the common Turning Lathe, 173 



distance little advantage has arisen, owing as well to the 

 expense of purchasing it, as to its incapability of turning 

 that variety which is necessary, since only screws similar 

 to those traced on the traversing manderil can be cut with 

 it ; and these are limited to a very small number. To those, 

 then, who find it difficult to cut screws without such an in- 

 strument, an apparatus is proposed, by the application of 

 which to the plain lathe that difficulty may be obviated. 

 The plate represents a perspective view of the tool ; and the 

 letters referring to the different parts of it may, perhaps, 

 make its description clearer. On the common manderil 

 A (Plate IV.), is screwed a chuck, B, to which may be 

 screwed the chucks of the lathe, as R. On the outside of 

 this chuck, B, is turned a screw, which is fitted to an inside 

 or female screw worked in the circular block C, from which 

 block extends an arm, D, as long as may be thought fit for 

 the purpose of permitting another arm, E, to slide up and 

 down it : a piece of iron should be screwed to the circular 

 block* C, of such a length as to be capable of moving in a 

 groove that may be cut in the collar, or adapted to it. The 

 rest, GFO, must not stand, as usual, parallel to the work, 

 in cutting an outside or male screw; but at right angles, as 

 when an inside or female screw is to be cut, in order that 

 the further arm of the rest, F, may be joined to the end of 

 the second or intermediate arm, E. It is necessary that this 

 second or intermediate arm, E, shall be capable of fastening 

 firmly the first arm, D, to any part of the rest, GF, as also 

 to have a joint at each end to admit in a horizontal plane 

 its free play. Thus, as the lathe turns to us or from us, the 

 arms must traverse forwards or backwards ; which gives a 

 similar motion to the tool H, that is held steadily or fixed 

 with a screw on the further arm, F, of the rest ; and thus a 

 screw is cut with a tool of a single point. It is unnecessary 

 lo mention that no joggling should arise from the motion of 

 the arms, as that would cause a failure in cutting a perfect 

 screw. If the centre of the rest should be drawn nearer to 

 us, and by that means bring the tool closer to the interme- 

 diate arm E, then a screw of a much larger size will be cut; 

 for as the rest, turning within its socket (the. thumb-screw 

 for fixing it being in this operation always withdrawn), 

 moves on a centre, tne further the tool is moved from this 

 centre the greater will be the radius of the circle described, 

 and consequently the coarser will be the screw; and, vice 

 versa , the nearer the tool is brought to the centre the smaller 

 will be the radius of the circle, and thus the screw will be 

 finer. Should the intermediate arm, E, be connected to 



the 



