Measuring and Cutting Operations
Domain 6 tests accuracy: measuring and marking, calculating allowances before cutting, producing square cuts, and removing burr so the joint seals first time.
Measure, then mark
- Steel tape for lengths, try square for squareness, spirit level and plumb bob for alignment.
- Mark with a scriber or soapstone and centre punch drill points.
- Wrap a strip of paper or a wrap-around round the pipe to mark a truly square line.
Allowances
- Cut length equals face-to-face length plus thread engagement at each end plus any expansion gap.
- A threaded socket takes roughly 12 to 15 mm of pipe per end on 25 mm GI.
- A solvent-weld socket needs the full insertion depth plus a little cement space.
Cutting
- Hacksaw with the right TPI, wheel cutter for copper, ratchet cutter for PVC.
- Ream and chamfer after cutting, then check the cut against a try square.
- Measure twice and cut once, and write a cutting list before starting.
Worked example
A 25 mm GI pipe must give 500 mm between two fittings. Reasoning: each threaded end engages about 12 to 15 mm, so cut about 500 + 2 × 13 = 526 mm, thread the ends, trial-fit, and trim if the run comes out long. Cutting exactly 500 mm would leave the run short by the socket depth at each end.
Traps
Forgetting fitting make-up; leaving burr inside the bore; cutting out of square so the thread runs crooked; trusting a worn tape hook.