Abstract: A method for determining the position of a part in an orthonormal frame of reference based on a structure of a numerically controlled machine tool to determine the trajectory of a tool for performing an operation on the part, the method including the steps of: holding the part in position on a fixed carrier inside an enclosure of the machine tool, so as to arrange a through-hole comprised in the part such that it faces a through-orifice included in the carrier, the hole opening out in its entirety inside the orifice, illuminating, using a light source, so as to emit light radiation from the carrier towards an optical measuring instrument arranged inside the enclosure of the machine tool, through the hole and the orifice, reading the part using the optical measuring instrument so as to determine the position of the part in the orthonormal frame of reference.