In the prior art, the elastic means described above take the form of a series of Belleville washers arranged coaxially around the tool-holding collet, which are in contact with a special part of the collet, on which they exert a predetermined pressure due to the abovementioned precompression, keeping it closed during machining. If an axial force is applied to the tool-holding collet, via a rod passing through the interior of the spindle cavity containing the Belleville washers, the latter are further compressed to bring about this opening action of the collet.
However, the system described above has certain troublesome drawbacks: firstly, at the extremely high rotational speeds of the spindle that are currently achievable (over 100,000 rpm), the said Belleville washers are drawn round by the rotation, creating effectsof unbalance which cause vibrations harmful to machining accuracy. Also, to accommodate Belleville washers with specifications suitable for the job, a large-diameter cavity must be formed in the spindle, which means that the spindle itself must have a large diameter and consequently a large mass.
Moreover, with the system currently in use, it is necessary, as already discussed, to operate from the outside with a rod which must move cyclically into contact with the end of the tool-holding collet and exert upon it a force (of the order of some 2,000 N) which, over time, owing in part to the situation of point loading that occurs, can deform the tip and/or alignment of the rod, causing problems of accuracy and service life.
In order to solve all the problems listed