Patent ID: 8166656

Claim:
A method of working a screw rotor by controlling at least two axes, that is, rotation of a work and turning of a tool at a time to form a screw tooth space, the method comprising a first step of using a first end mill as the tool to make a predetermined cut in a depth direction of the screw tooth space to perform engraving to a predetermined depth, a second step of using a second end mill, which has a smaller diameter than that of the first end mill, as the tool to perform rough working on a side of the screw tooth space, a third step of using a third end mill, which has a rounded tip end, as the tool to perform rough working on a bottom surface of the screw tooth space, a fourth step of using a fourth end mill, which has a cutting edge portion being shorter than a depth of the screw tooth space in a depth direction and a neck portion being smaller in diameter than the cutting edge portion, as the tool to perform finish working on the side of the screw tooth space, and a fifth step of using a shaving byte, which is shaped in conformity to the bottom surface of the screw tooth space, as the tool to perform finish working on the bottom surface of the screw tooth space, wherein the neck portion has a dented dimension determined by the following formula as an amount of interference between the fourth end mill and the screw rotor: amount of interference=tool diameter×[(cos((δ c− (δ a+δb )/2)/cos ((δ b−δa )/2)]−tool diameter, where δa represents a grooves side lead angle in a tooth bottom position at a maximum groove depth position, δb represents a grooves side lead angle in a location higher than the tooth bottom position by a length of the cutting edge portion, and δc represents a grooves side lead angle in an external surface position.