a tangent screw, so that on turning the ring by the screw. the three dies were simultaneously and equally advanced towards the

centre.

These screw stocks were found to cut very rapidly, as every circumstance was favorable to that action. For instance, on the principle of the triangular bearing, all the three dies were constantly at work; the original tap being slightly taper, every thread in the length of the die was performing its part of the work, the same as in a taper tap, every thread of which removes its shaving or share of the material; and the dies were narrow, with radial edges, which admitted of being easily sharpened.

The diestock has been abandoned in favor of the screw stock, which is represented in Fig. 481. The one die embraces about one-third of the circle, the two others much less; the latter are fitted into grooves which are not radial, but lead into a point situated near the circumference of the screw-bolt; the edges of the dies are slightly hooked or ground respectively within the radius, and they are simultaneously advanced by the double wedge and nut; the dies are cut over a large original, such as Fig. 467, that is, two depths larger than the screw. The large die serves to line out or commence the screw, and the two others act alternately; the one whilst the stock descends down the bolt, the other during its

ascent.

We will notice but one more screw stock. It is seen that the one die embraces about one-third the screw, the other is very narrow; the peculiarity of this construction is that a circular recess is first turned out of the screw stock, and a parallel groove is made into the same, the one handle of the stock (which is shaded), nearly fills this recess, and receives the small die. If the handle fitted mathematically true, it is clear it would be immovable, but the straight part of the handle is narrower than the width of the groove; when the stock is turned round, say in the direction from 2 to 1, the first process is to rotate the handle in the circle, and to bring it in hard contact with the side 1, this slightly rotates the die also, and the one corner becomes somewhat more prominent than the other. When the motion of the stock is reversed, the handle leaves the side 1, of the groove, and strikes against the other side 2, and then the opposite angle of the die becomes the more prominent; and that without any thought or adjustment on the part of the workman, as the play of the handle in the groove 1, 2, is exactly proportioned to cause the required angular change in the die.

The cutting edges of the die act exactly like turning tools, and therefore they may very safely be beveled or hooked as such; as when they are not cutting, they are removed a little way out of contact, and therefore out of danger of being snipped off, or of being blunted by hard friction. The opposite die affords during the time an efficient guidance for the screw, and the broad die is advanced in the usual manner, by the pressure screw made in con.

tinuation of the second handle of the diestock; the dies are kept in their places by a side plate, which is fitted in a chamfered groove in the ordinary manner.

There is less variety of method in cutting external screws with the diestocks, than internal screws with taps, but it is desirable in both cases, to remove the rough surface the work acquires in the foundry or forge, in order to economize the tools; and the best works are either bored or turned cylindrically to the true diameters corresponding with the screwing tools.

The bolt to be screwed is mostly fixed in the tail vice vertically, but sometimes horizontally, the dies are made to apply fairly, and a little oil is applied prior to starting. As a more expeditious method suitable to small screws, the work is caused to revolve in the lathe, whilst the diestock is held in the hand; and larger screws are sometimes marked or lined out whilst fixed in the vice, the principal part of the material is then removed with a chasing tool or hand-screw tool, and the screw is concluded in the diestocks. In cutting up large screw bolts, two individuals are required to work the screw stocks, and they walk round the standing vice or screwing clamp, which is fixed to a pedestal in the middle of the workshop.

For screwing large numbers of bolts, the engineer employs the bolt-screwing machine, which is a combination of the ordinary taps and dies, with a mandrel, driven by steam-power. In the machine the mandrel revolves, traverses, and carries the bolt, whilst the dies are fixed opposite to the mandrel; or else the mandrel carries the tap, and the nut to be screwed is grasped opposite to it. In another machine, the mandrel does not traverse, it carries the bolt, and the dies are mounted on a slide; or else the mandrel carries the nut, and the tap is fixed on the slide. The tap or die gives the traverse in every case, and the engine and strap supply the muscle; of course the means for changing the direction of motion and closing the dies, as in the hand process, are also essential.

The screwing table is a useful modification of the bolt machine, intended to be used for small bolts, and to be worked by hand. The mandrel is replaced by a long spindle running loosely in two

Fig4. 485

486.

bearings; the one end of the spindle terminates in a small wheel with a winch-handle, the other in a pair of jars closed by a screw. The jaws embrace the head of the bolt, which is presented opposite to dies tnat are fixed in a vertical frame or stock, and closed by a loaded lever to one fixed distance. In tapping the nut, it is fixed in the place before occupied by the dies, and the spindle then used is bored up to receive the shank of the tap, which is fixed by a side screw. This machine insures the rectangular position of the several parts, and the power is applied by the direct rotation of a hand wheels.

