I am using a ShuttleXpress USB jog control. It works with Chilipeppr to control the CNC xPro. The interface in Chilipeppr is really nice, even speaks the axis to you on button press. -- John
It's been supported by Mach 3 for a long time. Integration with GRBL is new. Instructions for Mach 3: http://www.machsupport.com/wp-content/uploads/2013/04/ShuttleProInstallation.pdf
Yeah, the source code is included, so you can change it as you desire. I'm not sure if that's a good idea though, as the outer dial is spring loaded, while the inner dial is free spinning.