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A countersink tool is cone type cutter used to remove the sharp edges left behind from a drilling or tapping operation.
It can also be used in relieving a drilled hole to allow the head of a countersunk screw or bolt to be flush or set just below the surface of the material around the hole.
Countersinks differ from countbores. A counterboring tool makes a flat bottom hole suitable for a socket head cap screw.
The countersink cutter can be used in any driven spindle in a downwards direction to produce a conical shape and sideways in a milling machine to produce a bevelled edge. Similar tools for milling are chamfering end mills or corner rounding cutter.
The crosshole countersink is coned shape with a crossways hole which generates a cutting edge. The cone section of the tool will have clearance to allow the edge to do its job.
The most common application for the crosshole countersink is in metal deburring (removing sharp edges) from a previous operation. These tools can be used to create a countersink relief in softer material such as wood or plastics.
For heavier chamfering the fluted countersink cutter is used to provide a tapered relief on a drilled hole. This may be required to allow a specific angled seat for a countersunk head screw or to provide the lead in for a second machining operation such as tapping. One of the common angles for countersink cutters is 90°however, they are also available in other angles such as 60°, 82°, 100°, 110°, or 120°. Typically countersunk-head screws of UN threads often have 82° heads whereas crews that follow the ISO standard may have a 90° angle. Countersunk fasteners used in aerospace typically have an angle of 100°.