Countersink is a type of machining operation in which a hole is drilled into a material at an angle, usually with a conical shape. The purpose of this operation is to create a recess in the material that will allow for the head of a screw or bolt to be flush with the surface. REAM is a machining operation that enlarges and smooths the inside of a previously drilled hole. It is often used in conjunction with countersinking to ensure a smooth, even surface.