This is an add on for turning your prusa i3 rework printer into a CNC mill. It bolts onto the carriage where the extruder would normally bolt on. it still uses the m4 screws and uses m4 screws and bolts to make the foredom stay in the holder. it has the spot to hook up a fan holder if you would like to clear any dust made by the milling. you can follow my blog to find out how to turn your prusa i3 into a milling machine if interested.