ment, in fact, in a kind of slide rest, the screw being handed round by levers." against the box or nut, but with the power of adjust- | originated by traversing the tool in a right line a a plain revolving cylinder. Sometimes the tool many points, and is guided by the hand alone: at ci times the tool has but one single point, and is gu mechanically, as will be explained hereafter in ta screw-cutting engine. Another method of originating a screw is to wrap a small wire in close coils round a larger wire, and to take an impression thereof between two pieces of hard wood. The hollow or counterpart thread thus indented serves as a guide in cutting the screw. Mr. Mallett, of Dublin, described the following plan in the Mechanics' Magazine for January, 1844:-" Two straight edges of equal length and width, and about ths of an inch in thickness each, are to be secured on a table, parallel to each other, standing on their edges, and distant from each other by nearly the length of the cylinder upon which the spiral is to be marked. Between these there is also to be secured in a diagonal direction, stretching from one to the other, a third straight edge, formed of two slips of deal glued together, with a slip of straight thick Bristol board between them projecting th inch at one edge. The entire height of the diagonal straight | edge, when standing on the table, must be a shade more than that of the two other straight edges. The three pieces being then thus arranged, the edge of Bristol board is charged with printers' ink. Then, on causing the cylinder to roll over the edges of the two parallel straight edges in the direction of their length, the diagonal slip of inked Bristol board will trace a spiral upon the surface of the cylinder with very considerable accuracy." By substituting a curved edge for the inclined straight edge variable screws will be obtained. the screw. Screws are also originated by indenting a smooth cylinder with a sharp-edged cutter placed across it at the required angle, the surface or rolling contact producing the rotation and traverse of the cylinder. "In the most simple application of this method, a deep groove is made along a picce of board, in which a straight wire is buried a little beneath the surface; a second groove is made nearly at right angles across the first, exactly to fit the cutter, which is just like a table-knife, and is placed at the angle required in The cutter, when slid over the wire, indents it, carries it round, and traverses it endways in the path of a screw; a helical line is thus obtained, which, by cautious management, may be perfected into a screw sufficiently good for many purposes. The late Mr. Henry Maudslay employed a cutter upon cylinders of wood, tin, brass, iron, and other materials, mounted to revolve between cutters in a triangular bar lathe; the knife was hollowed to fit the cylinder, and fixed at the required angle on a block adapted to slide upon the bar; the oblique incision carried the knife along the revolving cylinder. Some hundreds of screws were thus made, and their agreement with one another was in many instances quite remarkable; on the whole, he gave the preference to this mode of originating screws." The screw is also (1) Gill's Technological Repository, vol. vi. Mr. Holtzapffel thinks it probable that a gun-metal nut was cast upon this screw, for use, after the screw was finished. (2) Holtzapffel. "Mechanical Manipulation," vol. ii. The The external screws which are used for fasten pieces of wood, or wood and metal together, are calet wood screws, and in Scotland screw-nails. The blanks fr these screws were formerly forged by the nail-makes they being nearly the same as countersunk che nails, [see NAIL, Fig. 1489, No. 5], except that t ends are not pointed. The blanks were next nade of round rolled iron cut to the required lengths: :: heads being formed by pinching them while redbetween a pair of dies, and the threads were cat means of a file. But the method of making screws now most commonly adopted is that witnessed by the Editor in a factory at Birmingham, and described his work on the "Useful Arts and Manufactures Great Britain," already referred to. In the first place, a coil of wire fit for making screws is arranged so that it can be drawn into a machine as it is wanted; pieces of the proper length are cut off, and one end each is struck up to form the head, and the blacks! thus produced are then turned out into a box. By a second operation, the blanks are taken separately and placed in a lathe, where the heads and necks are pr perly shaped, by turning and cutting away superfluous metal. Thirdly, the notch or nick in the head of the screw for receiving the screw-driver is cut by means of a circular saw; the woman puts each screw inta a kind of clasp, which holds it firmly, and then, by means of a lever, raises it to the cutter; the nick is formed in an instant, the clasp is