ABOOK on the Lathe would hardly be complete without an explanation of that attachment briefly called "The Overhead." There are many forms of this, but that shown on the miniature screw-cutting lathe (Fig. 13, page 89) is as good as any other, and is the plan most commonly adopted. The main object of it is to enable a turner to connect the movement of the fly-wheel with the slide-rest, or with various pieces of apparatus held therein, such as drills and revolving cutters. The screw of the slide-rest may also be itself so connected, which will render it self-acting, thus imitating, in a small way, the large screw-cutting lathes. For this purpose the winch handle is removed, and replaced by a pulley of any desired size. The overhead consists of a revolving shaft, supported on a pair of light but firm standards, attached to the backboard of the lathe. Upon this shaft is a pulley like that on the mandrel, and a roller, or second adjustable pulley. A cord from the fly-wheel, or sometimes from the mandrel, passes over the first of these, and a second cord descends from the roller to the pulley of the slide-rest screw, or to that of the drill, or of any revolving cutter which may be in use. It will evidently be necessary to keep this cord tightly strained under all circumstances, whether the driving pulley of the apparatus be large or small, and whether it is advanced nearer to the work or withdrawn by the action of the cross slide of the rest. To attain this end, there is a bar just under the shaft of the overhead, acting partly as a tie-bar between the standards, but, secondly, as a support to a sliding block, capable of being placed at any part of the bar, about which, also, it can turn as a hinge. Through this block passes a round rod, of stout iron, which carries at one end a pair of pulleys, whose peripheries bear against the cord, the pressure depending upon a weight slung to the opposite end of the bar, as well as upon the leverage. For this reason, the entire affair is called the "tension apparatus." Sometimes a spring arrangement takes the place of the weight; but the latter, being constant in its action, is decidedly preferable, and if it is suspended near the floor there is nothing objectionable about it.

The principle of the overhead having been explained, it will be as well to enter briefly upon some of its uses, although we do not intend to set forth in extenso the details of ornamental turning, which, indeed, require an extra volume.

Let us suppose a drilling spindle securely held in the rest, having a round-ended drill in its socket, such drill pointing to a smooth cylinder of boxwood held between centres or in a chuck. Let the height of drill be level with the lathe centres. Now suppose the ordinary lathe cord removed, and that from the fly-wheel to the overhead to be arranged, and a second cord to lead down from the overhead to the driving pulley of the drill. If, by means of the cross slide, the drill is advanced, it will simply bore a hole through the cylinder. But now let us suppose that the cross slide advances the drill only about 1/10 in., while the other handle of the rest is steadily turned, so as to carry the entire drilling apparatus horizontally along the work, lengthwise. In this case, the result will be a rounded groove, or recess, or flute. The mandrel, it will be observed, is not revolving at all, because its cord was removed. In practice it is held fast by an index point, resting in one of the holes of a division plate fixed to the face of the mandrel pulley; the circles of holes for this purpose are, however, very frequently drilled in the face of the pulley itself. By these means we can advance the pulley as many divisions as wo please between each traverse of the drill, and make the flutes as near to each other or as far apart as we please.

