Sheet Metal Work
Sheet Metal Work tests layout, cutting, bending, drilling, forming and flat-pattern development. You need to read dimensions, allow for bend radius and thickness, and produce a part that fits without distortion.
Core rules
- Work from a datum edge and mark clearly; use a scriber, dividers and try square, and centre-punch hole positions.
- Sheet gauge affects stiffness and formability. Thicker sheet needs more force and a larger bend radius.
- Bend allowance depends on angle, inside radius, thickness and neutral-axis factor K. For a 90° bend, BA = (π/2)(R + K·T).
- Cut outside the line and finish to the line with a file or deburring tool; remove burrs before assembly.
- Drill pilot holes, support the sheet and use the correct speed; clamp parts to prevent tearing.
- Plan rivet or spot-weld spacing from edge distances and pitch, not by eye.
Worked example
A 90° bend has inside radius R = 4 mm, sheet thickness T = 2 mm and K = 0.5. BA = 1.5708 × (4 + 0.5 × 2) = 1.5708 × 5 = 7.85 mm. The flat blank must include this bend allowance between the two leg dimensions. If the legs are measured to the outside of the bend, use the bend deduction instead. Reasoning: ignoring the allowance makes the finished part short.
Traps
Do not measure from a damaged edge. Do not use outside dimensions as flat lengths without allowance. Do not leave burrs that prevent tight joints. Do not use tin snips for thick sheet or tight curves.