Machine-tools are ordinarily fixed to a floor, and the work is brought to them; but very big castings or forgings, machinery that has to be repaired, altered or put together in situ, etc., often cannot satisfactorily be treated in this way, and in such cases it is usually quicker, easier and cheaper to employ a portable machine that can be bolted to the work, or supported closely to it in position for the cutting. Parts of large engine beds and frames and of electrical machinery and turbines are conveniently drilled, bored, tapped, milled, planed, shaped, slotted or ground by appropriate machines driven with flexible shafts or electric motors. Workshops dealing with such large objects have floor-plates, huge cast-iron beds with tee-slots into which bolts may be set about anywhere over the surface. The casting to be machined is bolted on the plate, and the portable machines are appropriately bolted alongside, several being used simultaneously on the various faces. Repair operations in mills and factories and on board ship of ten demand the services of a portable machine-tool which is attached to a framing, or clamped upon a shaft or wheel, to drill holes or ream them, or to mill off surfaces or cut keyways; such work could be done with hand tools, but as time is usually a vital factor in repairs and breakdown cases, a hand or power-driven portable machine-tool is preferable. Many tools are not really attached, but are held and controlled by the hands, as explained under PNEUMATIC TOOLS, but there is usually some sort of support by the tool, otherwise the operator would not be able to guide the cutting action.
Plate and Sheet-metal Working Tools.—Although a certain amount of cut ting is done in the machine-shop on plates as well as castings and forgings, the great bulk of such machining is relegated to spe cial departments. The thicker iron and steel sheets and plates for boilers, bridges, buildings, ships, cranes and other construc tions are sheared, planed, drilled, punched, reamed and tapped in machines of special ized type, giving capacity for the fast working necessary in view of the enormous mass of detail involved. Multiple working is adopted as far as possible, i.e., a number of holes are punched or drilled simul taneously, some sheets requiring scores or even hundreds to be put through.
Sheet-metal working on the lighter side is done in shearing and punching machines of much lighter construction than those of the planing- and boiler-shop, and there is a whole range of presses using punches and dies for blanking or finishing all sorts of metal articles. Some machines perform more or less elaborate operations, including bending, folding, assembling and riveting, and are formative in the main.
The wood-working machine-tools are characterized by high speeds, keenness, and the rapidity with which the wood is fed for cutting. A simple kind of vice or clamp may be employed, cor rugated rollers, or just a guide plate or fence to keep the position right. As with metal-working machines, the tools for making holes and the various knives, bits and cutters are used in multiple whenever practicable. A problem which is more insistent than
with some of the metal cutting operations is the disposal of the masses of shavings which accumulate in a few moments; pneumatic suction pipes effect their removal.
It is in practice always essential to machine off a definite amount of metal to bring the piece of work to a dimension determined on the shop drawings and in many instances a second cut is necessary for accurate finishing, the machine taking a heavy cut nearly to the indicated size and then a light one to make a smooth and accurate surface. If the attempt is made to take off the whole of the cut at one traverse, accuracy is not so likely to result, for the tools, cutters and the machine parts tend to spring under the stress, and slight devia tions occur in the movements resulting in vibration and minor irregularities on the surfaces. But a finishing tool with only very light duty to perform is not subject to these faults, and it can be kept sharp and in the best condition of edge to give a fine finish. Some lathes and other machines rough and finish with the same tool, others with tools of rather different shapes, while a few (such as milling-machines) pass the articles under distinct rough ing and finishing cutters in the course of the one feed traverse. Or roughing is given to the lathe or planer or miller, and finishing to the grinding-machine. There is in some high-class products an interval between the two operations, such as for motor-car cyl inders and parts of scientific machines, when the rough skin of the casting has been taken off the whole thing may alter its form to a small degree owing to the release of surface tension. By leaving the pieces time to "season" after roughing there is avoid ance of warping taking place after the fine finishing process.
It is necessary to allow a predetermined amount for roughing, and in repeat orders all the sizes are marked on the drawings, so that the tool-maker and setter and the operator know just what to do. Sometimes there is no necessity for measuring while the cuts are in progress, in other instances frequent checking has to be made. The familiar hinged leg calipers have been mostly displaced in up-to-date factories in favour of more precise instruments.
Methods of adjusting edged tools vary; they may either be set tentatively and then tested and re-tested against the actual work, or they may be set by means of a thickness block or gauge which brings the cutter to exact position. Careful measurement or gauging of components turned out in large quantities is requisite at frequent intervals, so as to be sure that none of the numerous tools (as in an automatic screw machine) have altered, slipped, or become blunted. The tendency is always towards reduction of hand measurements in favour of automatic indication of sizes; of grinding-machines especially is this true, a magnifying arm or an electric indicator showing when the work reaches a specified size.