April 10, 1885.] 



♦ KNOWLEDGE ♦ 



303 



the hole in the plate which it is intended to use, select ii 

 piece of round metal having the diameter of the head of 

 the screw to be made ; then reduce one end of this piece 

 of metal to the length and outside diameter of the screw 

 (see Ex. XXIX ), tapering the metal for about an eighth 

 of an inch from the small end, in order that an entry into 

 the hole in the screw-j.late may be easily eUccted. Fix the 

 metal in the hand-vice (E.\. XXVIII.) ; then, having 

 etVected this entrance, commence carefully to screw the 

 metal into the screw-hole, turning it as long as it pro- 

 gresses with a moderate degree of freedom. Then turn it 

 backwards slightly to relieve the strain exerted upon the 

 thread under formation, and repeat the forward movement 

 Continue ihis process until the required amount of thread 

 is produced. Oil should be used with all metals, brass 

 excepted. 



To form the head of the screw, cut off sufficient of the 

 unthreaded portion of the metal to leave a head of the 

 necessary length, using a _frame or back saw (Ex. XIX. 



-^*=. 



Fig. 10. 



Fi' 20. 



Fig. 21. 



and XX.) for the purpose. Having smoothed the head 

 with a file, cut by means of either of these saws the usual 

 slot to carry the screw-driver.* 



TAPS. 



A tap, as above indicated, is a tool for producing a 

 thread in the hole into which a screw is to be driven. 

 It is, therefore, a cutting tool, and a most important 

 one too. It is generally made by cutting a thread on a 

 piece of wire, and then cutting away longitudinal sections 

 until cutting edges of the required depth, ifec, are provided. 

 A set of taps is usually sold with a screw-plate, but the 

 worker will speedily be impressed with the fact that they 

 are not, as a rule, as effectual as they might be. He will 



* The orthodoi method of making screws is by means of a lathe. 

 I am sorely tempted to dilate npon the uses and choice of such 

 a tool, but I am restrained from so doing by the fact that this 

 aeries of papers is intended for the young electrician who has no 

 tools to start with, and who is only able to expend a few shillings 

 at a time. 



often find it impossible to tap a shallow hole of uniform 

 diameter. Indeed, mo.st workmen prefer, and so it is 

 believed will our young experimenter, to use home-made 

 taps. 



E.\. XLVI. — Fig. 20 represents, on a considerably 

 enlarged scale, the type of tap frequently mot with. By 

 reference to the plan and cross section, it will be seen that 

 the tap is made by simply cutting away tlirc^c longitudinal 

 sections until the cross-section approximates somewhat 

 closely to an ((juilatcral triangle. The great defect per- 

 taining to this tool is that the cutting edges, instead of 

 cutting mcay the metal, ])arlially drive or push it away. 

 One great effect of this is the exertion of a considerable 

 strain upon the tap, with the result that it often gives way, 

 breaking otf in the hole. 



Ex. XLVir. — Another frequently sold form is one in 

 which four longitudinal sections are removed, giving a cross 

 section something like a .square. The same disadvantages 

 inherent in the triangular tap are present in this form, and 

 to an intensified degree. Moreover, the small amount of 

 metal removed from the tap afford little or no egress for 

 the cuttings produced by it. 



Ex. XLVIII. — A much better form is that illustrated 

 in Fig. "21, and is the one which the young electrician 

 is recommended to make for liimself. The usual thread 

 is cut in a piece of wire by means of the screw-plate 

 (Ex. XLV.). Tiie 1 est bright steel wire should be employed, 

 the same as that recommended for drills (Ex. XXIX.). 

 Such a size should be selected as will correspond with the 

 diameter of the screw which is to be diiven into the hole 

 after it is tapped. Should the diameter of the wire be too 

 great, reduce that portion of it which is to carry the thread 

 to the required thickness. To produce the cutting edges, 

 secure the unthreaded end in a vice. Then with a small 

 three-square tile cut three grooves, at equal distances apart, 

 throughout the entire length of the thread, and somewhat 

 deeper than the bottom of the thread. Leaving theenlarged plan 

 to explainitself,a little attention bestowed upon the sectional 

 view is necessary. It will there be seen that sides of the 

 groove are not eciual in depth, nor do they bear the same 

 angular relation to the surface of the tap. The arrow 

 indicates the direction in which the tap will revolve, whence 

 it will be seen that aaa will be cutting edges, and that they 

 will acluallij cut forward into the metal, while b h h take i o 

 part in the cutting. The groove between a and // afford a 

 means of egress for the metal cuttings produced by <i. The 

 advantages, then, which this form of tap possesses over the 

 previously-described forms are the acute cutting edges aaa, 

 and the increased facility for the removal of cuttings. These 

 features imply more rapid working and a considerable re- 

 duction of the nsk of breaking the tap. It is almost 

 superfluous to mention that this proneness for common taps 

 to break inside the hole is exceedingly tiresome, resulting 

 frequently in effectually spoiling the outcome of many 

 hours, or even days, of industrious labour. 



Before using this tap it will require hardening. Heat 

 the cutting end of the tap to cherry-red by either of the 

 processes described in Ex. XXX. Then dip it re.rlicalbj in 

 either oil or water. Do not remove it from the liquid until 

 it is cold. 



Preparatory to tempering, clean the grooves by means of a 

 I.ieceoffineemery-papt-r.bent once round a small three-square 

 file, care being taken not to injure the cutting edges. Next, 

 with a pair of pliers, hold the shank or unthreaded end in 

 a flame until the other end assumes a deep straw colour. 

 Then plunge the tap immediately into water or oil. 



When cool, file two flat and parallel faces on the shank 

 end, and the tap will be ready for use. 



( To he coiUiinifid.) 



