Engineering Drawing

What to remember

• Drawing is the language of engineers. Every line type, scale, sheet size and projection method follows a fixed convention so that any engineer can read it the same way.
• India uses first-angle projection. The object lies in the first quadrant, between the observer and the plane. The front view is drawn above the plan (top view), and the left-side view is drawn on the right of the front view.
• Scale and dimensioning decide accuracy. Representative Fraction (RF) = length of the drawing ÷ actual length of the object. Dimensions are written once, in millimetres, without units on the figure.

Drawing instruments, sheets and lettering

Common instruments are the drawing board, T-square or drafter, set squares (45° and 30°-60°), compass, divider, protractor, French curves, scales and pencils of grades H, 2H (hard, for construction lines) and HB, B (soft, for final lines).

Standard sheets follow the A-series. A0 has an area of 1 square metre. Each next size is obtained by halving the longer side of the previous one, so the ratio of sides is always 1 : √2.

Every sheet has a border and a title block at the bottom right corner. The title block carries the name of the drawing, scale, projection symbol, drawn-by and checked-by fields, and the drawing number.

Lettering should be plain, uniform and legible. The single-stroke Gothic style is used, either vertical or inclined at about 75° to the horizontal. The height of capital letters is measured in millimetres. The standard heights are 2.5, 3.5, 5, 7, 10, 14 and 20 mm. The space between two words is about the width of one letter or slightly more.

Lines, dimensioning and scales

Each line type has one meaning.

Dimensioning rules:

• Dimension lines are thin and end with arrowheads. The usual arrowhead is closed and filled, with the length about three times its width.
• Extension (projection) lines extend slightly beyond the dimension line and do not touch the object.
• Each dimension is shown once, outside the view where possible, and is read from the bottom or from the right side of the sheet.
• Dimensions are given in millimetres. The symbol Ø is used for a diameter, R for a radius and □ for a square.
• Overall dimensions are placed outside the chain of part dimensions.

Scales: the full size scale is 1 : 1, a reduced scale is 1 : 2 or 1 : 100, and an enlarged scale is 2 : 1 or 10 : 1. A building plan is commonly drawn at 1 : 100, a site plan at 1 : 500 or 1 : 1000, and machine parts at 1 : 1 or 1 : 2.

• Plain scale reads units and one subdivision.
• Diagonal scale reads three units, such as metres, decimetres and centimetres, by using diagonal division of a small rectangle.
• Vernier scale reads very small fractions, using the difference of one main division and one vernier division.
• Comparative scale shows two different units, such as miles and kilometres, on the same RF.
• Scale of chords is used to measure angles.

Worked example: A road 600 m long is drawn as 12 cm on a sheet. Actual length = 600 m = 60,000 cm. RF = 12 ÷ 60,000 = 1/5000. So the scale is 1 : 5000.

Geometrical construction and engineering curves

Basic constructions include bisecting a line and an angle, dividing a line into equal parts, drawing a perpendicular, and drawing regular polygons. The interior angle of a regular polygon with n sides is (n − 2) × 180° ÷ n. For a hexagon this gives 120°.

Conic sections are curves cut from a right circular cone. They are defined by eccentricity e = distance from the focus ÷ distance from the directrix.

Other curves:

• Cycloid: path of a point on the circumference of a circle that rolls along a straight line without slipping.
• Epicycloid and hypocycloid: the rolling circle moves outside or inside another circle.
• Involute: path of the free end of a thread unwound from a circle. Gear tooth profiles use this curve.
• Helix: the path of a point that moves along a cylinder and rotates around it at a constant rate. A screw thread is a helix.
• Spiral: a curve that winds around a fixed point and moves away from it.

Orthographic and isometric projection

In orthographic projection, the object is projected by parallel rays at right angles to the planes. The planes are the vertical plane (VP) and the horizontal plane (HP). The view on VP is the front elevation, and the view on HP is the plan. A third plane, the profile plane (PP), gives the side elevation.

