Laser cutting copper sheet design: holes, slots and kerf

A Ø1.5 mm hole in 2 mm copper looks like an ordinary detail on a drawing and behaves like a fault. The ratio is 0.75 to 1, the pierce takes longer than the contour, the melt does not clear before it freezes, and the hole arrives 0.15 mm undersized, tapered and half closed by dross. Laser cutting copper sheet design is mostly the discipline of keeping features large enough for the metal to clear itself.

Copper makes that harder than steel does. It conducts heat at roughly 400 W/mK, eight times mild steel, so the energy that should be cutting is pulled sideways into the sheet. The cut survives because a keyhole forms and absorptivity rises, but the window between a good cut and a failed one is narrow, and small features sit outside it.

The rules below are the ones we apply when a copper DXF comes in for quotation: minimum holes and slots by thickness, kerf, tabs, nesting and the tolerance the finished part can hold. The wider set of cutting guides is in the laser cutting and fabrication hub.

What minimum hole does laser cutting copper sheet design allow?

A hole should be at least 1.0 times the sheet thickness up to 1.5 mm, and 1.2 times the thickness above that. In 1.0 mm copper a 1.0 mm hole is a production feature. In 2.0 mm copper the floor is 2.4 mm, and a 1.5 mm hole belongs on a drill or in a different design.

Three things set that floor. The pierce has to open a keyhole wide enough for the beam to enter, and on copper that takes a controlled ramp of 1 to 2 seconds. The melt has to be pushed out of a small circle before the assist gas loses pressure down the kerf. And the lead-in has to fit inside the hole or start outside the part, which for a Ø1.0 mm circle is not possible.

Slots behave better than round holes at the same width because the cut runs in a straight line and the gas has an exit path. A slot can go to about 1.0 t at any thickness in the range, provided the slot is at least 3 times as long as it is wide, so the beam reaches a steady speed before the end.

Two consequences follow. A hole pattern designed for a 3 mm bolt on 8 mm centres in 2 mm copper is fine; the same pattern with 2 mm holes is not, and drilling 40 holes in a batch of 50 is slower than cutting them. And a drawing that shows a Ø2 mm hole in 3 mm copper will be quoted with a drilled hole or returned for a change, not quietly cut undersized.

Kerf, taper and why the pocket comes out small

Kerf on copper runs from 0.10–0.15 mm at 0.5 mm to 0.30–0.40 mm at 3.0 mm. The cut removes that width, centred on the toolpath, so a 20 mm square pocket cut on a 20 mm path finishes at 20 mm plus the kerf. Good CAM offsets the path inward by half the kerf and the part comes out at the drawn size.

The common error is a drawing that adds its own allowance — a 20.2 mm pocket dimensioned so the finished part is 20 mm — on top of a fabricator that also compensates. The part ends up 20.2 mm, which on a locating pocket in a busbar cover means a rattle rather than a fit.

Taper is the second effect. The melt front at the exit is wider than at the entry, typically 0.02–0.05 mm per millimetre of thickness, so a 3 mm copper cut has a 0.06–0.15 mm difference between top and bottom. On a hole that matters: a hole measured at the top face can pass a pin gauge and still be tight at the bottom. Measure holes at mid-thickness, or specify a go/no-go pin that has to pass through.

The third effect is dross. Copper in nitrogen leaves a light, friable dross at the exit face rather than the hard adherent dross steel produces, and a light deburr removes it. Where the part feeds a plating line, deburr before plating, because a plated dross nodule becomes a hard spot that fails a bend test later.

Heat, flatness and temper on thin copper

Copper's conductivity is the reason distortion is less of a problem than buyers expect. In 1.0 mm sheet the heat-affected zone stays within a fraction of a millimetre of the cut and the panel does not bow the way a 1.0 mm steel panel can after a dense nest.

Flatness problems come from somewhere else. Annealed (O temper) copper at 0.5–1.0 mm is soft enough to deform when it is lifted off the bed, stacked, or packed without interleaving. A panel that measures 0.8 mm out of flat at the machine can measure 2 mm out of flat after a day in a crate.

Half-hard (H02) temper costs no more to cut and holds flatness through handling, so specify it for flat parts and reserve annealed material for parts that will be formed after cutting. Where flatness matters, 1 mm over 500 mm is a realistic callout on 1.0–1.5 mm half-hard sheet; 0.5 mm over 500 mm is not, without a flattening operation we do not run.

Nesting: the geometry decision that beats cut speed

On copper the nest decides the price before the cutting speed does, because the material is most of the cost. A 2 mm C11000 sheet at 1,250 × 2,500 mm weighs 56 kg at 17.92 kg/m². Every part that does not fit on the sheet is copper bought, freighted, stored and scrapped.

Take an earthing link 250 × 100 mm in 2 mm copper, two Ø8 mm holes, nested with a 10 mm web and a 5 mm edge margin. With the 100 mm side across the 1,250 mm width, the sheet takes 11 columns and 9 rows: 99 parts. Rotated so the 250 mm side runs across the width, it takes 4 columns and 22 rows: 88 parts.

