325,350. Holbrook, R. E. Jan. 11, 1929. Backing-off lathes; attachments for turning; reversing-mechanism; screw-cutting.-In a relieving device for lathes to enable the backing-off of cutters and hobs to be performed, and which may be an attachment to an existing lathe, of the kind comprising a cam for reciprocating the tool-slide to and from the work, a train of changewheel gearing for rotating the cam and a second train of change-wheel gearing for imparting an additional rotation to the cam when spiral fluted hobs are being relieved, the support for the tool slide is adjustable about the axis of the cam to enable backing-off to be performed in different directions. A third gear-train imparts longitudinal motion to the tool-slide and cam, each train being independent so that the first is selected according to the number of flutes in the cutter or hob, the second according to the pitch of the flutes and the third according to the axial pitch of the teeth. A pinion 16, Fig. 1, on the headstook spindle 15 rotates, through an idler, a parallel shaft 17 connected by gearing 18, 20 to a shaft 21 within the backing-off gear casing 13. The shaft 21 carries one sun wheel 22, Fig, 2, of a differential gear 23 whose other sunwheel 34 is mounted on a shaft 35 connected by change gearing 38 to a non-circular shaft 14 passing through a bracket 50 of the tool-saddle. A worm 51, Fig. 5, on the shaft 14 drives a transverse non-circular shaft 53 and a worm on the latter rotates the cam 57 which engages a roller 61 on the transverse tool slide 60. Springs 62 maintain the roller in contact with the cam, and the retractive movement of the slide may be limited by a plate 67 thereon and an adjustable fixed stop 69. For backing-off hobs, &c. with straight flutes the cam is rotated by the gear train described and the tool-slide is fed longitudinally by the lathe feed screw rotated from the pinion 16, but for hobs having spiral flutes an additional rotation is imparted to the cam by the planet pinions 22<1> of the differential gear 23. The casing carrying the pinions 22<1> is rotated by a worm drive 27, Fig. 2, from a shaft 29 driven by change gearing 30, 31 from a shaft 26, Figs. 1 and 2, which is connected by skew gearing 24 and a reversing clutch to the shaft 17. Slide-rests.-The tool slide 60, Fig. 5, is slidable on a base 58 which is angularly adjustable about the axis of the cam 57, the support 54 for the base taking the place of the usual cross-slide of the lathe and engaging the transverse feed screw. The bolts 59 which secure the base may be inserted in a number of positions on the support 54 to enable the slide 60 to be adjusted to anv angular position and the tool-holder 63 mav be attached to either end of the slide sothat internal backing-off may be performed.