opened, the blank falls out, and another is inserted in its place, wita great quickness. The teeth of the circular saws used in this operation, require filing up after every halfhour's work: for which purpose they are taken out, softened by heating and slowly cooling, then filed and sharpened, next hardened by heating and suddenly cooling; after which they are again fit for use. Fourthly, the blanks are ready for worming, as the operation of cutting the thread is called. This is done in a lathe; and as the blank requires to be held therein very steadily, the nick just formed is made to assist in doing so. The arrangement is shown in Fig. 1921:-A steel spindle or mandril revolves be Fig. 1921. tween collars in two uprights, by the motion of a strap passing round the pulley, f: is a loose pulley to carry the strap when it is required to stop the machine. At b is an iron box made to hold firmly Chapter on the Screw in this work is of great value; it extends beyond 100 pages, and is illustrated by 109 wood engravings. and by the operation of the lever the cutters are made to act upon the shank of the screw, and to exert a greater or less degree of pressure, according to circumstances. There is also a lever which causes certain directing points, resembling the cutters, to close upon the regulating screw P, and the two levers being connected by a horizontal bar, can be depressed or raised together, and the cutters and directors applied at the same moment. In this way the inclination of the thread is determined by the pattern screw, and its shape by the form and position of the cutters. This arrangement being understood, the order of proceeding is as follows:-The workwoman fixes a blank in the chuck; she next transfers the strap from the loose to the fixed pulley, thereby causing the chuck to revolve; she then depresses the levers, and the guides, acting upon the regulator screw from behind, cause the chuck to move forward and force the shank of the blank screw between the cutters, which turn out a shaving of metal, leaving a sharp thread or worm. The heat occasioned by this operation would soon destroy the temper of the steel cutters were they not kept cool; for this purpose, while the woman holds the lever down with one hand, she takes up with the other a piece of wood, dips it in water, and applies it to the tools. When the thread is traced far enough, the levers are raised, the chuck falls back, and the screw is released. Another screw is inserted in its place, and thus the cutting or tapping proceeds. Screws are also cut by dies instead of cutters. The dies are arranged on a frame, and are opened and shut by a right and left-handed screw. As the dies regulate the size of the thread, there is no pattern screw, and of course the dies require to be changed for every variety of screw intended to be cut. Mr. Ryland's gimblet-pointed screws are thus made. This screw enters the wood easily and retains its hold firmly, and it is not liable to break where the thread ends, which the common screw will often do, especially in hard wood. Mr. Nettlefold's patent screws are also made on a similar principle. In them great attention is paid to the thread, the upper side of which is very flat or inclined, which causes a great resistance to its being forcibly pulled out of the wood; while the under side is considerably inclined, which enables it to go into the wood with great ease, and rendering it unnecessary, in soft elastic woods, to bore a hole for its reception. The form of this screw, and its action upon the wood, will be better understood from the following figures, in which Fig. 1924, represents the screw; Fig. 1925, the mould made by it in wood; and Fig. 1926 is a section of the common form of screw, in which the worm is shallow and imperfect. "The chief defects of common wood screws, besides bad threads, are the having, at the termination of the worm, a projecting bur, which is apt to tear away the wood before it, and leave little or no solid matter for the screw to hold by; the nicks in the heads being too shallow, or highest in the middle, preventing the screwdriver from taking an efficient hold to turn them in and out." Screw bolts and other screws for working in metal are cut by a die resembling a common iron nut; it is formed of well-tempered steel in two parts, which are fixed in an iron box or die-stock, with two long handles or levers, as in Fig. 1927, and there is also a set screw, s, by means of which the 2 halves of the die may be brought nearer together or removed from each other. The blank or iron pin on which the screw is to be cut, having been turned to the proper size is introduced by its narrow extremity into the dies, which are then closed so as to grip the pin with tolerable force. The pin is set in a vice, and the die is worked round upon the pin by means of the two handles, and when a tolerably good impression of the thread is made