4PCS #6 #8 #10 #12 Countersink Drill Bit Sets 1/4 Hex Shank Sets w/ Hexagonal Screw Material: HSS Drill 4241 Ideal for drilling carbon steel,sheet iron,insulating plate,PVC sheet,wood,etc. Drill a screw hole and a nut hole at the same time,after tightening the screws,the nut will inset the board. Drilling length can be adjusted for various length screws simply by loosening and tightening the setscrew. Set include 1 each: #6, #8, #10, #12.