LcheapoAssemblyCNC

Start with the laser, clearly (Your laser PCB may look different; this is the one with the ball switch to only allow the laser on when it's aimed downward)

Get a 12V power supply of at least 2.5 amps. Be sure to mark the polarity on output wires!

Remove the prongs...

... and replace with...

... some wire...

... then close the case.

Be sure that the 12V power supply has some wire length to it! There isn't much current on the AC side, so thin wires are OK.

Start opening up your CNC control box.

Once the control box is open, identify the rotary tool relay (middle right in this picture). It will be the largest relay.

Make some room for the laser power supply somewhere...

...and get its AC lead close to the relay.

You will need a SPDT switch rated at the current that the rotary tool takes.

You will also need to make room for it on the control panel...

... and label it properly.

Disconnect from the relay, the lead that goes directly to the rotary tool.

Connect that to the switch's center contact. Then make a short wire to connect one of the side contacts back to the relay.

The other side contact should go to one of the AC leads of the laser's power supply.

The other AC lead for the laser's power supply should be spliced into the other AC lead for the rotary tool...

...in a way that's visible, in case you need to go back to it and change it.

Now, the DC leads to the laser's power supply go to the laser. You did mark the polarity on the output wires, right?

This is what you're doing electrically, if you'd rather see a very simple diagram than a series of pictures. I asked someone with no knowledge of electronics to draw this, and they got it right the first time!

Mounting the laser will depend a lot on what your CNC mill looks like, but it should be able to move with the rotary tool, obviously.

Apply zip ties liberally to make sure the extra wire can't get itself into moving parts.

Enjoy!