Opto isolation board

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Z-axis homing & opto-isolation board

It is always uncomfortable bringing logic-level signals outside of the "box" and into the real world.  Any excess voltage can wipe-out your stepper controller board - and since the inputs are fairly high impedance, these lines are very susceptible to induced noise - especially from the nasty spikes caused during the driving of the stepper motors.
I designed a 5-channel opto-isolator board to keep the ugly outside world away from my HobbyCNC controller board.  I designed the board to run from around 12 volts, but I added a constant-current source to each input line.  This ensures 20ma flowing through the opto isolator LED.  The added benefit is I can add extra LED's to the 'chain' without affecting the signal and I can safely run the limit switch wiring through the same wiring harness as the stepper motor cables with no fear of interference.
I added a yellow LED on the board for each of the 5 inputs - this allows me to verify an input is working without needing to grab a voltmeter.  The constant current source also allows me to put another LED out where the work is done.
To complete the board is a green power LED and a fuse.  In the images, the opto chips are actually surface-mounted on the underside of the board - the one lit yellow LED is for the limit switches - this tells me all 6 limit switches are closed (good).

I created a special rig to set the z-axis home.  It uses an alligator clip and a precision brass 1/2" square bar.  I added an LED in the bar so I can confirm all is good before I press "home".  Any problem with this setup and the system will drive the tool into the workpiece - a real big issue if using small drills for PCBs.  The short video shows the Z axis home setup being tested to confirm continuity, then the video shows the opto board during the same test.

The new build - the base

I kept the base design similar to the original - using a torsion box design.  This provides a considerable amount of strength and rigidity with minimum mass.  It is important to have a very flat surface to assemble and glue-up the frame.  Even with all the care, I ended up in excess of 30 thousandths out of square.  This is excellent accuracy for a woodworking project, but it is not all that great for a milling machine.

Taking care at every step means less fine-tuning later.  Cut the components carefully, make sure to get the angles as square as possible.  Ensure the top (facing downward in these images) is flat-and-true against your flat surface.

Here is the finished torsion box.  I changed the design of the x-axis rails - both to provide extra strength and rigidity, and to get the x-axis rails inset just enough to keep them out of the way of most of the dust and debris created during milling.