UNIT 1
Fundamentals of 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.
Drawings prepared in one country may be utilised in any other country irrespective of the language spoken. Hence, engineering drawing is called the universal language of engineers. Any language to be communicative, should follow certain rules so that it conveys the same meaning to everyone. Similarly, drawing practice must follow certain rules, if it is to serve as a means of communication. For this purpose, Bureau of Indian Standards (BIS) adapted the International Standards on code of practice for drawing. The other foreign standards are: DIN of Germany, BS of Britain and ANSI of America.
Role of Engineering Drawing
The ability to read drawing is the most important requirement of all technical people in any profession. As compared to verbal or written description, this method is brief and clearer. Some of the applications are: building drawing for civil engineers, machine drawing for mechanical engineers, circuit diagrams for electrical and electronics engineers, computer graphics for one and all.
The subject in general is designed to impart the following skills.
1. Ability to read and prepare engineering drawings.
2. Ability to make free - hand sketching of objects.
3. Power to imagine, analyze and communicate, and
4. Capacity to understand other subjects.
Drawing instruments and their uses
To record information on paper (or another surface), instruments and equipment are required. Even for drawings made freehand, pencils, erasers, and sometimes coordinate paper or other special items are used. The lines made on drawings are straight or curved (including circles and arcs). They are made with drawing instruments, which are the necessary tools for laying down lines on a drawing in an accurate and efficient manner. To position the lines, a measuring device, a scale, is needed.
The various instruments will be described in detail later, but the opening of this chapter will serve as an introduction. To draw straight lines, the T square, with its straight blade and perpendicular head, or a triangle is used to support the stroke of the pencil. To draw circles, a compass is needed. In addition to the compass, the draftsman needs dividers for spacing distances and a small bow compass for drawing small circles. To draw curved lines other than circles, a French curve is required. A scale is used for making measurements.
- Drawing Paper
Drawing paper is made up of variety of qualities and manufactured in sheets or rolls. White drawing papers that will not turn yellow with age or exposure are used for finished drawings, maps, charts, and drawings for photographic reproduction. For pencil layouts and working drawings buff detail papers are preferred as they are easier on the eyes compared to white papers.
- Tracing Paper
Tracing papers are thin papers, natural or transparent, on which drawings are traced, in pencil or ink, and from which blueprints or similar contact prints can be made. In most drafting rooms original drawings are penciled on tracing papers, and blueprints are made directly from these drawings, a practice increasingly successful because of improvements both in papers and in printing.
- Thumbtacks
The best thumbtacks are made with thin heads and steel points screwed into them. Cheaper ones are made by stamping. Use tacks with tapering pins of small diameter and avoid flat-headed (often colored) map pins, as the heads are too thick and the pins rather large.
- Pencils
The basic instrument is the graphite lead pencil, made in various hardness’s. Each manufacturer has special methods of processing design to make the lead strong and yet give a smooth clear line.
In the left there is an ordinary pencil, with the lead set in wood and the other is a semi-automatic pencil with thinner leads. Both are fine but have the disadvantage that in use, the wood must be cut away to expose the lead (a time-consuming job), and the pencil becomes shorter and shorter until the last portion of it is to be discarded. Whereas semiautomatic pencils, with a chuck to clamp and hold the lead, are more convenient, has a plastic handle and changeable tip (for indicating the grade of lead).
Drawing pencils are graded by numbers and letters from 6B which is very soft and black, to 5B, 4B, 3B, 2B, B, and HB to F, the medium grade; then H, 2H, 3H, 4H, 5H, 6H, 7H, and 8H to 9H, the hardest. The soft (B) grades are used primarily for sketching and rendered drawings and the hard (H) grades for instrument drawings.
- Pencil Pointer
After the wood of the ordinary pencil is cut away with a pocketknife or mechanical sharpener, the lead must be formed to a long, conic point.
A pencil pointer is a tool for sharpening a pencil’s writing point by shaving away its worn surface.
- Erasers
The Ruby pencil eraser, large size with beveled ends, is the standard. This eraser not only removes pencil lines effectively but is better for ink, as it removes ink without seriously damaging the surface of paper or cloth.
Artgum or a soft-rubber eraser, is useful for cleaning paper and cloth of finger marks and smears that spoil the appearance of the completed drawing.
