Engineering Guides
Sheet metal is usually cheaper for uniform-wall enclosures and brackets, while CNC machining wins on varying thickness and tight tolerances. This guide compares both and explains bend radius and K-factor basics.
Choose sheet metal fabrication for enclosures, covers and brackets with a uniform wall thickness, bent flanges and moderate tolerances; it uses less material, needs less machine time per part and becomes much cheaper than machining as quantity grows. Choose CNC machining when the part has varying wall thickness, pockets, bosses, tight tolerances across several faces, or when you need a small number of parts with complex 3D geometry. Many good enclosure designs combine both: a bent sheet metal body with machined or inserted features where precision is needed.
Sheet metal fabrication starts with flat sheet, typically 0.5–6 mm thick. The flat pattern is cut by laser or punching, then formed on a press brake into flanges and boxes. Features are added with hardware insertion (self-clinching nuts, studs and standoffs), welding, tapping or countersinking. Higher volumes may use stamping dies.
CNC machining starts with a solid block, bar or plate and removes material with milling and turning tools until only the part remains. Wall thickness, pockets, bosses and threads can vary freely across the part.
The difference in starting material drives everything else: material use, cost structure, tolerance and design freedom.
| Factor | Sheet metal fabrication | CNC machining |
|---|---|---|
| Starting material | Flat sheet (typically 0.5–6 mm) | Solid block, plate or bar |
| Wall thickness | Uniform, equal to sheet thickness | Can vary across the part |
| Geometry | Flat panels, bends, flanges, cut-outs | Pockets, bosses, 3D surfaces, threads anywhere |
| Typical tolerances | Laser-cut features around ±0.1 mm; bent dimensions looser, often ±0.2–0.5 mm; bend angle around ±0.5–1° | ±0.05 mm standard; down to ±0.01 mm or tighter on critical features |
| Material use | Efficient (nesting of flat patterns) | Often removes most of the starting block |
| Setup / tooling | Low for laser + brake; high for stamping dies | Low (fixtures and programs) |
| Cost per part at low quantity | Low to moderate | Moderate to high for large parts |
| Cost per part at high quantity | Low, and very low with stamping | Decreases slowly |
| Weight | Light for large enclosures | Heavier unless heavily pocketed |
| Typical materials | 5052 aluminum, cold-rolled steel, galvanized steel, 304 stainless, copper | Almost any metal or engineering plastic |
| Best for | Enclosures, chassis, brackets, panels, covers | Precision housings, heat sinks, blocks, complex brackets |
Sheet metal has its own DFM rules. Getting them right avoids cracked bends, distorted holes and rework.
The inside bend radius should usually be at least equal to the material thickness (1T) for ductile materials such as mild steel and 5052 aluminum. Harder or less ductile materials need more. Heat-treated aluminum such as 6061-T6 tends to crack on tight bends and typically needs a radius of several times its thickness, which is why 5052 is the standard aluminum for bent parts. Using standard punch radii that your supplier already has also reduces cost.
When sheet bends, the material on the inside of the bend compresses and the outside stretches. Somewhere in between is the neutral axis, which keeps its original length. The K-factor is the position of the neutral axis expressed as a fraction of the thickness, measured from the inside surface:
CAD software uses the K-factor to calculate the flat pattern. The bend allowance (the arc length of the neutral axis through the bend) is:
BA = π × (R + K × T) × (A / 180)
where R is the inside bend radius, T the thickness and A the bend angle in degrees.
Example: 2 mm aluminum, inside radius 2 mm, K = 0.4, 90° bend: BA = π × (2 + 0.8) × 0.5 ≈ 4.40 mm.
In practice, the supplier adjusts the flat pattern to its own press brake tooling and material. Send a 3D model with the bend radius and thickness defined, and let the fabricator finalize the flat pattern unless you have a validated K-factor from the same supplier.
There is no single crossover quantity, because it depends on size, material and complexity. The general pattern:
The practical approach is to quote both options for borderline parts. A supplier that offers both processes can compare them on the same drawing.
Yes, and it is often the best answer:
Both processes accept the same main finishes: powder coating, painting, anodizing (for aluminum), electroplating, passivation and brushing. Note that:
Axisform offers laser cutting, sheet-metal bending, stamping and welding alongside 3-, 4- and 5-axis CNC machining, so both routes can be quoted from the same drawing. Browse products or send your design through contact. For machining-specific design rules, see our DFM guide.
For enclosures, brackets and panels with uniform wall thickness, sheet metal is usually cheaper, especially as quantity increases. For small, complex parts or parts with tight tolerances and varying thickness, CNC machining can be more economical.
K-factor values typically range from about 0.3 to 0.5, depending on material, thickness, bend radius and tooling. Tight bends tend toward the lower end; for accurate flat patterns, use the K-factor validated by your fabricator.
6061-T6 can be bent, but it tends to crack on tight radii and needs a generous bend radius. 5052 aluminum is the usual choice for bent sheet metal parts because it forms well with a radius close to its thickness.
Laser-cut features typically hold around ±0.1 mm, while dimensions across bends are looser, commonly ±0.2–0.5 mm, and bend angles around ±0.5–1°. For tighter tolerances, machine critical features after forming or use a CNC machined part.
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