Countersinks drill downwards a small amount in order to make a quality starting hole and a profile. Follow up with a secondary drill for the full depth of cut of the work-piece. They don't route along a slot. Combination drill/countersinks provides exceptional starting positioning for secondary drills, plus provide qualified Countersink dimensions for screw heads, etc. Premium aluminum titanium nitride (Altin) coating provides Quantum increases in productivity, either in terms of speed or longevity, with or without coolant, #51672, (D) diameter 5/16, (b) Cutting height 1/8, (D) shank 5/16, overall length (L) 2-1/8, flutes 2, D1 1/8, wood screw size #4, 51672 CNC 118 degree point center drill high performance Solid carbide 60 degree Countersink 5/16 dia x 1/8 x 5/16 shank Altin coated router bit