Rules to remember:

• A point is described by its distance from the VP (shown in plan) and from the HP (shown in elevation).
• A line parallel to both planes shows its true length in both views.
• A line parallel to one plane and inclined to the other shows its true length in the view of the plane it is parallel to.
• A plane parallel to the HP shows its true shape in the plan.
• For solids, the true shape of the base appears in the view where the axis is perpendicular to the plane.

Isometric projection shows three faces of an object in one view. The three isometric axes make 120° with each other. The vertical axis stays vertical and the other two are drawn at 30° to the horizontal. In true isometric projection, each length along the axes is shortened to about 0.816 times, so the isometric scale is 0.816 : 1 (cos 45° ÷ cos 30°, which equals cos 35°16′). In an isometric view (drawing), full lengths are used and the object looks about 22% larger. A circle appears as an ellipse, which is drawn by the four-centre method.

Sections: an imaginary cutting plane removes the front portion to show interior details. The cut surface is hatched with thin lines at 45° and uniform spacing. Parts like bolts, nuts, shafts, ribs and keys are not hatched when cut along their length. Types of section are full, half, offset, revolved and removed sections.

Development of surfaces means unfolding the surface onto a flat plane. The development of a right circular cone is a sector of a circle with radius equal to the slant height l and angle = (r ÷ l) × 360°, where r is the base radius. The development of a cylinder is a rectangle with sides equal to the circumference and the height.

Worked example: A cone has base radius 30 mm and slant height 90 mm. Sector angle = (30 ÷ 90) × 360° = 120°.

Building drawing and civil conventions

A civil drawing set contains the plan, elevation, section and site plan. A plan is the horizontal section of the building cut at about 1 m above the floor level, showing walls, doors, windows and rooms. The elevation is the external view of one face of the building. The section shows the foundation, floor, roof, plinth height and floor-to-ceiling height. A line plan shows the centre lines of walls only.

Common symbols and practices:

• Doors are shown by a quarter arc showing the swing. Windows are shown by thin parallel lines in the thickness of the wall.
• In the plan, walls are hatched or filled with a pattern for brick, stone or concrete.
• A working drawing gives dimensions and notes needed for construction. A foundation plan shows trenches, footing widths and column positions.
• A schedule of openings lists the doors (D), windows (W) and ventilators (V), their sizes and numbers.
• The north point is always marked on a plan so that orientation is clear.
• Water supply and drainage drawings use standard symbols for pipes, valves, traps, manholes and inspection chambers.

Computer-aided drafting (CAD) uses a computer to create drawings with commands such as line, circle, offset, trim, layer and dimension. Its main advantages are accuracy, easy editing, and storage of layers for a drawing set.

Exam traps

• First-angle projection (used in India) places the plan below the front view. Third-angle places it above.
• The thickest line is the visible outline. Dimension, extension and hatching lines are thin.
• Hidden edges are dashed. Centre lines are chain thin. Do not mix them.
• RF is a ratio of lengths and has no unit. A scale of 1 : 50 has RF = 1/50.
• Isometric scale (0.816) is not the same as an isometric drawing, which uses true lengths.
• Eccentricity: ellipse < 1, parabola = 1, hyperbola > 1, circle = 0.
• A sheet A3 is half of A2, not of A4. A4 is half of A3.
• Cut sections are hatched at 45°. Shafts, bolts and ribs are not hatched when cut longitudinally.

One-liners

• 1. A0 sheet size is 841 × 1189 mm.
• 2. A4 sheet size is 210 × 297 mm.
• 3. The title block is placed at the bottom right corner of the sheet.
• 4. Representative Fraction = drawing length ÷ actual length.
• 5. Dashed lines represent hidden edges.
• 6. Chain thin lines represent centre lines.
• 7. India follows first-angle projection.
• 8. In isometric projection the axes are 120° apart.
• 9. The isometric scale ratio is 0.816 : 1.
• 10. A cycloid is traced by a point on a rolling circle.
• 11. The involute is used for gear tooth profiles.
• 12. The plan of a building is a horizontal section at about 1 m above the floor.

