A machine tool adapted to perform operations, for example, machining, on a surface of a part of large size is provided. The machine tool comprises a longitudinal rail and a transverse arm, orthogonal to the longitudinal rail, slidingly fastened at a first end to the longitudinal rail. A second end of the transverse arm is provided with a unit comprising a holding device enabling the unit to be held in position on the part during an operation. The transverse arm bearing a tool is thus precisely positioned on the part on which the operations are carried out.