Joint Preparation: Most of a Weld Is Decided Before the Arc Is Struck

Mill scale, rust, paint, galvanising, oil and cutting fluid all end up in the weld pool if they are still there when the arc arrives. Scale and rust are oxides — they make slag inclusions and porosity. Paint and oil burn to gas under the arc and that gas is trapped as the pool freezes. Zinc from galvanising boils at 907 C, well below the pool temperature, so it comes off as a white fume that is genuinely dangerous to breathe (the hazard rung covers it). Clean 25 mm back from the joint on both faces, not just the edge, because the arc wanders and the pool is wider than the gap. A wire brush takes loose rust; a flap disc or grinding wheel takes mill scale. Wipe solvent off and let it dry — a solvent-wet joint is worse than a dirty one.

Two plates that meet with a 3 mm gap in one place and touch in another cannot be welded at one setting. Where they touch the arc burns a hole; where they gape the pool falls through. Cut both edges square and check them against each other, not against a rule — a pair of edges that match make a good joint even if neither is exactly straight. For a butt joint in plate up to about 5 mm, a root gap of roughly the electrode core diameter is the usual start: about 2.5 mm for a 2.5 mm rod. Less and the root does not fuse; more and the pool drops through. Clamp so the gap is held, and tack it (step 4 of the distortion rung explains the order).

Five joints cover nearly everything: butt, lap, tee, corner and edge. A **butt** joint puts the two parts in the same plane and is the only one that can be made as strong as the parent metal, because the weld replaces the section rather than adding to it. It is also the one that needs the most preparation. A **lap** joint overlaps the parts and welds along the exposed edges. It needs no edge preparation at all, which is why it is everywhere in sheet metal, but the load path bends through the overlap so it is poor in fatigue. A **tee** joint stands one part on another and is welded with fillets in the two inside corners. This is the workhorse of frames and brackets. A **corner** joint meets at the edges, and an **edge** joint welds two parallel faces along their common edge — useful for sheet, not for load. Choose by the load first, then by how much preparation the job is worth. A handrail bracket does not need a bevelled butt weld.