Abstract: A mechanism for individually positioning a drilling head in a multiple head drilling assembly. The assembly includes a plurality of spindle blocks arranged in side-by-side relation, and a spindle that is adapted to carry a drill head is mounted for rotation and axial movement in each block. Each spindle block is mounted for individual lateral movement in a direction normal to the axis of the spindle on guide rods, and is driven laterally by a rotatable lead screw that extends parallel to the guide rods. A recirculating ball nut is associated with each spindle block and is engaged with the lead screw. A clamping mechanism acts to selectively clamp each nut to the respective spindle block and by clamping the nut to the block, the spindle block will be moved along the guide rods on rotation of the lead screw, and by disengaging the nut from the respective spindle block, rotation of the lead screw will cause free rotation of the nut without movement of the block.