Now let us substitute for the round-ended drill one with a semicircular hollow at the end, such as would be made with a rat-tail file. This, if gently advanced by the rest, while kept in rapid rotation by the overhead, will not drill a hole, but will form a bead. It must not, however, be traversed along the work, because it would not form a flute. But if now we use the division-plate and index as before, it will be easy to drill a ring of beads round the work, close together, or at any desired distance apart. This is the principle of fluting and beading work, and also of cutting watch and clock wheels, a revolving saw or cutter taking the place of the drill. If this were the sole use of the overhead, it is evident that it would be a very valuable adjunct to the lathe; but there are other uses, to which our attention must be directed. It has been remarked that one great advantage of a self-acting lathe is that it enables a perfectly regular feed to be obtained in making a cut. The tool is advanced evenly and steadily, making a very beautiful finish to the work, if the tool cuts well, and is correctly placed. All that is here needed is a pulley, instead of a winch handle, on the feed-screw of the rest, and that this shall be driven at a suitable rate, by means of a cord from the overhead. The balance-weight of the tension apparatus must be so adjusted as to keep the cord tightly strained, and then there will be no slip, but the action will be as sure as if it was attained by cog wheels or gearing. Now let us go a step further: it was explained in the Chapter on Screw-cutting in the self-acting lathe, that we can obtain any desired pitch of screw by gearing the slide-rest with the mandrel, by means of cog wheels of varying sizes. We can do exactly the same by means of cords and pulleys of proportional size. If we have a pulley on the overhead of exactly the size of any one of those on the mandrel, and connect them by a cord, they will revolve at exactly the same speed; and if we have on the slide-rest screw a pulley equal to the roller in size, the mandrel, and overhead shaft, and slide-rest screw, will revolve exactly together, each turn of the mandrel giving one revolution to the other two, which are thus geared with it. Suppose the screw to have ten threads to one inch of its length; then, at each turn of the mandrel (and any work attached to it), the tool held in the rest, and actuated by the screw, will advance one-tenth of an inch. If, then, a pointed screw-tool is in the rest, and a cylinder in the chuck, a screw-thread of 1/10 pitch will be traced upon it, ten turns completing one inch. This will be a screw of ten pitch. Double the size of pulley on the slide-rest, leaving the others as before; and the tool will traverse at half the speed, cutting a twenty-pitch screw. The practical difficulty of this arrangement arises from the nicety required to size the pulleys with accuracy; but it is more theoretical than practical after all, as a little care will overcome it. A greater difficulty arises from the speed with which the mandrel is driven by the fly-wheel. The traverse is proportionally affected, and is correct; but it is too quick for most purposes. The remedy is to have one speed-pulley of the fly-wheel so small that the mandrel shall make but one turn to one of the crank axle, and then to have a large pulley on the overhead, to gear with the smallest on the mandrel. The same thing is done better by having a worm wheel and tangent screw fitted to the rest-screw or mandrel; but this will be more costly, and, unless for exceptional purposes, is not necessary.

We do not, indeed, advocate this plan for screw-cutting in a general way, unless a second shaft can be added, to facilitate the variations of speed. To take down the main shaft, in order to affix a pulley which again needs to be tested several times, and then, after each trial, to replace it, entails a deal of awkward and unsatisfactory work. Hence, in one form of overhead, a short, intermediate shaft has been added, which can be replaced between the lathe centres, in order to turn its pulleys, and again placed in its bearings without having to remove the main shaft at all. The latter is, in fact, too long to be thus mounted between the centres of its own lathe. Sometimes a shaft is added upon two short standards at the back of the lathe, instead of, or in addition to, the high ones which carry the long shaft and its pulleys. In that case, the lower shaft may be quite short, to receive the change wheels, or sized pulleys, which take their place.

From what was said about change wheels in the screwcutting lathe, it will be seen that a large variety of threads can be cut with two pulleys only; and here we can do without the intermediate idle wheel, as we can at once change the direction of traverse by merely crossing the driving cord. We can even reckon the sizes of pulleys by the same tables which give us the sizes or number of teeth in the wheels. A wheel of sixty teeth we may match with a pulley of 60in. circumference, or sixty half or quarter inches, and then one of twenty or ten teeth will match a pulley of 20in. or 10in., or half or quarter inches. Thus there will be no guesswork, and the pulley may even be turned accurately to size at once, without repeated testing.

Another use of the overhead is to drive emery wheels or grinders, held in a proper shank, similar to that used for drilling. By this a bar of iron or steel, between centres, can be ground to absolute truth after it has been turned, and even hardened. The lathe centres too, can at any time be ground true, and the points renewed when worn.

From the remarks we have now made, it will be seen that an overhead is such a useful addition to a lathe as to be well worth its original cost. It can, if desired, be also easily rigged up by the amateur with standards of wood, and an axle made of gas pipe, plugged at each end, and with centres drilled and hardened. At the same time, a properly-fitted one, entirely of metal, not only looks neater, but is unquestionably better. The roller should preferably be made hollow, so as to lighten it, and the standards should be substantial enough to check vibration as much as possible.

Before quitting this subject, it may be as well to point out that the drilling spindle, which is supplied with the overhead in the lathes of the Britannia Co., may be fitted to do eccentric work of interlacing circles, by the simple expedient of cranking the drills, so that they shall trace circles as they revolve. According to the length of the cranked part, so will be, of course, the size of circle, each requiring a different drill; but these are so easily made, that the defect in question is unimportant. The proper tool for such work is the eccentric outter, in which the little tool can be placed at any distance from the centre of rotation; so that it may be called a cranked drill, with self-contained power of adjustment. With this, or the simpler substitute, the index plate must, of course, be used to determine the distance between the several circles. Their position on the work is regulated by the slide-rest.