- Penholders and Pens
The penholder should have a grip of medium size, small enough to enter the mouth of a drawing-inkbottle easily yet not so small as to cramp the fingers while in use. A size slightly larger than the diameter of a pencil is good.
- Triangles
Triangles, are made of transparent hard or other plastic material. Through internal strains they sometimes lose their accuracy. Triangles should be kept flat to prevent warping. For ordinary work, a 6- or 8-in. 45° and a 10-in. 30-60° are good sizes.
- The T Squares
The fixed-head T square, is used for all ordinary work. It should be of hardwood, and the blade should be perfectly straight. The transparent-edged blade is much the best. A draftsman will have severalfixed-head squares of different lengths and will findan adjustable-head square of occasional use.
- Scales
Scales, are made in various number of graduations to meet the requirements of many kinds of work. For convenience, scales are classified according to their most-common uses.
Mechanical Engineer’s Scale
These are divided and numbered in such a way that fractions of inches represent inches. The most common ranges are 1/8, 1/4, 1/2, and 1 in. To the inch. These scales are known as the size scales because the reduced size also represents the ratio of size, as for example one-eighth size.
Mechanical engineer's scales are almost always "full divided"; that is, the smallest divisions run throughout the entire length.
They are generally graduated with the marked divisions numbered from right to left, as well as from left to right, as shown in Figure Mechanical engineer's scales are used mostly for drawings of machine parts and small structures where the drawing size is more than one-eighth the size of the actual object.
Civil Engineer’s Scale
These are divided into decimals with divisions ranging from 10, 20, 30, 40, 50, 60, and 80 to the inch. Such a scale is usually full divided and is sometimes numbered both from left to right and right to left. Civil engineer's scales are most used for plotting and drawing maps, although they are convenient for any work where divisions of the inch in tenths is required.
Metric Scales
Metric scales are supplied in all the styles and sizes and materials of other scales. They are usually full-divided and are numbered in meters, centimeters, or millimeters depending upon the reduction in size. Typical size reductions are: 1:1, 1:2, 1:5, 1:10, 1:20, 1:25, 1:33.3, 1:50, 1:75, 1:80, 1: 100, 1: 150. Also available are metric equivalent scales for direct conversion of English scales.
- Curves
Curve rulers, called "irregular curves" or "French curves," are used to draw curved lines other than circular arcs. The patterns for these curves are laid out in parts of ellipse and spirals or other mathematical curves in various combinations. For the student, one ellipse curve of the general shape or one spiral, either a logarithmic spiral is a useful small curve.
- Case Instruments
We have so far, except for curves considered only the instruments needed for drawing straight lines. A major portion of any drawing is likely to be circles and circle arcs, and the so called “case” instruments are used for these.
Divider:
It is used for laying off or transferring measurements.
Next is the large compass with lengthening bar and pen attachment. The three “bow” instruments are for smaller work. They are almost always made without the conversion feature.
The ruling pen is used for inking straight lines.
Protractor is used to measure angles upto 180°.
- Cautions in the use of Instruments
- Scales are not to be used as a ruler for drawing lines.
- Horizontal lines with the lower edge should not be drawn with a T square.
- The lower edge of T square should not be used as a base for drawing triangles.
- T square is not to be used as hammer.
- Never put either end of a pencil into the mouth.
- Never sharpen a pencil over a drawing board.
- Never oil the joints of compasses.
- Never begin work without wiping off the table and instruments.
- Never work on a table cluttered with unneeded instruments or equipment’s.
- Never fold a drawing or a tracing.
Lettering and relevant BIS conventions & standards
Lettering
Graphic representation of the shape of a part, machine, or structure gives one aspect of the information needed for its construction. To this must be added, to complete the description, figured dimensions, notes on material and finish, and a descriptive title—all lettered, freehand, in a style that is perfectly legible, uniform, and capable of rapid execution. As far as the appearance of a drawing is concerned, the lettering is the most important part. But the usefulness of a drawing, too, can be ruined by lettering done ignorantly or carelessly, because illegible figures are apt to cause mistakes in the work.
By far the greatest amount of lettering on drawings is done in a rapid single-stroke letter, either vertical or inclined, and every engineer must have absolute command of these styles.
