MACHINE TOOLS 155 FIG. 6. Screwing Machine. universally accepted. Immense loss and inconvenience were formerly caused by the absence of uniformity in this respect, but, thanks to the persevering manner in which the efforts of Maudslay and Clement to put an end to this evil have been followed up by Whitworth, it has now almost ceased to exist, and any bolt or nut can be sub stituted for any other of a like size, however different the processes by which, the two have been manufactured. The machine (fig. 6) is in fact a lathe with a few special features, such as the hollow man drel, which enables it to operate upon a bar of any length. Dies mounted on a modified form of slide-rest cut the thread to the full depth at a single traverse, and a simple arrangement enables nuts to be tapped with equal facility. In some other varieties of screwing machines, more particularly those in tended for hand power only, the outward resemblance to the turning lathe is less apparent, but if their action is looked into it will be found that in them as in almost all machine tools it is the principle of the slide which is mainly conducive to their success. Second only to the lathe in its importance stands the planing machine. Just as the slide lathe renders it easy to turn a cylindrical surface true from end to end, a task which before its introduction had been one of extreme difficulty, even for the most highly skilled workman, so the planing machine supersedes, by a method giving vastly superior results, the difficult and costly process of hand chipping and filing, by which flat surfaces of metal were formerly produced. Although it is a comparatively modern invention, its real origin is obscure. No drawings or description of any planing machine at all resembling those now in use were published in England previously to those of one made by Clement in 1825, which appeared in the Transactions of the Society of Arts. With this beauti ful machine, which was of considerable size, being capable of admitting articles measuring as much as 6 feet in height or width, he obtained results which would satisfy all ordinary requirements at the present day. The ordinary self-acting planing machine is shown in fig. 7. Its action bears no resemblance to the familiar FIG. 7. Planing Machine. process of wood planing, but is analogous to that by which the successive cuts of a narrow tool produce a cylindrical surface in a slide lathe. A traversing table carries the work and forces it against the tool, which is stationary while making its cut, but between the cuts has a slight "feed" motion along its horizontal slide. Perfectly parallel cuts are thus taken from every portion of the work in succession, the result being a surface, not indeed per fectly smooth and free from scores, but (what is generally far more important) possessing a general flatness and. free dom from twist which can be obtained only with a great expenditure of time and trouble by hand labour. The extent to which machinery has cheapened work of this kind will be appreciated from the fact that in 1826 the cost of rendering a square foot of surface true by hand chip ping and filing was 12s., whereas in 1856 it could be done in the planing machine at a cost of less than one penny. Planing machines, equally with lathes, are required not only to give good results but to give them quickly. Pro vision is therefore made for regulating the traverse of the table to suit the length of the cut, and for utilizing or accelerating its return journeys. The former is sometimes done by fixing the tool in a revolving tool-holder or "jim crow," so that its face can be always turned towards its cut, and for accomplishing the latter there are various arrangements which give a " quick return " to the table.. It is also a common practice to use two tools at once, as in turning. It will be observed that the size of the work which can be treated in a planing machine, such as fig. 7, is strictly limited by the clear width between the standards, and the height of the horizontal slide above the table when at its highest point. Although these dimensions are very considerable in the larger sizes, which can occasionally take in articles over 9 feet in width and height and 50 feet in length, yet it is sometimes desirable to be able to exceed them, and in these large machines the weight of the table and the power consumed in driving and reversing it become a serious consideration. It is therefore mechani cally preferable to keep the work at rest when it is large or heavy, and to give all the requisite movements to the tool. This view is now gradually gaining favour, and the makers of some recent machines have adopted a form of construction entirely different from the above, which has the advantage of enabling cuts either horizontal or vertical to be taken from any piece of work which can be secured to the base-plate, so that its full size is almost immaterial. An ordinary vertical drilling machine is represented in fig. 8, one of comparatively small size and single- geared having been chosen rather than a larger example with greater complication. When once properly started, this machine is self-acting, but for each hole the work has to be adjusted by hand so as to bring the required portion exactly under the drill spindle, and the small size of the table prevents its being at any great distance from the edge. These objections are remedied in larger machines, either by making the table cap able of horizontal adjustment, a good way of doing this being to pivot a circular table at the end of an arm which can revolve round the main standard of the machine, or by mounting the drill spindle on a radial arm, and enabling its distance from the standard to be varied. In the first case the tool is than distinguished as a "pillar " and in the second as a "radial" drilling machine. Either of these methods enables the drill to be brought to bear exactly upon the desired spot F IG . 8. Vertical Drilling
Machine.