\6o ^lerhanical Prcufsfor conJlriitHiig the Screw. 



If a number of deep marks, fiich as c d, be cut obliquely acrofs the thread with a tlirce- 

 I'ljuare file, in a fteel fcrew AIJ Figure I. Plate IX. of the kind here dcfcribcd, and the 

 fcrcw be then hardened, and tompored to the pale (haw-colour, it may be ufcd to form the 

 inltrunitnt Fig. 2. called a fcrcwtool. For tlie fcrcw-tool being rendered quite foft by an- 

 nealing, and fixed in a proper liandle, will be cut into teeth by prclhng the end A againd the 

 fcrew AB Fig. i, while turning in the lathe. It is fcarccly neceflary to fay that the fcrcw- 

 tool will be fupported by the Reft, and that oil or tallow mud be ufcd to prevent the work 

 iiom heating. 



It is evident that the fcrew-tool will be much more regular than the fcrew it was made 

 from ; becaufe all the irregularities of the fcrew will pafs in fucceffion through each of its 

 teeth. If the fcrew therefore be now foftencd, and the fcrew-tool hardened and tempered 

 to ftraw-colour, and wlietted on the upper face, the former operation in the lathe may be 

 repeated ; but with this difference, that the fcrew-tool will now cut the fcrcw, and render 

 it ftill more regular, for the reafon juil mentioned. After this reftification,- it is ufual to 

 ■deepen the groove between the threads by applying an acute angled file or tool to the fcrew 

 Mhile turning. 



The procefs of re£lification naturally fuggefts an improvement in the order of working. 

 It is certainly much more eafy to make a regular fcrew-tool by hand than a fcrew ; more 

 cfpecially when the thread is fine. Suppofe Fig. 2. to be fuch a fcrew-tool, and the fcrew 

 part of A 15 Fig. I. to be merely cylindrical. Jf the hard fcrcw-tool be then fteadily applied 

 againft the foft cylinder while turning Tii the lathe, i: will cut a number of equidiftant cir- 

 cular grooves ; but if, inftead of being held in one place, the tool be regularly moved along 

 the reft, the grooves will be helical; and the fucceeding teeth, falling into the marks made by 

 ihofe which went before, will render the threads equidiftant, and tend to produce more and 

 more of regularity. Such a fcrew may nevenhelefs vary in the obliquity of its threads on 

 jlifferent fides, and this imperfeclion will be very little amended by continuing the adlion of 

 the fcrew-tool. It would require too long a detail to ftate the manipulations by which 

 workmen acquire the facility of making fcrews in this way witliconfiderable regularity; and 

 it is befides probable that they would occur to any ingenious beginner as he proceeds. 



^Vhether the tap (which is the name given to a fcrew referved for making or tapping 

 female fcrews) be made and reflified in this or the other way, the next ordinary (tep 

 is to tap a pair of dies. Figure 5. Plate IX. reprefents a die for making fcrews. Its 

 thicknefs is fliewn in Fig. 6; and in Fig. 3. a pair of fuch dies are feen properly difpofed in 

 the frame A BCD, called a ftock. The thicknefs of the ftock is precifely equal to that of 

 the dies, which are well fitted to Hide in the opening GHIK. On the lower fide of the 

 {lock, as it now lies, is fixed, by rivets or fcrews, a flat plate of the dimcnfions of Fig. 4, 

 the width of the opening PPPP being fomewhat lefs than that of GHIK Fig. 3. It there- 

 fore affords a fupport to prevent the dies from falling through ; and the plate Fig. 4. being 

 applied on the upper fide of the ftock ferves effeiflually to prevent any motion whatever in 

 the dies, except the direft approach or reccfs with regard to each other by means of the 

 fcrews EF. The top plate is fecured by four fcrews LLLL, which pafs through holes of 

 the Figure rcprefented in the (ketch. The convenience of thefe holes is, that the plate 

 may be taken off to change the dies without a£lually drawing the four fcrews. A variety 

 of fa(hions prevail in the form of ftocks, which are fuppofed to have their rcfpcclivc advan- 



4 tages. 



