Safety, Measurement and Workshop Practice
Safety, measurement and workshop practice carries the marks that are easiest to lose: instrument least counts, correct reading, zero error, and the safety rule that fits the situation.
Safety essentials
Wear goggles when chipping, grinding or welding; button the overall; remove rings and ties; secure long hair. Guards must be in place on belts, gears and shafts. Never clean chips by hand - use a brush. Never leave a chuck key in the chuck. Use the correct extinguisher: CO2 or dry powder for electrical fires, never water. Know the first-aid box location and report every accident.
Measuring instruments
Steel rule 0.5 mm; vernier caliper 0.02 mm; micrometer 0.01 mm; dial gauge 0.01 mm; vernier height and depth gauges; try square for squareness; protractor and sine bar for angles; feeler gauge for gaps; surface plate as the datum.
Reading the vernier
Least count = 1 main scale division − 1 vernier scale division. With 1 mm main divisions and 50 vernier divisions, LC = 1 − 0.98 = 0.02 mm. Reading = main scale reading before the zero + (coinciding vernier division × LC).
Micrometer
LC = pitch / thimble divisions = 0.5/50 = 0.01 mm.
Worked example
A vernier shows 12 mm on the main scale and the 23rd vernier line coincides: reading = 12 + 23 × 0.02 = 12.46 mm. A micrometer shows 5.5 mm on the barrel and 25 on the thimble: 5.5 + 25 × 0.01 = 5.75 mm. If the vernier reads 0.04 mm with the jaws closed, subtract that zero error from every reading.
Traps
Using a steel rule for precision work. Forgetting the sign of the zero error. Reading the vernier from the wrong end. Using a micrometer as a C-clamp. Wearing loose sleeves near a lathe.