Ccnjl 111311011 of thi Scrftu. — Errors of Tapping. l6l 



tages. Steadinefs, accurate fitting, and a regular fair aflion of the end preflure, are the ef- 

 fential requifites of a good ftock. The drawing here given is one of the mod fimple. 

 When it is ufed for ftrong work, the ftock is fixed in a piece of wood terminating in a 

 handle on each fide. 



Suppofe the dies NN Fig. 3. to be foff, and a notch A Fig. 5, of about fixty degrees of the 

 circumference of the tap E Fig. i, not reckoning the thicknefs of the thread, filed in each. Let 

 them be placed in the ftock, two pieces of brafs OO being put in to defend them againft the 

 thumb-fcrews, and the whole fecured by the face plate. The dies are then ready for tapping. 

 The hardened tap AB muft be fixed in the vice, with its end B uppermoft, and the dies be then 

 made to bite gently upon its fcrew. When the ftock in this fituation (the cutting parts being 

 previoufly well oiled) is moved backwards and forwards on the axis of the fcrew, the tap 

 cuts an hollow thread in the dies, which becomes deeper and deeper as the fcrews E and F 

 are driven inwards. The dies muft be taken out and examined from time to time during the 

 procefs, which is to be continued until the thread is found to be as deep and perfect as may 

 be expedled. A notch C Fig. 6. is then to be filed in the bottom of the thread, the dies 

 hardened and tempered, and the face A Fig. 5. whetted on the Turkey hone. The dies 

 are in this ftate ready for tapping other fcrews. 



For the fame reafon why an irregular fcrew will cut a more regular fcrew tool (Fig. 2.), it 

 happens that the female fcrew is more regular than the male. The dies will therefore 

 reflify the tap to a ftill greater degree of accuracy ; and this tap, if thought neceflary, may 

 be again ufed to rectify the dies. 



Excellent fliort fcrews have been made by very flow and careful tapping with new ftiarp 

 dies. The principal fources of error in this procefs are the following : 



1. The ftop from change of hands produces a wave in the thread. This is frequently 

 perceptible to the eye, and may be partly remedied by the workman walking round his 

 ■work, or frequently (hifting the fcrew in the vice. 



2. Preflure in the direftion of the axis, by one hand more than the other, occafions a 

 periodical change in the obliquity of the thread, and the fcrew is what the workmen call 

 drunk. It is difiicult to avoid this error when the fcrew is very fliort, or the dies thin. 



Mechanical means have been ufed to remove thtTe two fources of error, by fixing the ftock, 

 and confining the tap to move by a regular rotation in the dire£lion of its own axis. 



3. Since the corners of tlic dies cut firft, there will be little difpofition in the firft ftrokes 

 to form the required fcrew. If the operation be not favoured by preflure in the direiSlion 

 of the axis, the dies will follow accidental irregularitics^of the furface or of the material; 

 and as probably cut circular rings, or a left-handed fcrew, as the right-handed fcrew fup- 

 pofed to be in the dies. In thefe circumftanecs, therefore, the thread frequently proves 

 undulated, with very little rife in the run of eacli corner of the die, until it fuddenly falls 

 into the cut made by the corner it follows. Each turn confifts accordingly of four waves, 

 which are amended as the dies fink deeper, and are led by their own flope. In extreme 

 ftriftncfs it may be qucftioncd whether a tapped fcrew be ever begun without tliis irregu- 

 laiity, or perfeftly corrcfted by the fubfequent operation. 



4. If the dies be not well fitted in the ftock, or the material to be cut be veiny or un- 

 equally hard, they will recede from the h.ird parts, and produce an irregular thread. Long 

 dies are leaft fubjeifl to tliis impcrfcQion. 



Vol,. I. — JfLY 1707. V ;. The 



