Window cutting tool for well casing

A method of milling a window in a selected portion of a well casing comprising the steps of:

• providing a milling apparatus comprising a rotary milling bit, motor means for driving the milling bit, and a guide device for positioning the milling bit in contact with a selected portion of the casing to be milled out, wherein said guide device comprises at least one elongate longitudinally extending well casing engagement member mounted on a body, which body is disposable, in use of the device, in-line with the drill string, so as to be movable relative to said body between a retracted position and a generally radially outwardly displaced position for well casing engagement, each said at least one engagement member having pressurised-fluid operable actuator means formed and arranged for driving said well casing engagement member between said retracted and well casing engagement positions, said device including pressurised fluid supply means formed and arranged for controlling the supply of pressurised fluid, in use of the device, to said actuator means for operation thereof whereby in use of the device said milling bit may be directed relative to the central longitudinal axis of the well casing in which said milling bit is disposed;
• placing the milling apparatus within the well casing;
• operating the actuator means of the cutting tool guide device so as to position the milling bit in contact with the selected portion;
• and
• driving the milling bit so as to form a window in the well casing.