be preferred.

Paper Napkins or Handkerchief

Paper napkins or a small handkerchief may be used to clean the drawing sheet and drawing instruments frequently.


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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.

1. 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.

2. 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.

3. Fix a drawing clip (Clip 1) at bottom right corner of the board. See INSET 1 for proper clip placement.

4. 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.


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5. 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.

6. 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.


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Preparing the Pencil and Lead Sticks

A penknife may be used to remove the wooden shell from the unlettered end of the pencil. Initially, around 35 mm length of shell should be removed to uncover approximately 10 mm length of lead, Fig. 1.22(a). The lead end should then be sharpened to a conical tip using a sandpaper. The sharp conical tip should be converted to a rounded tip. The lead sticks to be used in compasses may be sharpened using sandpaper in a similar way.


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Preparing the Compass

Loosen the screw of the pencil point of the compass. Insert a lead stick of appropriate length and prepared with tip as explained below. Adjust the needle and lead so that the needle tip extends slightly more than the lead tip.


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Working with Pencil

The pencil should be gripped at an approximate distance of 35 mm from the lead tip, Fig.1.25. The pencil is usually held inclined at about 60° with the paper. The slope of the pencil should be in the direction of the stroke of the line. For horizontal lines, the pencil should slope up toward the right-hand side. For vertical lines, it should slope up toward the user. The pencil may be rotated slightly while drawing a line to ensure the uniformity in line thickness.


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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. The positions of the set-squares are shown in Fig. 1.26.


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Drawing Margins and Title Block

Sufficient margins should be kept on all the sides of the drawing sheet. The margin widths at the four sides of A2 size (trimmed) sheet are shown in Fig. 1.27. A thick drawing frame should be drawn after fixing the margin width. Often, a longer frame line, say the bottom line, is drawn parallel to the corresponding edge of the sheet.


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The title block is located at the bottom right corner of the frame attached to the frame lines, Fig. 1.27. It typically includes information like, name of the organization, name of the designer or draftsman, drawing title, scale of the drawing, etc. The projection method symbol is also included in the title block.


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TIPS FOR GOOD QUALITY DRAWING

1. Sharpen the tip of the pencil from time to time by using a penknife and sandpaper.

2. Sharpen the lead tip inserted in the compass frequently by sandpaper.

3. Use a proper grade of the pencil and/or lead, i.e., H, 2H or HB as the case may be.

4. Avoid frequent use of eraser.

5. Complete a line, circle or arc in one stroke only. Avoid overdrawing.

6. Maintain constant hand pressure while drawing a particular line, circle or arc.

7. Check frequently the 0 setting of the drafter scale.

8. Don’t use a drafter to draw measured inclinations. Use a protractor for this purpose.

9. While moving the drafter scale from one point to another, care should be taken that it does not rub with the drawing sheet.

10.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 less than 5 mm.

11.Draw smooth curves (e.g., engineering curves, loci of points, sections of solids, development, curves of intersection, etc.,) initially very lightly by freehand and then use the French curve to make them sufficiently thick and uniform.


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12.Use a paper napkin or clean handkerchief to clean away the rubbed particles from drawing sheet.

13.Avoid the contact of drawing instruments with drawing sheet except during their actual use.

14.Your drawing sheet gets stained by dirt on the drawing instruments, drawing board and your hands. Keep all these always clean.

15.Protect your drawing sheet from all external factors which may spoil or make it dirty.

16.Before placing the drawing sheet inside the container, roll it properly and place a rubber-band over it.

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