Importance of accurate optical alignment

[greg_gran]

I'm just setting up a used K40 with LightObject upgrades - AWC608, Z table, air assist, etc. (Bought from another party who obviously never did get it setup right)
I've got a fair bit of optical experience (Amateur telescope maker), and it's clear to me that accurate alignment is everything.

Optical alignment entails not only mirror tilt, but also mirror position. You need to achieve perfect right angles in the laser path so that the beam is perfectly parallel with the gantry in each axis. If you don't, the position of the laser spot will vary depending on the X and Y axis positions. This can't be fixed with just mirror tilt adjustments alone. The tricky part is that position and tilt adjustments all interact, so the full alignment process is very iterative.

I discovered that I also needed to shim the height of the first and second mirror holders in order to maintain exact positioning of the laser spot vertically when moving the Y axis from one end to another. There is no other way to make this adjustment on a K40.

The next issue was the rotation adjustment of the 3rd mirror/focus lens assy. The beam needs to pass through the center of the focus lens. Best method I found was to put a small circle on a piece of material that matches the diameter of the nozzle. Run the table up until it almost touches and center it under the nozzle exactly. Then lower the table to the focus position and fire a test. If the spot is not perfectly centered in your circle, you need to adjust the tilt and/or rotation of the mirror/lens assy.

The final obstacle - one I'm still trying to get perfect - it getting the Z table perfectly co-planar with the focus plane. The Z table is a must-have upgrade, but getting it accurately setup is not trivial. Adjusting the brass nuts to get the table accurately level within its own frame is pretty easy. The harder part is getting it properly shimmed up from the bottom of the K40 so that the table surface is equally spaced from the bottom of the nozzle regardless of the X-Y position. My current approach involves using 1/4" thick bubble rubber blocks between the table frame and the bottom of the K40. By putting the mounting screws through these blocks, and adjusting their tension, the leveling of the entire Z table assy can be controlled to a certain extent. I also found that the plate of the Z table was not perfectly flat, and needed to be "tweaked" slightly to eliminate a slight bow it had it had.

I believe that attention to these details is important to getting good results from your machine. It's even more critical if you are using a short focus lens to achieve a small laser spot for finely detailed engraving. The depth of focus for shorter focus lenses is proportionally narrower, and demands tighter focus distance tolerances.

These are just my observations - comments welcome.