* Mr. Clement considers the many points to act with less risk than the single point, because in the processes of hardening, first the hob and then the screw tool, they both become slightly enlarged, or a little coarser than the pitch of the screw; consequently part of the teeth cut on one aide, and part on the other, but none of them on both sides of the points; which latter action gives rise to confnsion by interrupting the free escape of the shavings.

† In making a hob with rounded threads, it is usual to prove whether the top and bottom of the thread are equally rounded, by driving two different pieces of lead into the hob with a hammer; the two impressions will only fit together so as to exclude the light, when the departure from the simple angle is alike at the top and bottom of the hob, and that the thread is perpendicular or doss not lean. Master taps are similarly proved.

Angular thread screws are fitted to their corresponding nuts simply by reduction in diameter; but square thread screws require attention both as to diameter and width of groove, and are consequently more troublesome. Square thread screws are, in general, of twice the pitch, or double the obliquity, of angular screws of the same diameters; and, consequently, the interference of angle before explained as concerning the diestocks, refers with a twofold effect to square threads, which are in all respects much better produced in the screw-cutting lathe.

The ordinary tool for square thread screws is represented in three views in fig. 607: the shaft is shouldered down so as to terminate in a rectangular part which is exactly equal to the width of the groove; in general the end alone of the tool is required to cut, and the sides are bevilled according to the angle of the screw, to avoid rubbing against the sides of the thread. Tools which cut upon the side alone, are also occasionally used for adjusting the width of the groove. In either case it requires considerable care to maintain the exact width and height of the tool; the inclination of which should also differ for every change of diameter.

On Cutting Screws In Lathes With Traversing Tools  200112

To obviate these several inconvesions the author several years back contrived a tool-holder, fig. 608, for carrying small blades made exactly rectangular. In height, as at h, the blades arc alike, in width, w, they are exactly half the pitch of the threads, and they are ground upon the ends alone. The parallel blades are clamped in the rectangular aperture of the tool socket by the four screws cc; and when the screws ss, which pass through the circular mortises in the sockets, are loosened, the swivel joint and graduations allow the blades to be placed at the particular angle of the thread, which is readily obtained by calculation, and is estimated for the medium depth of the thread, or midway between the extreme angles at the top and bottom.*

One blade, therefore, serves perfectly for all screws of the same pitch, both right and left-handed, and of all diameters; as the tool exactly fills the groove, it works steadily, and the width of the groove and the height of center of the tool, are also strictly maintained with the least possible trouble. The depth of the groove, which is generally one sixth more than its width, is read off with great facility by means of the adjusting screw of the slide-rest; especially if, as usual, the screw and its micrometer agree with the decimal division of the inch.

The holder, fig. 608, has been much and satisfactorily used for screws from about 20 to 2 threads per inch; but when the screw is coarse and oblique, compared with its diameter, the blade is ground away to the dotted line in h, and is sometimes beviled on the sides almost to the upper edge, to suit the obliquity of the thread, but without altering the extreme width of the tool.

The tools for external screws of very coarse pitch, are necessarily formed in the lathe by aid of the corresponding wheels, and a revolving cutter bar resembling fig. 515, p. 569. The soft tool is fixed in the slide-rest, and is thereby carried against the revolving cutter bar, 515, which has a straight tool, either pointed or square as the case may be. The end of the screw-tool is thus shaped as part of an infernal screw, the counterpart of that to be cut; the face of the screw tool is filed at right angles to the obliquity of the thread, and the end and sides are slightly bevilled for penetration, previously to its being hardened.

Internal square threads of small size, are usually cut with taps which resemble fig. 548, p. 587, except in the form of the teeth. When internal square threads are cut in the lathe, the tool assumes the ordinary form, of a straight bar of steel with a rectangular point standing off at right angles, in most respects like the common pointed tool for inside work.

* For the mods of calculating the angles of screws, see foot-note, p. 657.

For very deep holes, and for threads of very considerable obliquity, cutter bars, such as fig. 515, p. 569, are used. The work and the temporary bearings of the bar, are all immoveably fixed for the time, and the bar advances through the bearings in virtue of its screw thread; or otherwise a plain bar, having a cutter only, and not being screwed, may be mounted between centers in the screw lathe, and the work, fixed to the slide-rest, may traverse parallel with the bar by aid of the change wheels. The cutter bar in some cases requires a ring to fill out the space between itself and the hole, to prevent vibration, and it is necessary to increase the radial distance of the cutter between each trip, by a set screw, or by slight blows of a hammer.

Very oblique inside cutters are turned to their respective forms with a fixed tool, in a manner the converse of that explained above; and some peculiarities of management are required in using them, in order to obtain the under-cut form of the internal thread, - but the consideration of which does not belong to this place.