When you’re aligning to an edge or scribe mark, you want the laser spot as small as it can possibly be, so you tune for best focus.
To locate the center of a hole, you first find the edge, then move toward the center by one radius… so you must know the diameter, too. It’s tricky to find an edge exactly on the X or Y axis, which means you generally resort to successive approximation. I did something like that there with good results.
If you defocus your laser aligner to produce a spot slightly larger than the hole, you can simply position the hole under the beam to produce a nice bright ring. Adjust the focus to make the spot barely larger than the hole and you can get pretty close to the center without any messy arithmetic.
Now, should you happen to own a real laser aligner, you might actually have a nice-looking defocused spot. My homebrew Orc Engineering aligner, as shown there, starts with the beam from a chip laser in a hacked carpenter’s level, so the defocused spot is rather, mmm, ragged, even after passing through the not-very-restrictive aperture behind the lens.
With the lens in the spindle, though, I can spin it at a few hundred RPM and persistence of vision blurs the beam into a nice, symmetrical disk. Jog to center the disk around the hole, twiddle the Z-axis position to adjust the focus / size / blobbiness, jog more slowly, tune for best picture, and it’s all good.
This obviously doesn’t produce jig-boring quality alignment, but, then, I’m not doing that sort of work. In the picture, I’m enlarging a 4-40 hole molded in a Pactec case to fit a 6-32 screw. Normally I’d do that by hand on the drill press, but this time I also had to enlarge the counterbore at the top and figured I’d use a quick G2 with an end mill after I had it aligned for the drill.
Maybe everybody else knows this trick, but I was delighted to find that it actually works pretty well…