SCREWS AND TWISTS.
Neither of these can be very accurately made without special and somewhat expensive apparatus; but both can with practice be done tolerably well by the young mechanic with ordinary simple means. I need not describe a screw, for all boys know what it is; and sporting boys, of which in these days there are many, know what sort of animal a screw is. Well, never mind. I am always riding a screw, I believe, for it is my hobby, and there is a great deal of science in a screw; and as for the variety of the manufactured article, there is plenty of it. There is the corkscrew, which is, after all, not a screw, but a twist,—and this is often the means of making men screwed; and the miserly screw, who skins fleas for the sake of their fat; and there is the mythical, invisible, moral (and im-moral) screw, which hard-fisted men inflict upon their weaker brethren; and there is the gigantic screw of the Great Eastern steamship; and the minute, microscopic screw of the lady’s tiny jewelled watch.
There are several modes of cutting screws, in the lathe and out of it. The small ones required for holding together the different parts of machinery, as well as larger ones for the same purpose, are always cut with stock and dies. The very small ones used by watchmakers, and all below one-eighth of an inch diameter, are made by the screw-plate. But when either large or small screws are required of great accuracy, they are invariably cut in the lathe, and with the aid of mechanical appliances of the most delicately accurate description. These are all metal screws. But the young mechanic will often wish to put screwed covers to his boxes, and to join various parts of his work by screwed connections instead of glue; and all these may be cut in the lathe by simple hand-tools skilfully applied, although the operation is sufficiently fraught with difficulty to require a great deal of practice before it can be done with certainty of success. At the same time, my young friends cannot possibly do better than practise this operation, for there are numberless cases in which screws cannot be conveniently cut in any other way, and it is, further, an accomplishment that will at once stamp them as skilful workmen.
Fig. 51.
The tools required are represented at A, B, Fig. 51. A is an outside, and B an inside screw chasing-tool. These are always made in pairs, of exactly the same pitch, i.e., the outside tool being applied to the inside, the respective notches and points will exactly fit into each other. If you were to examine the under side of these tools, shown at C, you would notice that the notches do not run straight, but slanting. They are in fact parts of screw threads; and you could make a tool of this kind out of a common screw nut, as I have shown you at D, only it would be too much hollowed out to make a good tool.
Now, supposing you were to hold the tool A flat on the rest, while a cylindrical piece of wood revolved in contact with it, you would cut a series of rings only; but if you were at the same time to slide the tool sideways upon the rest, so that by the time the wood had revolved once, the first point of the tool would have just reached the spot which was occupied by the second when you started, you would have traced a screw thread of that particular pitch. This is what you have to learn to do always, and with certainty, no matter what pitch of tool you may be using, and it is easy to understand how difficult the operation must be to a beginner. Indeed, there are numbers of otherwise good turners who have never succeeded in mastering this work. Nevertheless it can be done, and, although difficult, it is not so much so as might be supposed. Indeed, at first sight it would hardly be believed possible, because each different pitch of tool, and each different-sized piece of work, requires a different speed of traverse to be given to the tool. But a practised hand will strike thread after thread without failure, and those whose trade is to make all sorts of screw-covered boxes and similar articles, will execute the work with as much speed and apparent ease, as they would any ordinary operation of turning. I shall tell you by and by, however, of several ways to escape this difficulty of screw-cutting,—lathes being fitted in various ways to insure good work, in some cases by carrying forward the tool at exactly the right rate of traverse, and at others by moving along the work itself at the proper speed, while the cutting tool is held immovably fixed in one position,—and I will give one tool of great service which will guide you in starting the ordinary chasing-tool; and a good start is here truly “half the battle.”
The chasing-tool must run from right to left for an ordinary right-handed screw (and a left-handed one is very seldom required), so that the young mechanic need not trouble himself about it. Precise directions cannot be given further than to have a rest with a very smooth and even edge, which will not in the least hinder the traverse of the chasing-tool, and to get the lathe into steady, equable motion. Then hold the tool lightly, but firmly, keeping it at right angles with the work. Allow it only just to touch until you find you have got into the right swing. It is all a matter of knack and practice; and if you succeed quickly, you may congratulate yourself.
The inside chasing-tool is used in precisely the same way, running it from the outer edge of the hole inwards. To some this is an easier tool to use than the outside chaser. I cannot say that I find it so; especially as one has to work more in the dark; unless the work is of large diameter like the cover of a box, and even then the work is sufficiently difficult owing to the shallowness of the lid, which necessitates the instant stopping of the tool for a fresh cut. For you must understand that you have to deepen the screw-threads very gradually, and it will take several traverses of the tool to cut them to a sufficient depth.
The chasers require to be very sharp to cut wooden screws neatly, but observe you must only rub the upper flat face upon the oilstone, or, if a notch has been made by using the tools upon metal (they will cut brass well with care), grind them in the same way; the great secret being to hold the tool quite flat on the stone. You will thus, even by continual grinding, only thin the blade of the chaser, which will thus last for a long time. A practised hand will even cut a good thread with any flat piece of steel filed into equal notches, but a screw-chaser is the only tool really fit for the purpose.
Fig. 52.
