This application will let you design and create your own CNC mill. You can design the mill in a free manner, without having to pay anything. The software is open source, and if you want to learn how to use it, you can download it from GitHub. You can also make your own customizations to it.

It is a free and open source application, and it works on Android devices. 

It works as a controller to move the stepper motors in the Arduino Mega 2560 R3 board, so that the mill turns. You can design and make modifications to the mill in the same manner as the software, and then you can connect it to the Arduino Mega 2560 R3 board via USB.