406 



♦ KNOWLEDGE ♦ 



[May 15, 1885. 



THE YOU:XG ELECTRICIAX. 



By W. Slisgo. 



{Continued from p. 345.) 

 DIES. 



EX. LI. — A few words were said a, fortaight since on dies 

 when referring to small die-stocks, but something 

 further is necessary. Fig. 2-1: represents, as then stated, 

 the plan (A B) and elevation (C D) of a pair of dies. Those 

 here shown belong to what is known as the Whitworth 

 screw-cutting tackle. The principles involved in them are 

 identical with those pertaining to the small dies fitting into 

 the small die-stock (Fig. 2-3). A pair of dies in reality 

 act in a similar way to the hole in a good screw-plate, such 

 as that illustrated in Fig. 19. The dies are intended, how- 

 ever, fur work too heavy to be performed by a screw-plate. 

 There is also a most important advantage inherent in the 

 die.s, viz , that the thread is produced gradually, or by a 

 succession of varying cuts, instead of being produced by a 

 single thrust through the plate. This gradual formation of 

 the screw is obtained by variations in the adjustment of 

 the thumb-screw, so as to briog the dies closer together at 

 each adjustment. 



A little amplification is, perhaps, necessary to simplify 

 the matter. When it is intended to cut a screw with 

 the aid of the dies and stock, the movable die is drawn 

 sufficiently far from the stationary one to allow the lower 

 portion of the work to be placed between them. The 

 movable die is then screwed up until a small indentation is 

 made on the metal by the die threads. The metal is then 

 screwed down through a distance equal to the length of the 

 intended screw (or the metal may be held in a vice, ic, 

 and the die-stock revolved). In this way a portion of 

 the thread is formed. To increase its depth, set the 

 thumb-screw a little further forward and then turn the 

 metal (or die-stock) backwards through the length of the 

 screw. The other end being reached, repeat the operation, 

 and continue the series of cuttings until the requisite 

 depth is attained. 



This gradual or stepby-step production of the thread 

 insures the manufacture of a screw, the pitch of which cor- 

 responds with something approaching to accuracy to the pitch 

 of the thread on the dies from which it is cut. A second 

 advantage possessed by dies over a screw-plate is, that in 

 the event of the tap used to make the female thread, into 

 which the to-be-made screw is to fit, being partially worn 

 or by some other means rendered a trifle smaller than it 

 should be, the dies may be so adjusted as to produce a 

 screw proportionately smaller than the normal ^ize. This 

 will sometimes be found to be an important feature. It 

 is, thirdly, noteworthy that when a screw is made by a 

 single thrust, as in the screw-plate, there is a tendencv to 

 elongation on the part of the metal, resulting in an increase 

 in the pitch. 



Now for the dies themselves. In Fig. 24 the plans B 

 and A are stamped with the figures 1 and 2 respectively, 

 showing the relative positions which they should hold 

 in the stock — that is to say, Xo. 2 should be the movable 

 die actuated by the thumbscrew. Whatever the figures may 

 be, the higher one should always be the adjustable die. 

 These figures, moreover, are placed on one side of the upper 

 face of the die, and corresponding figures are stamped along 

 the face of one side of the stock. The object of this is to 

 indicate the right way to insert the dies. The figures on 

 them should be on the same side and on the same face 

 as those on the stock. This is most important, for, if not 

 carried out, the dies wUl not match and a screw cannot be 

 formed. 



The construction of the dies may be gathered from the 

 illustrations, aaa being the cutting edges, the spaces between 

 affording ready means for the escape of the cuttings. The 

 peculiarities of the threads we can deal -with presently. 

 It may, however, be observed that the dies are cut from a 

 master-tap which is larger in diameter than the screw to 

 be cut by them, the difference being equal to the depth of 

 the threads. It is important that we should understand 

 the reason of this. Were the thread on the dies equal in 

 diameter to that on the intended screw, it will be apparent 

 that the smaller diameter of the dies (or the distance be- 

 tween two opposed a, a, Fig. 24) would be less than the 

 diameter of the metal to be cut, seeing that that diameter 

 is equal to the diameter at the bottom of the die-threads. 

 Now prior to any cut being made upon the surface of the 



Fis. 



C 

 A 



. ^ 





■ . % 



B 

 D 



Fig. 24. 



Ficr. 26. 



Fig. 25. 



metal, the approach of the dies to it will result in their 

 only touching it at the two corners of the cutting-edges on 

 each die. To make this clearer (in an exaggerated sense) 

 we may suppose a semi-circle of 1 inch diameter to approach 

 a circle of li inches diameter. It is evident that the 

 semi-circle can only touch the circle at its two extreme 

 points. The effect of such an arrangement, were it applied 

 to dies, would be that the comers of the cutting-edges 

 would have far too great a strain upon them, and would 

 soon be broken. 



WHITWOHTH STOCK. 



Ex. LII. — This stock, which is illustrated in Fig. 25, is 

 used when heavy work has to be done. It is rarely tised 

 for screws less than a quarter of an inch in diameter, the 

 threads being coarser than those with which the dies tised 

 in the small stock (Fig. 23) are provided. The frame is 



