The Sienci Mill One uses Grbl 1.1v firmware on an Arduino Uno. The firmware can be flashed onto the Arduino Uno with the Arduino IDE, Grbl 1.1v with default settings for the Sienci Mill One, and by following the instructions on the Grbl wiki page. This version is meant for Mill Ones set up for ⅛ microstepping.
Please note that Grbl 1.1v works best with the latest nightly builds of Universal Gcode Sender, however note that the nightly builds may have the potential to have bugs since they’re the most recent version of UGS.
If you are using an Apple computer with the latest OS (10.12 Sierra), you may experience crashes when plugging in CH340 based Arduino Unos. Please use this driver if you have trouble connecting your Arduino to your computer or if you experience these issues. Windows users should not have this problem.