For engineering manufactures, including all sorts of steel con struction (boilers excepted), ships, agricultural machinery, tanks, cisterns, piping, fencing, railway carriage and wagon frames and so on, the punching machines are either single or multiple, or are combined in one frame with a shearing machine, so that one drive actuates each slide (see the sketch). The punch slide has a disengaging device, rendering the punch inoperative until the attendant has accurately located the plate beneath it, whereupon he pulls a little lever and the punch descends. The purpose of the flywheel is to give a reserve of energy to carry the punch or shear through the metal. The larger machines possess self contained jib cranes, enabling heavy plates to be slung and moved along as necessary without keeping the shop crane en gaged. Punching-machines only have an eccentric action to the slide (similar in principle to that of the diagram) and are often double-ended, using a differently sized punch in each slide. In stead of eccentric motion, powerful levers are often utilized in collaboration with a central eccentric which rocks them and so moves each slide. Sometimes a third slide is included in a dou ble-ended punching and shearing machine to cut angle-iron, which requires a special shape of blade. Provision for shearing off angles, tees, channels, girders or H-irons is incorporated in some machines, there being apertures of corresponding section through which the shapes are pushed and sheared by a knife behind. The framings of such machines are frequently made in either cast steel or steel plates riveted together, to afford extra strength without such weight as is necessary for cast-iron framings.
Multiple punching is greatly in favour because of the economy of production. The regular machines may have an attachment to take two or three punches, but otherwise a special machine has to be used, with a wider slide than usual. The punches are either adjustable as to distance apart, or fixed in this respect. Plates and bars for tanks, agricultural machines, carriages and wagons and various machinery may have any number up to 1 oo holes pierced at a stroke. If several rows must be put through a
sheet (as for mine screens) the sheet can be handled rapidly on an automatic spacing table, which feeds it accurately to the successive positions past the punches. Alternatively the plate may remain stationary and the punching head travel intermit tently to the successive positions.
The massive shears required in steel works and those making big boilers are actuated by powerful gearing and eccentrics, or by hydraulic cylinders. The parts must be extremely strong in order to withstand the enormous force exerted at the moment of shearing and powerful spring buffers are often provided to take the shock as the blade goes through the plate. In these, as in most other shears, the blade is set at an angle so that it only shears progressively from one end to the other.
Hydraulic action furnishes a simpler mode of actuating a punch or shear slide than gearing in some of the shipyards, for punching big holes necessary in some of the plates. Portable machines for either punching or shearing are often used in the yards, being either carried about or transported on wheels. The working of a lever operates the pump and sends water to the actuating cylinder. Small machines are termed punching bears.
Rotary shears are so named because bevel-edged discs are used for the shearing. Some cut straight, others in circles, the plate being rotated on a central stud. Both circles and rings are sheared thus in large numbers for the tinware industry and other branches requiring these forms to build up shapes, or to stamp to further outline in dies. Some are built as slitting shears; that is suitably to make a long straight cut to divide a plate in two, or shear it up into strips, and there are also scroll shears which will cut intricate outlines between the discs, the attendant suitably manoeuvering the sheet as he feeds it along. (F. H.)