PUNCHING AND SHEARING MACHINES. The quickest method of making holes in, or severing, materials is by punching or shearing, and there are few forms of manufacture that do not utilize machines for these operations. Paper, card, leather, cotton, cloth, slate, tin, brass, iron and steel all undergo such treatment. Circular and other shaped holes are made, pieces are punched out to give various outlines, and various shapes are sheared off. The most striking examples are in steel works, where blades of I2 feet in length will shear through 2 in. cold steel plate, or hot slabs 43 in. by 20 in. thick are cut through. Many indus tries are suited by hand-worked machines, others require belt, electric motor or steam-engine drive, while the largest punches and shears are worked hydraulically. Hand machines are employed by tinsmiths and other light metalworkers, also by boxmakers, and these machines rest on the bench. Larger punches or shears are actuated by treadle, the latter machines having blades up to about 4 feet in length. Some have blades formed to cut curves in tinplate or iron plate, instead of using a die to stamp the out lines, and corrugated blades are used for shearing corrugated iron. Blacksmiths and others make use of a combined type of machine, with a single lever moving the punch and the shear blade. This principle is also applied in many big machines of the en gineering works, but using power drive.
Punching and shearing are alike in action which amounts to a rather violent separation of the fibres of the material. In soft substances there is little injury, because the cutting edges can be made of a keen angle, but iron and steel become more or less torn unless very thin, and the edge reveals minute cracks. For certain purposes these would be undesirable, as tending to increase under stress; consequently plates and strips for steam-boilers are not punched, but drilled for reasons of safety. And where struc tures are subject to heavy loads of a fluctuating nature, the best plan is undoubtedly to ream punched holes, the reamer tak ing away the cracked and torn walls and leaving a smooth and whole surface. Shorn plates for boilers and certain other con
structions are also machined on the edges, by turning, planing or milling, until a clean faultless metal is left. Obviously in order to ensure neat punching or shearing, as well as to minimize cracks and damaged edges, the tools should be kept ground up sharp, and set so that the material is well supported at the moment of im pact. In a great many cases a slotted finger, termed a stripper, is fixed a fraction of an inch above the material, so that as the punch rises after the stroke the material cannot be pulled up with it, but is automatically stripped off. Some substances, especially when laid in piles for simultaneous treatment, must be clamped down firmly to prevent faulty results, while the thicker steel plates are held immovably by a mechanically-operated clamping device which comes into play just before the descent of the slide. Hydraulic vice rams perform this function in the large hydraulic shears of the steel-works. In rolling-mills, plates and slabs are transported under the shear on sets of power-driven rollers.