With Mirror 1 and Mirror 2 aligned, the next step is positioning the laser head to put the beamline at the center of both the aperture and Mirror 3 inside:
Raising the laser tube by 5 mm put the head’s Z axis screws in the middle of their slots. This had the additional benefit of letting me rotate the head slightly around the X axis to make it perpendicular with the bed, thus fixing its mysterious from-the-factory misalignment.
Centering the beamline horizontally required a few iterations of Mirror 2’s position along the Y axis, but eventually produced this result:
Those are five manual pulses with the head at the corners and center of the platform. I put the 3M target on the mirror rotated 90° from the proper orientation with the stretched scale aligned vertically and parallel to the slightly oval beam mark.
The F target shows the beam position inside the head just above the focus lens:
The little target in the middle gets centered on the nozzle by feel and shows the beam position within a 2 mm circle. The initial position was off against the side of the nozzle, but slight twiddling of the Mirror 3 screws centered it.
I centered the lower F target at the beam position using the red dot aiming pointer, then pulsed the laser to put a pinhole almost exactly at the graticule center. The larger scorch shows the beam size with the platform lowered 10 mm from the focus level. The Z axis leadscrews are not particularly precise and the platform moves by about a millimeter in X and Y as they rotate, so that’s about as good as it gets.
After all that, the laser behaves at least as well as it ever did and I feel better about having the beamline actually travel along the center of the optical path.
Now, back to cutting out interesting shapes …
The Smell of Molten Projects in the Morning 
Comments
2 responses to “Laser Cutter: Moving Mirror 3”
Nice reward for your bravery! Mine arrived out of alignment (admittedly there was one small incident while moving it downstairs which may have contributed to this) so my newbie trial by fire was getting it aligned. I too hit some end-of-travel situations but did not have the fortitude to do that level of modification yet, but after seeing your results, might someday. It is amazing the contradictions in the online advice (“you must be in the center of everything” vs. “center matters not, consistent dot placement is everything”). It is certainly a fun puzzle with many movable parts.
It took me a while to realize the common alignment advice presumes the mirrors sit in the correct spots and need only rotational adjustments. After clearing that conceptual hurdle, the rest was easy!