Workshop Practice and Practical Skills
Domain 9 tests bench work and measurement - the marks a practical candidate should never lose.
Measuring instruments
Steel rule 0.5 mm; vernier caliper least count 0.02 mm (0.05 mm on some); micrometer 0.01 mm; dial gauge 0.01 mm; feeler gauge for gaps; bore gauge for cylinder wear; torque wrench for specified tightening. Check zero before measuring and read at eye level to avoid parallax error.
Bench work
Mark from a datum with a scriber and centre punch before drilling. Tap drill size = major diameter − pitch, so M10 × 1.5 needs an 8.5 mm drill. Use the correct spanner - never pliers or a pipe wrench on a hex nut. Clean and oil tools after use, store them dry, and never leave the chuck key in the drill.
Worked example
Vernier reading: main scale 12 mm, and the 7th vernier line coincides with a main-scale line; least count is 0.02 mm. Reading = 12 + (7 × 0.02) = 12.14 mm. If the jaws are closed and the vernier zero sits 0.04 mm past the main-scale zero, that zero error is subtracted: 12.14 − 0.04 = 12.10 mm.
Traps that cost marks
Ignoring zero error; over-tightening a micrometer instead of using its ratchet; measuring a hot component; mixing up torque units (N·m and kgf·m differ by about ten times); using a torque wrench to slacken a fastener; reading the vernier from an angle.