The most effectual remedy for the screw-cutting difficulty, is unfortunately rather expensive in its best form. But in another, it is by no means costly; and although it may not look so well as the first, it is equally effective, and extensively used by the turners at Tunbridge Wells, who make those beautiful little inlaid boxes and other articles. I shall explain this to you, therefore, first:—
A, is a lathe-head, something like the one I have already described, but you will notice that the mandrel is a much longer one, and has several short screws cut upon it, each one being of a different “thread” or “pitch.”[1] This mandrel runs through two collars, so that, besides turning round, it can be pushed endwise. Now, supposing I was to hold the point of a tool firmly against either of the screws, and at the same time was to turn the pulley and mandrel, you will understand that it would run backwards or forwards in its collars, at such a rate as the screw-thread compelled it to move. This is the plan of the traversing mandrel; and now supposing that you had a box held as usual in a chuck, and while the mandrel was compelled to move endwise as described, you were to hold a pointed tool against it, the tool would evidently cut a screw-thread of exactly the same pitch as that upon the mandrel against which the pointed tool first spoken of was applied. But in practice, a single-pointed tool held against the mandrel would not answer very well, and so the following plan is adopted instead, which answers perfectly.
Fig. 52, C, is called a half-nut. It has a set of screw-threads, cut where the semicircular hollow is, which threads fit one of the screws on the mandrel. A whole row of these half-nuts are fitted to turn at one end upon a long bar, so that either one can be raised up at pleasure to touch the screw upon the mandrel, which has threads of the same pitch as itself, B. These, then, are ranged under the mandrel, and when it is desired to make it traverse in its collars, one of these half-nuts is raised and kept up by a wedge placed underneath it. When no screw is required, a somewhat similar half-nut, but with merely a sharp edge instead of a thread, is raised, and this edge falls into a notch or groove turned upon the mandrel, or sometimes a back centre-screw is added like D, and when no screw has to be cut, this is run up against the mandrel like an ordinary lathe.
In the more expensive traversing mandrels, although the principle is the same, there is a little difference in the arrangement of the different parts. The mandrel is not very much longer than usual; and it has no screw-threads cut upon it. But a number of ferules like K, are made each with a screw upon its edge, and one of these of the desired pitch is slid upon the end of the mandrel at b, fig. P, and is there held by a nut or otherwise, so that it cannot move out of its place. The half-nut is seen at a. It consists of a piece of brass or steel of the form shown with a hole in the middle, and a screw cut upon each hollow, so that it is a circle or set of half-nuts of different pitches. This slips over a pin at a, and when the screw b is turned, it draws up this pin and the nut attached, until the latter comes in contact with the ferule upon the end of the mandrel. This is very neat but expensive. Now, by far the cheapest and best way for the young mechanic, is to set boldly to work to conquer the difficulty of chasing screws by hand. There are even disadvantages in the expensive form of a traversing mandrel, which render it by no means a desirable mode of fitting up a lathe, and after all, the length of screw which it enables one to cut is very limited, and in addition, it is not every day, nor probably once a month, that screw-cutting will be necessary at all. My advice, therefore, is, do not get a traversing mandrel until you can cut screws well with the chaser. When you can do this, you will be able to judge of the advantage or disadvantage of one.
By far the greater number of common screws are cut without the lathe, by screw-plates, or stocks and dies, and the nut, or hole into which such screws are to fit, is cut with a tap. A screw-plate is a simple affair,—a mere flat plate of steel, in which several holes are drilled, which are afterwards threaded by screwing into them taps, or hard cutting steel screws of the size required; the plate is then hardened by being heated red-hot and suddenly cooled, after which being much harder than brass, iron, or steel which has not gone through such process, it will in turn cut a thread upon any of these by simply screwing them into it. But although this will answer for small and common screws, it is not at all suitable for better ones, because the thread is burred up, not cut cleanly as it would be with a proper tool. A far better plan is a stock and dies; the latter being practically a hardened steel nut sawn in half, and fitted so that the two halves can be pressed nearer and nearer together as the screw thread becomes deeper. The dies are screwed up by means of a thumbscrew opposite to the handle.
To use it, a piece of iron is filed up or turned to the required size, which must be exactly that of the finished screw. The dies are then loosened and slipped on to the end of this screw-blank as it is called, and are then slightly tightened upon it. All that is now required is to keep turning the tool round and round upon the pin, which it will soon cut into a screw thread. When the stock is at the bottom or top, you may tighten the dies, and so work up or down; but never tighten them in any other part. If iron or steel is to be cut, use oil with the tool, but brass may be dry. If the screw is of steel, heat it red-hot and let it cool very gradually, to make it as soft as possible.
The hole or nut, into which the screw is to fit, is to be drilled so as just to allow the taper tap to enter about a couple of threads; a wrench, or, if small, a hand-vice is then applied to twist it forcibly into the hole, when the thread will be completed. Take great care to hold the tap upright, or else, if it is a screw with a flat head which has to fit into it, it will not lie correctly, but one side of the head will touch while the other is more or less raised.
There are other modes of screw cutting, but at present I need only mention one, which is used for wooden screws alone. It is called a screw-box, and is only made to cut one size, a tap being always sold to match. You can, however, purchase any size you like, from a quarter of an inch to 2 or 3 inches; but the latter are only intended for very large screws, such as are used for carpenters’ benches and various kinds of presses. A screw-box looks like a small block of wood with a hole in it, but if you take out two screws you will find a blade of a peculiar shape, which forms the thread by cutting the wood as it is screwed into the hole in the box.
