clutch brake 33 is controlled according to the time ratio of a power supply and the clutch brake 33 produces a variable braking torque corresponding to the drive voltage the drive gear 32 clutch brake 33 controller 45 and the electromagnetic clutch driver 46 as a whole constitute braking control means 47 in addition to the same effect as that of the embodiment of fig1 the braking torque control can be more accurately performed by the drive voltage control of the clutch brake 33 in the embodiment of fig2 therefore a feed bar member 11 that performs the feed motion of a drill not shown is rotated and axially moved at a desired speed so that the number of rotations of the drill and the feed speed thereof can be controlled simply and variably fig3 schematically illustrates a third embodiment of the motion converting mechanism constructed in accordance with the present invention the same reference numerals will be applied to the pars substantially identical to corresponding parts of the fig1 embodiment and therefore a description of the corresponding pars will not be given in the third embodiment instead of the clutch brake 33 of the embodiments of fig1 and 2 a know torque limiter 51 is provided between a support structure 18 and a drive gear 32 on the basis of the position signal of a rotational position detector 44 a torque limiter controller 52 variably controls the limit value of a braking torque which is imparted to the screw nut member 12 by the torque limiter 51 as required therefore braking control means 53 in the third embodiment.