Why is my laser-cut kerf tapered — wider at the top than the bottom?
Updated:
Short answer
Taper means the beam is not delivering a straight-walled cut through the full thickness: usually focus sitting at or above the surface on thick material, speed slightly too high, or a worn lens spreading the beam. It shows up mostly on plate over 8–10 mm, where the beam has more depth to travel through.
What causes taper
| Symptom | Likely cause | First action |
|---|---|---|
| Taper on every part, all thicknesses. | Focus set too high for the material and gas combination. | Move focus deeper for nitrogen cutting, or verify the correct offset for oxygen — see the focus position page. |
| Taper worse on thick plate only, thin sheet is fine. | Normal to a degree — beam divergence over depth grows with thickness. | Use a longer focal-length lens, or accept the table’s expected taper range for that thickness. |
| Taper increased gradually over weeks. | Lens wear or contamination spreading the focused spot. | Inspect and clean or replace the focusing lens on schedule. |
| Taper direction reverses — wider at the bottom. | Focus set too deep below the surface. | Bring focus back up toward the table value; too-deep focus overcorrects. |
| Taper on one side of the contour only. | Beam not centred in the nozzle, or head slightly tilted. | Re-centre the beam and check head squareness to the table. |
Focus and speed settings that hold the kerf straight — inside the app. Open SBM Laser Help →
Or take it with you:
How to check for taper
- Cut a straight test line in production material and thickness.
- Measure kerf width at the top surface and again at the bottom.
- A difference under roughly 10–15% of kerf width is normal; more than that points to focus or optics.
- Re-run the focus test from the focus position page before changing anything else.
Related questions
- How much taper is acceptable?
- For most fabrication work, a slight taper invisible without calipers is fine. Parts that mate tightly or get welded from both sides need it checked against the drawing tolerance.
- Does taper affect part strength?
- Not directly — it is a dimensional issue, not a metallurgical one. The concern is fit, not weakness.
- Can taper be fixed by slowing down?
- Sometimes it helps a little, but taper is mainly a focus and optics issue. Treat speed as the last lever, not the first.