Sheet Metalworking Craft Practice
The Sheet Metalworking Craft Practice domain tests the hand and machine skills that turn flat sheet into accurate components: marking out from a datum, cutting, bending, rolling, seam and rivet work, and material identification.
Marking out and allowances
- Work from a datum edge or centre line; do not chain dimensions from a previously cut edge.
- Calculate bend allowance rather than adding outside dimensions when developing a flat pattern.
- Use a scriber on bare metal, a fine marker on coated sheet, and witness marks that survive folding.
Cutting and forming
- Snips cut curves and trims; a guillotine or nibbler handles straight or internal cuts. Keep blades sharp and clamp the sheet firmly.
- Fold on a brake or folder with the bend line aligned to the die; allow for springback and inside radius.
- Rolled cylinders need equal passes and a check against a template.
Worked example
A 90° bend in 1.5 mm mild steel has an inside radius of 3 mm and a K-factor of 0.33. Bend allowance = (π/180) × angle × (R + K×T) = 0.01745 × 90 × (3 + 0.495) = 5.49 mm. Add this allowance to the straight flange lengths measured to the tangent points; do not simply add the outside dimensions.
Traps
- Mixing inside and outside dimensions.
- Ignoring K-factor or springback.
- Marking from a cut edge that is not square.
- Using blunt snips and distorting the edge.