I was having some issues last weekend removing noise on the paint management system axis limit switches. The consequence of noise being that even when none of the limit switches were pressed, grbl would stop the machine an indicate a hard limit switch alarm. This seems to be a fairly common issue among grbl users when wiring the limit switch signal wires directly to the Arduino. The grbl wiki recommends using an RC circuit or better yet opto isolation - https://github.com/gnea/grbl/wiki/Wiring-Limit-Switches. I tried the RC circuit method with the recommended resistance and capacitance values but still got false hard limit switch alarm trips. After a few hours of troubleshooting, I discovered that by routing the limit switch signal wires away from the stepper motors and stepper motor power wiring the false alarm trips would no longer occur. The paint management system has operated well since rerouting the signal wires, but I am skeptical whether the machine will be able to operate without false alarm trips for durations up to several hours without a robust signal filter or isolation solution. Though at the moment, I am grateful to have found a simple solution.

After resolving the noise issue on the limit switch signal wires, I was able to set the homing sequence and perform the homing operation on both machines. Previously I had issues with getting the grbl homing operation to work. Using the "show verbose output" feature of the Universal Gcode Sender open source application (https://winder.github.io/ugs_website/), I discovered that the unused axis limit switch pins where being treated as "activated". Connecting all the unused limit switch pins to ground solved the issue and the homing operation now works great. 

The six axis brush control cnc machine homes to X-, Y-, and Z+. The homing sequence is Z+ then X- and Y-.

The paint management system homes to X-, Y+, Z-, and B-. The homing sequence is X- and Y+, Z-, then B-.

During the process of developing the homing sequence, I realized that some of the dimensional control parameter datums needed adjustment. For example, on the paint management system I originally planned on homing to Y- but at that end of the machine there isn't any room to home the Z axis (the water dish is in the way). So after established the appropriate homing sequence for each machine, I spent some time updating the dimensional control parameter values that tell the machine where in the workspace objects (i.e. water dish, paint palette, canvas, etc.) are located. All of the new values are captured in the If Then Paint Image2Gcode code on GitHub (https://github.com/johnopsahl/IfThenPaint_Image2Gcode).