The difference is 11 parts, or 4.9 kg of copper on one sheet, and 12.5% of the material cost of the job. It costs nothing to achieve except a nesting decision made at the quotation stage rather than at the machine.

The second nesting lever is the web, and it only pays when it changes the column or row count. Dropping from a 10 mm web to 6 mm on this link gains nothing because the pitch already divides the sheet; on a part 20 mm wider it would add a column. There is a floor in any case: below about 4 mm the web can lift and jam between the slats. Common-line cutting, where adjacent parts share one cut, saves a further 8–12% of cut length but leaves a dross bridge at the shared edge and is not used where the edge will be plated or visible.

Copper should also never share a sheet with steel. The offcuts have to stay segregated by alloy and temper to keep their scrap value, and steel parts in the gaps turn a priced remnant into mixed scrap. How nesting reduces sheet metal cutting cost walks through the same arithmetic on a steel bracket, where the tonnage is lower and the percentage loss costs less.

Tabs, part loss and handling

A 250 × 100 mm link in 2 mm copper weighs 0.45 kg, which is light enough for two tabs of 0.4 mm width to hold it through the cut. A 400 × 300 mm busbar cover weighs 2.1 kg and needs three or four tabs, positioned so the part does not tip into the slats when the last contour closes.

Tab position is a design decision because the tab leaves a witness mark and a nib that has to be dressed. Put tabs on an edge that will be hidden, or on an edge that gets deburred anyway, and keep them off a mating face. On plated parts, a tab on a visible face shows after finishing, and no amount of polishing removes the step.

The other handling rule is stacking. Cut copper parts scratch each other and mark where the dross has been removed. Interleaved packing, or a coat of protective film on visible faces, belongs in the quotation scope; without it the first container arrives with scratches that look like a supplier quality problem and are really a packing decision.

The tolerances a copper part can hold

Cut features hold ±0.10 mm on 0.5–1.0 mm sheet and ±0.15 mm on 1.5–2.0 mm, both tighter than the ±0.5 mm typical sheet metal tolerance that applies to formed parts. Hole position follows the same figures when the part is cut flat and not formed afterwards.

Two tolerances are worth calling out separately. Hole diameter should be specified as a range with a pin gauge requirement rather than a nominal with a general tolerance, because taper means the diameter varies through the thickness. Flatness should carry a value and a measurement condition — on the bench, unrestrained — because a clamped part measures flat regardless.

Where the part is formed after cutting, the bend tolerance takes over and the cut tolerance becomes irrelevant: a 90° bend on 2 mm copper springs back 1–3°, less than CRCA at the same thickness but enough to change the angle on a short flange. The minimum bend radius guide gives the radius for each alloy, and the formed dimension is what the assembly feels.

The 12 kW line that cuts the copper

Copper runs on the 12 kW fibre laser with the 40 ft × 16 ft bed, which is long enough to cut a 3 m busbar in one piece rather than in two and bolted. The two 6 kW and two 3 kW support machines take the thinner work, and the same cell cuts stainless, aluminium and brass, so a mixed assembly can be cut in one release.

We cut copper to about 3 mm and brass to about 6 mm. The practical constraints that set those ceilings — pierce time, absorptivity, dross — are covered in the guide to laser cutting copper and brass, and they are the reason a 5 mm copper job is quoted as a slow cut rather than a normal one.

Two things we do not do. We do not cut beryllium copper, because the fume is a health hazard the shop is not set up to control. And we do not cut copper foil below 0.5 mm, where handling dominates the process and a different route is cheaper.

What to put on the drawing

• Alloy designation and temper, such as C11000 H02, rather than the word copper.
• Thickness with its tolerance, and whether the sheet is bought to that tolerance or to a standard mill range.
• Holes at finished size, with any hole below 1.2 t either enlarged or marked as drilled.
• Pockets at finished size, with no kerf allowance added.
• Flatness with its measurement condition, and the temper that makes it achievable.
• Edge requirement: burr limit, deburr, and whether the cut edge will be plated or lacquered.
• Tab positions, or a note that tab marks are acceptable on a named face.
• Packing requirement where visible surfaces matter.
• Quantity and release pattern, because the nest and the remnant plan depend on it.

When laser cutting is the wrong route for copper

Above 3 mm, the cut slows to the point where another process wins, and above roughly 5,000 pieces a year a stamping tool recovers the material more efficiently and removes the pierce cost entirely. A copper part at that volume should be quoted as a stamping, and the copper and brass pressed components guide sets out what a tool can hold that a laser cannot.

A part with a fold, a drawn cup or a coined contact face is not a laser part at all, however thin the sheet. And a design that relies on laser-cut edges for electrical contact should be checked with the customer's conductivity requirement before the drawing is released, because a cut edge is not a rolled surface.

Send the DXF with the alloy and temper stated and the minimum features visible, and the cut check takes an hour rather than a week. The nest and the pierce count are what move the price. Custom copper and brass work is described under copper, brass and aluminium fabrication, and the RFQ route takes the DXF, alloy and quantity in one form.