Practice questions

1. What is the standard size of an A0 drawing sheet?

   1. 420 × 594 mm
   2. 841 × 1189 mm
   3. 594 × 841 mm
   4. 297 × 420 mm
   Answer

   B. 841 × 1189 mm

   A0 is 841 × 1189 mm and has an area of 1 m².

2. Which method of projection is followed in India for engineering drawing?

   1. Perspective projection
   2. Third-angle projection
   3. Oblique projection
   4. First-angle projection
   Answer

   D. First-angle projection

   Indian standards adopt first-angle projection.

3. Hidden edges of an object are shown in a drawing by

   1. continuous thin lines
   2. chain thin lines
   3. dashed lines
   4. continuous thick lines
   Answer

   C. dashed lines

   Hidden edges use dashed lines.

4. Centre lines in a drawing are shown by

   1. continuous thick lines
   2. chain thin lines
   3. dashed medium lines
   4. freehand wavy lines
   Answer

   B. chain thin lines

   Axes and centres of circles use long-short-long chain thin lines.

5. The eccentricity of a parabola is

   1. equal to 1
   2. less than 1
   3. greater than 1
   4. equal to 0
   Answer

   A. equal to 1

   For a parabola the distance from the focus equals the distance from the directrix, so e = 1.

6. The title block of a drawing sheet is normally placed at the

   1. top right corner
   2. centre of the sheet
   3. bottom right corner
   4. top left corner
   Answer

   C. bottom right corner

   Standard practice puts the title block at the bottom right corner.

7. The curve traced by a point on the circumference of a circle rolling along a straight line without slipping is a

   1. parabola
   2. cycloid
   3. helix
   4. involute
   Answer

   B. cycloid

   This is the definition of a cycloid.

8. In true isometric projection, the angle between any two isometric axes is

   1. 90°
   2. 135°
   3. 60°
   4. 120°
   Answer

   D. 120°

   The three isometric axes are 120° apart.

9. Hatching lines in a sectional view are normally drawn at

   1. 45° to the horizontal
   2. 90° to the horizontal
   3. 10° to the horizontal
   4. 15° to the horizontal
   Answer

   A. 45° to the horizontal

   Section lines are thin lines at 45° with uniform spacing.

10. Which curve is commonly used for the profile of a gear tooth?

    1. Cycloid
    2. Parabola
    3. Involute
    4. Helix
    Answer

    C. Involute

    Involute teeth keep a constant velocity ratio and are standard for gears.

11. A road 600 m long is shown as 12 cm on a drawing. The Representative Fraction is

    1. 1/50000
    2. 1/5000
    3. 1/500
    4. 1/2500
    Answer

    B. 1/5000

    600 m = 60,000 cm; RF = 12 ÷ 60,000 = 1/5000.

12. A right circular cone has a base radius of 30 mm and a slant height of 90 mm. The angle of the sector in its development is

    1. 90°
    2. 60°
    3. 180°
    4. 120°
    Answer

    D. 120°

    Sector angle = (r ÷ l) × 360° = (30 ÷ 90) × 360° = 120°.

13. The interior angle of a regular hexagon is

    1. 135°
    2. 120°
    3. 108°
    4. 144°
    Answer

    B. 120°

    (n − 2) × 180° ÷ n = 4 × 180° ÷ 6 = 120°.

14. A wall 6 m long is drawn to a scale of 1 : 50. Its length on the drawing is

    1. 600 mm
    2. 60 mm
    3. 120 mm
    4. 12 mm
    Answer

    C. 120 mm

    6000 mm ÷ 50 = 120 mm.

