Like the milling-machines these have a rotary drive to a many-toothed tool, but in the sawing-machines the latter is comparatively narrow and the feed only has to cause a penetration partly or completely through the object. The cutting serves to make slots as well as severance, while in wood-working certain kinds of saws perform quite a range of cutting of various outlines. Reciprocating saws are an adaptation of the hand saw, accurately guided, and with control of the object. Files are also worked in machines so increasing accuracy and output.
All grinding was formerly done in the lathe, and its chief purpose was to correct the bearing portions of spindles which had been hardened and had warped slightly in the process. There is still a large amount of grinding of small hard ened components in the fine instrument-makers' lathes, but in general engineering practice the grinding-machines are distinct designs, more suited for running the wheels at the requisite great speeds without vibration, and with special provisions for pre venting damage to bearings and slides from the grit and from the water used in cooling the work. For a good while only finishing was attempted, the major portion of the work being done on the lathe, a trifle only being left for the wheel to remove ; mechanical improvements have altered this, and now the grinder can deal with rough castings, f orgings and rolled bars and will take off all the metal necessary to reach the finished size. Parts of stoves, doors, dampers and other boiler details, forgings for cars and engines, and many items for agricultural and domestic machinery are now cast or forged very nearly to the final dimension, and grinding is a quick and economical method of completing the work.
Practically all running parts of good-class machinery, such as shafts, spindles, bushes, washers, ball and roller bearings, gears, nuts, pins and pivots and such-like are finished to a close degree of accuracy by grinding, and mass production is favoured thereby.
Bef ore the possibilities of milling were apparent all heavy and much light cutting was effected on one or other of the reciprocating machines using a tool with a single edge as distinct from a rotary many-toothed cutter. The objections to these machines are that no work is performed on the return stroke as the tool only cuts on the forward stroke, and difficult contours, which are easy to mill with formed cutters, are not so con veniently planed or slotted, nor is the fin ish usually so good. At one time the mill ing-machines competed with the recipro cating machines in the speed at which metal could be removed, but the designers of the latter soon remedied this. Stronger con struction, higher rates of cutting, and a much faster rate of return on the non-cutting stroke were introduced, and electric driving helped in increasing the speeds for big machines. Further, by arranging switches along the table edge of the large planers the speed could be accelerated suddenly at any spot, and de celerated to the proper cutting rate again the advantage being that the table can rapidly jump any gaps between pieces of work bolted on the table, or openings in the surfaces of work.
Economy in planing is best secured by employing a very long table filled with pieces of work all to pass under the tools, instead of a short table carrying only a few articles, or by using a machine that carries several tools, one or two operating along the top of the work, and one at each side.
Shapers and slotters are utilized for relatively short cuts, the one in the horizontal direction the other in the vertical; both are capable of a wide range of action, and both—especially the slot ters—will effect heavy removals of metal. The latter are well adapted to cut on the ends of rods and long specimens, as well as to slog off thick shavings from forgings which require a good deal of machining to bring them to form. Internal slotting is also easy, an operation not so convenient as a rule on the planers, shapers and millers ; but the newer machine-tool, the broaching machine, will at one stroke undertake even the most difficult internal cutting in a fraction of the time occupied by the many reciprocations of the slotter.
In cutting the tooth spaces of gears it is essential that the curves of the teeth are cut with exactitude and that the spacing or pitch between the teeth is uniform throughout. Otherwise the gears will not mesh well, and the run ning will be noisy and cause vibration. The dividing mechanism must be perfect to ensure accurate pitching, and the cutters or tools must maintain their shape and produce like profiles on all the teeth. The earliest and most obvious way to attain this was to make a revolving milling-cutter with flanks identical in shape to the tooth curves, and with it plough through the metal of the wheel rim. This is still the procedure in a great many machines, the feed and quick return ready for another traverse being given automatically to the slide which carries the cutter spindle, while the wheel is indexed automatically by one tooth space before the next cut commences. Gear-cutters with planing or shaping tools instead of milling-cutters are employed for two reasons. One is that bevel gears can only be cut with correctly shaped teeth by a tool which travels in a path corresponding to the taper of the teeth, from the big to the small end ; a cutter with fixed curves obviously cannot produce such a tapered shape. The other reason is that a planing type of machine with its simple flat-edged tools does away with the necessity for a large stock of cutters to suit each size of tooth, and the shape of the tooth is correctly formed or generated by mechanism incorporated in the construction of the machine. For the best workmanship, such as is necessary in automobile and other high-grade gears, the teeth are finished by grinding so as to ensure perfect contact and noiseless running.