Method of Cutting' Screws. , f 275 



part, which will, however, depend on the length and diameter 

 of the screw. 



After the screw is brought to a sufficient depih, the steel 

 cutters are to be taken out and copper grinders to be put in- 

 to their places, and the screw to be worked through till it be 

 perfectly adjusted. Two or three sets of the copper grinders 

 may be necessary. 



By these methods I have cut screws from twelve to forty- 

 eight inches in length, the accuracy of which has been proved 

 by tests of the severest description. 



Make a nut to fit the screw to be proved ; fix the nut to 

 the under side of a board or bench, and introduce the screw, 

 which must now be furnished with a micrometer head or in- 

 dex. The screw is now to be made fast at both ends upon a 

 table, but free to revolve, and upon this table is fastened a 

 stage rising above the bench, and at right angles to the axis 

 of the screw, the stage having a sliding point. Place now on 

 the upper board two straight slips of brass, laid edge to edge ; 

 set the index, and draw a line across the brass slips ; turn the 

 index, say 90° or J of a revolution, draw another line and re- 

 peat the operation till you have a sufficient number of lines. 

 Let now the slips be shifted, and bring any two lines to coin- 

 cide; if the whole lines always coincide after the slips are re- 

 peatedly shifted, the screw is perfect. Parallel straight lines 

 may thus be produced at any distance. 



The leading screw E of the circular dividing machine. Fig. 7, 

 is made by the process already described of making a screw for 

 producing equidistant parallel lines, but it must afterwards 

 receive a certain curvature. Cast a lead wheel one diameter 

 of the leading screw larger than that in which the screw is 

 destined to work, and considerably thicker. Turn a semicir- 

 cular groove into the periphery of the lead wheel to embrace 

 one-half of the screw, which is now to be worked into the 

 groove till a full impression is made in the lead, then ground 

 with emery till it have acquired the same curvature, and it is 

 then to be connected with the other parts of the machine re- 

 presented in the drawing, the purpose of which is to produce 

 the screw G, the centres of the threads of which are not pa- 

 rallel ; but if produced, would converge and meet at a point. 