The term "single-stroke," or "one-stroke," does not mean that the entire letter is made without lifting the pencil or pen but that the width of the stroke of the pencil or pen is the width of the stem of the letter.
Single Stroke lettering
By far the greatest amount of lettering on drawings is done in a rapid single-stroke letter, either vertical or inclined, and every engineer must have absolute command of these styles.
The term "single-stroke," or "one-stroke," does not mean that the entire letter is made without lifting the pencil or pen but that the width of the stroke of the pencil or pen is the width of the stem of the letter.
Guidelines
Always draw light guide lines for both tops and bottoms of letters, using a sharp pencil. Figure 1 shows a method of laying off several equally spaced lines. Draw the first base line; then set the bow spacers to the distance wanted between base lines and step off the required number of base lines. Above the last line mark the desired height of the letters.
Lettering in Pencil
Good technique is as essential in lettering as in drawing. The quality of the lettering is important whether it appears on finished work to be reproduced by one of the printing processes or as part of a pencil drawing to be inked. In the first case, the penciling must be clean, firm, and opaque; in the second, it may be lighter.
Single - stroke Vertical Capitals:
The vertical, single-stroke, commercial Gothic letter is a standard for titles, reference letters, etc. As for the proportion of width to height, the general rule is that the smaller the letters are, the more extended they should be in width.
A low extended letter is more legible than a high compressed one and at the same time makes a better appearance.
Vertical lower-case letters:
The single-stroke, vertical lower-case letter is not commonly used on machine drawings but is used extensively in map drawing. It is the standard letter for hypsography in government topographic drawing. The bodies are made two-thirds the height of the capitals with the ascenders extending to the capital line and the descenders dropping the same distance below.
Single strokes Inclined capitals:
Many draftsmen use the inclined, or slant, letter in preference to the upright. The order and direction of strokes are the same as in the vertical form.
Single stroke, Inclined lower-case letters:
The inclined lower-case letters have bodies two-thirds the height of the capitals with the ascenders extending to the capital line and the descenders dropping the same distance below the base line. The lower-case letter is suitable for notes and statements on drawings because it is much more easily read than all capitals.
The loop letters are made with an ellipse whose long axis is inclined about 45° in combination with a straight line.
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.
IS 10714 (Pint 20): 2001 (General principles of presentation on technical drawings) and SP 46:2003specify the following types of lines and their applications:
1. 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.
2. Dimension Lines: (Continuous narrow Lines) Dimension Lines are drawn to mark dimension.
3. Extension Lines: (Continuous narrow Lines) There are extended slightly beyond the respective dimension lines.
4. Construction Lines: (Continuous narrow Lines) Construction Lines are drawn for constructing drawings and should not be erased after completion of the drawing.
5. Hatching / Section Lines: (Continuous Narrow Lines)
Hatching Lines 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.
6. Guide Lines: (Continuous Narrow Lines)
Guide Lines are drawn for lettering and should not be erased after lettering.
7. Break Lines: (Continuous Narrow Freehand Lines)
Wavy continuous narrow line drawn freehand is used to represent break of an object.
8. Break Lines: (Continuous Narrow Lines with Zigzags)
Straight continuous arrow line with zigzags is used to represent break of an object.
9. 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.
10. Center Lines: (Long-Dashed Dotted Narrow Lines)
Center Lines 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.
11. 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.
12. Border Lines
Border Lines are continuous wide lines of minimum thickness 0.7 mm.
Figure 1 Types of lines
Table 2 Types of lines and their applications
Line | Description | General application |
| Continuous thick or continuous wide | Visible outlines, visible edges, main representation in diagrams, maps; system lines |
| Continuous thin (narrow) (straight or curved) | Imaginary lines of intersection; grid, dimension. Extension, projection, reference lines and hatching. |
| Continuous thin (narrow) freehand. | Limits of partial or interrupted views and sections, if the limit is not a chain thin line. |
| Continuous thin (narrow) with zigzags (straight) | Long break line |
 | Dashed thick (wide) | Line showing permissible of surface treatment. |
 | Dashed thin (narrow) | Hidden outline; hidden edges |
 | Chain thin Long-dashed dotted (narrow) | Centre line; lines of symmetry; trajectories; pitch circle of gears, pitch circle of holes. |
 | Chain thick or long - dashed (dotted) wide | Indication of lines or surfaces to which a special requirement applies. |
Line widths:
Line width means line thickness
Choose line widths according to the size of the drawing from the following range: 0.13,0.18,0.25, 0.35, 0.5, 0.7 and 1 mm.
