Counter Bore/Sink

Counter Bore/Sink

Counter-sinking is often done to accommodate heads of flat head screws. However as can be seen from the figure, there is a sideways component of the thrust which could split the countersink due to the generated hoop stresses

Counter-boring is done to accomodate pan-head, fillister-head or round-head screws or other screws with flat-bottomed undersides.

Counter-bored screws exert only force in the axial direction, thus operate mostly under compression, with no sideward component to the applied force vector. Such design is inherently more robust than counter-sinking.