Set screw

“ Another step in the direction of economy depends on the removal of all screw-cutting, except those screws which are of several feet in length, from the ordinary lathe to the special chasing and screwing machines. The screw-cutting arrangement of an engineer's lathe is a cumbrous apparatus to fit up and set in motion for the cutting of screws of small dimensions. When there was no other method available except that of common dies operated by hand or carried in a screwing machine, there was good reason why a true cutting tool should be operated in the lathe through change wheels. ”

“ Screw-bound is a common fault often overlooked by the amateur. It is caused by using screws of which the [Pg 111] heads are too large for the countersunk holes in the hinge, and may be avoided by slightly sinking the holes in the brasswork with a countersink or rose-bit.
Fig. 223.—Gauging.
Alignment.—Another fault that is fairly common is having the axes of the hinges out of alignment. Especially is this the case when three hinges are used to hang a wardrobe or other large door. ”

“ Then set the cylinder adjusting screw, and insert the wooden screws in lock face and drive them home with the screwdriver. Hold and adjust the escutcheon so that the knob will rotate back and forth automatically without friction, starting the screw holes with an awl. If the doors are made of hardwood, an automatic drill will have to be used to bore the screw holes. In such a case, follow the directions just given regarding escutcheons ”

“ Another device for use on lathes specially designed for screw-cutting is shown in Fig. 586, in which a represents the cross feed screw. It is fast to the notched wheel b, and is operated by it in the usual way. c is a short screw which provides journal bearing for the screw a by a plain hole. It is screwed on the outside, and the plate in which it fits acts as its nut. It is fast to the handle d, and is in fact operated by it. ”

“ Nevertheless, these threads are made to occupy the same length, in both screw and nut, as though they did enter each other: so that the two parts running parallel to each other, leave a square interstice b, all along both nut and screw: into which balls of brass, or soft iron are [82] introduced, which at once restore the screw-property without it’s friction: a friction so considerable in the common screw, that it always surpasses the effective power, since it remains closed, (in a vice for example) while holding any object squeezed with all the force a man can apply. ”

“ To make the teeth of a pair of endless screws fit correctly and work smoothly, a hardened steel screw is made of the figure of the smaller screw, with its thread or threads notched so as to form a cutting tool; the larger screw, or “wheel,” is cast approximately of the required figure; the larger screw and the steel screw are fitted up in their proper relative position, and made to rotate in contact with each other by turning the steel screw, which cuts the threads of the larger screw to their true figure. ”

“ The ordinary screw, cut by dies or in the screw-cutting lathe, is found, on applying comparatively coarse tests, to be far from accurate in pitch, while the thread may be wavy or “ drunken " and the diameter may not be uniform at all points. There are several methods of correcting the errors in screws; the principal one is that of retarding or accelerating the traverse motion of the screw-cutting tool by means of a compensating lever bearing on a compensating bar, which is formed after observations have been made on the degree of accuracy of the leading screw used to propel the tool carriage. ”

“ A lead screw should preferably be as near as possible to the middle of the lathe shears, and as close to the surface as possible, so as to bring it as nearly in line with the strain on the tool as possible, but on account of the cuttings, which falling upon the screw would cause it to wear rapidly, it is usual to locate it on one side, so as to protect it from the cuttings. ”

“ The screw is a spiral thread or groove cut round a cylinder, and everywhere making the same angle with the length of it. The force of a power applied to turn a screw round is to the force with which it [297] presses upward, or downward, setting aside the friction, as the distance between two threads is to the circumference where the power is applied; or the advantage gained is as much as the circumference of a circle described by the handle of the winch exceeds the interval, or distance, between the spirals of the screw. Hence the force of any machine turned by a screw can readily be computed ”

“ In withdrawing nails, a block under the hammer will often aid greatly, and also protect the surface of the work.
Screws.
Screws usually need holes properly bored to receive them; a large hole first, the size of the screw above the threads, a small hole next, the size at the roots of the threads (in hard wood somewhat larger) , and a place for the head made with a countersink. Usually the screw should slip easily thru the first piece of wood and be tight in the second. The screwdriver should always be held in the line that the screw is going, and it ought fairly to fit the slot in the head. ”

“ While more expensive than a solid nut, it neatly circumvents the need (and saves the time involved) to reverse the screw in order to get the tool back to the point of beginning preliminary to taking another cut. David Wilkinson’s lathe of 1798 (fig. 17) which was developed in Rhode Island at the same time shows the same method of mounting and driving the master screw. At least in the United States, this method of changing the lead screw instead of using change gears remained popular for many years. ”

“ We now come to the most difficult operation in turning, that of cutting a screw; to perform it well and easily is a proof of the workman’s skill and proficiency in the art. There are many ways of doing it; we shall therefore give the best and least expensive. The screw tools, figures 14 and 15, plate 1, must fit exactly one into the other. Fig. 15 is an outside, Fig. 14 an inside tool. As the threads may be required to be cut coarse or fine, according to the work you are engaged upon, the small grooves in the tools are made to suit. ”

“ At this stage the screw has to be balanced. This is of great importance, since the screw that is unbalanced loses a great amount of efficiency owing to the consequent vibration when it rotates. In full-size practice it would be highly dangerous to use a screw that is not balanced.
A piece of wire is passed through the hole previously drilled, and the heavier blade carefully glasspapered down (with No. 00 glasspaper to finish) until the screw poises in a horizontal plane. Fig. 47 shows the sort of brush to use for polishing, and Fig. 48 the finished screw. ”

