April 24, 1885.] 



♦ KNOWLEDGE ♦ 



345 



shown in Fig. 22, which will be found to have certain 

 advantages over the ordinary plate, amongst them being 

 the fact that the dimensions of the screw may be accu- 

 rately obtained : the smooth hole near one end of the plate 

 being equal to the size of the screw at the top of the thread ; 

 and the smooth hole at the otlier end, equal to the size at 

 the bottom of the thread ; the two side lioles are provided 

 with cutting edges (Ex. XLIV.) and threads of which the 

 other two holes are the dimensions. The use of such an 

 arrangement is that with the aid of the larger smooth hole 

 the wire from which the screw is to be made may be of 

 e.xactly the proper gauge, this hole affording also a ready 

 means for ascertainiug which is exactly the right tsp (so 

 far as its diameter is concerned) to be employed for pro- 

 ducing the corresponding female thread. This hole oflers a 

 further advantage, inasmuch as it euables one to determine 

 the accurate dimensions of the drill necessary to provide 

 smooth holes through which it may be necessary to pass 

 the screw. The small hole affords an opportunity for mea- 

 suring the size of the drill to be employed in making the 

 hole which is to be provided with tlie thread. 



To make this convenient little plate, 

 procure a piece of bar steel, about three- 

 eighths or half -an -inch wide, and an 

 eighth-of-an-inch thick. With the aid 

 of a three-square or half-round file, cut 

 off a piece two inches or thereabouts in 

 length (Ex. XXIV.). In the event of such 

 a piece of metal not being at hand, an 

 old flat file of approximately similar 



AT 



Fig. 22. 



dimensions may be used instead, first 

 softening it by carefully and uniformly 

 heating it to a cherry red, then placing it 

 in a quantity of hot ashes and leaving it 

 there until practically cold. When softened 

 it may be cut to the required size, and 

 the holes drilled in positions as shown in 



Fig. 23. 



Pijr. 2i. 



the illustration (Fig. 22). The requisite diameter of the 

 smaller hole may be determined by passing a drill or a 

 piece of wire through the hole in the standard screw- 

 plate. The diameter to be given to the larger hole may be 

 determined by measuring the external diameter of the 

 corresponding tap with a pair of callipers (see next article), 

 or by cutting a notch in a piece of sheet-metal just large 

 enough to admit the thread of the tap. These measure- 

 ments, of course, give the width of the blade of the drills 

 to be employed for boring the holes. The drill used for 

 boring the smaller hole must also be employed for boring 

 the two side holes, the thread being produced by the tap, 

 and the cutting edges by a three-square file. Prior to making 

 the cutting edges, the burr which will have been produced 

 by the tap on each face of the plate, should be removed by 

 means of a gentle application of a comparatively large 

 blade drill. It is possible that the file in producing the 

 cutting edges may leave them somewhat ragged. This 

 mtist be overcome by passing the tap through the hole 

 g&rUly. It would be well before doing this to smooth down 



the groove produced by the above-mentioned file by means 

 of another of a very fine cut. The face of the plate may 

 then be smoothed by means of a fine file. The screw-]ilato 

 being thus completed only requires hardening (Ex. XXX.) 

 and tempering to a Iljlil straw colour (E.\. XXXI.). 



SMALL DIE STOCK. •" ■ 



Up to a certain point — say for screws of an eighth of an 

 inch in diameter — the screwplate is all that could be 

 desired, or nearly so. It is to be noticed that the metal 

 from which a screw is being made is strained and E(juee/,ed 

 in the formation, resulting in diijhthj stretching the thread, 

 and so increasing the pitch. Again, where long screws are 

 being made, they are aiit to be bent to a greater or less 

 extent. So far, however, as we shall require it, the screw 

 ))late will answer our purpose (Ex. XLI.). Another method 

 for making fine threaded scicws, and one which we shall have 

 to resort to when making screws of more than an eighth of 

 an inch in diameter, is to employ a "die stock " and "dies " 

 (Figs. 23 and 24). The stock consists of an iron frame 

 provided at one end with a fixed handle. At the other 

 end is a long thumbscrew which passes through the frame 

 projecting inwards. 



The inner portion of the frame demands a little con 

 sideration, so far as its longer sides are concerned. For a 

 short distance at the end, near the screw, these sides are 

 fiat, but for the remainder of the distance they are provided 

 with bevelled extensions — that is to say, there are two 

 longitudinal faces, making an angle of about 120° with 

 each other, and 30° with what would otherwise be the fiat 

 inner side, of the frame. In this way two solid V-shaped 

 guides are produced, the use for which we shall see in 

 di^cu6sing dies. In Fig. 24, A and B, and C and D are 

 plans and elevations respectively of a pair of dies, which 

 are pieces of steel furnished with cutting edges and 

 thread for making screws. The V shaped tides shown 

 in A and B afibrd a ready for securing the dies in 

 the stock. They are introduced by first withdrawing the 

 thumb-screw, so as to allow them to effec't an entrance, when 

 they are pushed along the Vgnides, and the thumb-screw 

 driven in to the requisite distance. In the form of appa- 

 ratus shown (Fig. 23) there are dies of various sizes, and 

 each piece of steel (excepting the two end pieces) is provided 

 with two different-sized threads. 



Our remarks on dies and stocks must, for want of space, 

 be for the pre.>-ent suspended, but we shall find occasion for 

 somewhat considerable auiplification at the next oppor- 

 tunity. 



CRITICAL METHODS OF DETECTING 

 ERRORS IN PLANE SURFACES. 



. • By John A. Brashear. 



( Continued Jrom p. 329.) 



NOTWITHSTANDING these defects of the two 

 systems I have mentioned, in the hands of the 

 practical workmen, they are productive of very good re- 

 sults, and very many excellent surfaces have been made by 

 their use, and we are not justified in ignoring them, because 

 they are the stepping- stones to lead us on to better ones. 

 In my early work Dr. Draper suggested a very excellent 

 plan for testing a flat surface, which 1 will briefly describe. 

 It is a well-known truth that if an artificial star is placed in 

 the exact centre of curvature of a truly spherical miiTor, 

 and an eye-jiiece be used to examine the image close beside 

 the source of light, that the star will be sharply defined and 

 will bear very high magnification. If the eye-piece is now 



