The machine is equipped with a double-spindle unit mounted on a slide with oleo-pneumatic advancement and a double vice for external clamping of the rear dropouts. Two milling cutters are precisely mounted on the two spindles on axis and opposed to one another. During the work phase, these tools advance on the slide and enter the dropout slot and mill the dropout slot bottom, upper part or lower part, depending on the error automatically detected in the search phase. Thus a perfect alignment axis of the two slots is obtained for the wheel center. For the error searching, base points of reference can be selected: bottom bracket, seat tube, or seat tube plus down tube. The vertical reference plane is preset to accept any frame position (for the various rear dropouts types).