No joke this is the best alignment video i have ever watched… hands down, the BEST.
Actually I should add in a 4th thing that “alignment tutorials” usually omit.
If your head isn’t squared off the gantry, you can still adjust the #3 mirror so it goes through the cone orifice dead center. Sure. But, when you cut thick materials, they will be cut on a slant.
So, what I do is find something thick- like 3/4" acrylic- and cut it into 1" wide test bars.
Fix a straightedge on the bed so it’s parallel with the gantry. Lay the bar against the straightedge. Make 1 cut all the way through along the y (front of bed to back or vice versa) about 1/4" from the edge of the bar.
Flip the bar left to right (keep same side up) and make the same cut on the right side of the bar, beside the first., 1/4" further in. They’re going to be parallel from the top if your straightedge is correct. But caliper the difference in width from top to bottom. If it’s a parallelogram, you did something wrong. If it’s a rhombus, then you need to adjust the head. Typically this is done by shimming. I cut a strip of aluminum coke can, fold it over as necessary, and shim it under the screws attaching the head to the carriage. What you need to do varies greatly by the machine’s construction. Some machines have the wheels/linear rail on the side and don’t shim like that.
Now, repeat by using a straightedge on the Y axis and taking a horizontal cut through. But if THIS one is off, god help you. It’s pretty machine-specific to figure out. In many cases it means unbolting the XY joints on the sides of the gantry and shim or bolt down differently.
But it can be worse- you actually want to start by doing this test on both the left and right sides of the bed first. Because the gantry CAN be twisted, in which case you may need to make different adjustments on either side.
In doing that, you should also take care to not unlevel the gantry. That is, say we accidentally mount the right side 2mm higher than the left. Now when we focus the machine in the center, it’ll be +1mm too close on the left and 1mm too far on the right. But, to do this right, actually you want check that the bed is level before doing that.
Larger machines (1390 etc) typically have one stepper motor driving two Z-screws on the left, and another on the right. When powered up, they’re driven together, but when powered down, they freewheel and can get out of sync, screwing on the left/right leveling. Get this level against the frame first, then work on the XY joints if needed. All you gotta do is power the machine down, grab a belt, and use it to lift one side without the other until it’s level. Then power back up.
Or, usually we’re not messing with the XY gantry joints so we’d just level against the gantry. Just check the distance from cone tip to bed on the left and right and move the Z screws on one side as needed.