upon the pin the two halves of the die are set closer together, and the operation is repeated whereby the thread is more defined, and it is cut deeper by setting the dies still more closely together. In such a case the worm is not formed by merely cutting away the metal, but partly by compressing it and squeezing it up, as it were, into the thread. In cutting such a thread by machinery the dies are ir 4 pieces, the die-frame is fixed and the bolt or screw pin is made to revolve. The die-stocks in common use are known as double | die-stocks for all screws exceeding about last chamfered and single chamfered. In the former about diameter. one-third of the length of the chamfer is filed away at one end for the removal of the dies laterally, and one at a time, and the aperture is about as long as 3 of the dies. In the single chamfered die-stock, the aperture is about as long as 2 of the dies, and these are removed by first taking off the side plate P, Fig. 1927, which is either attached by its chamfered edges as a slide, or by 4 screws, which when loosened allow the plate to be slid endways, when the screws escape from the grooves at g, and the screw heads from the holes h. The single chamfered die-stock is preferred for large threads. The form Small metal screws are cut by a steel tap plate or screw-plate, Fig. 1929, wormed and notched, but furnished with several holes varying slightly in size, and the worm is formed progressively by using holes gradually diminishing in size. From 2 to 6 holes are In cutting large screws, especially when the thress is square, a steel cutter may be used with the d In cutting long screws in wood a steel cutter is fx-. in the female screw or screw-box, as it is called, shown in section Fig. 1930, and in plan Fig. 1931, through the line a, and is thus described by Mr. HoltzapfelIt consists of two pieces of wood, accurately attached by two steady pins and two screws, so as to admit of separation and exact replacement: the ends of the thicker pieces are frequently formed into handles by which the instrument is wore! A perforation is made through the two pieces of Fig. 1931. Fig. 1930. wood; the hole in the thinner piece is cylindrical and exactly agrees with the external diameter cf the screw or of the prepared cylinder; and the hole in the thicker piece is screwed with the same ta that is used for the internal screws or nuts. Th cutter or V has a thin cutting edge sloped externally to the angle of the thread, usually about 60, and thinned internally by a notch made with a triangular file; the cutter is inlaid in the thicker piece of wood and fastened by a hook-form screwbor and nut. In placing the cutter form different conditions require strict attention. Its angular ridge should lie as a tangent to the inner circle; its edge should be sharpened on the dotted line 6, or at an angle of about 100° with the back; its point should exactly intersect the ridge of the thread, in the box: and it should lie precisely at the rake or angle of the thread, for which purpose it is inlaid deeper at i's blunt extremity. The piece of wood for the screw is turned cylindrical and a little pointed: it is then twisted into the screw box, the cutter makes a notch which catches upon the ridge of the wooden worm Fig. 1929. intended for each thread, and are arranged in groups for the purpose as indicated by the short lines shown in the figure. The cutting edges are formed by making 2 or 3 small holes, and connecting them by the lateral cuts of a thin saw. In making the screw the wire should be fixed in a vice, the end tapered off, and after being moistened with oil screwed into the holes of the plate in succession. Although the screw plate is sometimes used for common screws as large as from to inch diameter, it is better to use Fig. 1932. immediately behind the cutter, and this carries the work forward exactly at the rate of the thread. The whole of the material is removed at one cut and the shavings make their escape at the aperture or mouth m." The cutter is shown in Fig. 1932. Screws of half an inch diameter and upwards are fixed in the vice and the screw box is handed round like the dic-stock: but for large screws exceeding 2 or 3 inches in diameter, two of the V's or cutters are placed in the box so as to divide the work and lessen the risk of breaking the keen edge of the cutter. The screw box is used for wooden screws of 4, 6 and 8 inches diameter, but these large screws are now seldom made, metal screws having taken their place. A steel tap is used for cutting interior screws, as already noticed. This tap is commonly a screw with a considerable portion of the worm removed by filing two or more flat faces along its whole length, the angles left by the operation forming a series of obtuse cutters. The tap is made somewhat conical in order to enter the hole with facility and cut the worm gradually. A tap is shown in 3 views in Fig. 1933. Two