Cutting, Forming, and Fabrication
Cutting methods and their edges
- Oxy-fuel cutting suits carbon steel: preheat, then pierce away from the finished edge so blow-back does not gouge the part. Kerf width, drag lines and edge squareness change with tip size, gas pressure and travel speed.
- Plasma cuts faster on thin plate and on stainless and aluminum; the kerf is wider and slightly bevelled, so fit-up allowances differ from oxy-fuel.
- Mechanical methods - shear, band saw, abrasive saw, waterjet - each leave a different edge. A sheared edge is cold-worked and may need dressing before bending or welding.
Forming
Press-brake work depends on bend allowance: BA = θ(rad) × (R + K·t), where K is the neutral-axis factor, roughly 0.33–0.5 for common cold bending. Rolling depends on the mean diameter: developed length of a shell = π × D_mean.
Worked example: flat blank for a 90° bend
6 mm plate, inside radius 12 mm, K = 0.33, legs dimensioned to outside corners. Outside setback = R + t = 18 mm. θ = 1.5708 rad. BA = 1.5708 × (12 + 0.33 × 6) = 1.5708 × 13.98 = 21.96 mm. Bend deduction = 2 × 18 − 21.96 = 14.04 mm. Flat length = sum of outside leg dimensions − 14 mm. Ignore the deduction and the part comes out 14 mm long, and every hole moves with it.
Fit-up and distortion
- Weld preparation must match the procedure: bevel angle, root face and root gap all control penetration.
- Shrinkage is predictable; compensate with preset, layout allowance, balanced sequence and backstep. Strongbacks hold what welding wants to move.
- Tacks are welds: qualified procedure, matching consumable, small, and consumed into the root.
Traps
- Over-rolling and then correcting with heat in the wrong place; measure springback on the first pass and compensate.
- Cutting to the line and losing the kerf, leaving the part undersize.
- Bending across an undressed sheared edge and starting a crack.