Measuring and marking out
Measuring and marking out
This domain tests accuracy: reading the right instrument, allowing for fittings and threads, and marking a line that survives the vice and the hacksaw.
Instruments
- Steel rule / measuring tape: tape for long runs, rule for bench work; read at eye level to avoid parallax.
- Try square: checks squareness and marks across a pipe or sheet.
- Vernier caliper: least count usually 0.02 mm; measures outside, inside and depth - useful for checking pipe OD and wall thickness.
- Micrometer: 0.01 mm, for sheet and wire thickness.
- Spirit level and plumb bob: establish level and vertical when setting fixtures and marking pipe routes.
- Marking gauge, dividers, centre punch, scriber, chalk line: transfer and preserve the marked line.
Nominal bore versus outside diameter
GI pipe is sold by nominal bore (15, 20, 25, 32 mm and so on), but the OD and wall thickness are larger and vary between light, medium and heavy classes. Never order pipe by measuring the OD.
Worked example
A 25 mm GI line runs between two 90 degree elbows, centre-to-centre 1500 mm. Each elbow takes 32 mm of pipe, and each end must stand 3 mm proud for the thread. Reasoning: cut length = 1500 − (2 × 32) + (2 × 3) = 1500 − 64 + 6 = 1442 mm. Mark, centre punch, cut on the waste side of the line, then ream and thread.
Traps that cost marks
Measuring OD and ordering NB; forgetting fitting take-up so the pipe ends up short; marking with a thick pencil line and cutting on it; ignoring vice jaw width; letting a tape sag over a long span.