In none of these was the slide-rest combined with change gears and a power-driven lead screw. It was this combination which formed Maudslay’s great contribution, together with improvements in proportion and in mechanical design which raised the device from an ingenious but cumbersome mechanical movement to an instrument of precision and power. Jesse Ramsden, a famous instrument maker, is said to have made a small lathe in 1775, which had change wheels and a sliding tool holder moved by a lead screw. The writer has been unable to find any illustration or description of it, and if such a lathe existed, it certainly did not exert a very wide influence. The combination was anticipated in Bentham’s famous patent of 1793. In this patent Bentham says: “When the motion is of a rotative kind, advancement (of the tool) may be provided by hand, yet regularity may be more effectually insured by the aid of mechanism. For this purpose one expedient is the connecting, for instance, by cogged wheels, of the advancing motion of the piece with the rotative motion of the tool.”[35] This patent contained no drawings, and the suggestion was not, so far as is known, embodied in any definite construction. [35] See the British patent records. Patent No. 1951, dated April 23, 1793. Many men were working at the problem of generating an accurate screw thread. The use of dies was quite well known, but their design and workmanship was of the crudest order and their product of the same character; and they were inadequate for the making of any large threads. Holtzapffel’s book on “Turning and Mechanical Manipulation,” published in London, 1847, describes some of the attempts of the earlier mechanics to devise other means.[36] At the famous Soho works in Birmingham a workman by the name of Anthony Robinson cut a screw 7 feet long and 6 inches in diameter with a square, triple thread. After the cylinder had been turned, paper was cut and fitted around it, removed, marked in ink with parallel oblique lines, then replaced on the cylinder and the lines were pricked through with a center punch. The paper was again removed and dots connected by fine lines with a file. The alternate spaces between the lines were then cut out with a chisel and hammer and smoothed by filing. A block of lead and tin, as a temporary guide nut, was then cast around the partially formed screw. Adjustable cutters were fixed upon this guide nut and it was used as a kind of tool-holding slide-rest, being rotated around the screw by hand levers, thereby cutting the finished thread. In other words, a lead screw was cut on the piece itself and the temporary nut was used as a tool holder to finish the work. [36] Holtzapffel. Vol. II, pp. 635-655.