Technical Drawing and Measurement
The Technical Drawing and Measurement domain tests drawing interpretation and dimensional control: orthographic views, first-angle projection, dimensioning and tolerance, development, and correct use of measuring instruments.
Drawing conventions
- In first-angle projection, the view from the front is placed above the plan; the left view goes on the right.
- Dimensions should be given from a datum, with tolerances and surface finish where needed.
- Centre lines, hidden detail and section hatching must be read consistently.
Measuring
- Vernier calipers: main scale plus vernier coincidence. Micrometer: sleeve plus thimble.
- Check zero error before use; measure at the same temperature as the workpiece where possible.
- Use a try square and protractor for angular work; use dividers or trammels for arcs.
Worked example
A metric micrometer reads 7.5 mm on the sleeve and 22 divisions on the thimble. With a 0.01 mm thimble, the reading is 7.5 + (22 × 0.01) = 7.72 mm. If the zero error is +0.02 mm, subtract it: 7.70 mm.
Traps
- Confusing first- and third-angle projection.
- Reading the vernier from the wrong zero.
- Parallax when reading a scale.
- Forgetting tolerance and measuring only the nominal size.
- Using a damaged or uncalibrated instrument.