Encyclopedia
Sheet metal and plate work processes
Cutting, bending, punching, rolling and joining: principles, orders of magnitude and pitfalls to avoid.
Cutting
Three processes share sheet cutting. The fibre laser is the most widespread up to 25 or 30 mm of steel: a fine beam, a kerf of 0.1 to 0.3 mm, cutting with oxygen for steel and with nitrogen for stainless and aluminium, with no oxidation. Plasma remains economical on thick steel, with a less square cut. Abrasive waterjet cuts any material cold, up to 200 mm.
- Minimum laser hole diameter: about the sheet thickness (smaller holes lose quality).
- The cutting DXF follows the true outline with no kerf compensation: the machine applies it.
- Micro-joints hold the parts in the skeleton during cutting.
Bending
In air bending, the sheet rests on the two shoulders of the V-die and the angle depends on how deep the punch goes. The resulting inside radius is about 15 % of the die opening for mild steel: the die makes the radius, not the punch.
Choosing the die
Common rule: a die opening of 6 to 12 times the thickness (8 × t on average). Too small a die requires a very high force; too large a die gives a large radius and a longer minimum flange.
| t (mm) | V (mm) | Inside R (mm) | Min. flange (mm) | Force (t/m, S235) |
|---|---|---|---|---|
| 1 | 6 – 8 | ≈ 1 | 4 – 5 | 4 – 6 |
| 2 | 12 – 16 | ≈ 2 – 2.5 | 8 – 10 | 10 – 14 |
| 3 | 20 – 24 | ≈ 3 – 4 | 13 – 15 | 16 – 22 |
| 4 | 25 – 32 | ≈ 4 – 5 | 17 – 20 | 22 – 30 |
| 6 | 40 – 50 | ≈ 6 – 8 | 25 – 32 | 35 – 45 |
| 8 | 60 – 80 | ≈ 9 – 12 | 35 – 50 | 55 – 70 |
Indicative values for S235 steel; check against your tool maker's charts.
K factor and flat length
During bending, the outer fibre stretches and the inner fibre is compressed. The neutral fibre keeps its length; it lies at K × t from the inside. The developed length of a bend is angle (in radians) × (R + K × t). K is 0.3 to 0.35 for a small radius (R ≈ t) and tends to 0.5 for large radii (DIN 6935).
Springback
When the punch rises, the sheet opens by a few degrees: 0.5 to 2° for mild steel, 2 to 4° for stainless, more for high-strength steels. The press bends further to compensate; the value measured on a 90° test piece remains the reference.
Punching
A punch press cuts by shearing between a punch and a die, with a clearance of 10 to 20 % of the thickness depending on the material. It excels at repeated holes, forms (louvres, bridges, tapping) and grids; combined laser-punch machines bring both together.
Plate work: flat patterns and rolling
Cylinders, cones, square-to-rounds and hoppers are made from flat patterns calculated on the mean line. A cylinder of mean diameter D has a flat length of π × D. A cone develops into an annular sector; a square-to-round combines four flat triangles and four cone portions, divided into generators.
- Rolled: the rolls give a continuous shape and the flat pattern is exact.
- Bent in facets: each generator is a press bend and the flat pattern follows the chords.
- Dimensions: state whether diameters are inside, outside or on the mean line.
Welding
MIG/MAG for productivity on steel, TIG for stainless and fine, clean sheet work, and now handheld laser welding, which distorts the sheet little and reduces grinding. Welding distortion is prevented at the design stage: tack points, alternating sequences, stiffening flanges.
Glossary
| Flat pattern | Flat shape of the part, before bending or rolling. |
|---|---|
| Flange | Leg bent along an edge of the part. |
| Relief | Notch that prevents tearing at the end of a bend. |
| V-die | Opening of the bending die. |
| K factor | Relative position of the neutral fibre within the thickness. |
| Cylinder (shell) | Rolled cylindrical plate-work element. |
| Square-to-round | Transition between a square or rectangular section and a round section. |
| Faceting | Splitting a warped surface into flat facets bent on the press brake. |
| Nesting | Placing parts on the sheet before cutting. |