Thumb screw

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The tailstock or poppet-head screw is a right hand because the nut moves in this case. The object of employing right-hand screws in some cases, and left-hand ones in others, is that it comes most natural in operating a screw to move it from right to left to unscrew, and from left to right to screw up a piece, this being the action of a right-hand screw, left-hand screws being comparatively rarely used in mechanism, save when to attain the object above referred to. ”

“ 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. ”

“ 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. ”

“ Holdfast and Saw Table.
II. Holdfasts.—Clamps or Cramps, Fig. 7. These cramps [6] are small iron frames, like three sides of a square, with a screw in the under limb. They are used on the edge of the table to hold the work firmly down to its surface; two or more are always employed. Their fault is that they indent and damage the work; a piece of waste wood may be interposed between the work and the upper limb to prevent this, but such a guard is generally in the way and otherwise objectionable. Hand Screws, Figs. 8 and 9, are a far better tool, entirely free from the above-named objection. ”

“ 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. ”

“ There is no amount of handwork that will produce a proper finish on the round head of a brass screw. Therefore, when small pieces of metal are to be shaped up with the file and polished, simply set them in a three-jaw chuck of a hand drill, and then clamp the drill in a vise, as shown in the illustration. Operate the drill with the left hand and hold the file, or emery paper, with the right, and a finish will be obtained that can in nowise be distinguished from lathe work. It is surprising how different round-head brass screws appear when treated in this manner. ”

“ There are three classes of chucks ordinarily used on the lathe, known as the independent, universal and combination types. The independent chuck is so named because each jaw can be adjusted in or out independently of the others by turning the jaw screws S with a wrench. The jaws of the universal chuck all move together and keep the same distance from the center, and they [37] can be adjusted by turning any one of the screws S, whereas with the independent type the chuck wrench must be applied to each jaw screw. ”

“ 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. ”

“ Figure 2.—Cross-slide for the thread-cutting lathe of Das mittelalterliche Hausbuch, shown in figure 1. It is remarkable not only for its early date, but also for its high state of development with a crossfeed screw which had not become universally accepted 300 years later. The cutter, shown out of its socket, is obviously sharpened for use on wood. ”

“ The threaded extremity of the screw engages a female screw in the loose jaw, and while the same are engaged the screw cannot be released from the embrace of the fixed collar, as it requires considerable lateral movement to accomplish this. ”

“ One fixed, the other revolving, impart a motion equal to the difference of pitch of the two screws (see Section 31) . See No. 1430.
1381. Screw, with half nut; the bearings of the screw being fixed act as a fulcrum for the motion of the half nut, which may be attached to any sliding device; employed for jaw chucks.
1382. Screw and worm gear, used for screw jacks, &c. The worm gears with a worm wheel having a central nut running on the main screw.
1383. Mutilated screw and nut. In one position the nut can slide on the screw, and a partial turn locks it. Used for instantaneous grip vices, &c. ”

“ 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 ”

“ 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. ”

“ Determine the size of screws that are to be used in fixing the brass corner clamps. Let us say No. 7 is decided on; and if brass screws are used, then get a piece of brass, Fig. 4, the exact diameter of the screw-head, and a little longer than the thickness of the framing. If iron screws are to be used, then this piece must be iron. Now bore a hole into which this bolt will fit closely, right through the framing at D, Fig. 2. ”

“ The nipple used in military arms makes a good substitute for a broken plunger nipple, as the thread is nearly the same as that of some plunger nipples.
How to Remove Rusted Screws, Broken Nipples, etc.—Sometimes it so happens that a screw is so rusted in a lock or other part of a gun, or a rusted nipple refuses to start from its seat, and by repeated trials the sides of the screw-head adjoining the slot are worn away or the squares of the nipple are forced off, and the removal of either screw or nipple an almost impossibility with the hand screw-driver or nipple wrench. ”

“ It will be observed that one has uniform pitch, another increasing pitch, and the third compound increasing pitch. In order to test the efficiency of my screws I made the apparatus shown in Fig. 12. The power for running the screw was transmitted by means of a belt to the straight cylindrical pulley c, c. Shaft b, b was of steel, rather small in diameter, and ran smoothly, and practically without friction, through the two bearings d, d. When the first screw, a, a, was run at a high velocity, the axial thrust pushed the shaft b, b back and elongated the spiral spring e. ”

“ If made of polished wood, the friction is so extremely small as to be almost unmeasurable. The diameter of a well-made screw running in air is therefore not limited in any degree by skin friction, as is the case with a screw running in water; in fact, it is rather a question of its weight, and its efficiency ought to increase in direct ratio with its diameter, because the area of the disc increases with the square of the diameter. The screw slip is therefore reduced by one-half by simply doubling the diameter of the screw. ”

“ Fig.40 Bench Holdfast The old-fashioned hand-screw cramp (Fig. 41) is made of wood entirely. It is a very useful tool in the joiner's shop, and is used for holding together pieces of wood when glued for thicknessing up. It is indispensable when glueing up face veneers for shop fittings, etc.; these screws are made in different sizes suitable for heavy and light work. Iron G-cramps are a very useful form, the smaller sizes being made with a thumbscrew (Fig. 42) and being used for light purposes. The stronger and larger kinds will take in work up to 12 in. ”

“ Care should be taken to ensure that the shaft is quite central sectionally and diametrically.
A method of securing carved screw shafts is shown by Fig. 55, and is self-explanatory. When the elastic skein is in tension it has a tendency to pull the hooks out straight, so releasing the skein, with sometimes painful consequences to the hand. The safety hook shown by Fig. 56 has a brass-tube collar which slides over the end. All the hooks should be covered with valve tubing, to prevent the elastic cutting through. ”

“ 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. ”

“ A screw D, binds the jaws together, and hinders the selvage from going inwards. Upon the side cross beam of the loom, seen in section at I, a bolt is fixed which carries a nut F at its end, into which a screwed iron rod E enters, on one of whose ends is the handle B. The other extremity of the screw E is adapted by a washer and pin to the back of the pincers at the point H, so that by turning the handle to the right or the left, we draw onwards or push backwards the pincers and the stuff at pleasure. The warp of the web is made of black linen yarn. ”

“ Of at least equal importance with the lead screw and work and their relationship to each other is the tool-support with its screw-adjusted cross-slide (fig. 2) . Just how this was attached to the frame of the machine so that it placed the tool at a suitable radius is again a questionable point. The very well-developed cutting tool is sharpened to a thin, keen edge totally unsuited for cutting metal but ideal for use on a softer, fibrous substance: undoubtedly wood, in this instance. ”