Precedence of Lines
1. When a Visible Line coincide with a Hidden Line or Center Line, draw the Visible Line. Also, extend the Center Line beyond the outlines of the view.
2. When a Hidden Line coincides with a Center Line, draw the Hidden Line.
3. When a Visible Line coincides with a Cutting Plane, draw the Visible Line.
4. When a Center line coincides with a Cutting Plane, draw the Center Line and show the
Cutting Plane line outside the outlines of the view at the ends of the Center Line by thick dashes.
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.
Principles of Dimensioning
Some of the basic principles of dimensioning are given below.
1. All dimensional information necessary to describe a component clearly and completely shall be written directly on a drawing.
2. Each feature shall be dimensioned once only on a drawing, i.e., dimension marked in one view need not be repeated in another view.
3. Dimension should be placed on the view where the shape is best seen (Fig.7)
4. As far as possible, dimensions should be expressed in one unit only preferably in millimeters, without showing the unit symbol (mm).
5. As far as possible dimensions should be placed outside the view (Fig. 8).
6. Dimensions should be taken from visible outlines rather than from hidden lines (Fig. 9).
Figure 2
Figure 3
Figure 4
7. No gap should be left between the feature and the start of the extension line (Fig.10).
Figure 5 Marking of Extension lines
8. Crossing of center lines should be done by a long dash and not a short dash (Fig.11).
Figure 6 Crossing of Centre lines
Execution of dimensions:
1. Projection and dimension lines should be drawn as thin continuous lines. Projection lines should extend slightly beyond the respective dimension line. Projection lines should be drawn perpendicular to the feature being dimensioned. If the space for dimensioning is insufficient, the arrow heads may be reversed, and the adjacent arrow heads may be replaced by a dot(Fig.12). However, they may be drawn obliquely, but parallel to each other in special cases, such as on tapered feature (Fig.13).
Figure 7 Dimensioning in narrow spaces
Figure 8 Dimensioning a tapered feature
2. A leader line is a line referring to a feature (object, outline, dimension). Leader lines should be inclined to the horizontal at an angle greater than 30°. Leader line should terminate,
(a) with a dot, if they end within the outline ofan object (Fig.2.21a).
(b) with an arrow head, if they end on outside of the object (Fig.2.21b).
(c) without a dot or arrow head, if they end on dimension line (Fig.2.21c).
Figure 9 Termination of leader lines
Dimension Termination and Origin Indication
Dimension lines should show distinct termination in the form of arrow heads or oblique strokes or where applicable an origin indication (Fig.15). The arrow head included angle is 15°. The origin indication is drawn as a small open circle of approximately 3 mm in diameter. The proportion length to depth 3: 1 of arrow head is shown in Fig.16.
Figure 10 Termination of dimension lines
Figure 11 Proportions of an arrow head
Methods of Indicating Dimensions
The dimensions are indicated on the drawings according to one of the following two methods.
Method I (Aligned method)
Dimensions should be placed parallel to and above their dimension lines and preferably at the middle, and clear of the line. (Fig. 17).
Figure 12
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.18a and except where unavoidable, they shall not be placed in the 30° zone. Angular dimensions are oriented as shown in Fig.18b.
Figure 13 Angular dimensioning
Method 2(Uni-directional method)
Dimensions should be indicated so that they can be read from the bottom of the drawing only. Non-horizontal dimension lines are interrupted, preferably in the middle for insertion of the dimension (Fig.19a).
Angular dimensions may be oriented as in Fig.19b
Figure 14 Uni-directional method
Simple Geometrical constructions
Engineering drawing consists of many geometrical constructions. A few methods are illustrated here without mathematical proofs.
1. To divide a straight line into a given number of equal parts say 5.
Construction (Fig. 20)
Figure 15 Dividing a line
i. Draw AC at any angle θ to AB.
Ii. Construct the required number of equal parts of convenient length on AC like 1,2,3.
