Fabrication, Layout, and Assembly
Fabrication, Layout, and Assembly
This domain tests turning a drawing into a fit-up: layout, cutting lists, jigs, tacking, alignment, and weld sequence. The exam looks for control of accumulation error, squareness, root gap, and shrinkage.
Core rules
- Establish a datum and work from it. Avoid stacking many small measurements; accumulated error grows. Use centrelines, baselines, and matched diagonals.
- Square a rectangle by equal diagonals. For a frame, the diagonal is √(L² + W²); if both diagonals match, the frame is square (assuming opposite sides are equal).
- Fit-up: check bevel angle, root face, root gap, alignment, and hi-lo. Tacks must be the correct size, clean, and strong enough to resist welding stresses. Cracked or porous tacks must be removed.
- Jigs and fixtures reduce distortion and speed assembly, but they must not prevent access or force a part into place with excessive stress.
- Weld sequence controls shrinkage: backstep, skip welding, balanced welding about the neutral axis, and welding from the restrained area toward the free edge reduce bowing.
- Allow for kerf, shrinkage, and machining allowance in cut lists. Check overall dimensions after assembly, not only individual parts.
Worked example
A rectangular frame is specified 1200 mm × 800 mm outside. The diagonal should be √(1200² + 800²) = √(1,440,000 + 640,000) = √2,080,000 = 1442.2 mm. If one diagonal measures 1445 mm and the other 1439 mm, the frame is out of square even though the sides may measure correctly. The fix is to adjust the corners and recheck diagonals before final welding; clamping and welding an out-of-square frame locks in the error.
Traps
- Measuring each part from a different reference.
- Tacking only at the corners and ignoring root gap.
- Using tacks that are too small and crack during welding.
- Welding both sides continuously without a balanced sequence.
- Forgetting kerf when cutting a finished length.
- Assuming a jig eliminates the need to check dimensions.