Trade Theory: Tools, Materials and Workshop Principles
Core concepts
Trade theory tests understanding of the tools, materials, measuring instruments and standard procedures of the candidate's specific trade. Regardless of trade, expect questions on measuring and marking out, cutting and shaping tools, joining methods, material properties, tolerance and fits, and reading simple workshop drawings.
Common traps
- Tool purpose confusion: a try square checks squareness, not length; a vernier caliper measures internal, external and depth dimensions, while a micrometer measures external dimensions to finer accuracy.
- Least count: the smallest measurement an instrument can read. For a vernier caliper with 0.02 mm least count, record two decimal places, not one.
- Material properties: hardness resists scratching, toughness resists shock, ductility allows drawing into wire, and brittleness fractures without deformation.
- Safety in procedure: always isolate power and wear eye protection before drilling or grinding.
- Drawing conventions: first-angle and third-angle projections place views differently; check the symbol before answering.
Rule of thumb
Before any measurement, zero the instrument and check for parallax error by viewing the scale straight on. Record the reading with the correct number of decimal places for the least count.
Worked example
A vernier caliper has a main scale reading of 12 mm and the 7th vernier division coincides with a main scale line. With a least count of 0.02 mm, vernier reading = 7 × 0.02 = 0.14 mm. Total = 12 + 0.14 = 12.14 mm.