Sheet Metal Work and Fabrication
What the domain tests
Sheet Metal Work and Fabrication tests layout, development, cutting, bending, folding, seaming and assembly. You must allow for bend allowance, select snips and machines correctly, and produce accurate flat patterns.
Core rules
- Sheet metal is cut by shearing, snips or nibbling. Use straight snips for straight cuts and left- or right-hand snips for curves; do not twist the blades.
- Mark the flat pattern from a datum and include bend allowance. The neutral axis lies between the inside and outside surfaces, so a bend consumes material.
- For a 90° bend, a simple estimate is bend allowance = (π/2)(R + T/2), where R is inside radius and T is thickness. Use the drawing’s formula if one is specified.
- Fold with the bend line aligned to the machine or folding bars; clamp evenly and bend progressively. Holes near a bend distort, so keep edge distance.
- Join by riveting, spot welding, soldering or mechanical fasteners, depending on strength, appearance and service.
Worked example
A 90° bend has inside radius 4 mm and sheet thickness 2 mm. Using the neutral-axis estimate: BA = 1.571 × (4 + 1) = 1.571 × 5 = 7.85 mm. Add 7.85 mm to the flat length for that bend, then check the drawing’s required outside dimensions and tolerances before cutting.
Traps
Forgetting bend allowance makes the finished part short. Cutting exactly on the line removes the line and can lose size; cut just outside and finish. Do not use snips for thick plate or force a fold that causes cracking. Watch for holes stretching near bend lines.