Positioning mechanism for a multiple head drilling assembly

A positioning apparatus for positioning a head of a multiple head working apparatus, comprising a frame, a plurality of spindle blocks mounted in side-by-side relation on the frame, guide means for guiding the spindle blocks in movement on said frame in a given direction, a spindle to carry a working member and connected to each spindle block, means for rotating the spindle about its axis, means for moving the spindle axially relative to said block, screw means extending in said direction, nut means associated with each spindle block and engaged with said screw means, and clamping means for selectively clamping each nut means to the respective spindle block, whereby rotation of the screw means when the nut means is engaged with the respective spindle block will cause movement of the spindle block in said direction and rotation of said shaft means when said nut means is disengaged with said spindle block will not cause movement of said spindle block, said clamping means including a first clamping element disposed to clamp against the outer surface of said nut means fluid cylinder means including a cylinder member and a piston member, a first of said members being connected to said first clamping element, actuation of said fluid cylinder means moving said first clamping element in a generally radial direction into engagement with the outer surface of said nut means, a second of said members being mounted for sliding movement relative to said block, a second clamping element secured to said second member, and mounting means for mounting said second clamping element for radial movement relative to said block, said fluid cylinder means and said mounting means being disposed in alignment, initial actuation of said fluid cylinder means acting to move said first clamping element into engagement with said nut means and further actuation of said fluid cylinder means acting to move said second member relative to said block to thereby move said second clamping element in a direction toward said first clamping element and into engagement with said nut means to thereby prevent relative movement between said nut means and said block.