Sheet metalwork and fabrication
This domain covers layout, cutting, folding, seaming, riveting and the bend allowance needed to make a flat pattern. The exam tests whether you can develop a component accurately and choose a suitable joint.
Core rules
- Sheet thickness is often given by gauge; always confirm the actual thickness before calculating allowances.
- Mark out from a datum edge, allow for bend radius and material thickness, and cut on the waste side of the line.
- The neutral axis does not stretch or compress. Bend allowance is the arc length along that axis: BA = (pi/180) x bend angle x (R + K x T), where R is inside radius, T is thickness and K is the K-factor, often about 0.33-0.5.
- Fold with the grain direction where possible; bending across a sharp grain can crack the sheet.
- Use lap, seam, rivet or folded joints appropriate to the load. Rivet holes should be slightly larger than the rivet, and rivet spacing should be even.
Worked example
A 90-degree bend is required in 1 mm mild steel with an inside radius of 3 mm. Use K = 0.4. BA = 1.5708 x (3 + 0.4 x 1) = 1.5708 x 3.4 = 5.34 mm. If the two straight flange lengths measured to the bend tangent points are 40 mm and 20 mm, the developed length = 40 + 20 + 5.34 = 65.34 mm. Reason: the flat blank must include the arc length of the neutral axis, not the outside corner, or the finished part will be short.
Traps that cost marks
- Adding flange lengths measured to the outside mould line without a bend deduction.
- Forgetting material thickness in the allowance.
- Cutting exactly on the layout line and losing size.
- Riveting too close to an edge or with uneven spacing.
- Bending against a pronounced grain direction and causing cracks.