Abstract: A replaceable gang head machine tool unit having a driving motor, the unit being mounted on a slide table arranged to be driven and advance in a given direction wherein: there is provided an annular rail comprising a rear stationary rail and a front movable rail disposed on a machine base, a plurality of gang heads disposed around the annular rail and supported thereon, the plurality of gang heads being arranged to be rotated on the annular rail by an index table to permit any desired gang head to be selected therefrom on the movable rail. When the working unit is in a retreated position in the machine base, the selected head on the movable rail may be joined to the unit for travel therewith together with the movable rail.