Kerf and cut tolerance: how accurate a laser really is
The beam has a width, and the cut edge is not a perfect line. What kerf is, why you draw the finished part and not the cut path, the tolerance a fiber laser actually holds — and where that last tenth of a millimetre starts to matter.
The cut has a width
A laser doesn’t part metal along an infinitely thin line — it melts a narrow channel and blows it clear. The width of that channel is the kerf, and it’s real material that leaves as vapour and dross. On a fiber laser it runs from about a tenth of a millimetre on thin sheet to half a millimetre on thick plate. Every dimension on your part is cut with that width in play, which is the root of both questions people ask about laser accuracy: does the part come out to size, and how tight can I hold a fit?
| SHEET t | TYPICAL KERF | NOTE |
|---|---|---|
| 1 mm | ≈ 0.10 mm | Barely there |
| 3 mm | ≈ 0.15 mm | Still fine |
| 6 mm | ≈ 0.25 mm | Noticeable on a fit |
| 10 mm | ≈ 0.35 mm | Plan for it |
| 20 mm | ≈ 0.50 mm | And it tapers — see below |
Kerf grows with thickness and shifts a little with gas and speed. These are working figures, not a guarantee.
You draw the part, not the cut path
Here’s the part that trips people up: if the beam removes 0.2 mm of width, wouldn’t every hole come out 0.2 mm too big and every outline 0.2 mm too small? It would — if the machine cut down the middle of your line. It doesn’t. The CAM offsets the toolpath by half the kerf, to the correct side of every contour, so the finished feature lands on your nominal dimension. You draw the part you want; kerf compensation is the machine’s problem, and it’s solved automatically.
The tolerance a laser holds
With kerf compensated, what’s left is the accuracy of the machine and the metal. On thin gauge a fiber laser holds a cut feature to roughly ±0.1 mm, and it does it at no premium — hole sizes, hole-to-hole spacing and outside profile on a flat part are the cheapest place on the whole job to be accurate. Tolerance opens up as the plate gets thicker and hotter, because taper and thermal movement grow with it.
| SHEET t | CUT TOLERANCE | LIMIT COMES FROM |
|---|---|---|
| ≤ 3 mm | ≈ ±0.1 mm | Machine positioning |
| 3 – 8 mm | ≈ ±0.15 mm | A little taper, some heat |
| 8 – 15 mm | ≈ ±0.2 mm | Taper and thermal growth |
| > 15 mm | ≈ ±0.3 mm+ | Heat moves the plate as it cuts |
Datum critical dimensions off cut features on a flat part — that’s where the laser is tightest and cheapest.
The edge isn’t perfectly square
The kerf is slightly wider at the top than the bottom — the cut leaves a small taper. On thin sheet it’s invisible; on 10 mm and up it can be a couple of tenths across the thickness, which matters for a press-fit or a pin that has to be square to the face. There’s also a heat-affected zone, a very narrow band beside the cut where the metal saw high temperature. On a fiber laser it’s a fraction of a millimetre — negligible for almost everything, worth knowing only if you’re fitting hardened parts or machining the edge afterward.
Repeatability beats single-part accuracy
The number that matters for a production run isn’t how close one part is to nominal — it’s how close part 1,000 is to part 1. Here the laser shines: same program, same nest, same result, part after part, with none of the drift you get from a hand operation. Two features cut in the same setup hold their relationship to each other far tighter than either holds to absolute nominal. So if a fit has to be right, put both mating features on the same cut part where you can — the laser guarantees the gap between them.
A laser is more repeatable than it is absolutely accurate. Design so the fit lives between two features it cut together, and you get the best of it for free.
When the tenth matters, say so
Most parts never need to think about any of this — the laser is tighter than the job requires and kerf is handled before you ever see it. It’s only the few features that have to fit that are worth a word. Flag those and we’ll tell you what we can hold, and design around the kerf and taper where the number is tight.
- 01 Which dimension has to fit Not all of them — the bore, the mating slot, the hole pattern that has to line up.
- 02 The thickness and material Tolerance and taper both scale with it — a fit in 2 mm is not a fit in 15 mm.
- 03 Whether the edge gets machined after If a bore is reamed or a face is milled, we cut it under and leave the accuracy to that step.
A dimension that has to be exact? Send the part and flag the feature that has to fit — we’ll tell you the tolerance we can hold on it, and where kerf or taper needs designing around.