On Cutting Internal Screws With Screw Taps Part 2 20086

The principle of chamfering, or relieving the taps, must not however, be carried to excess, or it will lead to mischief; in explanation of which the diagrams 544, 545, and 546 may be considered parallel with the forms 429, 430, and 431, of page 532. For example, the tap, if sloped behind the teeth as in 544, would be much exposed to fracture; and the instrument being entirely under its own guidance, the three series of keen points would be apt to stick irregularly into the metal, and would not produce the smooth, circular, or helical hole, obtained when the tool 545 is used, which may be considered parallel with the turning tool fig. 430. The relief should be slight, and the surfaces of the teeth then assimilate to the condition of the graver for copperplates (see page 532), and thereby direct the tap in a very superior manner.

The teeth sloped in front, as in figs. 546, would certainly cut more keenly than those of 545, but they would be much more exposed to accident, as the least backward motion or violence would be liable to snip off the keen points of the teeth; and therefore, on the score of general economy and usefulness, the radial and slightly relieved teeth of fig. 545, or rather of 542, are proper for working taps.

It appears further to be quite impolitic, entirely to expunge the surface-bearing, or squeeze, from the taps and dies, when these arc applied to the ductile metals; as not only does it, when slight, greatly assist in the more perfect guidance of the instrument, but it also serves somewhat to condense or compress the metal.*

The transverse sections hitherto referred to, are always used for those taps employed in screwing the inner surfaces of the nuts, and holes required in general mechanism. The longitudinal section of the working tap, is taper and somewhat like a broach, the one end being small enough in external diameter to enter the blank hole to be screwed, and the other end being as large as the screw for which the nut is intended.

Fig. 547.

On Cutting Internal Screws With Screw Taps Part 2 20087


On Cutting Internal Screws With Screw Taps Part 2 20088

In many cases a series of two, three, or four taps must be used instead of only one single conical tap, and the modifications in their construction are explained by the above diagrams; namely, fig. 547, the tap formerly used for nuts and thoroughfare holes, and fig. 543 the modern tap for the same purposes; the dotted lines in each represent the bottoms of the threads.

* Unlees the taps cut very freely, it is the general aim to avoid the necessity for tapping cast-iron, which is a granular and crystalline substance, apt to crumble away in the tapping, or in the after use. The general remedy is the employment of bolts and nuts made of wrought-iron, or fixing screwed wrought-iron pins in the work, by means of transverse keys and other contrivances, and sometimes by the insertion of plugs of gun-metal, to be afterwards tapped with the screw-threads. In general also, the small screws for cast-iron, are coarse and shallow in the thread compared with those for wrought-iron, steel, and brass.

In the former kind, the thread was frequently finished of a taper figure, with the screw tool in the lathe; after which either the four or three plane surfaces were filed upon it, as shown by the section at s; the neck from f to g was as small as the bottom of the thread, and the tang from g to h was either square or rectangular for the tap-wrench. The tang, if square was also taper, the tap-wrench then wedged fast upon the tap; the sides of the tang, if parallel, were rectangular, and measured as about one to two, and there were shoulders on two sides to sustain the wrench.

In the modern thoroughfare taps for nuts, drawn to the same scale in fig. 548, the thread is left cylindrical, from the screw-tool or the dies: then from a to b, or about one diameter in length, is turned down cylindrical until the thread is nearly obliterated; from d to /, also nearly one diameter in length at the other end, is left of the full size of the bolt, and the intermediate part, b to d equal to three or four diameters, is turned to a cone, after which the tap is fluted as seen at s. The neck f g, as before, is as small as the bottom of the thread, and the square g h, measures diagonally the same as the turned neck.

In using the modern instrument fig. 548, the hole to be tapped is bored out exactly to fit the cylindrical plug a b, which therefore guides the tap very perfectly in the commencement; the tool is simply passed once through the nut without any retrograde motion whatever, and the cylindrical part d f, takes up the guidance when the larger end of the cone enters the hole; at the completion, the tap drops through, the head being smaller than the bottom of the thread. The old four square taps could not be thus used, for as they rather squeezed than cut, they had much more friction; it was necessary to move them backwards and forwards, and to make the square for the wrench larger, to avoid the risk of twisting off the head of the tap. In taps of modern construction of less than half an inch diameter, it is also needful to make the squares larger than the proportion employed in fig. 548.

In tapping shallow holes, as only a small portion of the end of the tap can be used, the screwed part seldom exceeds two diameters in length, and as they will not take hold when made too conical, a succession of three or four taps is generally required, The screwed part of the first may be considered to extend from a to b of fig. 547, of the second, from c to d, of the third from e to f; so that the prior tap may, in each case, preparefor the reception of the following one. The taps are generally made in sets of three; the first, which is also called the entering or taper tap, is in most cases regularly taper throughout its length; the second, or the middle tap, is sometimes taper, but more generally cylindrical, with just two or three threads at the end tapered off; the third tap, which is also called the plug or finishing tap, is always cylindrical, except at the two or three first threads, which are slightly reduced.