The machining of large dimension part presents issues of accuracy and of part handling. Machine-tools of large capacity are classically used, but they are very expensive. The paper investigates original solutions with mobile milling machines. A comparative study of a machining robot and of a portable milling machine is performed. Instrumented experiments of face milling operation were carried out and the resulting quality was analyzed. Finally, the manufacturable precision for the large part is assessed by the tolerance stacking model that is proposed.