Sheet Metal Design Guide
Forming rules for thickness, bend radius, holes, flat patterns and tolerances
Sheet metal fabrication cuts flat stock and forms it with press brakes, punches and welds. Cost and quality hinge on a small set of rules: thickness range, minimum bend radius, hole-to-edge distances, bend allowance math and the flat-pattern geometry derived from the 3D model. A correctly designed sheet part quotes fast; one that violates bend or hole rules must be redesigned or run on custom tooling.
1. Stock and cutting
Supported thickness range is roughly 0.6–6.35 mm (0.024–0.250 in) in steel, aluminum, stainless and some copper alloys. Laser cutting holds ±0.13–0.3 mm with kerf of 0.1–0.3 mm; waterjet cuts thicker and heat-free but slower; stamping and punching dominate high volume with dedicated dies.
Minimum hole diameter is about 1× the sheet thickness for punching and 0.8 mm for laser; holes smaller than thickness plug with slug or deform. Keep hole centers and edges ≥ 1.5× thickness from adjacent cuts to avoid tearing.
2. Bends and flat patterns
Air bending achieves an inside bend radius down to the sheet thickness; radii smaller than thickness require special tooling and risk fracture on the outer surface. Bend lines should run parallel to grain for aluminum where possible; multi-directional bends are fine but cost more in tooling setup.
The flat pattern is derived from the 3D model using the neutral-axis K-factor, typically 0.30–0.50 depending on material and tooling (0.33 is a common default). The platform quotes from the flat; inconsistencies between model and drawing (e.g. modeled bends vs. flat dimensions) are the top cause of sheet-metal rejects.
3. Forming, hardware and tolerances
Standard sheet-metal tolerance is ±0.3 mm on cut features and ±0.5 mm on formed (bent) dimensions, because springback varies with material temper and bend angle. Precision laser and punch-tape work can tighten cut tolerances to ±0.13 mm.
Captive hardware (press-in PEM studs/nuts, clinch nuts, rivets) is installed in a secondary press operation; keep hardware 2× thickness from edges and specify the correct panel thickness range in the standard tables.
4. Finishing and assembly
Powder coating (25–100 µm), painting, anodizing (aluminum), zinc plating and passivation (stainless) are the common finishes; they cover cut edges, so edge burrs must be removed first for a clean line. Countersinks for flush fasteners should follow standard 82°/90° angles.
Welding (MIG/TIG/spot) joins brackets and enclosures; weld distortion pulls on thin material, so design weldment tolerances looser (±0.5–1.0 mm) and specify weld seams on non-visible faces. Stiffening ribs, hemmed edges and corner reliefs (3–6 mm) improve rigidity and formability.
Quick reference
| Rule | Typical value |
|---|---|
| Thickness range | 0.6–6.35 mm (0.024–0.250 in) |
| Min inside bend radius | = sheet thickness (air bending) |
| Hole diameter | ≥ 1× thickness (punch); ≥ 0.8 mm (laser) |
| Hole-to-edge distance | ≥ 1.5× thickness |
| Cut tolerance | ±0.13–0.3 mm (laser) |
| Formed tolerance | ±0.3–0.5 mm (springback) |
| K-factor default | 0.33 (0.30–0.50 range) |
| Corner relief | 3–6 mm recommended |
Sources & references
- ISO 2768-1:1989 — General tolerances
- ASME Y14.5-2018 — Dimensioning and Tolerancing
- ASTM B209 — Aluminum sheet and plate
- ASTM A1008 — Steel sheet, cold-rolled
- Xometry Learning Center — Sheet Metal Design Guide (public engineering reference)