Collet actuating mechanisms

A collet actuating mechanism comprising a hollow work spindle tube with an internal frusto conical surface at an operative end thereof, a collet tube co-axial with and slidably engaged within the work spindle tube, the collet tube having a collet threadably engaged with its end, the collet having a complimentarily frusto conical external surface engagable with the work spindle, the end of the collet being divided into a number of segments to that it can contract inwards to grip a work piece, a toggle mechanism to perform axial sliding movement between the work spindle tube and collet tube, a sleeve slidable around the work spindle tube to cause the toggles to act between the slidable tube and an assembly including springs reacting against a nut fixed relatively to the collet tube, spring means acting on one of a pair of axially relatively movable elements which, in turn, act on the toggle and on the fixed abutment on the collet tube, respectively, and adjustable cam means also acting between said elements and arranged to alter the effective between the toggle and said abutment, the arrangement being such that the collet will grip work pieces of varying diameter with substantially the same gripping force.