Optics and Exhaust

It may seem weird that I’ve grouped these together but there is a reason.  Both are relatively simple to install but should be done near the end of the build.



I think the mirrors and lens should be installed after the wiring, water and air lines, and final squaring are all complete.  It should be the last thing before mounting the actual laser so we don’t dirty up the mirrors and lens.  A laser pointer should be used for alignment before installation of the tube.

Alan had some suggestions on this in the project notes on the wiki:

“For the “optically inclined” folks: – Mount and align the mirrors and final stage lens. Tools to help this along include a couple of white plastic disks (in a ziplock bag, I think) and a standard red laser pointer (I believe I put one in the bin). The white plastic disks (printed curtesy of Catherine Clements) should fit into the laser tube brackets and be held by the set screws. The one with the larger center hole is designed to hold the laser pointer and should be installed in the right-most laser tube bracket. Turn on the laser pointer and align the mirrors sequentially (as described on a number of the laser cutter build sites). Using the white disks and the red laser pointer lets you easily get a reasonably good alignment before the final alignment using the -invisible- CO2 laser. I’ve been told that thermal cash register paper can be used to see where the CO2 laser is hitting. (The other one or two white printed plastic items in the ziplock bag are designed to mount a red laser to the lid so that it swings down into the optics path when the lid is opened – might help with setting up the material to be cut.”


After the mirrors are pre-aligned in this manner, the tube should be installed and then the steps in the google doc starting on page 18 should be followed.  This includes test firing the laser and should only be done with safety glasses on and all necessary precautions taken.

The instructions in the google doc are reproduced below.

Test Tube Danger warnings here….If this is your first time, you are going to be amazed at the power of the beam.  It will instantly burn anything in it’s path.  Be very careful.

  1. Fire at a target like a thick piece of wood.  Keep it close enough to the tube so it is sure to hit the target, but far enough away so smoke or debris will not contact the exit mirror of the tube.

Install Mirrors

  1. The first step is to do your best to get all the mirror centers as square as possible with the frame.
  2. Install the gantry end mirror in the center of the mounting slots and as close to 45° as you can get it.
  3. Use the mirror adjustment screws to move the mirror out evenly about 1/16”.  This will give you some adjustment forward and back when aligning the mirrors.
  4. Now loosely install the lens carriage mirror.  When attaching the mirror holder, be sure it is mounted so it is exactly perpendicular to the beam.  Otherwise, the beam will angle slightly towards the front or back when it shoots down towards the table.
  5. Use some sort of reference plane to make sure the center of the mirror is aligned front to back with the gantry end mirror.  I suggests moving the gantry to the front of the enclosure and using a front horizontal extrusion.
  6. Install the mirror at the back of the enclosure.
  7. Use the adjustment screws to move the mirror 1/16” out.
  8. Adjust the mirror so it is aligned with the center of the tube and the gantry end mirror.

Align Mirrors


  • You are going to be firing the laser with the mirrors unaligned.  Be very careful.
  • Make sure everyone has laser safe safety glasses and you have a fire extinguisher ready.
  • Never have your hands within 6 inches of the tube when firing it.  In many respects, an unfocused beam is more dangerous than a focused one.  It has a very small divergence angle and will carry dangerous amounts of power density very far.  It could easily start a fire or do damage far across the room and you might not immediately notice.
  • Always fire the laser at something (target) that will absorb the laser energy in front of each mirror until you are sure it will hit the center.  I like to use small wooden blocks, some people like to use thermal fax or calculator paper.
  1. Place your target in front of the first mirror the beam will hit.
  2. Manually fire the laser and observe the mark made.
  3. Keep adjusting the screws on the tube until the beam will strike the center of the mirror.  Ideally the beam is perfectly horizontal and square with the frame, but it is not required.  Since the first mirror never moves, the beam should always hit the center and the mirror can adjust for the out of squareness.

Note: Do you best to keep it square to limit the amount of adjustment required.  The low cost mirror mounts move the mirror center as they adjust, so you do not want to have to adjust them too much.

You are now going to adjust a mirror to a moving target.  While you want to hit close to the center of the mirror, that is not the ultimate goal here.  The goal is to adjust the beam so it is perfectly square and does not rise up, down, left or right along the length of the path.  If it does, you beam may still strike the workpiece, but your shapes will be distorted and not the correct size.

  1. Move the gantry close to the rear.
    Place a large target in front of the gantry mirror.
  2. Adjust the rear mirror until it would be close to the center of the mirror.
  3. With the target attached to the gantry move the gantry to the front.
  4. Make another spot and observe the difference.  If the far spot is lower, it means the beam is dropping along the path and needs to be adjusted up.

There are many strategies, but mine is to

  • Make a spot close to the mirror you are adjusting
  • Then move to the far end and adjust until you are close to the first spot.

Since the first spot was not made exactly at the mirror you are adjusting, it is also off, so it helps to over adjust a little along the line between the two spots.  This will compensate the first spot also.

Do the “first spot…far spot…adjust” routine over and over again until both spots or right on top of each other.

If the resulting location does not hit the mirror in a decent location, you have to move the mirrors.  No amount of adjustment can fix mirrors in the wrong location.
The first alignment of a machine can take well over an hour.  Once the mirrors are in the right locations, future alignment checks can be done in less than 20 minutes.



We need to decide on a final location before mounting the tube, as we shouldn’t move the entire thing much with the tube in place or it may fracture.  I don’t know if we are planning on using the 2nd output of the blower connected to the CNC router or if there will be a separate one going out the vent hole in the door near the laser’s current location.  Either way, venting considerations will affect the final location of the machine.

The blower can be powered and switched on externally.  Alternatively, we could use one of the relay switches ( TB5 ) in the interface wiring diagram to control it as suggested.



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