Most flat sheet-metal blanks are cut on a fibre laser or a CNC turret punch. For many simple parts either will do. The differences start to matter when a part has hundreds of holes, small holes in thick material, louvres or tapped extrusions, or an edge that has to be welded, painted or handled without gloves.
How the two processes work
A fibre laser focuses a high-power beam to a small spot and melts a narrow channel through the sheet, while an assist gas blows the melt out of the cut. Oxygen, used mainly on thicker mild steel, adds heat by burning the iron and leaves a thin oxide layer on the cut face. Nitrogen, the usual choice for stainless steel, aluminium and thin mild steel, leaves a bright, oxide-free edge. The head follows the contour, so any drawn shape can be cut without tooling.
A turret punch holds a magazine of punch and die sets. The sheet moves under the ram and each hit shears out a slug the shape of the tool. Larger outlines are built up from many overlapping hits, called nibbling. Because the tool presses rather than melts, the same machine can also form the sheet.
Edge quality, kerf and heat
The laser removes a strip of material called the kerf. On thin sheet it is typically in the region of 0.1 to 0.3 mm wide and grows with thickness. The CAM software compensates for it, but it sets a lower limit on how narrow a slot can be cut. The cut face carries fine striations and sometimes a little dross on the underside.
The heat affected zone (HAZ) next to a laser cut is narrow, usually a fraction of a millimetre on sheet, but it is there. It rarely matters, except in heat treated materials or where an oxygen-cut oxide layer would spoil paint adhesion or weld quality. Nitrogen cutting avoids the oxide, and many shops blast or grind oxygen-cut edges before powder coating.
A punched edge has no HAZ, but it is not square either. The punch first pulls the top surface down into a small rollover, then shears a smooth burnished band, often around a third of the thickness when the clearance is right. The rest of the thickness breaks away as a rougher, slightly angled fracture zone, and a small burr is left on the die side. Burr height grows as tools wear, so the burr side should be marked on the drawing if it faces a hand, a seal or a cable.
Punch and die clearance
The die is always slightly larger than the punch. As a rule of thumb, total clearance (both sides added together) is in the order of 10 to 25 percent of the thickness, lower for soft aluminium and higher for stainless steel. The fabricator chooses it from the tooling supplier's tables. For the designer, a punched hole is sized by the punch and is slightly larger on the die side.
The force needed for each hit is the sheared perimeter times the thickness times the shear strength of the material:
F = L × T × τ
for a round hole: F = π × D × T × ττ is the shear strength. A common rule of thumb is about 0.8 × the tensile strength; around 350 N/mm² is a typical assumption for mild steel sheet.
A 10 mm hole in 2 mm mild steel needs about π × 10 × 2 × 350 = 22,000 N, or 22 kN. A 50 mm hole in 3 mm needs about 165 kN, which is a large hit for many turret punches. Large openings are therefore often nibbled or cut with a laser, and turret punches are generally used on sheet up to a few millimetres thick. Check the machine capacity with your fabricator.
Minimum hole size
The most useful rule for punching is that the hole should be at least as large as the sheet is thick: hole diameter ≥ T. A smaller punch is slender compared with the force it carries and tends to buckle or break. The same applies to slot widths and narrow webs. For stainless steel and other strong materials, many shops ask for more, often around 1.5 to 2 × T.
The laser has no punch to break, but small holes in thick material can come out tapered or rough because the pierce and the heat are large compared with the hole. As a rule of thumb, holes down to around the material thickness cut well; smaller holes are possible on thin sheet, but confirm with your fabricator.
Features only a punch can make
A turret punch can form features that a laser cannot:
- Louvres for ventilation, lanced and raised in one hit.
- Extruded holes, where the material is drawn into a collar so that a thin sheet has enough thread length to tap.
- Formed countersinks for flush countersunk screws in sheet too thin to machine a countersink.
- Knock-outs for cable glands, sheared almost through and held by small ties so the installer can remove them on site.
- Embossed ribs, dimples, lances, bridges, card guides and part marks.
Each needs a dedicated tool, so check your fabricator holds it, and keep formed features away from bend lines.
Nibbling marks and contour quality
When a turret punch cuts an outline larger than its tools, the edge carries small scallops or steps at each hit. For a round punch the scallop height follows from the geometry:
h = R − √(R² − (p ÷ 2)²)With R = 5 mm and p = 2 mm, h = 5 − √24 = 0.10 mm. Halving the pitch to 1 mm gives about 0.03 mm, at twice the number of hits.
Straight edges are usually cut with a rectangular tool, which leaves small witness marks instead. On a visible edge or a sealing face a laser cut is normally the better choice.
Speed and cost
For parts with many identical holes, punching is usually faster: each hole is one hit, and multi-tools can punch several at once, while the laser must pierce and trace every hole. For long or complex outlines, few holes or thicker material, the laser is usually faster and needs no tool set-up. Special-shape punches cost money and take time to make, so they pay off only on repeat work.
Combination punch-laser machines put both heads on one frame: the laser cuts the outline, the punch makes holes, forms and louvres, and the part comes off finished in one set-up.
Side by side
| Aspect | Fibre laser | CNC turret punch |
|---|---|---|
| Tooling | None, any drawn shape | Punch and die per shape; specials for unusual shapes |
| Edge | Square, fine striations, narrow HAZ | Rollover, burnish, fracture zone and burr |
| Heat | Small HAZ; oxide layer if oxygen cut | None |
| Minimum hole (rule of thumb) | About T; smaller on thin sheet with care | Diameter ≥ T; often 1.5 to 2 × T for stainless |
| Formed features | Not possible | Louvres, extrusions, countersinks, knock-outs, embosses |
| Many identical holes | Each hole pierced and traced | One hit each; cluster tools for patterns |
| Complex outlines | Clean, any shape | Nibbled, visible marks |
| Thickness | Thin sheet to thick plate | Generally sheet up to a few mm |
A simple decision guide
- Does the part need louvres, extrusions, formed countersinks or knock-outs? Punching, or a punch-laser machine.
- Is it a perforated panel or a part with a large number of identical holes? Punching is usually quicker.
- Is the outline long, curved or visible, or does the edge seal against something? Laser.
- Is the material thicker than your fabricator punches, or are there holes smaller than T? Laser.
- Is it a one-off or a short run of an unusual shape that would need a special punch? Laser.
- Will the edge be welded or painted? Laser with nitrogen, or punched and deburred, rather than oxygen-cut and left as is.
Where possible, specify the result rather than the process: edge condition, burr side, cosmetic faces and the tolerances that matter. The fabricator can then choose the cheapest way to meet them.
Checklist
- Holes, slots and webs at least as large as T, more in stainless.
- Formed features checked against the fabricator's tool list.
- Burr side marked where it matters.
- Edge finish stated for welded, painted or visible edges.
- Process left open on the drawing unless there is a reason to fix it.
If you would like a second pair of eyes on a part before it goes for cutting, send us your drawings and we can check them against the process you plan to use.