MACHINE PHOTOGRAVURE: see PHOTOGRAVURE (Machine).
These lie at the basis of all modern industrial production, for they are not only necessary for the manufacture of every class of engine and kind of mechanism but every manufactured product, whether fabrics or textiles, metal goods, soap, foodstuffs, building materials, scientific instruments, etc., must be made either on a machine-tool, or on a machine which is constructed with their help. Little can be done by purely casting or forging processes; there is nearly always need to finish certain surfaces truly and smoothly to size in order to produce accurate fitting, or to enable the parts of a mechanism to work.
Machine-tools correct the inaccuracies in cast, forged, or rolled metal articles due to warping and other causes.
The term machine-tool strictly includes wood-working ma chinery, and there are many resemblances between some of these and metal-working machines. As a rule, however, the latter hold the pieces of work with powerful clamps, vices, chucks and fix tures, while wood can be presented to the cutters with far less trouble, direct by hand, or against guides, or with feeding rollers.
The necessity for firmly holding or driving a casting, forging, or a piece of bar or plate while the machine-tool is cutting it affects the design in many ways, and there are comparatively few machines which can dispense with mechanical holding devices. These are chiefly simple grinding or polishing machines (the operator pressing the articles against the wheels by hand) and punching and shearing machines (the plate resting on a bed and being steadied and moved about as required to make the cuts). Most other machines either take the object between point centres (lathes and grinding-machines) or grip it against jaws or flat surfaces.
In wood-working machinery the tools are very keen for soft woods, rather less so for hard woods, but thin edges such as these are quite unsuited for use against metal. Metal-working machine tools therefore use tools and cutters with more obtuse angles, and the speed of cutting has to be set so that these will not be destroyed rapidly. For example, brass and similar alloys can be cut at high speed without injury to the tools, but the same rate on iron and steel would abrade and burn the tools. The use of a
lubricating and cooling medium such as oil, soapy water or an emulsion greatly relieves the tools and enables a higher speed to be attained than by dry working; and there are special steels which will perform very severe duties without failure, even on hard materials such as chilled rolls and railway wheel tyres, which latter develop very hard spots by running on the rails; stellite will cut at an astonishing rate compared with the performance of a steel tool. This question of capacity affects the running of a machine-tool for definite periods on similar work, for if a tool edge requires frequent regrinding the machine cannot run auto matically for many hours or days without attention. Consequently the tool must be made of a suitable steel and so designed that it will resist wear for the required length of service, and the speed of the machine must be regulated to suit the class of metal being machined.
A large group of machine-tools, the grinding-machines, do not use metal tools, but operate with high-speed wheels of emery, corundum, carborundum and other abrasives. The kind or condi tion of metal makes no difference to these wheels—they grind off the hardest steel without difficulty—but there are many limita tions to their use; thick cuts possible with steel tools cannot be taken off castings or forgings with these, and surfaces of certain shapes cannot be readily dealt with. But the grinding wheel of ten acts as a finishing agent of ter forms have been roughed out on some other sort of machine-tool; highly accurate results, some times to within one ten-thousandth part of an inch and finer still in making gauges, can be attained by their use. After pieces have been hardened also the grinding-machine is the only means of further correction, and all sorts of tools and cutters which have been hardened or tempered need their use for final truing up and subsequent sharpening at intervals. The demands of the motor industry for immense quantities of relatively small hard ened parts have led to the design of new types of machines which either handle these rapidly in succession, or grind a large number at one pass of a wheel all to similar thickness.