Joint Design, Fabrication and Fit-Up
This domain tests joint types, groove preparation, fit-up and fabrication sequence. A good weld cannot compensate for a badly fitted joint.
Core rules
- Basic joints: butt, lap, T, corner and edge. Groove types include square, single-V, double-V, U and J. Choose the joint for access, thickness and loading.
- Fit-up controls root gap, root face, bevel angle, alignment and mismatch. Use gauges and clamps; do not force distorted parts into place.
- Tack welds should be made by a qualified welder, cleaned, and fused into the final weld or removed if defective.
- Fabrication sequence controls shrinkage. Balance welds about the neutral axis, use backstep or skip sequence, and allow for shrinkage.
Worked example
A 6 m long butt weld in mild steel is planned with an allowance of 1.5 mm/m longitudinal shrinkage. Allowance = 6 × 1.5 = 9 mm. The parts are cut 9 mm longer or the sequence is adjusted so the finished length is correct. The reasoning is that weld metal contracts on cooling and pulls the parts together.
Traps that cost marks
- Excessive root gap filled with wire instead of correcting fit-up.
- Tack welds left cracked or unfused.
- Welding from one end continuously on a long joint.
- Ignoring shrinkage allowance until after machining.