The new optics arrived. Their quality is great. The old optics are shown on the left, and you can see the new parts are a lot less bulky than my previous set. The vernier adjust is going to make height adjustment a lot easier.
Collimation. The first thing I did was adjust the collimator. The beam on lasers like mine spread out like a flashlight. The purpose of the collimator is to reduce beam divergence and to control the beam into a nice cylindrical beam, and is also useful because it allows you to expand the size of the beam (see: link). My collimator has two lenses and the precise distance of the two lenses influences overall expansion. To adjust these collimator, I rotated the laser sideways and pointed the beam at burn paper at a distance of 6 inches. Then I put a beam stop about 80 inches away and tested the diameter of the beam. A nice collimated beam should have the same diameter from a distance away. The collimator has some graduated lines on the side, and my burn card shows the beam size for each line. The graduated line for .5 inches seemed pretty good so I locked that position into place.
Centering the beam. Once the collimator was ready the laser was reoriented so the beam was shot downwards. The vernier adjust portion and cutting head was threaded on to the collimator. The optics assembly has a elbow bend with adjusting screws that allow you to center the beam. It was also useful to put some rusty carbon steel on the table, and used a cheap USB microscope to look at the position of the beam. I also used a bit of tape in the retaining ring of the cutting head, and hit the laser at low power for 0.04 sec duration to put a little hole in the tape. The beam was centered after a six or so iterations.
Height adjustment. After the beam was centered I started working on adjusting the height of the beam. This picture shows the goal of height adjustment. The issue is that the beam forms a waist and the most power of the laser occurs at the minimum waist diameter. The sweet spot of the beam waist can be placed in path of the beam by adjusting the height of the cutting nozzle.
To find the best height for minimum beam diameter, I used the thermally sensitive paper and looked at the beam diameter as a function of height. The first pass I took at this was by crudely changing the height while the shot a short duration laser pulse at burn paper. The smaller the dot on this burn test the better. This was repeated again to by starting at the general height from round one and using relative small turns on the vernier adjust. Up until this point the height adjustment was done without the copper cutting nozzle on the cutting head. I put the nozzle on the system and did more tests with the burn paper. If you look at this pic, you can see very odd things happen to the beam with nozzle on the cutting head. The beam is shifted around at the elbow using the adjusting screws until the crescent shapes around the central portion of the spot made by the beam is removed.