16 exterior to bearing block 30 is equipped with a knob 31 to facilitate manual turning by virture of the aforesaid arrangement of control rod and chuck assemblies rotation of the knob 31 in one direction causes the chuck assemblies to approach each other rotation of the control knob in the opposite direction causes the chuck assemblies to separate from each other locking bolts 15 threadably penetrate base 14 at two sites and engage the tongues 27 of each chuck assembly a knob 33 positioned upon the exterior extremity of each bolt 15 permits manual tightening of each bolt 15 to lock the chuck assemblies in desired positions along track 22 upper portion 25 of housing 24 of each chuck assembly is provided with centered chamber 34 whose upper extremity is provided with circular opening 39 the chamber confines a multitude of ball bearings 35 a retainer disc 60 having a circular center aperture is affixed by bolts 61 to top surface 62 of the housing in centered disposition above opening 39 an internally threaded hollow cylindrical hub 36 is welded to retainer disc 60 is centered relationship about the circular aperture in said disc the diameter of said aperture being at least as great as the internal diameter of hub 36 six slots 40 within upper portion 25 and communicating with chamber 34 are oriented in vertical planes radially emergent from chamber 34 the slots are bounded in part by flat bottom surfaces 70 tangentially emergent from the bottom of chamber 34 first holding means in the form of.