In particular, there is one absolutely vital modification that everyone should make before powering-up their laser. Without this, it's only a matter of time before someone blinds themselves while building their machine.
Laser Pull-down Resistor
When I first connected the power to my A3X, the laser fired up on full power for about a second. I wasn't wearing the protective glasses and if the laser had been pointing towards my eyes, I would now be blind.
Luckily the laser was already mounted pointing down, and I had a protective board over the dining room table. Otherwise there would now be a small black hole in the walnut, and my wife would not be happy. Thanks, Eleksmaker.
This happened again and again, when powering up or connecting and disconnecting the USB cable. Checking the Eleksmaker forum showed that this was an issue for many people, and also that there was a simple solution.
The laser is controlled by the voltage on a pin on the controller, which usually holds the voltage down to zero (ground). But when the controller is powered up, it takes about a second to get round to initialising the pin output, and meanwhile the voltage "floats". The laser sees a non-zero voltage, and does what it is supposed to - turns on.
This is actually a very common issue in electronics, and the solution is simple. You add a pull-down resistor to ground, which holds the pin voltage at zero until the controller initialises the pin. You need a 1K or 2K resistor soldered to the back of the controller board like this:
Given the danger involved, I think this is a serious defect. I strongly encourage Eleksmaker to add the pull-down resistor before shipping their controller board, but until they do so, please make sure you do this before powering up your A3X.
Laser Carriage WheelsThe laser carriage consists of two plates that you bolt together with wheels between them. When I assembled my plates, the upper and lower wheels were slightly too close together, so the carriage didn't run smoothly along the beam. Even worse, after they sat still on the beam for some time they developed indents. I could feel a jerk as the carriage rolled along the beam, every time the wheels made one revolution and the indents hit the beam again. This would result in small X-axis errors when burning.
My solution is to file away a small amount of the top side of the top two holes of each plate slightly, so the top bolts sit slightly further away from the bottom bolts. They only need to be a fraction of a millimetre higher, so do this in small increments and test each time to see how the carriage moves. I used a small round file.
Drive Belt TensionYou can read my separate post about my problems with fastening the drive belts, so I won't duplicate that here. But the drive belt tension is important - if it is too tight the servos will have difficulty driving the carriages; if it is too loose the carriages will slop about, making your positioning much less precise.
The instructions don't mention cabling. My kit came with three lengths of plastic trunking that clip into the beams, but I only used two of them (for the Y-axis cables). You need to put the cable into the trunking first, then push both into the beam. If you put the cable into the beam then try and clip the trunking in, you'll be there all day.
The Y-axis cables should go along the top of the rear beam (under the trunking); under the side beams and out; cable-tied to the top of the corner plate; over and cable-tied to the Y-axis carriages:
I drilled an extra hole in the carriage plates to cable-tie the Y-axis cables so any strain did not work against the servo plug. This should prevent these wires unplugging or breaking due to repetitive flexing.
Controller MountingThe back plate for the controller board did not fit with the new layout with the back rail under the side rails (the old A3 Pro had the back rail over the side rail). I drilled and tapped a couple of holes in the rear rail so I could mount it higher up.
I hope these tips helped with your build. Please leave a comment below if you have other suggestions, or better ways to solve the problems I came across.