GRBLDroidBT is a CNC Controller that works with the grbl 0. 9+ firmware. It does not work with the GRBL 0. 8 or GRBL 1. 1 firmware. It is the Bluetooth version of the normal GRBLDroid. Using a Bluetooth Android tablet you can pilot your grbl CNC machine via wireless. GRBL is the most common firmware used in the open source community for CNC machines.