how can I convert the whiteAnt from the 3D printer mode to the CNC milling mode?

To convert the whiteAnt to a CNC machine, you will want to lower the upper structure to the lowest position to create the most rigidity overall. You will see that there are many holes below the sides of the upper structure that are evenly spaced. This is so you can adjust this lower. You will also want to fasten the lower sides to a substrate (piece of wood, or something) with l-brackets or something that has a bit of adjustment capability. This will keep the sides more rigid as well. Use a Dremel for the spindle. If you want to use a another spindle, let me know and we can product a mount for that spindle since the Dremel has poor runout (TIR). This is important if you are wanting really precise milling.