15. On a plan drawn to a scale of 1 : 100, a length of 40 mm represents an actual length of

    1. 40 m
    2. 0.4 m
    3. 400 m
    4. 4 m
    Answer

    D. 4 m

    40 mm × 100 = 4000 mm = 4 m.

16. At a scale of 2 : 1, an object edge of 15 mm is drawn with a length of

    1. 30 mm
    2. 45 mm
    3. 15 mm
    4. 7.5 mm
    Answer

    A. 30 mm

    An enlarged scale of 2 : 1 doubles the length: 15 × 2 = 30 mm.

17. The interior angle of a regular pentagon is

    1. 108°
    2. 72°
    3. 120°
    4. 100°
    Answer

    A. 108°

    (5 − 2) × 180° ÷ 5 = 108°.

18. A right circular cone has a base radius of 20 mm and a slant height of 80 mm. The sector angle of its development is

    1. 60°
    2. 45°
    3. 90°
    4. 120°
    Answer

    C. 90°

    (20 ÷ 80) × 360° = 90°.

19. The development of the curved surface of a cylinder of diameter 40 mm and height 50 mm is a rectangle of size about

    1. 40 mm × 50 mm
    2. 251.3 mm × 50 mm
    3. 62.8 mm × 50 mm
    4. 125.7 mm × 50 mm
    Answer

    D. 125.7 mm × 50 mm

    The length equals the circumference π × 40 = 125.7 mm; the width is the height, 50 mm.

20. A wall 3 m long is drawn at a Representative Fraction of 1/20. Its drawn length is

    1. 60 mm
    2. 150 mm
    3. 15 mm
    4. 1500 mm
    Answer

    B. 150 mm

    3000 mm ÷ 20 = 150 mm.

21. A line of true length 100 mm along an isometric axis appears in true isometric projection with a length of about

    1. 81.6 mm
    2. 86.6 mm
    3. 100 mm
    4. 70.7 mm
    Answer

    A. 81.6 mm

    The isometric scale is 0.816, so 100 × 0.816 = 81.6 mm.

22. How many A4 sheets have the same total area as one A2 sheet?

    1. 4
    2. 2
    3. 6
    4. 8
    Answer

    A. 4

    A2 is twice A3 and A3 is twice A4, so A2 = 4 × A4.

23. In first-angle projection, the front view is placed

    1. to the left of the plan
    2. below the plan
    3. to the right of the plan
    4. above the plan
    Answer

    D. above the plan

    In first-angle projection the plan lies below the front view.

24. Which of the following is normally NOT hatched when cut along its length in a sectional view?

    1. A base plate
    2. A bearing bush
    3. A shaft
    4. A casting body
    Answer

    C. A shaft

    Solid parts like shafts, bolts, keys and ribs are shown unhatched when sectioned longitudinally.

25. A diagonal scale is used to read

    1. only angles
    2. very small fractions using a vernier
    3. areas of figures
    4. three units such as metres, decimetres and centimetres
    Answer

    D. three units such as metres, decimetres and centimetres

    Diagonal division of a small rectangle gives a third, smaller unit.

26. A scale of chords is used to

    1. read areas
    2. measure and set out angles
    3. measure two different units on one scale
    4. draw ellipses
    Answer

    B. measure and set out angles

    The scale of chords is a device for angles.

27. The plan of a building is a horizontal section taken at a height of about

    1. the lintel level
    2. the ceiling level
    3. 1 m above the floor
    4. the plinth level
    Answer

    C. 1 m above the floor

    The cutting plane passes through doors and windows, at about 1 m above the floor.

28. Which projection shows three faces of an object in a single view?

    1. Sectional elevation
    2. Isometric projection
    3. Orthographic front view
    4. Plan alone
    Answer

    B. Isometric projection

    Isometric view shows the top and two sides together.

29. A screw thread is based on which curve?

    1. Helix
    2. Cycloid
    3. Parabola
    4. Ellipse
    Answer

    A. Helix

    A thread is a helix wound on a cylinder or cone.

