Laser cutting tolerances for OEM parts: kerf, holes and flatness

Laser cutting tolerances for OEM parts are set by thickness, feature size and what happens after the cut, not by the machine's positioning accuracy alone. On a modern fibre laser, a 2 mm CRCA profile holds about ±0.15 mm by agreement and a 20 mm plate about ±0.50 mm, and both figures degrade the moment a dimension sits on a hole smaller than the sheet thickness.

That gap explains most first-off arguments. A drawing note of ±0.5 mm is honest for a formed assembly and loose for a cut blank. A drawing note of ±0.05 mm is not achievable across a production batch, and asking for it pushes the part into a secondary operation that nobody has costed.

The sections below give the numbers by thickness, show where each error comes from, and set out what belongs on the DXF so the blank arrives ready for the brake and the weld fixture.

Which laser cutting tolerances for OEM parts are realistic?

A profile dimension up to 3 mm thickness holds about ±0.10 mm to ±0.15 mm by agreement, and the same dimension on 12–20 mm plate holds about ±0.50 mm, which is the house figure. Hole positions follow the profile for features above one thickness; below that, the cut drifts because the beam cannot clear the kerf.

Machine positioning contributes roughly ±0.05 mm on a well-maintained fibre laser. Everything else in the table is physics and file preparation: kerf width, taper through the thickness, heat input on heavy plate, and how flat the sheet was when it was loaded.

Tolerances by thickness, with kerf and minimum hole

The table below is the working reference for mild steel and CRCA on a 12 kW fibre laser. The profile figures are quoted cut tolerances for a named part, not a house tolerance: every figure tighter than ±0.5 mm is agreed on the drawing before the nest is released. Stainless and aluminium sit in the same bands for profile, with a slightly wider kerf on aluminium and a rougher edge on thick stainless.

Read the profile tolerance as a cut-edge figure. Once the part is bent, the controlling error is the formed tolerance — ±0.5 mm typical on a press brake part, with flange lengths held tighter by agreement. The laser figure still matters, because a cut blank that is 0.3 mm long eats the bend allowance before the first stroke. Any cut figure tighter than ±0.5 mm in the table is quoted for the features named on the drawing; it is not the tolerance the next batch holds by default.

Where the millimetres actually go

Four sources account for almost all of the error between a DXF and a delivered part.

Kerf compensation. The beam removes a path, so the CAM program offsets the tool path by half the kerf. If the DXF has already been drawn at nominal size and the operator compensates again, holes come out undersize by a kerf. Leave compensation to the machine and send geometry at nominal.

Taper through the thickness. The beam is focused to a spot, so the cut widens towards the top or bottom depending on focus position. At 2 mm the taper is negligible. At 16 mm it can be 0.1 mm across the thickness, which is enough to make a 12 mm hole a press fit in one direction and a loose fit in the other.

Thermal movement and distortion. Long parts and dense nests heat the sheet. A 3,000 mm rail cut from a cold sheet can move 0.2–0.5 mm as the cut progresses if the sheet is not clamped and the nest is not sequenced to balance heat. This is the usual cause of a long part that measures correctly at one end.

Sheet flatness and loading. A sheet with a 3 mm bow does not sit flat on the slats. The focus distance changes by the same amount, and the cut quality drops on the bowed section before the dimensional error appears.

Holes, slots and narrow webs

Minimum hole size and slot width follow the sheet thickness: a clean hole is roughly one thickness up to 3 mm and about twice the thickness at 12–20 mm. Holes, slots and small features owns the full rule, including webs between cuts and when to drill or ream instead.

A nesting example that changes the tolerance question

Nesting changes material cost and heat input, not the cut tolerance itself; how nesting reduces sheet metal cutting cost works through the utilisation arithmetic on this same canopy panel.

Flatness and twist on long parts

A 40 ft × 16 ft bed exists for parts that will not fit anywhere else, and long parts bring a tolerance problem that a small nested part never shows: twist.

