Your laser is burning uneven cuts and you know the mirrors are off, but you’d rather rebuild the whole machine than break out a protractor.
Why This Matters
Mirror alignment sounds technical but it’s just geometry you can see with tape. If your beam isn’t hitting the center of each mirror at every corner of the bed, your cuts will be weak on one side or burn through on the other. You waste material. You waste time. You ruin jobs.
I avoided mirror alignment for months when I first got my CO2 laser. I thought I needed special tools or had to understand physics. Then I burned through a full sheet of Baltic birch because the beam was clipping the third mirror housing. That was expensive enough to make me figure it out.
The tape test shows you exactly where the beam hits. You don’t need to measure angles or do calculations. You just look at the burn mark and adjust until it’s centered. That’s it.
Steps
- Put on your laser safety glasses. The beam will fire multiple times during this process. Do not skip this.
- Cut small squares of masking tape. About 1 inch square. You need at least 8 pieces. Cheap blue painter’s tape works fine.
- Stick tape over the entry point of mirror 1. This is the first mirror after the tube. Make sure the tape is flat and covers the whole mirror opening.
- Pulse the laser. Use the test fire button or send a very short pulse from your software. You should see a small burn mark on the tape. If you don’t see anything, your power is too low. Try 20% power for 10ms.
- Check if the burn is centered on the tape. If it’s dead center, mirror 1 is good. If it’s off to one side, adjust mirror 1 using the knobs on the back of the mirror mount. Turn in very small increments. A quarter turn moves the beam more than you think.
- Repeat the pulse test after each adjustment. Replace the tape if the burn gets messy. Keep going until the mark is centered.
- Move the laser head to the far left corner of the bed. Stick tape over the entry point of mirror 2. Pulse again. Check if the burn is still centered. If it moved, go back to mirror 1 and adjust again. The goal is for the beam to hit the center of mirror 2 no matter where the gantry is positioned.
- Move the laser head to the far right corner. Tape and pulse again on mirror 2. The burn should still be centered. If it’s not, adjust mirror 1 until the beam hits center on both the left and right corners. This can take several tries.
- Stick tape on the entry of mirror 3. Move the head to all four corners of the bed. Pulse at each corner. If the burn mark moves around, adjust mirror 2. You want the beam hitting the exact same spot on mirror 3 no matter where the head is.
- Finally, tape the laser head lens tube. Move to all four corners again and pulse. The burn should be perfectly centered every time. If it’s not, adjust mirror 3. Small turns. Test after every change.
- Run a test cut on scrap. Cut a small square in each corner of your bed. If the cuts are consistent, you’re done. If one corner is weak or burns too deep, go back and check your tape tests on that side.
Common Mistakes
- Turning the adjustment knobs too much at once. A quarter turn is a lot. Start with 1/8 turn or less.
- Not replacing the tape between pulses. A dirty burn mark makes it hard to see where the new pulse landed.
- Skipping corners. You have to test all four corners or you won’t know if the beam stays centered when the gantry moves.
- Forgetting to tighten the lock screws after adjustment. Your mirrors will drift and you’ll have to do this again in a week.
- Using the laser without safety glasses during alignment. The beam can reflect in unexpected directions when mirrors are misaligned.
What I Do in My Shop
I keep a small plastic bag of pre-cut tape squares in my tool drawer. When I notice uneven cuts, I run through all three mirrors in about 15 minutes. I don’t wait for it to get bad.
Last month I was cutting 1/4 inch acrylic and the left side of every piece had a melted edge while the right side was clean. I knew it was mirror 2 because that’s the one that moves with the gantry. I pulled out my tape, pulsed at the far left and far right, and saw the burn was off by about 2mm on the left. Three small adjustments and it was centered. The next acrylic piece cut perfect.
I also check alignment any time I move the laser or if I bump the machine. It takes longer to ruin a job than it does to run a tape test.
Next Step
Put on your safety glasses and grab some masking tape. Start with mirror 1 and work your way through the system. If you get stuck on one mirror, take a photo of the tape burn and check if the machine is level. Sometimes the whole laser is tilted and no amount of mirror adjustment will fix it.