Notes on a targeting laser

A couple of years ago, after I had finished rebuilding my K40 laser cutter to have a larger bed (separate topic), I decided to add a red-dot targeting laser so I could see more easily where the main laser would cut. There were designs around that added a laser to the cutting head (sometimes two lasers to give a cross) but I was not that keen on any of them – I thought I had a better idea. Why not use the main laser’s mirrors to direct the targetting laser beam too.

There’s two ways that could be done. The most obvious is probably to add a “beam combiner” into the optical train, using a semi-transparent mirror where the main cutting laser passes straight through and the targeting laser is deflected into the same path. The problems with this are:

1. It requires a special mirror, which costs money
2. Some of the main cutting laser beam gets reflected, which reduces its power, and (more significantly) you need to worry where it goes – somewhere that can absorb the beam without creating dangerous heat or dangerous reflections.
3. It requires somewhere in the optical train to PUT that mirror. There’s no space in my modified K40 to put such a mirror.

So I went on to build the other way – a small red-dot laser that moves into the optical path when required, and out of the way when cutting. I used a hinged plate, pulled out of the way by a string when the lid is closed, and dropped into the optical path when the lid is opened. It worked ok, for a while, though the string kept breaking, and when I replaced the mirrors and cutting head with a better set from CloudRay I did not reinstall the targeting laser system.

The new cutting head, with an air-assist nozzle integrated, acts to some extent as its own indicator pointer, so the targeting laser is not needed as much. But I still missed it a bit, when trying to line up cuts near the edge of material, so a couple of weeks ago I decided to set about reinstating it.

My first attempt was just to reinstall what I had before, but I was quickly reminded of all the reasons I had not bothered to reinstall it last time. The main problems with it were

1. Very hard to align. I had made several attempts at building aligmnet mechanisms into it but none worked well
2. Not that reliable at moving out of the way when the lid was closed. Firing the cutting laser when the targetting laser is still in the way is not a good idea.
3. The string tended to pull the laser out of aligmnent.

So I decided to redesign it. The big changes were

1. A new aligmnent mechanism – basically using the same system of screws and springs as the mirrors, which is tried and tested, except using grub screws so I could adjust from the “back” (which faces the main chamber of my laser cutter).
2. A new lift mechanism, avoiding the unreliable string

The new targeting laser mount with adjusters worked pretty well first time. I had to play about a bit with the bearings and mount to make sure that it landed repeatably at the same position, but no big changes.

The lifting mechanism proved trickier to get working – lots of trial and error getting the lengths and angles right – but the final version works smoothly and I am happy with the result. It’s very convenient that there happens to be a gap in the metal case between the main compartment and the laser tube compartment just at the point that the lifting mechanism needs to pass through.

And here is a photo of the final assembly in place. This is in the “down” position (i.e. the lid is open) – when the lid is raised the targeting laser pivots up and out of the way of the main cutting laser’s beam.