A 3,000 mm elevator rail section cut from 2 mm CRCA is a flat blank with a 3,000 mm window of movement. If the sheet was bowed 2 mm before cutting, the released stresses and the cut sequence can leave 3–5 mm of bow over the length, which is more than ten times the profile tolerance and invisible on a 150 mm vernier. Check flatness on a surface plate or a granite table, not on the laser bed, and measure it after the part has cooled for 15 minutes rather than at the exit conveyor.

Thick plate behaves in the opposite way. A 16 mm bracket stays flat because there is enough section stiffness, but the pierce point leaves a 1–1.5 mm crater and a dross lump on the underside if the gas pressure is wrong. That crater is a tolerance problem on any face that will be machined or welded, so thick parts are pierced off-part and lead in, or the pierce is placed on a sacrificial tab.

How cut tolerance meets the brake and the weld fixture

The laser figure is an input to the next two operations, and both consume it.

On a press brake, the flat pattern length is the outside dimension minus the bend deduction. A 300 mm outside flange on a 2 mm part with a 2 mm inside radius bends from a flat of about 296 mm. If the blank is 0.3 mm over, the flange finishes 0.3 mm over, and the hole that was 12 mm from the bend edge moves by 0.3 mm plus whatever the bend deduction assumed. This is why the cut file and the bend table are reviewed together rather than approved separately.

At a weld fixture, the tolerance that matters is the gap. Two laser-cut parts that butt along a 400 mm edge meet within ±0.15 mm along most of the length, which is inside the 0.5 mm gap a MIG weld on 2 mm sheet can bridge. Where the joint is a lap or a corner, a 0.3 mm mismatch shows as a step on the visible face even after coating, so the edge that carries the step is specified as a reference edge and cut in the same nest as its mating part.

When to stop asking the laser to do the work

A laser is the right process when the part is flat, the volume is under a few thousand pieces a year, or the geometry is still moving. It is the wrong process when a dimension genuinely needs ±0.05 mm, when the hole is smaller than the thickness and must be functional, or when the edge will be a sealing face.

Those parts need a milling, drilling or fine-blanking step that the laser cannot supply. Stating that up front is cheaper than a reject sheet. Thick plate above 20 mm and parts that need a ground finish also leave the laser, and heavy structural plate is cut on the iron worker up to 30–40 mm.

What Ganesh runs, and how the route is checked

Ganesh cuts on a 12 kW fibre laser with a 40 ft × 16 ft bed, supported by two 6 kW and two 3 kW machines for thinner nests. The material range covers mild steel, CRCA, stainless, aluminium, brass and copper, with plate to 20 mm on the laser and 30–40 mm on the iron worker.

The cut file is reviewed against the next operation before the nest is released. On a bent part, the flat pattern, bend deductions and hole-to-bend distances are checked together, because a hole 4 mm from a bend line on a 3 mm sheet will distort even when the cut is perfect. On a welded assembly, the review checks that edges which meet another part are cut square and that the fixture references a laser-cut edge rather than a sheared one.

Repeat parts run with first article inspection and an approved sample. The house tolerance on formed parts is ±0.5 mm, with tighter critical features agreed on the drawing rather than assumed. Coatings, welding and assembly are held under one release, so a part that passes at the laser is not re-checked under a different datum at the weld fixture.

What to put on the DXF

• One part per file, geometry at nominal size, with kerf compensation left to the machine.
• Material grade and thickness as a layer name or a note, not only in an email.
• Critical dimensions marked individually, with a datum for hole patterns. Do not tighten the title block.
• Hole sizes stated as cut size or finished size, and whether powder coating will pass through them.
• Grain direction where bending follows, because a laser cut has no rolling direction effect but the bend does.
• Any edge that will be visible, welded or sealed, so deburring and edge quality are priced rather than assumed.

Send the flat pattern to the laser cutting capability and the team will return a route with the achievable tolerance stated per feature. The laser cutting cost estimator gives a first view of how thickness and cut length move the price, which is useful before deciding whether to thin a section or change material.

For the edge-quality side of the same question, mild steel thickness and edge quality covers what a cut edge looks like at each gauge, and holes, slots and small features goes deeper on the minimum-feature rules. If the blank is going straight to the brake, read deburring and edge quality after laser cutting before approving the corner condition. The rest of the laser cutting and fabrication hub covers nesting, machine choice and cost.