2-Countersinking, Counterboring, and Spotfacing - Countersinking
Countersinking is the operation of producing a taper or cone shape surface at the entrance of a hole for the purpose of having the head of a flat head screw, aviation rivet or other similar fastener sit flush or below a surface. This cone shape is machined with a countersink.
Countersinks are available as a single flute or multi flute. A variety of sizes and included angles of: 60°, 82°, 90°, 100°,110°, and 120° are available. Drawing specifications will determine the angle required. Micrometer stops (left) are available for controlling depth of cut.
Indicating chamfer gauges can be used to accurately control chamfer depth
A 90° countersink is commonly employed for removing burrs or sharp edges of holes. Also a 90° countersink is commonly used prior to tapping or machining an internal thread. Unless otherwise specified, countersink to a diameter .010-.015" larger than the basic size of the thread. 80 (or 82) degree and 100 degree counter sinks are most common for flat head screws.

Countersinking tools come is a variety of sizes (below left) and even in specialty shapes like the reverse countersinking tool (below right)

Most flat head screws heads have either an 80/82° or 100° included angle. Therefore some machinists call those countersinking tools and the 30°, 45°, 60°, and 90°, angles are called chamfering tools. R.S.