30. The ratio of the sides of the A-series drawing sheets is

    1. 1 : 2
    2. 1 : √2
    3. 2 : 3
    4. 1 : √3
    Answer

    B. 1 : √2

    Halving the longer side keeps the ratio of sides at 1 : √2.

31. Which pencil grade is best for light construction lines?

    1. HB
    2. 2B
    3. 2H
    4. 6B
    Answer

    C. 2H

    Hard grades give thin, light lines for construction.

32. The external view of one face of a building is called the

    1. plan
    2. foundation plan
    3. line plan
    4. elevation
    Answer

    D. elevation

    An elevation shows an outside face of the building.

33. Which of the following is a standard height for capital letters in drawing?

    1. 8 mm
    2. 5 mm
    3. 6 mm
    4. 4 mm
    Answer

    B. 5 mm

    Standard heights are 2.5, 3.5, 5, 7, 10, 14 and 20 mm.

34. Which of the following statements is/are correct? 1. An A0 sheet has an area of 1 square metre. 2. An A4 sheet is half of an A3 sheet.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    A0 has 1 m² area and each next size is half the previous one.

35. Which of the following statements is/are correct? 1. Hidden edges are shown by dashed lines. 2. Centre lines are shown by continuous thick lines.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Centre lines are chain thin lines, not continuous thick.

36. Which of the following statements is/are correct? 1. The eccentricity of an ellipse is less than 1. 2. The eccentricity of a hyperbola is less than 1.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Hyperbola has e greater than 1.

37. Which of the following statements is/are correct? 1. In first-angle projection the plan lies below the front view. 2. India follows third-angle projection.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    India follows first-angle projection.

38. Which of the following statements is/are correct? 1. The isometric axes are 120° apart. 2. The vertical isometric axis is drawn at 30° to the horizontal.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The vertical axis stays vertical; the other two are at 30° to the horizontal.

39. Which of the following statements is/are correct? 1. The Representative Fraction has no unit. 2. RF = length on the drawing ÷ actual length.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    RF is a pure ratio of lengths.

40. Which of the following statements is/are correct? 1. Dimensions on engineering drawings are generally given in metres. 2. Extension lines touch the object outline.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Dimensions are in millimetres and extension lines stop just short of the object.

41. Which of the following statements is/are correct? 1. An epicycloid is traced by a circle rolling inside another circle. 2. An involute is traced by the free end of a thread unwound from a circle.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    An epicycloid is made by rolling outside; a hypocycloid by rolling inside.

42. Which of the following statements is/are correct? 1. Hatching lines are drawn as thick lines. 2. The cutting plane is shown by a chain line that is thick at the ends.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Hatching lines are thin.

43. Match the lines with their use: (a) Visible outline (b) Hidden edge (c) Centre line. Types: (1) Chain thin (2) Continuous thick (3) Dashed

    1. a-3, b-2, c-1
    2. a-1, b-3, c-2
    3. a-2, b-1, c-3
    4. a-2, b-3, c-1
    Answer

    D. a-2, b-3, c-1

    Visible outline is continuous thick, hidden edge is dashed, centre line is chain thin.

44. Match the curve with its eccentricity: (a) Ellipse (b) Parabola (c) Hyperbola. Values: (1) e = 1 (2) e < 1 (3) e > 1

    1. a-3, b-1, c-2
    2. a-2, b-3, c-1
    3. a-1, b-2, c-3
    4. a-2, b-1, c-3
    Answer

    D. a-2, b-1, c-3

    Ellipse e < 1, parabola e = 1, hyperbola e > 1.

45. A 2 m length is shown as 25 mm on a drawing. The scale of the drawing is

    1. 1 : 8
    2. 1 : 800
    3. 1 : 80
    4. 1 : 50
    Answer

    C. 1 : 80

    2000 mm ÷ 25 mm = 80, so the scale is 1 : 80.