7 Sheet Metal Design Rules That Cut Fabrication Cost in Half

Sheet metal is one of the most common fabrication methods in mechanical design, yet surprisingly many engineers draw parts without proper training. The result? Calls from fabricators saying "we can't make this." Follow these 7 rules to cut fabrication costs in half while improving quality.

1. Minimum Bend Radius (R) — At Least Equal to Thickness (t)

If the inside radius is smaller than the sheet thickness, the outside develops cracks. Minimum values: mild steel (SPCC) R ≥ 1t, stainless (SUS304) R ≥ 1.5t, aluminum (A5052) R ≥ 1t. Hardened materials or bends perpendicular to the rolling direction require larger R.

2. Hole-to-Edge Distance ≥ 2t

Holes too close to the edge cause edge deformation during punching. Minimum distance is 2× sheet thickness. Example: For t=2mm sheet, maintain at least 4mm from hole center to edge.

3. Bend Line to Hole Spacing ≥ 2t + R

Holes near bend lines deform into "teardrop" shapes. Maintain at least 2t + R from the hole edge to the inner bend line. If unavoidable, add relief slots at the bend line.

4. Specify Burr Direction

Laser/punch cutting produces burrs on one side. For safety-critical surfaces (exposed panels, hand-contact areas) and sealing surfaces (gasket faces), specify burr direction on drawings. Default burr direction is die-side (bottom), but bending flips orientation.

5. Flat Pattern Dimension Formula

Post-bend dimensions differ from flat pattern. The unfolded length considering neutral axis:

• L = A + B + BA (A, B: flange lengths, BA: Bend Allowance)
• BA = (π/180) × bend angle × (R + K × t)
• K factor: mild steel 0.33, aluminum 0.33, SUS 0.4 (empirical)

6. Use Taps + Bolts Instead of Welding

Don't default to welding for attaching brackets to sheet metal. Welding causes thermal distortion and adds post-processing costs (grinding, painting). Where possible, PEM nuts (clinch nuts), rivet nuts, or tapped holes with bolts are cheaper and allow disassembly for maintenance.

7. Consider Material Utilization (Nesting)

Sheet stock typically comes in 1219×2438mm (4×8 ft) standard sizes. Calculate nesting efficiency for your flat patterns. If utilization falls below 70%, reconsider the design — resize or split the part.