British Standards -- shared drawing conventions
British Standards (BS) are the agreed conventions for how a formal (working) drawing is presented, so any skilled third party can read it and make the product exactly as intended. Three strands matter: line conventions (a different line for each kind of information), dimensioning (sizes in millimetres, stated once each) and recommended scales (ratios for reduction and enlargement). Following BS is what makes a drawing usable for manufacture, not just neat.
BS line types -- one line per job
BS gives every kind of information its own line, so a reader can tell an edge from a centre line at a glance. Continuous thick = visible outlines. Continuous thin = dimension, projection and leader lines. Thin dashed = hidden detail (edges out of sight). Thin chain (long-short) = centre lines and axes of symmetry. A fold line on a development has its own convention. Using one weight for everything, or the wrong line for a feature, loses easy marks.
Dimensioning rules and scale ratios
Dimensioning: projection lines run from the feature but do not touch it, the dimension line between them has an arrowhead at each end, and the figure sits above the line. Give sizes in millimetres, state each once, keep them off the outline. Scales are ratios written drawing : real -- reductions 1:2, 1:5, 1:10, 1:20, 1:50, 1:100; enlargements 2:1, 5:1, 10:1. On a reduction, real = drawn x scale (40 mm at 1:5 = 200 mm) and drawn = real / scale (3000 mm at 1:20 = 150 mm).
Drawn from real examiner reports.
Line conventions drawn as plain solid lines
A repeated loss: the shape is right but line conventions are ignored. Hidden detail is drawn solid instead of thin dashed, a centre line is left out or drawn solid instead of thin chain, and on a development the fold-line convention is missed. Marks are reserved for the correct line type -- choose the right line for each feature.
s23 P51 B5b(i); s23 P52 A2a
Every line the same thickness
A second line fault: drawing everything at one thickness, so outlines, dimensions and centre lines cannot be told apart. Apply the thick/thin distinction -- continuous thick for outlines, continuous thin for dimension and projection lines. Examiners also report thick/thin applied only to the outer edge of a mould, not the internal parts. Vary the line weight deliberately.
w23 P53 B5d(i)
Right shape, wrong scale or size
Working to the stated scale and exact dimensions is the base competence, yet candidates draw the right shape at the wrong size or height. A drawing at 1:2 whose lines are not half the real size is not usable -- the part is made the wrong size. Read the scale, convert every real size with the ratio, and measure accurately; neatness cannot rescue a wrong-size drawing.
s23/w23 P51-53 A1
Scale ratio read the wrong way round
Scaling calculations go wrong when the ratio direction is misread. A scale is written drawing : real, so 1:5 is a reduction (one fifth of real) and 5:1 an enlargement. Candidates read 1:5 as five times bigger, or multiply when they should divide (and vice versa). Fix the direction first: drawing to real on a reduction, multiply; real to drawing, divide.
s23 P52 A3; s22 P5 A2c
Dimensioning not to BS rules
Dimensioning marks are lost through avoidable slips: the wrong unit (all sizes are in mm), repeating a dimension so two figures could later disagree, projection lines touching the outline, or the figure below the dimension line instead of above it. State each size once, keep dimensions clear of the object, and nest small dimensions inside larger ones.
Using a non-standard scale
A scale must come from the recommended list -- reduction 1:2, 1:5, 1:10, 1:20, 1:50, 1:100; enlargement 2:1, 5:1, 10:1. Candidates invent an arbitrary ratio such as 1:3 or 1:7 to make a shape fit the paper, which is not a recognised BS scale. Choose the nearest recommended scale that lets the whole object fit, then draw every length to that exact ratio.
1:2 read as twice real size
Reading 1:2 as twice as big. A scale is written drawing : real, so 1:2 means the drawing is half the real size, a reduction. The larger the second number, the smaller the drawing: 1:100 is much smaller than 1:20. Enlargement is the reverse -- 2:1 means the drawing is twice real size. Read the ratio backwards and you scale the wrong way.
Bank the line and dimension marks
Formal-drawing marks are won on presentation. Choose the right line for each feature -- thick for outlines, thin dashed for hidden detail, thin chain for centre lines, thin for dimensions. Dimension to BS: sizes in mm, each stated once, arrowheads at both ends, the figure above the line.
Draw to the exact stated scale
Work to the exact stated scale -- the right shape at the wrong size scores nothing. Convert every real size with the ratio, and draw with instruments rather than estimating. If you choose the scale, pick one from the recommended list that fits the object on the paper.
Scale calculation -- fix the direction
In a scaling calculation, fix the ratio direction first -- a scale is drawing : real. Drawing to real on a reduction, multiply by the scale number; real to drawing, divide. Show the working (a method mark is available even if the figure slips) and state the unit.
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