a spindle and z - axis unit comprises a spindle housing and a hollow rotor shaft rotatable and axially movable with respect to the spindle housing . the rotor shaft carries a collet at its lower end for chucking a machine tool such as a drill bit . the tool is changed in the collet by pneumatic actuation of a diaphragm which actuates a push rod within the rotor shaft to move the collect axially against a spring bias to clamp and unclamp the tool . the shaft is moved in the z - direction along with a thrust bearing assembly by a screw nut which actuates a road to move the rotor shaft in the z - direction to drill a workpiece after a pressure foot has first been moved to clamp the workpiece with respect to the spindle . thus , the moving weight or mass previously attributable to parts such as a spindle saddle and spindle housing can be eliminated without impairing the reliability and chucking force of the collet mechanism .