The Sheet Metal Part That Cracked (Sharp Bend)
We designed a sheet metal bracket (a laser-cut, bent plate). The bend was a 90° (sharp, inner radius = 0). On the floor (after bending), the part cracked at the bend (the metal was stretched too much). The problem: the inner bend radius was too sharp (0). Sheet metal needs a minimum bend radius (the material thickness, or more). We changed the bend radius (to 1× the material thickness, e.g., 1 mm for 1 mm sheet). The bend didn’t crack. The mistake was designing a sharp bend (0 radius) without considering the material’s limit.
Sheet metal design (DFM) — design for bending. This article covers the rules.
The Bend Radius
Sheet metal bends (a press brake). The bend has an inner radius (R). A sharp bend (R = 0) cracks (the material stretches). A minimum radius (R ≥ material thickness, t) is safe. For 1 mm sheet, R ≥ 1 mm. For 2 mm sheet, R ≥ 2 mm. The radius depends on the material (aluminum needs a bigger radius, steel is more forgiving).
Step 1: The Bend Allowance (K-Factor)
When the sheet bends, the flat (the blank) is calculated (the bend allowance). The K-factor (the neutral axis) determines the blank size. A typical K-factor is 0.33–0.4. Use the manufacturer’s table (the blank size, for the radius and the angle). Don’t guess (the part won’t fit).
The sheet metal rule: Use a minimum bend radius (R ≥ t). The bracket that cracked had R = 0 (sharp). Use R = t (1 mm for 1 mm sheet). Use the bend allowance (K-factor) for the blank. The part bends (no crack).
Step 2: The Hole Distance (From the Bend)
A hole near a bend (within the bend zone) distorts (the bend stretches the hole). Keep the hole at least 2× material thickness (2t) from the bend. If the hole is too close, it becomes oval (distorted). Move the hole (or use a bigger distance).
Step 3: The Bend Angle (90°, or Other)
A 90° bend (standard). An acute angle (sharp) needs a special tool. An obtuse angle (gentle) is easy. Design for 90° (standard, cheap). Avoid a complex angle (it needs a special tool, expensive).
Step 4: The Material (Thickness, Type)
- Steel (cold-rolled): 1–3 mm. Easy to bend. Cheap.
- Stainless: Harder (needs a bigger radius). Expensive.
- Aluminum: Soft (easy, but needs a bigger radius — it cracks).
| Rule | Value |
|---|---|
| Inner bend radius (R) | ≥ material thickness (t) |
| Hole from bend | ≥ 2t |
| Angle | 90° (standard) |
| Blank | Use K-factor (0.33–0.4) |
A Sheet Metal Checklist
- Is the bend radius right? (R ≥ t?)
- Is the hole far from the bend? (≥ 2t?)
- Is the angle 90°? (Standard?)
- Is the blank calculated? (K-factor?)
- Is the material right? (Steel/stainless/aluminum?)
- Does it crack? (Test?)
- Is the laser cut right? (No burr?)
- Is the bend angle right? (90°?)
- Does the part fit? (Assembly?)
- Is the cost right? (Standard tools?)
The Bottom Line
Sheet metal design (DFM) uses a minimum bend radius. The bracket that cracked had R = 0. Use R = t. Keep the hole away (2t). Use the K-factor (blank). The part that didn’t crack wasn’t the sharpest design — it followed the bend rules.
