4PCS 3mm-6mm HSS 5 Flute Countersink Drill Bit Set Reamer Woodworking Chamfering Machine Special designed steel counter sleeve countersinks or counter bores pilot hole. Adjustable counter sleeve depth with wrench. Drilling length can be adjusted for various length screws simply by loosening and tightening the setscrew. Drill a screw hole and a nut hole at the same time,after tightening the screws,the nut will inset the board. Set include 1 each: 3mm,4mm,5mm,6mm