A hole made to receive a flathead screw or rivet should be countersunk deep enough for the head to be flush with the surface or up to .015 in. below the surface. A flathead fastener should never project above the surface. The included angle on commonly available countersinks are 60,82,90 and 100 degrees. Most flathead fasteners used in metal working have an 82-degree head angle, except for in the aircraft industry, where the 100-degree angle is prevalent. The cutting speed used when countersinking should always be slow enough to avoid chattering.