Plate cutting above 20 mm: drill, form and finish

Twenty millimetres is where the fibre laser stops, and it is not a soft limit. A 25 mm mild steel plate weighs 196 kg per square metre, a 40 mm plate 314 kg, and at that weight the handling, the cut and the edge preparation become one problem rather than three.

Plate cutting above 20 mm is a different process family: oxy-fuel or plasma for the profile, shear and punch for straight work, and a separate set of rules for holes and bends. The numbers below are the ones that decide which of those applies, and the point at which the answer is a design change rather than a process change.

The first decision is the cut. The second is the hole. The third is the bend, and the third is where most thick-plate designs fail.

How is plate above 20 mm cut?

Oxy-fuel for mild steel where the edge must be square, plasma where speed matters more or the material is stainless or aluminium. Both are thermal cuts, both leave a heat-affected zone, and both are outside what a 12 kW laser does on our floor.

Oxy-fuel works by heating the steel to about 1,200 °C and then burning it in a jet of pure oxygen. The oxide runs off as slag, and the kerf is narrow for the thickness: 1.5–2.5 mm at 25 mm, 2–3 mm at 40 mm. The cut is slow, 150–300 mm/min at 30 mm, but it is square and straight, which is why it is still the default for structural and base-plate work.

Plasma melts the material with a constricted arc at 20,000 °C or more and blows the molten metal out. It is four to eight times faster than oxy-fuel on the same thickness, cuts stainless and aluminium that oxy-fuel cannot touch, and pays for the speed with a wider kerf, 3–5 mm at 25 mm, and a more pronounced bevel on the exit side.

A shear is not a profile process, but it is often the right answer for a rectangular blank. Shearing 20 mm mild steel on an iron worker is fast, leaves a square edge and no heat-affected zone, and produces a blank that can then be punched and formed. The limit is shape: straight cuts only, and the rated thickness applies at a rated width, not across the full blade.

Cutting process limits at 20, 30 and 40 mm

Values are for mild steel unless stated. The Ganesh column records what is in our plant, including the route where the answer is no.

The weight row is not filler. A 1,250 × 2,500 mm blank at 40 mm weighs 981 kg, which is beyond a two-person lift, beyond most vacuum lifters and inside the range where the part needs a crane and a certified sling before anyone cuts it. Handling often decides the nest layout and the batch size more than the machine does.

The squareness row decides whether the cut edge can be a functional face. An oxy-fuel edge at 30 mm typically carries a 2–3° bevel, which on a 30 mm thickness is 1–1.6 mm of offset across the face. If the part registers on that edge, it needs machining or the drawing needs to accept the bevel and register elsewhere.

Holes in thick plate: punch or drill?

A hole in 20 mm plate is a punching job up to a point, and a drilling job past it. The crossover is set by force, by tool strength and by the hole-to-thickness ratio.

The force is easy to calculate. Punching force in kilonewtons is the cut perimeter in millimetres times the thickness in millimetres times the shear strength in megapascals, divided by 1,000. For mild steel at 450 MPa tensile, shear strength is about 0.62 of tensile, or 280 MPa.

A 20 mm hole through 20 mm plate has a perimeter of π × 20 = 62.8 mm. The force is 62.8 × 20 × 280 / 1,000 = 352 kN, or 36 tonne-force. A 40 mm hole in the same plate doubles the perimeter to 125.7 mm and the force to 704 kN, or 72 tonne-force.

Those numbers fit inside a 250 T punching press and a 300 T press with room to spare. What does not fit is the tool. A punch smaller than the plate thickness buckles under the load, and a punch at 0.8 times thickness in 25 mm plate is 20 mm in diameter with a 25 mm working length, which is a tool design problem before it is a tonnage problem.

The hole-to-thickness rule

Punch a hole no smaller than the thickness, and preferably 1.2 times the thickness, in plate above 12 mm. Below that ratio the punch is slender, the slug is thick, stripping force rises and the hole wall carries a visible shear band that runs a third of the way through the thickness.

Drilling handles the smaller holes, the deeper ones and the ones that need a true cylindrical wall. A drilled hole in 30 mm plate holds position to ±0.1 mm in a good setup, where a punched hole carries taper and a break-out burr. The trade is time: drilling a 20 mm hole through 30 mm plate takes minutes, and punching it takes a fraction of a second.

On our floor, holes are punched on the 250 T press or the iron worker, and tooling is designed in-house. Where the hole is below the punch ratio at that thickness, the honest answer is that it is a drilled feature and not a punched one, and it is better to say so at quotation than to build a tool that cannot hold it.

Forming plate above 10 mm

Our 200 T press brake is rated to 10 mm over a 4,450 mm bed, and even that rating needs reading carefully. Air bending force rises with the square of thickness and falls with the V-die opening, so the tonnage per metre is what sets the bend length.

The working formula for air bending mild steel is 1.42 × tensile strength × thickness squared, divided by the V-die opening, in kilonewtons per metre. At 10 mm thickness, 450 MPa tensile and an 80 mm V-die, that is 1.42 × 450 × 100 / 80 = 799 kN per metre, or about 81 tonne-force per metre.

