SCALE IS THE RATIO OF THE DRAWN LENGTH TO THE ACTUAL LENGTH: SCALE = DRAWING SIZE : ACTUAL SIZE THE THREE CATEGORIES: REDUCTION (1 : n) — the drawing is smaller than the object. Standard values: 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500, 1:1000. Used for buildings, site plans, large assemblies. FULL SIZE (1 : 1) — drawn actual size. The safest choice when the sheet allows it, because nothing is being mentally rescaled. ENLARGEMENT (n : 1) — the drawing is larger than the object. Standard values: 2:1, 5:1, 10:1, 20:1, 50:1. Used for small components, threads, electronic parts. THE STANDARD VALUES ARE MULTIPLES OF 1, 2 AND 5 TIMES A POWER OF TEN. That is deliberate: those are the ratios a person can divide mentally when reading a drawing. A scale of 1:3 or 1:7 is not used, because reading a dimension off it requires arithmetic rather than inspection. WORKED EXAMPLES, since scale questions are always numerical: 1. A wall is 12,500 mm long, drawn at 1:100. drawn length = 12,500 / 100 = 125 mm At 1:50 it would be 250 mm — too wide for the A4 drawing space (which is roughly 190 mm across after margins), so 1:100 IS THE CORRECT CHOICE FOR THAT SHEET. THE SCALE IS CHOSEN TO FIT THE SHEET, and that reasoning is the answer examiners want. 2. A bolt of diameter 8 mm drawn at 5:1 drawn diameter = 8 × 5 = 40 mm 3. A drawing at 1:20 shows a beam as 315 mm. actual length = 315 × 20 = 6,300 mm = 6.3 m THE RULE THAT OVERRIDES EVERYTHING ELSE: DIMENSIONS WRITTEN ON THE DRAWING ARE ALWAYS THE TRUE DIMENSIONS OF THE OBJECT, NEVER THE MEASURED LENGTH ON PAPER. A 12,500 mm wall drawn at 1:100 is labelled 12500 — not 125. THE SCALE AFFECTS THE PICTURE, NOT THE NUMBERS. This has an important corollary: A DRAWING SHOULD NEVER BE MEASURED WITH A RULER to obtain a dimension that is not written down. Prints stretch, photocopies scale, and drawings are revised; the written figure is the only authority. Drawings often carry the note "DO NOT SCALE FROM THIS DRAWING" for exactly this reason. THE SCALE IS STATED IN THE TITLE BLOCK. If a particular view uses a different scale from the rest of the sheet — a magnified detail, for instance — THAT VIEW CARRIES ITS OWN SCALE NOTE beside its title, e.g. "DETAIL A SCALE 5:1". "NTS" (NOT TO SCALE) marks a diagram drawn only for arrangement, where proportions carry no information.

THE FOUR ELEMENTS OF A DIMENSION: ├─────── 45 ───────┤ ┊ ┊ ┊ ┊ 1. EXTENSION (projection) LINES — ─┴──────────────────┴─ thin lines projecting from the feature feature feature, starting with a SMALL GAP (about 1 mm) so they are not confused with the outline 2. DIMENSION LINE — thin, parallel to the measured feature, drawn BETWEEN the extension lines 3. ARROWHEADS — solid, about 3 mm long, ratio roughly 3:1 length:width 4. THE DIMENSION VALUE — the number THE RULES, and each is examinable: 1. UNITS ARE MILLIMETRES BY DEFAULT and are NOT written after every number. State "ALL DIMENSIONS IN MM" once in the title block. Writing "45 mm" on every dimension is a common error. 2. DIMENSION EACH FEATURE ONCE ONLY. A dimension repeated in two views is a chance for the two to disagree after a revision — and if they disagree, the drawing is unbuildable. 3. DIMENSIONS GO OUTSIDE THE VIEW where possible, so they do not clutter the outline. 4. PLACE THEM BETWEEN VIEWS when the feature relates to both. 5. SMALLEST DIMENSIONS NEAREST THE VIEW, larger ones further out, so that extension lines do not cross dimension lines. 6. NEVER LET DIMENSION LINES CROSS EACH OTHER. Extension lines may cross where unavoidable, and are not broken. 7. THE CENTRE LINE AND THE OUTLINE ARE NEVER USED AS DIMENSION LINES, though a centre line may be extended to act as an extension line. 8. ALIGNED versus UNIDIRECTIONAL SYSTEM: ALIGNED — the value is written parallel to the dimension line and read from the bottom or the right-hand side of the sheet. UNIDIRECTIONAL — every value is written horizontally regardless of the dimension line's direction. UNIDIRECTIONAL IS PREFERRED IN MODERN PRACTICE AND IN ALL CAD, because it needs no rotation of the sheet to read and no rotation of text when the drawing is revised. 9. THE SYMBOLS, which must be used rather than words: ⌀ or Ø diameter, written BEFORE the value: ⌀25 R radius, before the value: R12 □ square section × for repeated features: "4 × ⌀8" means four 8 mm holes — one dimension replacing four SR spherical radius ∠ angle, dimensioned in degrees 10. A CIRCLE IS DIMENSIONED BY DIAMETER, AN ARC BY RADIUS. This is not a stylistic preference: a full circle is made by a drill or bore whose size is its diameter, while an arc is produced by a cutter of a given radius. THE DIMENSION REFLECTS THE MANUFACTURING OPERATION. THE THREE DIMENSIONING SYSTEMS: CHAIN DIMENSIONING — each feature measured from the last. ├─20─┼─30─┼─25─┤ SIMPLE BUT DANGEROUS: TOLERANCES ACCUMULATE. If each step is ±0.1 mm, the last feature can be out by ±0.3 mm relative to the first. Use only where the individual gaps matter more than the overall position. PARALLEL (DATUM) DIMENSIONING — every feature measured from ONE common datum face. ├─20─┤ ├────50────┤ ├───────75───────┤ NO ACCUMULATION: every feature's error is ±0.1 mm from the datum, independent of the others. THIS IS THE PREFERRED METHOD, and it also matches how a machine tool actually works, since a CNC machine positions from a fixed origin. ORDINATE DIMENSIONING — the same idea with the datum at 0,0 and values written at the features without dimension lines at all. Used for plates with many holes, where conventional dimensioning would be unreadable. TOLERANCES, since a dimension without one is an unachievable demand: No part can be made exactly to size, so the drawing states the acceptable range. 45 ± 0.1 bilateral, the common case 45 ⁺⁰·² ₋₀·₁ unilateral limits 44.9 / 45.1 limit dimensions, stated as two figures A GENERAL TOLERANCE NOTE in the title block covers every dimension without an individual one. THE ENGINEERING JUDGEMENT: TIGHTER TOLERANCES COST MORE, steeply and non-linearly. Specifying ±0.01 where ±0.5 would function is one of the most common and most expensive novice mistakes in design.