This is a better view of the alignment process that I endure once a year when I haul my Sherline mill back from Cabin Fever. The whole thing depends on a laser level that I’ve gutted and clamped to the floor joists over the mill, as described there.
The first step uses a plumb bob to position the hacked laser level lens directly over the Sherline’s spindle bore. I’ve shimmed the countertop under the mill to be pretty much level, so a vertical line from the bore determines where the lens must be.
Then I fiddle around to get the beam directly in the middle of the spindle bore, using a slip of paper to figure out where it’s going. The top picture shows the result.
Having done this a few times, the laser level starts out pretty much aligned, but the first setup required quite a bit of back-and-forth twiddling of the screws.
Then I put a mirror flat on the Sherline’s table / tooling plate to reflect the beam back up the spindle. More fiddling around gets the reflected spot pretty close to the outgoing beam; this picture shows the spot just off-center near the top (actually, toward the base of the laser level’s frame) of the aperture.
When the outgoing and reflected beams converge, then I put the bushing (without the polarizing filter) in the top of the spindle bore to reduce the beam size and fine-tune the positions & angles.
Surprisingly, it stays in position quite solidly. I do twiddle it every now & again, but as long as the beam gets through the bore it’s close enough.