Counterbores are tools for cutting recessed holes in a workpiece, typically on center with a smaller hole. Counterbores are frequently cut in order to allow screw head seating in the recessed hole, so that the screw head does not protrude above the material surface. Counterbores may also be used to square the screw head’s seating surface perpendicular to the axis of the hole, an operation known as spot facing. Counterbores are available in both English and metric screw sizes.
When determining which of the available styles of counterbores are best for a given application, there are a number of important specifications to consider. Foremost among these are the diameter of the counterbore; the diameter of the integral pilot (or pilot-hole should the counterbore feature interchangeable pilots, or completely lack them); the diameter, hex size, or nominal taper size of mounting arbor or shank; and the number of flutes or cutting edges desired.
Counterbores are available in a number of general configurations, which are simply delineated based upon alphabetical designations common to cutting tools. The most common designations are Type B, Type CB, Type CR, Type M, Type MS, Type R, and Type X. In addition, specialty counterbores are available, which are designed for specific counterboring tasks. These configurations include aircraft counterbores, cap screw counterbores, fillister head screw counterbores, blade counterbores, and back counterbores.
Aircraft counterbores typically have a standard chucking shank size such as 1/4", 1/2", etc., and are often configured for nonferrous materials, such as aluminum, that are frequently found in aircraft.
Cap screw counterbores provide a recess or spot face for the head of a standard socket head cap screw. Similarly, fillister head screw counterbores have a recess or spot face for the head of a fillister head screw.
Blade counterbores use straight blade-like protrusions from the side of the body to cut the counterbore.
Back counterbores are designed so that the bore extends through the body of the material, so that the counterbored surface is on the opposite side of the counterbore chuck or holder.
Counterbores are available in a wide range of material designs, some of which are better at performing specific cutting functions. High-speed steel is the most common material used because it is an efficient, cost effective cutter for general applications. Cobalt steel counterbores are somewhat harder than high-speed steel and are often used for counter boring in stainless steel and other alloys. Carbide counterbores and carbide-tipped counterbores much harder than high speed steel, and therefore last longer and wear less. Industrial diamond grades such as polycrystalline diamond (PCD) provide very long life, and are used for special applications such as very high-speed cutting of hard materials.