Bench fitting, hand tools and measurement
Bench fitting, hand tools and measurement covers marking out, sawing, filing, scraping, drilling and tapping by hand, plus the correct use of calipers, micrometers, dial gauges, surface plates and angle standards.
Core rules
- Mark from a datum, not from a damaged edge. Use a surface plate, height gauge, V-block and angle plate to establish references.
- A file cuts on the forward stroke; keep the file flat and use chalk on a loaded file. Never use a file without a handle.
- Tap drill size for a metric thread is approximately major diameter minus pitch; use cutting fluid and reverse to break chips.
- Vernier least count = smallest main-scale division divided by number of vernier divisions. For a 0.02 mm caliper, each vernier division represents 0.02 mm.
- Micrometer reading = barrel reading + thimble division times least count; check zero before use and use the ratchet.
- Limits and fits describe permitted variation; clearance, interference and transition fits are selected by function.
Worked example
A 0.02 mm vernier caliper reads 12 mm on the main scale, and the 13th vernier line coincides. The vernier contribution is 13 times 0.02 = 0.26 mm, so the reading is 12.26 mm. If the caliper has a zero error of plus 0.03 mm, subtract it: 12.26 minus 0.03 = 12.23 mm.
Traps
Parallax, tilted calipers, a dirty micrometer face, excessive measuring force and reading the wrong vernier line all cause small errors that fail a tolerance check. Do not use calipers as a spanner or scriber.