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CO2 mirror alignment: square the machine first, then touch the screws
Cutting well on the left and badly on the right is the classic CO2 alignment symptom. The mistake is starting at the mirror screws - if the frame and gantry are not square, no alignment will hold.
Check your job time →1. Mechanics before optics
Before any mirror adjustment, check the machine is square: the gantry perpendicular to the rails, the rails parallel, the bed flat and parallel to the head's travel. On budget CO2 machines these are genuinely often out of the box. If the head sits 2 mm lower at the far corner, the beam path length changes as it travels and you will chase that with mirror screws forever without fixing it.
2. The tape test
Stick masking tape over the receiving mirror or nozzle, pulse the laser at low power, and look at where the burn mark lands. Do it with the head near and far, left and right - four corners. The goal is the same spot in all four positions. Different spots tell you which stage is drifting. Always use the lowest power that leaves a visible mark, and never look into the beam path.
3. Align in order: 1, then 2, then 3
Mirror 1 sits at the tube and sends the beam down the long axis. If it is off, everything downstream is off, so it is always first. Mirror 2 rides the gantry and sends the beam across. Mirror 3 is on the head and turns the beam down through the lens. Adjust one mirror at a time and re-run the four-corner tape test after each - fixing mirror 3 while mirror 1 is wrong just hides the problem in one position.
4. Centred through the nozzle
The final check is that the beam passes through the middle of the nozzle aperture. A beam clipping the edge loses power, scatters, and heats the nozzle. Tape over the nozzle, pulse, and confirm the mark is central. If it is not, that is mirror 3 or the lens seating - not a reason to go back to mirror 1.
5. When alignment is not the problem
If power is weak everywhere rather than in one region, suspect the tube, the power supply, dirty optics or cooling instead. Alignment produces uneven results across the bed. Uniform weakness is a different fault - start with the optics and the water.
Rule of thumb: Safety: CO2 beams are invisible. Never place your hand or eye in the beam path, always pulse at the lowest usable power, and keep the lid interlocks working.
Open the calculator →FAQ
Why does my K40 cut on one side but not the other?
That is the classic alignment or squareness symptom. Check the gantry is square and the bed parallel first, then align mirrors 1, 2 and 3 in order using a four-corner tape test.
Which mirror do I align first?
Mirror 1 at the tube, always. It sets the beam path for everything downstream, so aligning mirrors 2 or 3 while 1 is off only hides the error in one head position.
How do I check CO2 mirror alignment?
Tape over the receiving mirror, pulse at the lowest power that marks the tape, and compare where the mark lands with the head near, far, left and right. The same spot in all four positions means the stage is aligned.
Does alignment cause weak power everywhere?
No. Misalignment causes uneven results across the bed. Uniformly weak power points at dirty optics, cooling, the power supply or an ageing tube.