TOOLS FOR SCREW-CUTTING. 93 



screw, and may be altogether changed when the distance 

 between two threads, compared with the diameter of the 

 cylinder, becomes so great as in the screw fig. 65, A. 

 To avoid such wide threads, several threads are often 

 wound in parallel spirals round the cylinder. In fig. 

 65 B, there are four such parallel threads. 



In the construction of physical apparatus as well as for many other 

 mechanical purposes, the screw finds many and various applications ; 

 the student who wishes to construct his own apparatus should there- 

 fore acquire practice in cutting the necessary screws. Very small 

 screws are generally cut by means of a tool, called a screw-plate, 

 while the larger kinds are cut in a peculiarly constructed turning- 

 lathe. The tool used for cutting screws of average size, which will 

 be described here, is called a screw-stock or die-stock. It need not be 

 very large for our purpose, but must be really good, if satisfactory 

 and speedy work is expected from it. 



The screw-stock represented in fig. 67, J., consists of a rectangular 

 iron frame, having a central rectangular aperture, and being pro- 

 vided at two opposite corners, PP, with handles. The ' dies,' or 

 half-nuts by means of which the screws are cut, are inserted in 

 aperture, and may be squeezed together by the screw S, the 

 of which is perforated for the reception of a pin by means of 

 rhich the screw is turned and adjusted as required. The sides of 

 the aperture are bevelled or * chamfered ' above and below, so that the 

 dies may rest on the projecting ridge : this is shown in fig. 67, B, 

 which is a section across the screw-stock along the dotted line d d 

 in A. For about one- third of its length the chamfer is filed away 

 and the aperture enlarged, for the removal and insertion of the dies 

 laterally. The stock shown in the figure will cut screws of three 

 sizes, viz. having an external diameter of 6 mm> 5, and 3 to 4 mm , with 

 'pitches' or distances between the threads of l mm< 25, l mm '0, and O mm *9 

 respectively. Three pair of dies are therefore required, and to avoid 

 mistakes, they are marked by points, the number of which corre- 

 sponds to a like number marked on the stock, as shown in the figure 

 where the dies are marked 6, 6, and . The inner surface of 

 each die includes from a third to nearly the half of a circle, and a 

 notch is made at the central part of each die, so that the pair of dies 

 present four arcs and eight series of cutting points or edges. 



Hollow screws are cut by means of screw-taps, represented in 

 fig. 67, (7, which also shows sections of the tool at three different 

 points. A screw-tap is simply a screw of which great part of the 