taps are in some cases used, only the first of Mr. R. Jones has con| ternal angles as in d and e." trived a tap in which a steel cutter can be inserted as shown in g the cutter is made to project a little, so that the tap follows it without difficulty. Mr. Gill has described a tap for cutting a square threaded screw: it consists of a hollow steel screw with a hole drilled obliquely from the front end of the thread to the hole in the centre of the tap: the edges of this oblique hole are sharp, and cut their way through the wood, while the hole forms a passage for the escape of the shavings. Mr. Siebe's tap for cutting right hand or left hand internal screws in wood, according to the direction in which it is turned, is represented in Figs. 1935, 1936. The tap is formed by Fig. 1933. square, which is tapered. The head of the tap is for fitting into the middle of a long handle or tap-wrench which is used for working it into the nut. The taps for cutting screws in wood are usually fluted at the side to allow the cuttings to escape. The simplest form of tap is produced by filing 4 planes upon the screw, as in Fig. 1834, a; but the edges thus produced are very obtuse, and form the thread rather by raising or burring up the metal than by cutting. A better form for cutting is obtained by filing only 3 planes on the screw as in b, but even then the angle is as great as 120°. Taps of very small size cannot well be formed of a more favourable section; but for general purposes the best angle for the edges of screw taps and dies is the radial line or an angle of 90°. Sir John Robison obtained such an angle in his half round tap c, and he states that such a tap "will cut a full clear thread, even if it be made of a sharp pitch, without making up any part of it by the burr, as is almost universally the case when blunt edged or grooved taps are used." Mr. Holtzapffel remarks that when two-thirds of the circle are allowed to remain as in d, this, "although somewhat less penetrative than the last, is also less liable to displacement with the tap wrench. Fig. 1934. It is turning a wooden screw of the required size, cutting a longitudinal slit along its centre, and inserting a plate of steel of the length and breadth of the screw: the edges of this plate are now filed into notches corresponding with the worm: the plate is then removed and the wooden thread is turned away, leaving the wood in the form of a plain cylinder. The steel plate is then reinserted and firmly secured, as shown in the figures, which give a side and an end view of the tap. The steel plate is made to taper a little, so as to ensure a gradual cutting action. A groove cut on each side of the plate, affords a channel for the escape of the turnings: the upper end of the cylinder is made square for the insertion of the wrench. Such a tap may also be formed without the assistance of the worm on the stock, simply by notching the steel plate, taking care that a tooth on one side shall coincide with a hollow on the other. Mr. Siebe proposes similar taps with only two cutting edges, for cutting screws in metal.' SCREW-CUTTING ENGINE. In the method of cutting screws by the lathe as already described, the pattern screw regulates the pitch of the thread, and screws of different make require each its own pattern. In the Woolwich Dockyard is an engine for cutting large screws accurately to any required pitch, from one pattern. This engine is represented in elevation and plan in the steel engraving. A B, Fig. 1, is one side of a cast-iron lathe-bed, both sides of which are represented in Fig. 2. PQ are two cast-iron puppets. CD an axle turning on a point at c, its end being countersunk to receive the point, the other end bearing on a slightly conical collar in the puppet q. 2 bevelled wheels w w' fit one part of the axle, and either of them may be thrown in or out of with gear the other bevelled wheel w', Fig. 2, which is turned by the steam-engine shaft. On the side of the bed are 2 standards ss, in which the screws 8 s turn in The much more usual to employ 3 radial cutting edges instead of one only; and as in the best forms of taps, they are only required to cut in one direction, or when they are screwed into the nut, the other edges are then chamfered to make room for the shavings, thereby giving the tap a section somewhat like that of a ratchet wheel, with either 3, 4 or 5 teeth," as in e and g. "It is more common, however, to file up the side of the tap or to cut by machinery 3 concave or elliptical flutes as in f; this form sufficiently approximates to the desideratum of the radial cutting edges, it allows plenty of room for the shavings, and is easily wiped out. What is of equal or greater importance, it presents a symmetrical figure, little liable either to accident in the hardening, of distortion from Encyclopædia Metropolitana, for the drawings and descriptions of unequal section as in c and d, or of cracking from in с (1) Transactions of the Society of Arts, vol. xli. (2) We are indebted to the volume on "Manufactures," in the this engine. collars, both screws being seen in Fig. 2. 