“ Piles with a single turn of the screw, it seems to me, are the best, although the double-threaded screw may be right in soft marshy ground; but the usefulness of a double thread is doubtful, for I believe it breaks up the ground for no good, although some state that the screw threads work in parallel lines, and that a double-threaded screw is steadier; for they say a single-threaded pile is always likely to turn on the outside edge of the blade, and that the double-threaded is not, as it has a lip on both sides. ”

“ Maudslay's love of accuracy also led him from an early period to study the subject of improved screw-cutting. The importance of this department of mechanism can scarcely be overrated, the solidity and permanency of most mechanical structures mainly depending on the employment of the screw, at the same time that the parts can be readily separated for renewal or repair. Any one can form an idea of the importance of the screw as an element in mechanical construction by examining say a steam-engine, and counting the number of screws employed in holding it together. ”

“ If a screw rotates, the number of threads which pass a fixed point in one revolution is the number of threads in the screw.
A pair of convex screws, each rotating about its axis, are used as an elementary combination to transmit motion by the sliding contact of their threads. Such screws are commonly called endless screws. At the point of contact of the screws their threads must be parallel; and their line of connexion is the common perpendicular to the acting surfaces of the threads at their point of contact. ”

“ The method of work with the screw is to first rig a brace, and then drill a hole in the safe, cut a thread in the hole and then insert a female screw. Then, with a long steel screw with a handle so long that two men can turn it, the screw is inserted in the female screw, and by turning it goes in until it strikes the back of the safe. ”

“ While the discussion was still going on, one of the distance judges brought in the toe weight, and also the screw which held it, but instead of the screw being broken, or the threads worn, it was seen that the screw had only a head, and very short shank, and that the latter showed marks of its having quite recently had the end of the screw shank filed off, so that there was only a head, and a little bit of shank. ”

“ They are seldom or never removed and their diameters are made dissimilar only for the purpose of giving to the guide screw a slower rate of motion than that of the mandrel whereby it may be made of coarser pitch than that of the screw to be cut and its wear materially lessened. ”

“ When a lead screw is long it requires to be supported, otherwise, either its weight will be supported or lifted by the feed nut in gear, or if that nut does not lift the screw, the thread cut will be finer than that due to the pitch of the lead screw, by reason of its deflection or sag. ”

“ A machine screw is a small screw, such as in Fig. 387, the diameter of the body being made to the Birmingham wire gauge, the heads being formed by upsetting the wire of which they are made. They have saw slots s for a screw driver, the threads having special pitches, which are given hereafter. The forms of the heads are as in Fig. 387, a being termed a Fillister, b a countersink, and c a round head. The difference between a Fillister head of a machine screw and the same form of head in a cap screw is that the former is upset cold, and the latter is either forged or cut out of the solid metal. ”

“ Mounting of Screws.—The mounting must be devised most carefully, and is indeed more difficult to make without error than the screw itself. The principle which should be adopted is that iro workmanship is perfect; the design must make up for its imperfections. Thus the screw can never be made to run true on its bearings, and hence the device of resting one end of the carriage on the nut must be rejected. Also all rigid connexion between the nut and the carriage must be avoided, as the screw can never be adjusted parallel to the ways on which the carriage rests. ”

“ These pencil lines are for locating the holes for the screws, so that they will all be on the same level. Bore a hole on each line with a bit or drill, large enough so that the body of a round-head blue screw either 21⁄4 or 21⁄2 inches long will just slide through.
Before assembling, bevel or round the top of each leg about 1⁄8 inch. Fasten the four legs to the top with the screws, slip the shelf into position, and make fast in the same manner. Stand the tabourette on a level surface, and if it needs levelling, proceed as explained in the making of saw horse. ”

“ The screws used by carpenters, smiths, and others, for fastening wood or metal-work together, are generally made by machinery. A piece of wire of the required thickness and length is first cut off, which is then placed in a hole or mould, with its upper orifice “countersunk” as in the annexed cut, and the head of the screw is formed by beating the upper part of the piece of wire into the counter-sinking of the mould. ”

“ Figure 21 shows a plan view [7] of the machine, and figure 22 a detailed view of the sine-bar mechanism actuated by the master screw, 6, to modify the effective pitch of the lead screw in accordance with the realities of practice as stated in the preamble of the patent:
This invention relates to material working machines, and particularly to machines such as hob grinders and the like, wherein the work is reciprocated through the agency of a lead screw. ”

“ His slides are not, of course, associated with the spindle; neither is there any natural law which compels them to guide the tool exactly parallel with the axis of revolution. In this sense the screw-cutting lathe in the Hausbuch is superior because it is in harmony with natural law and can generate a true cylinder, whereas Maudslay’s lathe can only transfer to the work whatever accuracy is built into it. ”

“ To make this, procure two screws 3⁄4 inches long, also a piece of brass tube that will just slip on the plain part of the screw without shaking. Counter sink out the ends until the screw heads are flush; cut two pieces off the tube three-sixteenths and file up true—these pieces are best held by sinking them with a bit in a piece of hardwood. Now when about to strike an ellipse, drive these screws in with the collars on to half major and 125 minor, measured from the point of the trammel to the inside of the collar for the major, and to the outside of the collar for the minor. ”

“ The screw f is made of 3⁄8-inch round iron sufficiently long to give an action of about 1 inch. The top at n is filed square to receive a wheel such as is used on a common gate valve. A long thread should then be cut on this screw. The clamp c is made a half circle in shape, so as to clear the cover with ease. It should be 3⁄4 inch wide, flat on the under side, while the top side 29should have a rib cast on it to prevent springing. The top at g should be reinforced with metal and made heavy enough to stand drilling and tapping to receive the screw f. ”