CHAPTER- 1

(INTRODUCTION TO ENGINEERING DRAWING)

Engineering drawing is a two dimensional representation of three dimensional objects. In general, it provides necessary information about the shape, size, surface quality, material, manufacturing process, etc., of the object. It is the graphic language from which a trained person can visualize objects.

Drawing Instruments and aids:

The Instruments and other aids used in drafting work are listed below:

Drawing board
Set squares
French curve
Template
Mini drafter
Instrument box
Protractor
Set of scales
Drawing sheets
Pencils

Drawing Board:

Until recently drawing boards used are made of well seasoned softwood of about 25 mm thick with a working edge for T-square. Nowadays mini-drafters are used instead of T-squares which can be fixed on any board. The standard size of board depends on the size of drawing sheet size required.

Mini-Drafter:

Mini-drafter consists of an angle formed by two arms with scales marked and rigidly hinged to each other .It combines the functions of T-square, set-squares, scales and protractor. It is used for drawing horizontal, vertical and inclined lines, parallel and perpendicular lines and for measuring lines and angles.

Instrument Box :

Instrument box contains 1. Compasses, 2. Dividers and 3. Inking pens. What is important is the position of the pencil lead with respect to the tip of the compass. It should be at least 1 mm above as shown in the fig. because the tip goes into the board for grip by 1 mm.

Figure.1.2

Pencils:

Pencils with leads of different degrees of hardness or grades are available in the market. The hardness or softness of the lead is indicated by 3H, 2H, H, HB, B, 2B, 3B, etc. The grade HB denotes medium hardness of lead used for general purpose. The hardness increases as the value of the numeral before the letter H increases. The lead becomes softer, as the value of the numeral before B increases.

HB Soft grade for Border lines, lettering and free sketching
H Medium grade for Visible outlines, visible edges and boundary lines
2H Hard grade for construction lines, Dimension lines, Leader lines, Extension lines, Centre lines, Hatching lines and Hidden lines.

Drawing Sheet:

The standard drawing sheet sizes are arrived at on the basic Principal of x: y = 1: 2^ (1/2) and xy = 1 where x and y are the sides of the sheet. For example AO, having a surface area of 1 Sq.m; x = 841 mm and y = 1189 mm. The successive sizes are obtained by either by halving along the length or doubling the width, the area being in the ratio 1: 2. Designation of sizes is given in the fig. For class work use of A2 size drawing sheet is preferred.

Figure. 1.3

figure.1.4

Title Block:

The title block should lie within the drawing space at the bottom right hand comer of the sheet. The title block can have a maximum length of 170 mm and width of 65mm providing the following information. Title of the drawing.

Drawing number.
Scale.
Symbol denoting the method of projection.
Name of the firm, and
Initials of staff, who have designed, checked and approved.

Lines:

Just as in English textbook the correct words are used for making correct sentences; in Engineering Graphics, the details of various objects are drawn by different types of lines. Each line has a definite meaning and sense to convey.

Visible Outlines, Visible Edges: (Continuous wide lines) the lines drawn to represent the visible outlines/ visible edges / surface boundary lines of objects should be outstanding in appearance.
Dimension Lines (Continuous narrow Lines): Dimension Lines are drawn to mark dimension.
Extension Lines (Continuous narrow Lines): There are extended slightly beyond the respective dimension lines.
Construction Lines (Continuous narrow Lines): These are drawn for constructing drawings and should not be erased after completion of the drawing.
Hatching / Section Lines (Continuous Narrow Lines): These are drawn for the sectioned portion of an object. These are drawn inclined at an angle of 45° to the axis or to the main outline of the section.
Guide Lines (Continuous Narrow Lines): These are drawn for lettering and should not be erased after lettering.
Break Lines (Continuous Narrow Freehand Lines):Wavy continuous narrow line drawn freehand is used to represent break of an object.
Break Lines (Continuous Narrow Lines With Zigzags): Straight continuous narrow line with zigzags is used to represent break of an object.
Dashed Narrow Lines (Dashed Narrow Lines):Hidden edges / Hidden outlines of objects are shown by dashed lines of short dashes of equal lengths of about 3 mm, spaced at equal distances of about 1 mm. the points of intersection of these lines with the outlines / another hidden line should be clearly shown.
Center Lines (Long-Dashed Dotted Narrow Lines): These are draWn at the center of the drawings symmetrical about an axis or both the axes. These are extended by a short distance beyond the outline of the drawing.
Cutting Plane Lines: Cutting Plane Line is drawn to show the location of a cutting plane. It is long-dashed dotted narrow line, made wide at the ends, bends and change of direction. The direction of viewing is shown by means of arrows resting on the cutting plane line.
Border Lines: Border Lines are continuous wide lines of minimum thickness 0.7 mm.

CONVENTIONAL REPRESENTATION OF MATERIALS -

LETTERING

Lettering is defined as writing of titles, sub-titles, dimensions, etc., on a drawing.

Importance of Lettering:
To undertake production work of an engineering component as per the drawing, the size and other details are indicated on the drawing. This is done in the form of notes and dimensions. Main Features of Lettering are legibility, uniformity and rapidity of execution. Use of drawing instruments for lettering consumes more time. Lettering should be done freehand with speed. Practice accompanied by continuous efforts would improve the lettering skill and style. Poor lettering mars the appearance of an otherwise good drawing. Size of Letters:
Size of Letters is measured by the height h of the CAPITAL letters as well as numerals.
Standard heights for CAPITAL letters and numerals recommended by BIS are given below: 1.8, 2.5, 3.5, 5, 6, 10, 14 and 20 mm
Note: Size of the letters may be selected based upon the size of drawing.

Guide Lines:

In order to obtain correct and uniform height of letters and numerals, guide lines are drawn, using 2H pencil with light pressure. HB grade conical end pencil is used for lettering. The following are some of the guide lines for lettering

o Drawing numbers, title block and letters denoting cutting planes, sections are written in 10 mm size. o Drawing title is written in 7 mm size.

o Hatching, sub-titles, materials, dimensions, notes, etc., are written in 3.5 mm size.

o Space between lines = 3/4 h

o Space between words may be equal to the width of alphabet M or 3/5 h.

Procedure for Lettering:

1. Thin horizontal guide lines are drawn first at a distance ' h' apart.
2. Lettering Technique: Horizontal lines of the letters are drawn from left to right. Vertical, Inclined and curved lines are drawn from top to bottom.
3. After lettering has been completed, the guidelines are not erased.

Dimensioning:

Drawing of a component, in addition to providing complete shape description, must also furnish Information regarding the size description. These are provided through the distances between the Surfaces, location of holes, nature of surface finish, type of material, etc. The expression of these Features on a drawing, using lines, symbols, figures and notes is called dimensioning.

Methods of Indicating Dimensions:

The dimensions are indicated on the drawings according to one of the following two

methods. Method - 1 (Aligned method):

Dimensions should be placed parallel to and above their dimension lines and preferably at the middle, and clear of the line. Dimensions may be written so that they can be read from the bottom or from the right side of the drawing. Dimensions on oblique dimension lines should be oriented as shown in Fig.2.26a and except where unavoidable, they shall not be placed in the 30° zone. Angular dimensions are oriented.

Engineering Drawing (DPT-III)