2 is a sliding frame with a nut on each side, through which the screws 88 pass, so that as the screws revolve, this sliding frame is carried from one end of the bed to the, other, and in this frame is fixed the tool for cutting the threads of the screw. The cutter is secured in its proper position by means of plates and nuts, Fig. 2. Another cutter may be fixed on the opposite side, so that one cutter may act while the slider is passing from A towards B, and the other as it returns from B to A. This plan, however, does not produce very good results in practice, so that one cutter only is employed, and the carriage returns free. The cutter is adjusted to its proper place by the screw and lever cd. TV is a sliding puppet carrying the back centre: this puppet may be placed in any part of the bed, and there screwed fast, while any small adjustment for distance may be made by the screw G, Fig. 2. The bolt or cylinder on which the screw is to be formed being turned perfectly true to two centres, one at each end; this is applied to the points in the mandrel and back centre, and a proper degree of contact is attained by the screw G. The end of the cylinder next to the mandrel D is properly secured: an iron chuck is placed on the mandrel, and rotation is given to the cylinder by means of the driver, and in this form it may be used for any purpose in which a powerful lathe is required. But for cutting screws, a wheel w is fixed on the mandrel, and 2 equal wheels on the ends of the screws 88, so that while the cylinder revolves by means of the power communicated to the bevelled wheel by the engine, the wheel w gives rotation to the wheels ww. A horizontal motion is thus given to the slide-rest or sliding frame from one end of the bed to the other, and the cutter being pushed forward by means of the screw, a first impression of a thread is formed in the revolving cylinder. When the frame arrives at the end of the cylinder, it presses on the stud n on the lever 1, and throws the wheel out of gear. The tool is then brought back, the motion is reversed by bringing the other bevel wheel in gear, and the cutting frame returns. The wheel is now thrown out of gear by the frame itself, as at the other end; the tool is readjusted and made to cut deeper, and thus the operation is repeated until the screw is completed. If the screw to be cut is to have the same pitch as the permanent screw which carries the slide-rest, the 3 wheels wo, w', wo", are all equal to each other, so that one revolution of the axle advances the cutter one thread; but by changing these wheels, so that the wheel w exceeds or falls short of the other two in any ratio, the thread of the new screw may be made to differ from those of the permanent screw in the same ratio. Screws of any required pitch may, therefore, be cut by having a variety of wheels so arranged that the sum of their two radii shall be a given quantity. For cutting a screw with a double thread, one thread is first cut, and then the driver is applied to a hole in the iron chuck directly opposite to its firs: position, by which means a semi-rotation is given to the new screw without producing any motion in t cutting frame, and consequently the cutter is now applied to a point midway between the two former threads, when by the same process as before tas intermediate thread may be formed. If a trip: thread be required, the driver is applied to a be in the chuck, which is distant from its original pos tion one-third of a circumference, and then again to another at two-thirds the distance, and in a simiar manner any number of threads may be formed. The apparatus for cutting the nut consists of a revolving cutter, turned on the principal axis while te nut itself is fixed in the slide-rest, so that while the cutter revolves in its place, the nut gradually advances upon it, instead of the cutter advancing upun the thread, as in cutting the screw. The SCREW, as a mechanical power, will be noticed under STATICS. SCREW OF ARCHIMEDES. A machine for raising water, invented by Archimedes: it is also called the Fig. 1937. ARCHIMEDIAN SCREW. spiral pump, and in Germany the water-snail. Its structure and application will be understood by referring to Fig. 1937, in which w is a wheel moved in the direction of the arrow by the fall of water F, which need not be more than 3 feet. The axle a of the wheel may be raised so as to make an angle of between 44° or 45° and 60° with the horizon, and on the top of the axle is a wheel w' which turns a similar wheel w", having the same number of teeth, the axle A' of this wheel being parallel to the axle a of the two Fig. 1938. former wheels. The axle A is cut into a double threaded screw, Fig. 1938, which must be right |