Why Is My Laser Not Cutting Through?
A laser that does not cut through is usually out of focus, moving too fast, set to too little power, or fighting a dirty lens or glue-heavy plywood. Check focus first, then clean the optics, slow down by 20–30% or add a pass, and turn on air assist. Test every change on scrap.
You send the job, the laser works through the whole outline, you lift the piece out and it clings to the sheet at three corners. A knife and an annoyed sigh later, the piece is out, and there is a charred, weak cut line along the bottom. The question is simple: why is my laser not cutting through?
A laser that does not cut through is almost always out of focus, running too fast, short of power, or blocked by dirty optics or tough material. The table below lists the nine usual causes in order of how often they happen, with a quick test and fix for each.
Cause and fix table
| # | Cause | How to check | Fix |
|---|---|---|---|
| 1 | Out of focus | Cut line is wide and brown, edges scorched on top | Refocus on the material surface, see how to focus a laser |
| 2 | Speed too high | Cuts partly, then gets through at slow speed | Lower speed 20–30% or add a pass |
| 3 | Power too low | Settings show less than 100% on diode | Use 100% on diode; raise CO2 power in steps of 5–10% |
| 4 | Dirty lens or mirrors | Smoke film, brown spots, power dropped over time | Clean with proper optics cleaner and lint-free swabs |
| 5 | No or weak air assist | Flames at the cut, soot, charred bottom | Turn on air assist, see air assist |
| 6 | Warped or uneven sheet | Cuts on one side but not on the other | Flatten and tape, use hold-downs |
| 7 | Glue or resin pockets in plywood | Random stuck spots | Add a pass, buy better plywood |
| 8 | Wrong material for the laser | Clear acrylic with a diode | Choose a compatible material or machine |
| 9 | Aging laser source or misaligned beam | Gradual decline, uneven power | Align mirrors (CO2), service the source |
Start with focus
The beam is a cone that is thinnest at the focal point. A millimetre off, and the energy per square millimetre drops sharply. On diodes the focus is fixed by a spacer, so a small change in material thickness shifts it. Run a ramp or line test as described in the focus guide before you touch speed or power.
Check speed and power against a baseline
Compare your settings with typical values for the material. For 3 mm plywood, a 10 W diode usually needs about 3–6 mm/s at 100% power in 2–4 passes, and a 40 W CO2 about 8–15 mm/s at 60–85% in one pass. If you are outside these ranges, you have found a suspect. See LightBurn settings for 3 mm plywood.
| Laser | Typical 3 mm plywood | If it does not cut through |
|---|---|---|
| Diode 10 W | 3–6 mm/s, 100%, 2–4 passes | Slow to 2 mm/s or add passes |
| Diode 20 W | 6–15 mm/s, 100%, 1–2 passes | Slow by 20–30% |
| CO2 40 W | 8–15 mm/s, 60–85% | Raise power 5–10% or slow by 20% |
| CO2 100 W | 20–40 mm/s, 45–70% | Lower speed, check lens and alignment |
A worked example
Say you have a 10 W diode and a sheet of 3 mm plywood. You use 8 mm/s at 100% power with 2 passes. The cut does not reach the bottom. Check focus: it is fine. You slow to 5 mm/s and keep 2 passes, and the pieces still hold at the corners. Dropping to 4 mm/s with 3 passes, with air assist on, finally frees the piece. The lesson: change one thing at a time, and record the final settings.
Pro tip: If a job needs more than 4 or 5 passes on 3 mm plywood, stop adjusting and clean the lens first. A smoky lens quietly steals a large part of the power.
Glue, grain and bad plywood
Plywood is layers of veneer glued together. A hard or thick glue line makes a hard spot in the cut. Cheap hardware-store plywood can also contain voids, knots or even a heavy filler. Laser plywood from suppliers who sell it for laser cutting is usually much more consistent. See laser cutting plywood.
Common mistake: Moving the plywood between passes. Even a 0.2 mm shift means the second pass cuts a new line next to the first, and you get a wide, ragged cut that still does not go through.
Material that a laser cannot cut
A diode laser cannot cut clear or lightly coloured acrylic well, because the 445–455 nm light passes through. Reflective metals are not cut by hobby diodes or CO2 lasers either. Dark opaque material is a better match. Check can a diode laser cut acrylic before buying material.
A short checklist before every job
- Material flat and taped down.
- Lens and mirrors clean.
- Focus measured from the top surface.
- Air assist on.
- Settings checked against a known good test cut.
FAQ-style quick checks
If the problem appeared suddenly, look at what changed: a new sheet, a moved lens, a different file, a different nozzle. If it appeared slowly, suspect dirty optics or an aging source.
Questions and answers
Why does my laser cut in some places but not others?
Uneven focus (warped sheet or tilted bed), glue pockets in plywood and uneven material thickness are the usual causes. Flatten the sheet and refocus.
Can I just add more passes?
Often yes, especially on a diode, as long as the material does not move and focus stays right. If it takes more than 4–6 passes on 3 mm, look for the real cause.
Why did my laser stop cutting what it cut last week?
Typical reasons are a dirty lens or mirror, a different sheet, higher humidity, or slowly losing power from an aging CO2 tube or a worn diode.
Can a diode laser cut clear acrylic?
No, not well. Clear and most blue or green acrylic let the diode beam pass through. Dark or black opaque acrylic can be cut, slowly. See our guide on diode lasers and acrylic.
Is it the file or the machine?
Run a small square from scratch with known settings. If that cuts, the problem is in your file settings. If not, it is the machine, focus, or the material.
How can I check my laser power?
A power meter is the accurate way. Without one, run a material test grid on a known material and compare it with your earlier results. A big drop points to the optics or the source.