A 200 T brake can therefore bend about 2.4 m of 10 mm mild steel in one hit, not the full 4,450 mm bed. The 10 mm rating is a thickness at a length, and a drawing that needs a full-length 10 mm bend needs a bigger press or a segmented hit.

Above 10 mm the tonnage climbs out of reach quickly. At 16 mm with a 128 mm V-die, the same formula gives 1.42 × 450 × 256 / 128 = 1,278 kN per metre, and a 200 T brake can bend 1.5 m of it. At 20 mm with a 160 mm die it is 1,598 kN per metre, which leaves 1.2 m of bend in one hit — and at that tonnage the tooling, not the press, is the limiting item.

That is why thick plate is rarely bent on a press brake. It is hot formed, rolled, or fabricated from a welded section. A 25 mm bend on a 2 m part is a different process with a different supplier, and a design that assumes the brake can do it will be quoted twice.

Forming direction and the cut edge

If a thick plate is formed at all, the bend line should not sit on a sheared edge. A shear leaves a cold-worked, slightly hardened edge with a burr on one face, and a bend that puts that edge in tension starts a crack. Keep the bend line 2–3 times the thickness away from a sheared edge, or machine the edge before forming.

The same applies to a plasma or oxy-fuel edge. A thermal cut leaves a heat-affected zone that is harder and less ductile than the parent plate, and a bend line placed in that zone cracks earlier. On 20 mm plate, keep the bend 10–15 mm clear of the cut edge unless the edge is machined.

Thick plate and the coating route

A 25 mm part does not go through the same preparation as a 1.6 mm panel. The cut edge carries mill scale, oxide and a rough profile, and the seven-tank line will clean it without removing the oxide.

Blasting comes first. The 16 ft × 16 ft shot blast chamber takes parts up to that envelope, and blasting removes the scale and gives the conversion coating a surface it can grip. On an oxy-fuel edge at 30 mm, the oxide layer is 1–3 mm deep including the heat-affected zone, and no alkaline cleaner removes it.

The weight and the racking matter too. A 200 kg part cannot hang from a wire in the same way as an enclosure panel, so it goes through on a purpose-built rack or a trolley, and the masking is planned around how it will sit. DFT is checked per ISO 2808 after cure, and the edge is where the reading is lowest.

Where the part is a base frame, a platen or a structural bracket, the coating is often a primer rather than a powder coat. That is a different specification and a different conversation, and it belongs on the drawing rather than in a purchase order note.

What Ganesh actually runs

The cutting envelope stops at 20 mm on the 12 kW fibre laser, with a 40 ft × 16 ft bed and two 6 kW and two 3 kW machines behind it. Past that, the iron worker shears and punches plate to 30–40 mm, which covers straight edges, rectangular blanks and holes at the rated blade width. The 250 T punching press and the 20–300 T press range handle the punching and forming work below that, with press tools built in our own tool room.

What we do not do is cut a 30 mm profile. There is no plasma table and no oxy-fuel profile machine on the floor, so a curved 30 mm bracket leaves our route. We will say that at review rather than accept the order and send it out unannounced. The laser cutting capability page gives the machine envelope, and how to select press tonnage covers the force calculation for the punching side.

The 200 T, 5-axis press brake on a 4,450 mm bed is rated to 10 mm, and that rating is the one to design to. Parts above it are quoted as welded fabrications or referred out, depending on the geometry.

What to put on the drawing

1. Material grade and thickness, with the plate standard it is bought to. Mill tolerance on plate thickness is not the same as sheet tolerance.
2. Which edges are functional faces. A cut edge with a 2–3° bevel is acceptable on a cover and not on a register.
3. Hole size and position tolerance, plus whether the hole may be punched or must be drilled and bored.
4. The bend line location relative to cut edges, and whether the bend is in the parent plate or the heat-affected zone.
5. Weight and lifting points, for anything above about 100 kg, so the route can be planned before the first cut.
6. Surface preparation before coating: blast, primer or bare, and the standard the finish is measured against.
7. Whether the part may be made as a weldment instead of a single plate. Often the honest answer, and it changes the price by more than a process choice does.

When thick plate is the wrong material

A 25 mm plate is sometimes a design decision that was never tested against a thinner one. Three alternatives are worth pricing before the plate is ordered.

Folded thin section with gussets. A 3 mm folded channel with welded ribs can carry the same load as a 20 mm flat plate in bending, because stiffness rises with the cube of depth rather than linearly with thickness. The folded part weighs less, coats in the standard line and needs no special handling.

Laser-cut plate up to 20 mm. If the design needs 18 mm, it is inside our envelope and can be cut, punched and coated on the standard route. The difference between 18 mm and 22 mm is the difference between a normal part and a subcontract.

A casting or a forging. For a bracket with complex geometry and a long run, a casting can remove the cutting, the welding and the machining in one step. It is a different supplier and a different tooling cost, and it is worth comparing before a fabricated plate is frozen.

Send the drawing with the thickness, the smallest hole and the functional edges to request a quote. If the part sits above the laser envelope, the reply will name the route it needs rather than a price for a process we do not run. The laser cutting and fabrication hub covers the thickness bands and edge quality below 20 mm, including what the 12 kW machine does at each band.

Related reading: the Aluminium sheet laser cutting: process, gas and edge quality and the Deburring and edge quality after laser cutting take the same problem further.