This is to mount the extruders on the top rails of Steve Grabers' excellently designed Cerberus Delta Printer found here: http://grabercars.github.io/CerberusDrill a hole in the center of the 1" aluminum rail. Use 2 machine screws and nuts to secure the extruder in place (I used M4). Note: The 12mm in the filename indicates that the hole has a 12mm offset from the top of the ~25mm (1") aluminum railing.