Figs. 168, 169.
Figs. 170, 171.
In [Fig. 171] the half nuts slide up and down on the front of the saddle, and are moved by a circular plate on the outside. The action is very similar to that of [Fig. 167], intervening links, attached to the plate and to the halves of the nuts, giving motion to the latter as required. Cams and eccentrics of varied forms are also made use of for the same purpose, every lathe maker devising new and improved movements from time to time. There is indeed little difficulty in arranging a satisfactory method, the best being that which is under easy control, of adequate strength, and not likely to be easily disarranged by the rough treatment it is liable to receive at the hands of the workman. The leading screw may either be placed on the outside, in front of the bed next to the workman, or inside between the bearers. The former is preferable as being more accessible, but it is rather more exposed to injury. When between the bearers the large saddle of the slide rest serves to protect it from the falling chips and shavings of metal which, mixing with the oil, are apt to clog the threads and add to the friction of the parts, besides increasing considerably the wear and tear, and no accidental blow is likely to reach the screw thus protected on both sides by the lathe bed. Nevertheless, all the various parts of a machine should be made as accessible as possible, and, with the exercise of ordinary care in its preservation, the position outside is on the whole the best. If the lathe bed is very long, it may be desirable to support the screw by allowing it to rest on friction wheels fixed to the saddle at both ends, or by allowing it to pass through a pair of brasses. Thus, as the saddle is often two feet in length, such bearings at each end, with the split nut between the two, form points of support which go far to prevent the screw from jagging or bending in the middle from its own weight.
The general arrangement of self-acting tools is similar with all the makers. [Fig. 172] is a drawing of such a tool from a sketch of Eades & Son, an old firm in Lichfield-street, Birmingham, the screw being here placed in front. Whitworth, the eminent mechanician of Manchester, makes many lathes with the screw between the beds. He also uses a double slide rest, with a tool working at each side of the piece to be turned, which cannot thus spring away from the cut. This is called a duplex slide rest. By this method double the cut is taken, and the strain upon the work lessened. In addition to the screw there is usually a rack attached to the lathe bed, on the furthest side, and a cog wheel working upon this, attached to the back of the slide rest saddle, is turned by a handle in front of the rest, [Fig. 173]. This enables the workman, after having, by means of the screw, completed the cut to the end of the work, to run the rest back again very quickly, having first released the split nut from the screw. The tool is thus brought into position for the next cut far more rapidly than could be managed by reversing the motion of the screw. This is called, therefore, the quick return motion, and is almost always attached to the larger class of machine-lathes. Before entering upon the details of other forms of gearing, and the arrangement of change-wheels for cutting screws or for plain turning, we shall introduce a few remarks upon the often-repeated question as to the advisability of self-acting lathes for the purposes of the amateur. It must be remembered that the chief object of such a lathe is the manufacture of large screws, long shafts, and such work as the piston rods of engines, requiring perfect accuracy from one end to the other. Boring cylinders and similar work is now generally done in an upright or vertical boring machine, which is for many reasons more convenient. This class of work, of course, the amateur has nothing to do with. The traverse of an ordinary slide rest is generally adequate for surfacing or boring to a length of six inches, which is sufficient for most purposes, and can readily be done by turning the leading screw of the rest by hand. Then, as regards screw cutting in the lathe, it is questionable whether it is not easier to turn up the blank and cut with stock and dies. The thread thus formed is a copy of that of the tool, and the latter, as made by Whitworth and other first-class makers, is as perfect as machinery directed by talented workmen can produce. If it is really desirable, however, to make use of the lathe for this purpose, the plan already suggested is generally applicable, namely, gearing the leading screw of the slide rest to the mandrel, either by means of change-wheels attached to an arm, or by connection with the overhead pulleys. There is, in addition, a plan of attaching a pair of dies to the tool holder of the rest, after detachment of the screw from its nut, which may be available in some cases. There are also screw plates made in two halves, the plates being divided lengthwise and clamped together at pleasure, admitting of application to any screw-blank while revolving in the lathe; and, lastly, pairs of dies, fitted like pliers, can be similarly used by hand without any slide rest. On the whole, then, it will hardly pay to give £50, or £60, for a screw-cutting lathe that is merely destined for small work; that can be done sufficiently well in a common lathe with hand motion to the rest. For cutting short screws, such as those of boxes, a convenient and practically useful contrivance is still a desideratum. The traversing mandrel is, no doubt, the best, but it is an expensive pattern, if well made, and it is necessary to sacrifice, in a measure, general utility to questionable and certainly partial advantage. Turning is, moreover, an art, and to attain skill in it, perseverance and practice are necessary—hence the zest experienced in its pursuit by the real lover of it. This perseverance pays, and to attain the skill required to trace a short screw with the chasing tool is within the reach of any one who desires to accomplish it. The art once acquired, the desire for a traversing mandrel ceases; for no good workman would accept a contrivance of the kind when he can more easily and quickly accomplish his end without it. Writing this for the special benefit of amateurs, we would strongly advise them not to throw away money to purchase the means of doing what after all they will probably never, and certainly only seldom, carry into practice. The pleasure afforded by all the mechanical arts is greatly enhanced by meeting with and triumphantly surmounting all sorts of difficulties. Let it be a standing rule of our readers to make use of the appliances at hand instead of seeking others which only save trouble and render skill unnecessary. What can be more aggravating than for an amateur, with his hundred guinea lathe and chests of tools, to be obliged to take his work to a mechanic, and to see him, whose whole stock might be bought with perhaps a tenth part of the money, take in hand and finish with ease what has baffled the skill of his more wealthy patron? The common fault of the amateur is undue hurry. To him time is seldom an object, yet the mechanic, to whom it is so precious, readily spends more upon his work. He never hurries, never compromises, but with lathe and file fits the several parts of his work with the most patient care and practised skill. The result is at once seen when his productions are placed side by side with those of the zealous but too hasty amateur.
It remains, as previously described, to arrange certain cogwheels gearing with each other upon the spindle of the mandrel and upon the end of the screw, to enable the workman at pleasure to vary their ratio of speed, in order to give him the means of regulating the pitch of thread to be cut. For this purpose a set of change wheels is supplied with the screw-cutting lathe, the number of cogs in which usually commencing at fifteen, increase by five, until the number one hundred and twenty is reached, then increasing by ten up to the complete set. Duplicates of some numbers are also convenient. The method of finding the proper sized wheels is, of course, based upon the proportion borne by the required pitch to that of the leading screw and one form of calculation for the purpose has already been given. Thus, supposing the leading screw to have two threads to the inch, it must revolve twice to move the tool that distance, and if we wish to cut a screw of ten threads to the inch, the work on the mandrel must revolve ten times while the leading screw revolves twice, or, which is the same thing, the mandrel must revolve five times while the screw revolves once. The cogs in the wheels must therefore be in the proportion of five to one, say fifty on the screw and ten on the mandrel, with an idle wheel on the stud to cause the motion of the tool to be such as will cut a right-handed thread, or to cause the mandrel and screw to take the same direction. Now, it seldom happens that two wheels and an idle one will give the requisite speed to the tool, and the number of cogs required soon mounts to inconvenient numbers. In this case, therefore, according to a principle already laid down, a stud-wheel with a pinion attached to the same, is placed in the train of wheels, and the object readily attained. A modification of the rule of calculation may make the system of change wheels still clearer. Let the pitch of guide screw be as before, namely, two threads to the inch, and let five be required. Place the numbers thus, 2-5, and divide both into their factors, twice one, or 2 × 1, and twice two and a half, or 2½ × 2.
2 1 / 2½..2