Countersink drill bits come from different materials, for instance high-speed steel, carbide, cobalt or titanium. Every type works for particular materials and needs. Picking the right size matters for a smooth finish in every work
Countersink bits come in many diameters and angles, that match the usual heads of screws: usually 82°, 90° or 100°. Most commonly you use 82° or 90°, but also exist 60°, 100°, 110° and 120°-angled bits. The diameter of the countersink must match the head of the screw for good support.
How to Choose and Use Countersink Drill Bits
Like this you avoid that the head sticks out, and you get a safe, smooth result.
Countersink bits are not the same as pilot holes. It is wider than the shaft of the screw. Ideally it has the size of the head.
You do pre-drilling with a drill of same diameter as the shaft between the threads, so around 2.5 to 3 mm for 4 to 5 mm screw. It matters to countersink to not rip the wood.
If you have no countersink bit, a normal drill can work. First make a pilot hole with a bit smaller than the screw (without the threads), later use a drill of size as the head, but only to the depth of the head. For instance, 2.8 mm drill works for 4.0 mm screws in soft wood, or 3.5 mm for 3.5 mm screws in hard wood.
Free charts for countersink bits exist with diameters of holes, drills and 82°/90°-angles for flat head screws from #2 to 1/2 inches. Such charts help match the countersink with a taper point or straight drill according to the size of the used screw. For #8 screw the pilot hole is around 0.170 to 0.193 inches, and the countersink diameter around 0.330 inches.
Screws with a small head can require a smaller countersink. Similarly, very wide heads require extra space for fit. Unlike pilot holes, countersink holes do not work on the tight side.
If in doubt, follow the recommendation.
Modern sets of countersink bits come in sizes for usual screws as #6, #8 and #10. Some bits countersink holes to 1 inch diameter, but they work only for wood. Snail countersinks give a smoother cut and come in three or four sizes.
Zero-flute countersinks, or also called cross-hole, do not have flutes, but instead have angled holes. A well sharpened zero-flute countersink easily makes a smooth countersink in almost every material. They work in a drill press, a hand-held power drill or even a hand drill.