It will be gathered from the foregoing remarks, that the die

stock is an instrument of most extensive use, and it would indeed almost appear as if every available construction had been tried, with a general tendency to foster the cutter, and to expunge the surface friction or rubbing action; by the excess of which latter the labor of work is greatly increased, and risk is incurred of stretching the thread.

Figures 483 and 484 show a shaping machine, built at the Lowell Machine Shop, Lowell, Mass. Many of the machines built at the Lowell Machine Shop, were much improved by W. B. Bennet. This shaping and planing instrument will plane either flat or curved surfaces.

The tool bar is moved by a variable crank adjustable to any length of motion not exceeding eight inches. It has a self-acting horizontal and circular feed motion, with a hand-feed motion for internal curves.

Figs. 485, 486 show a gear-cutting machine manufactured at the Lowell Machine Shop, Lowell, Massachusetts. The dividing plate is forty-eight inches in diameter. This machine will cut every number of teeth up to 133, and every even number to 268, also 272, 276, and 360 teeth.

The cutter stock is so arranged as to move either horizontally or vertically, or at any angle, so as to cut bevel, spur, and spiral wheels and gearing.

SCREWS CUT BY HAND IN THE COMMON LATHE.-Great numbers of screws are required in works of wood, ivory and metal, that cannot be cut with the taps and dies, or the other apparatus hitherto considered. This arises from the nature of the materials, the weakness of the forms of the objects, and the accidental proportions of the screws, many of which are comparatively of very large diameter and inconsiderable length. These, and other circumstances, conspire to prevent the use of the diestocks for objects such as the screws of telescopes and other slender tubes, those on the edges of disks, rings, boxes, and very many similar works.

Screws of this latter class are frequently cut in the lathe with the ordinary screw tool, and by dexterity of hand alone; there is little to be said in explanation of the apparatus and tools, which then consist solely of the lathe with an ordinary mandrel incapable of traversing endways, and the screw tools or the chasing tools, with the addition of the arın rest.

The screw tool held at rest would make a series of rings, because at the end of the first revolution of the object, the points A B C of the tool would fall exactly into the scratches A B C commenced respectively by them. But if, in its first revolution, the tool is shifted exactly the space between two of its teeth, at the end of the revolution, the point B of the tool drops into the groove made by the point A, and so with all the others, and a true screw is formed, or a continuous helical line, which appears in steady lateral motion during the revolution of the screw in the lathe.

It is likely the tool will fail exactly to drop into the groove, but

if the difference be inconsiderable, a tolerably good screw is nevertheless formed; as the tool being moved forward as equally as the hand will allow, corrects most of the error. But if the difference be great, the tool finds its way into the groove with an abrupt break in the curve; and during the revolution of the screw, as it progresses it also appears to roll about sideways, instead of being quiescent, and is said by workmen to be "drunk :" this error is frequently beyond correction.

It sometimes happens that the tool is moved too rapidly, and that the point C drops into the groove commenced by A; in this case the coarseness of the groove is the same as that of the tool, but the inclination is double that intended, and the screw has a double thread, or two distinct helices instead of one; the tool may pass over three or four intervals and make a treble or quadruple thread, but these are the results of design and skill, rather than of accident.

On the other hand, from being moved too slowly, the point B of the tool may fail to proceed so far as the groove made by A, but fall midway between A and B; in this case the screw has half the rise or inclination intended, and the grooves are as fine again as the tool; other accidental results may also occur which it is unnecessary to notice.

ON CUTTING SCREWS IN LATHES WITH TRAVERSING MANDRELS. -One of the oldest, most simple, and general apparatus for cutting short screws in the lathe, by means of a mechanical guidance, is the screw-mandrel or traversing-mandrel, which appears to have been known almost as soon as the iron mandrel itself was introduced.

Fig. 487.

Figure 487 is copied from an old French mandrel mounted in a wooden frame, and with tin collars cast in two parts; the upper halves of the collars are removed to show the cylindrical necks of the mandrel, upon the shaft of which are cut several short screws. In ordinary turning, the retaining key k, which is shown detached in the view k', prevents the mandrel from traversing, as its angular