Lettering, Dimensioning and Geometrical Construction
This domain tests single-stroke lettering, the rules of dimensioning to BIS/ISO practice, and the geometrical constructions used to build accurate drawings. Expect questions on letter heights, dimension-line rules, and constructions such as bisectors, hexagons and tangents.
Lettering
Use single-stroke Gothic capitals and numerals. Standard heights are 2.5, 3.5, 5, 7, 10, 14 and 20 mm; the ratio of height to width is about 7:4 for style B. Guide lines are drawn lightly with a 2H pencil. Inclined lettering slopes at 15° to the vertical, which is 75° to the horizontal.
Dimensioning rules
- Extension lines start 2–3 mm from the object and extend 2–3 mm beyond the dimension line.
- Dimension lines are continuous thin, parallel to the dimension, with arrowheads at both ends; numerals sit above the line and are read from the bottom or right-hand edge.
- Use R for radius, Ø for diameter, SR for spherical radius, SØ for spherical diameter, □ for square, and × for repeated features (4 × Ø10).
- Do not repeat a dimension, dimension hidden lines, or place dimensions inside hatched areas. Smaller dimensions go nearer the view.
Construction
Bisect a line with arcs of equal radius from both ends; bisect an angle with an arc from the vertex and equal arcs from the sides. Construct a regular hexagon by stepping the radius around a circle. Draw a tangent from an external point by bisecting the line joining the point to the centre and using the midpoint as centre.
Worked example
To dimension a 40 mm line: draw extension lines 2 mm clear of each end. Draw a thin dimension line 8–10 mm below, parallel to the line, with arrowheads touching the extension lines. Write 40 above the dimension line, centred. For a 20 mm radius, draw a leader from the arc through the centre, mark the arrow on the arc, and write R20. The R is mandatory because 20 alone would mean a diameter or a linear size.
Traps
Forgetting the R or Ø prefix loses marks. Arrowheads must touch the extension lines, not stop short. Do not use the object outline as a dimension line, and do not let dimension lines cross.