Processes in the Useful Arts. 34,1 



a contrivance (called the blind man's guide) which, when once set to that 

 distance accurately, is stopped by a bolt, and the plate can be turned all 

 round by it one of the said distances at a time, without the trouble of 

 looking at the divisions again. Towards the top of the frame which holds 

 the mandrill and dividing plate, there is a horizontal dovetail slide of brass 

 running back from the mandrill about six inches, on which slide an up- 

 right back (or cock) holding the cutter frame is drawn backwards and for- 

 wards by means of a screw in a horizontal direction from the top of the 

 mandrill, where the wheel is so that the cutter may enter the wheel any 

 required depth to form the tooth. In the drawing, Plate I. Fig. 4, (given 

 in last Number,) a, a, represents the top of this upright back, and to two 

 brass chocks, b, b, the cutting frame is fixed as represented, but does not 

 come forward farther than c, c, and is formed in the direction of the dotted 

 line. Between d, d, a steel axis is swiftly turned round by a lathe, and on 

 this axis is the cutter. This cutter frame may be moved upwards and down- 

 wards in the chocks b, b, turning thus by means of steel hollow cones, into 

 which the inner ends of the screws J] J) enter on the inside of the chocks — 

 after the cutter frame is brought forward on the dovetail slide, so that the cut- 

 ter will enter the wheel on the top of the mandrill, the requisite quantity, 

 the frame is then turned a little up, and the lathe set in motion, and the cut- 

 ter is by this means turned swiftly round in the frame ; the frame is then 

 turned downwards, and the cutter passes through the circumference of the 

 wheel cutting into it the required depth. 



I now propose, that after the first cut into the wheel is thus made, the 

 cutter-frame c, c, c, c, is taken out of the chocks b, b, for which purpose 

 the screws f,f, g, g, may be for convenience made with large round heads, 

 with milled edges, similar to h, h, as they can then be turned by hand; 

 the upright back and chocks are then to be drawn back on the dovetail 

 slide, and the frame k, k, is to be fixed in the chocks b, b, exactly the 

 same as the frame now used was done, taking out the screw m, and open- 

 ing the part out marked n, n, which has a hinge at o, and adjusting the 

 hole p, to the mandrill, by the screw to the dovetail slide, — then shut up 

 the mandrill in the hole p, by closing the part n, n, and screwing fast the 

 screw m, — this plate k, k, is to be of solid brass, and about half an inch 

 thick. The frame k, k, when thus fixed, must be in a plane parallel to 

 that of the surface of the dividing plate, the top of the mandrill, with the 

 wheel on it standing nearly an inch above the upper surface of the frame. 



On this frame, I propose to fix a pentagraph, constructed to diminish in 

 any required ratio, — the tracing end to be towards the chocks b, b, and the 

 marking end towards the mandrill and wheel at />, — at the end next p, the 

 cutter is to be fixed, so that its edge next p, is precisely in the marking 

 point of the pentagraph,— at the end of the plate next the chocks b, b, 

 where the tracing end of the pentagraph comes, an epicycloidal curve, si- 

 milar to that required for the teeth of the wheel, is to be deeply cut ; 

 when, therefore, the tracing end of the pentagraph is carried through this 

 curve, the edge of the cutter describes, or is carried through a similar curve, 

 but smaller in any required ratio to which the pentagraph may be 



