Fitting, Assembly, and Alignment
What this domain tests
Fitting questions check root gap, alignment, tack quality, and distortion control. Assembly questions check sequence, jig use, and verification of square, level, and plumb.
Fit-up essentials
- Root gap, root face, and bevel must match the WPS; a tight gap causes lack of penetration, a wide gap causes burn-through and excess shrinkage.
- Hi-lo, the internal misalignment, is limited by code; check with a hi-lo gauge before tacking.
- Clean the joint faces; tacks must be made with approved filler, be of adequate size, and be feathered at the ends so they fuse into the root.
- Do not hammer or wedge to close a gap; that stores stress that reappears as distortion.
Alignment and verification
Measure square with equal diagonals, level with a spirit or laser level, plumb with a level or plumb line, and long straight runs with a piano wire or laser. Verify before welding, after tacking, and after cooling, because hot steel moves. Pipe fit-up checks bolt-hole alignment across the flange centreline, gap, and rotation. Shaft alignment uses a straightedge and feeler, dial indicators, or laser alignment.
Distortion control
Welding shrinks; a typical allowance for carbon steel butt welds is about 1-2 mm per metre, more for heavy multi-pass work. Control methods:
- Balanced welding: alternate sides of the neutral axis.
- Backstep and skip sequence to spread heat.
- Preset the joint against the expected pull.
- Minimum passes, correct bead size, and controlled interpass temperature.
- Restraint, strongbacks, and chill bars where the procedure allows.
- Do not peen the root pass; peen only where permitted.
Assembly sequence
Dry-fit, check dimensions, mark match lines, disassemble, clean, reassemble, tack, verify again, then weld in the planned sequence. Leave final machining and hole drilling until after welding where possible.
Worked example
A 1500 mm by 900 mm frame must be square. The diagonal equals the square root of 1500 squared plus 900 squared, about 1749 mm. Measured diagonals are 1749 mm and 1755 mm, a 6 mm difference. Rack the frame to split the error, clamp, and re-measure. Then expect welding to pull the frame: preset the diagonals a few millimetres long on the side that will shrink, and weld with a balanced sequence.
Traps
- Tacks that are too small or made with the wrong filler.
- Checking square while the frame is still hot.
- Clamping a joint into place and welding under restraint without allowing for stress.
- Ignoring weld shrinkage when setting diagonals.
- Welding a long seam from one end only.