Tapping is a machining operation which cuts a screw thread into a hole. A tap is cross-shaped and located in an imaginary cylinder. At the end of the cross at the circumference there are cut grooves in the form of a wire. The depth of the cutting grooves increses when the tap gets deeper. Therefore it is not necessary to cut complete screw thread in a rotation. If a material is too sturdy one can drain and tap it in several steps. This implies that more than one tap is used where the groove depth increases after each tap.