How do I know if my CO₂ laser mirrors are out of alignment?

The classic sign is a laser that cuts in one area of the bed and not another with the same settings. A near/far tape test on each mirror confirms it.

Which mirror do you adjust first?

Start at the tube end. Use mirror 1 to fix the beam at mirror 2, mirror 2 to fix it at mirror 3 in all four corners, then mirror 3 to center it down the nozzle.

What power should I use to align laser mirrors?

The lowest setting that still fires, so the pulse marks the tape without burning through. That was 15% on Edmonds Woodshop’s OMTech; yours may fire at 10%.

Why does my laser cut double lines?

One common cause is the beam clipping the inside of the nozzle cone. Tape the nozzle tip, pulse, and adjust mirror 3 until the beam exits centered.

Which mirror is hardest to align?

Mirror 2 to mirror 3. It has to stay on target in all four corners, so expect it to take the most patience.

Laser Cutting & EngravingMaintenance & AlignmentBeam alignment

How do you align the mirrors on a CO₂ laser?

Your laser cuts clean through in the front left corner and barely scores the back right. Same file, same settings. That’s almost always CO2 laser mirror alignment. The beam bounces off three mirrors on its way to the lens, and if it drifts as the gantry travels, power and focus drift with it.

Alignment means firing an invisible Class 4 CO₂ beam with covers open. Wear glasses rated for the CO₂ wavelength (10,600 nm), keep hands and faces out of the beam path, use the lowest power that marks the tape, and stay clear of the tube’s high-voltage end. Tape targets can catch fire, so keep an extinguisher in reach.

⚡ Rule of thumb

Align one mirror at a time, in beam order: pulse a tape target at the near and far end of travel, and adjust the mirror before the target until both marks land on the same spot.

Starting point

Lowest power that fires (15% on Cam’s OMTech) · ⅛ turn or less per adjustment

Measured

Pulse marks on painter’s tape: near vs. far, then all four corners

Confidence

Widely accepted

▶ Source video — How to Align Your CO2 Laser Mirrors — Edmonds Woodshopwatch on YouTube ↗

Applies to

Process: CO₂ lasers with a mirror-and-lens beam path (OMTech and similar)
Not for: Diode and galvo lasers (no flying mirrors to align)
When: Cuts weaker on one side of the bed, after a move, a knock, or a new tube
Tools: Painter’s tape, a small wrench for the lock nuts, CO₂-rated glasses, scrap wood

Why it matters

A CO₂ beam travels tube → mirror 1 → mirror 2 (rides the gantry) → mirror 3 (rides the head) → lens. If the beam isn’t parallel to the rails, it lands in a different place on mirror 2 and 3 as they move. Near the tube it’s fine. Far from it, the beam clips the edge of a mirror or enters the lens off-center.

That’s why a misaligned laser feels haunted. The part in one corner cuts, the same part in the opposite corner doesn’t, and you start blaming the wood.

The last step matters too. Cam at Edmonds Woodshop points out that a beam clipping the nozzle cone gives you double lines in your designs.

Aligned, the beam hits every mirror and the lens in the same spot no matter where the head is. Same power, same focus, whole bed.

What good looks like

Front and rear pulses on mirror 2 land on top of each other
Pulses on mirror 3 land on top of each other in all four corners
The beam exits the nozzle centered, never touching the cone
A pulse close to the nozzle and one 2 in lower land in the same spot

How to apply it

Glasses on. Drop max power to the lowest setting that still fires (15% on Cam’s machine, maybe 10% on yours).
Mirror 1: tape over the hole in its frame and press a ring into the tape with your finger so you can see the opening. Pulse. Anywhere in the general area of center is fine.
Mirror 2: tape its hole. Pulse with the gantry all the way forward, then all the way back. If the two marks don’t overlap, adjust mirror 1.
Mirror 1 has three screws: up/down, left/right and diagonal. Two usually do it. Loosen the lock nut, turn ⅛ of a turn or less, then re-check front and rear.
Mirror 3: tape its hole and move the head to any corner. Pulse, then adjust mirror 2 until the mark sits centered.
Pulse in the other three corners. All four marks should stack on top of each other. Keep adjusting mirror 2 until they do.
Nozzle: tape over the bottom of the nozzle and pulse. The mark should sit centered in the hole, not on the rim. Adjust mirror 3’s screws if not.
Vertical check: raise the bed until a scrap almost touches the nozzle, pulse, lower it about 2 in, pulse again. The two marks should land together.
Clean the mirrors and lens when you’re done. All that tape leaves residue on the optics.

Test it on your setup

Cut the same small square in all four corners and the center of the bed at your normal settings. If all five drop out, you’re aligned.

Exceptions & tradeoffs

Screw directions vary by mount. On Cam’s mirror 2, the far lower screw was left/right (counterclockwise went left) and the upper one was up/down. The head’s screws sat in a flipped layout but moved the same way. Test one small turn before you trust a direction.

Diode lasers have no mirror path, so this whole page skips them. A weak corner on a diode is usually focus, a warped bed, or a dirty lens.

Galvo heads steer the beam with internal mirrors you don’t touch. Their version of this problem is timing, not alignment.