A rotor mountable fixture (30) is used for guiding a cutting tool (62) powered by a hand held air motor to machine a gas turbine engine rotor (10) without removing the rotor from its mountings in the engine. The fixture includes a root (45) having the same contour as a blade root slot of a rotor (10), a base having a platform attached to the root, and a machining guide in the form of a bushing (50) restrained in a guide slot (38) operable to receive and guide a hand held air motor powered cutting tool (62) for machining the rotor. This provides an accurate means to machine a gas turbine engine rotor without removing the rotor from its mounting. The hand held air motor may be used to power a machining tool to cut a blade damper pocket (20) in the dovetail slot of a pressure compressor rotor.