Orthographic projection, dimensioning and standardization
This domain covers the standardized language of the technical sheet: European orthographic views, dimensioning per UNE-EN ISO 129-1, sheet formats, scales, tolerances and surface finish. The exam usually gives a piece and asks for the missing view, the section, or the correct dimensioning of a badly dimensioned drawing.
Core rules
- Views (UNE-EN ISO 5456-2, first angle / sistema europeo): the planta goes below the alzado and the left-hand profile goes on the right. Draw the minimum number of views that defines the piece; suppress repeated hidden lines when a section is used.
- Sections and cuts: hatching at 45°, constant spacing, never hatched across a shaft or a bolt; a half-section for symmetrical pieces.
- Dimensioning: each element is dimensioned once and once only - no closed chains and no duplicated values. Dimensions are classified as funcionales, no funcionales (de fabricación) and auxiliares. Values in millimetres without the unit; ⌀ for diameters, R for radii, □ for squares, SR for spherical radii. Figures above 180° are dimensioned by diameter.
- Formats and scales: series A (A0 = 841 × 1189 mm, ratio 1:√2, each fold halves the area); preferred scales per UNE-EN ISO 5455 (1:2, 1:5, 1:10, 2:1...), with a title block and a graphic scale on the sheet.
- Tolerancing and finish: UNE-EN ISO 1101 symbols for form (straightness, flatness, roundness, cylindricity), orientation (parallelism, perpendicularity, angularity) and location; roughness per UNE-EN ISO 1302.
Worked example
A plate 240 × 120 × 40 mm must be drawn on A4 (210 × 297 mm, usable area about 190 × 277 mm with 10 mm margins and an 80 mm title strip). At 1:5 the plate measures 48 × 24 mm - too small. At 1:2 it measures 120 × 60 mm, which leaves room for the elevation, the planta below it and the profile on the right, plus dimensions. Answer: scale 1:2, planta under the alzado, total length dimensioned once.
Traps that cost marks
Over-dimensioning (a duplicated dimension removes the manufacturing tolerance); writing units after every figure; dimensioning to hidden lines; dimension text that cannot be read from the bottom or the right; using a non-normalized scale such as 1:7; confusing first- and third-angle placement; hatching at an arbitrary angle.