A countersink is a cone-shaped hole cut into a board or plane. A countersink is also the name of the cutter (or bit) used to cut such a hole. The common use of a countersink is for when the project calls for a flush finished surface – the countersink bit is used to create this effect. The tip of the bit drills through the working surface and the the countersink portion (the part closest to the base of the bit) acts as the cutting mechanism to create the cone-shaped hole desired. Really a must have for when the project requires fine craftswomanship!
Countersink Bit Explained February 15, 2011