Iii. Join the last point 5 to B
Iv. Through 4, 3, 2, 1 draw lines parallel to 5B to intersect AB at 4',3',2' and 1'..
Drawings of small objects can be prepared of the same size as the objects they
Represent. A 150 mm long pencil may be shown by a drawing of 150 mm length.
Drawings drawn of the same size as the objects, are called full-size drawings. The
Ordinary full-size scales are used for such drawings.
A scale is defined as the ratio of the linear dimensions of element of the object
As represented in a drawing to the actual dimensions of the same element of the
Object itself.
Scales:
The scales generally used for general engineering drawings are shown in table 4-1[SP: 46].
All these scales are usually 300 mm long and sub-divided throughout their
Lengths. The scale is indicated on the drawing at a suitable place near the title. Thecomplete designation of a scale consists of word scale followed by the ratio, i.e.scale 1: 1 or scale, full size.
It may not be always possible to prepare full-size drawings. They are, therefore,
Drawn proportionately smaller or larger. When drawings are drawn smaller than theactual size of the objects (as in case of buildings, bridges, large machines etc.) thescale used is said to be a reducing scale (1: 5). Drawings of small machine parts,mathematical instruments, watches etc. are made larger than their real size. Theseare said to be drawn on an enlarging scale (5:1 ).
The scales can be expressed in the following three ways:
(1)Engineer’s scale: In this case, the relation between the dimension on the
Drawing and the actual dimension of the object is mentioned numerically in the
Style as 10 mm = 5 m etc.
(2) Graphical scale: The scale is drawn on the drawing itself. As the drawing
Becomes old, the engineer's scale may shrink and may not give accurate results.
However, such is not the case with graphical scale because if the drawing shrinks,the scale will also shrink. Hence, the graphical scale.is commonly used in survey maps.
(3) Representative fraction: The ratio of the length of the object represented
On drawing to the actual length of the object represented is called the RepresentativeFraction (i.e. R.F.).
When a 1 cm long line in a drawing represents 1 metre length of the object, the
R.F. Is equal to
and the scale of the drawing will be
1 : 100 or1/100 full size. The R.F. Of a drawing is greater than unity when it is drawnon an enlarging scale. For example, when a 2 mm long edge of an object is shown in a drawing by a line 1 cm long, the R.F. Is 1 cm/2 mm = 10 mm/2mm = 5.
Such a drawing is said to be drawn on scale 5: 1 or five times full-size.
Scales on Drawings:
When an unusual scale is used, it is constructed on the drawing sheet. To construct
a scale the following information is essential:
(1) The R.F. Of the scale.
(2) The units which it must represent, for example, millimetres and centimetres,
Or feet and inches etc.
(3) The maximum length which it must show.
The length of the scale is determined by the formula:
Length of the scale = R.F. x maximum length required to be measured.
It may not be always possible to draw as long a scale as to measure thelongest length in the drawing. The scale is therefore drawn 15 cm to 30 cm long,longer lengths being measured by marking them off in parts.
The profile of number of objects consists of various types of curves. This chapter
Deals with various types of curves which are commonly used in engineering practice
As shown below:
1. Conic sections 4. Evolutes
2. Cycloidal curves 5. Spirals
3. Involute 6. Helix.
Figure 16
(i) When the section plane is inclined to the axis and cuts all the generators onone side of the apex, the section is an ellipse [fig. 16].
(ii) When the section plane is inclined to the axis and is parallel to one of thegenerators, the section is a parabola [fig. 16].
(iii) A hyperbola is a plane curve having two separate parts or branches, formedwhen two cones that point towards one another are intersected by a planethat is parallel to the axes of the cones.
The conic may be defined as the locus of a point moving in a plane in such a waythat the ratio of its distances from a fixed point and a fixed straight line is alwaysconstant. The fixed point is called the focus and the fixed line, the directrix.
The ratio 
is called eccentricity and isdenoted by e. It is always less than 1 for ellipse, equal to 1 for parabola and greater
Than 1 for hyperbola i.e.
(i) ellipse e < 1
(ii) parabola : e = 1
(iii) hyperbola : e > 1.
The line passing through the focus and perpendicular to the directrix is calledthe axis. The point at which the conic cuts its axis is called the vertex.
