Chapter 11: Engineering Drawing and Graphics
Drawing is the language of engineering. A drawing tells the manufacturer or builder exactly what to make, using standard conventions so that anyone trained can read it (Giesecke et al., 2016).
Instruments and standards
Traditional instruments: drawing board and T-square or drafting machine, set squares, compass, dividers, scales, pencils (H, HB, 2H), and templates. Drawings follow national and international standards (for example the Sri Lanka Standards and ISO drawing standards) for sheet sizes (A0–A4, each half the size of the one before), title blocks, line types, lettering, and symbols. Line types: thick continuous (visible outlines), thin continuous (dimension and projection lines), dashed (hidden detail), chain thin (centre lines), and chain thick (cutting planes).
Scales and dimensioning
A scale relates drawing size to actual size: 1:1 (full size), 1:20 (reduction), 2:1 (enlargement). A 5 m wall drawn at 1:20 is 250 mm long. Dimensions are written in millimetres unless stated, avoid repetition, show each size once, and are placed outside the view where possible with arrows, extension lines, and clear numerals. A drawing must be complete enough that nothing has to be measured with a ruler.
Orthographic projection
An object is shown in views at right angles: front view, plan (top), and side view. In first-angle projection (widely used in Sri Lanka and the UK) the object is placed between the observer and the plane of projection, so the plan appears below the front view and the left side view appears on the right. In third-angle projection (common in the USA) the plan appears above the front view. A projection symbol on the drawing shows which has been used.
Pictorial drawing
Isometric drawing shows three faces with axes at to the horizontal and true length along the axes; oblique drawing shows one face true and depth at , often at half scale; perspective shows depth as the eye sees it. Sketching freehand quickly with proportion is a skill used on site and in the workshop.
Sections and details
A sectional view cuts the object along an imaginary plane to show internal detail; cut surfaces are hatched with thin lines at . Rules: ribs, shafts, and bolts cut lengthwise are normally not hatched. Auxiliary views show true shapes of inclined faces. Assembly drawings show how parts fit, with a parts list; detail drawings give full information for one part.
Tolerances and fits
No part can be made exactly to size, so a tolerance states the permitted variation. A clearance fit leaves a gap (a rotating shaft in a bearing); an interference fit has the shaft larger than the hole (pressed bearings); a transition fit may give either. Example: hole mm and shaft mm. The hole ranges 20.00–20.02 mm and the shaft 19.96–19.98 mm, so the minimum clearance is mm and the maximum clearance is mm: a clearance fit. Surface finish and geometric tolerances (flatness, parallelism) are also specified.
Electrical, civil, and CAD drawings
Electrical drawings use standard symbols for sources, switches, lamps, and motors and show circuit diagrams, wiring diagrams, and layout plans. Civil drawings include site plans, floor plans, elevations, sections, and structural details. Computer-aided design (CAD) replaces the drawing board with 2D and 3D software that draws, edits, dimensions, and shares drawings accurately, links to 3D printers and CNC machines, and supports building information modelling (BIM) in construction.
Common mistakes
Mixing first- and third-angle conventions.
Over-dimensioning, or leaving out a dimension.
Hatching the wrong areas or using inconsistent line weights.
Practice questions
A 3.6 m wall is drawn at 1:30. What length is it on the drawing? [120 mm]
A shaft is mm and the hole is mm. Find the maximum and minimum clearance. [0.07 mm; 0.00 mm]
State two differences between first-angle and third-angle projection.