Workshop Tools and Measuring Instruments
This domain covers hand tools, torque wrenches, micrometers, vernier callipers, bore gauges, dial indicators, feeler gauges and tyre pressure gauges. The exam tests correct selection, least count, zeroing and safe use.
Core rules
A micrometer has a 0.01 mm least count on a metric thimble; a vernier calliper commonly has 0.02 mm. Read the main scale first, then add the thimble or vernier value. Zero the instrument and check for wear before measuring. Use the ratchet or friction thimble for consistent pressure. Torque wrenches are for tightening to a specified torque, not for loosening; they should be stored at the lowest setting. A dial indicator measures runout and end-play; a bore gauge compares bore diameter with a micrometer setting.
Worked example
A metric micrometer reads 12.5 mm on the barrel and 28 divisions on the thimble. The least count is 0.01 mm, so the thimble value is 28 x 0.01 = 0.28 mm. The reading is 12.5 + 0.28 = 12.78 mm. If the instrument has a zero error of +0.02 mm, the true measurement is 12.78 - 0.02 = 12.76 mm.
Traps
Reading the thimble from the wrong reference line; ignoring zero error; using a torque wrench to undo a stubborn bolt; measuring a hot or burred component; and using a vernier without checking that the jaws close fully at zero. Parallax and dirt on the anvils are common sources of small errors.