Abstract: A machining center or machine tool comprises a bed, structure to support and position the workpieces, and a rotary spindle fed with worktools by an automatic tool changer and carried by a spindlehead which also carries the driving motor therefor. The spindlehead moves back and forth along a rectilinear path under the control of a circulating-ball screw and nut assembly, the screw of which is rigid with the spindlehead and has its axis intersecting that of the spindle. The lead nut of the same assembly is caused to rotate by a motor and is kept axially fixed.