rollers bearings 7 the sleeve 6 surrounds the pin 8 and is slidable relative thereto an auxiliary pin 10 is slidably mounted in the pin 8 an expansion rod 11 is screwed into the lower end of the pin 10 and connects with an expansion cone 11a fig2 of the expansible grinding tool a bushing 12 extends transversely through the pin 10 and engages in longitudinal ports 8b of the pin 8 the auxiliary pin 10 is thereby rotated by the pin 8 but can slide axially relatively to it a shaft 13 extends axially through the bushing 12 and the ends of this shaft engages in the sleeve 6 the sleeve 6 has a shoulder 6a disposed between two needle bearing abutments 14a and 14b these abutments are housed in a widened portion 5a of the screw 5 and are retained there by a toothed wheel 15 fixed to the bottom portion of the screw 5 it will thus be seen that the sleeve 6 rotates with the pin 8 but can move axially thereof in unison with axial movement of the screw 5 to move the expansion rod 11 axially axial movement of the screw 5 is effected by rotating the screw 5 relative to the fixed nut 3 to this end the toothed wheel 15 meshes with a very long pinion 16 which itself meshes with a pinion 17 integral with a toothed wheel 18 the toothed wheel 18 meshes with a pinion 19 keyed on the output shaft 20 of a digitally controlled stepping electric motor 21 when the output shaft 20 is turned by one step the screw 5 turns through a certain angle due to the nut 3.