1.Forging milling machine is a casing opening tool which integrates casing cutting and section milling functions. The section milling tool goes down into the casing with the drilling tool combination to cut the casing at the specified position firstly, and after the casing is completely cut off, it directly conducts downward section milling on the casing from the position, and completes the task of casing window opening after reaching a certain length. 2. Simple structure, easy to operate and use, is a very effective casing windowing tool. 3. In the process of using, forging milling tool with the drilling tool combination down to the casing in the specified position, start the turntable after the opening of the pump pressure, the pressure to push the tool inside the piston downward, the piston lower taper from the tool inside the tool to promote the cutter body outward opening cutting the casing, when the cutter body opening the maximum diameter, at this time the casing is completely cut off, the piston bypass valve open, the pump pressure drops, in the continuous pump pressure supply, the cutter body to maintain the open state, can be directly down the section of milling Casing. 4. After stopping the pump, the piston is reset under the action of spring, and the cutter body is automatically retracted.