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.