Shearing machines and punches are similar in general construction and opera tion, excepting that in the former cutting edges which pass each other like the blades of a pair of scissors take the place of the punching tool. Their name and action sufficiently describe the purposes for which they are used.
Milling of great import ance in the manufacture of iron work of mod erate dimensions such as parts of sewing ma chines, automobiles, rifles and pistols. In gen eral construction a frame-work carries a spindle actuated by a pulley, and a table upon which chucks or holding devices are arranged. Gear wheel attachments to the cone pulley provide for changes of speed or greater driving power at the cutters. The advantages gained by their use are as follows: The rotary cutters insure a continuous cutting operation, with a con stantly changing cutting edge; the outlines of the work and the form of the cutting edges are exactly similar; once adjusted, all work is turned out uniform in size and shape, and any unlimited variety of shapes may be obtained by simply varying the form of the cutters, while the only special operating skill required is to maintain the form of the cutters and set the work.
The modern milling machines have been brought to a high degree of perfection and pro duce work in a truly wonderful variety. The milling cutters are worked singly or in ((gangs* of many forms on the same shaft. The tables are provided with longitudinal feed and cross feed, and the tools with vertical feed as well. Some of the numerous operations performed are planing of plain and curved surfaces, grooving, slotting, dove-tailing, cam cutting, ratchet wheel and gear cutting— spur, spiral and helical, as well as bevel and worm gearing, sawing, slitting, making of taps and reamers, etc. The automatic milling machines reproduce with great accuracy the form of any pattern in serted as a control, practically without atten tion.
The recent development of grinding machines has placed them in the front rank of metal working machinery as the fin ishers. They take work rough finished from the lathe and the milling machines and bring it to the finest degree of perfection. Not only do they •finish the work of the other machines, but they also grind to exquisite form the tools with which the other machines do their work. In the automatic section the work is ground accurately to size regardless of the wear of the wheels during the process, beginning the grind ing with a coarse feed and throwing in the fine feed automatically as the piece approaches its true size. The machine once set, piece after
piece is automatically machined to pattern size without attention except the removal of the finished work and the placing of another rough piece. A great deal of work formerly turned to extreme accuracy on lathes is now finished direct from the forging on the supersensitive automatic grinder and with a very considerable saving of time. See GRINDING Riveting machines are made in two forms, stationary and portable; the former used in heavy work are operated by pressure generated by steam or hydraulic power, while in the portable form the moving die is actuated by steam or compressed air, with hammer-like blows. They are employed to drive and head hot rivets in boiler work, bridge building and other structural work with results far superior to hand work in rapidity and efficiency. The usual form of the stationary type is a U-shaped frame or yoke. The ends of the arms are pro vided with dies projecting inwardly, one being fixed while the other has a reciprocating mo tion. The rivet being placed in the hole is compressed endwise between the dies; the pres sure forces the metal of the rivet into the ir regular edge of the hole, clamps the plates to gether and forms the rivet head.
Drop Hammers.— Heavy drop hammers are used for forging and also for welding, and are operated by hydraulic or steam power. They consist of an anvil upon a solid base of steel blocks laid over oak timbers to give elasticity to the machine. Above the anvil vertical housings capped by steel arches sup port the hammering ram and the platforms upon which the piston cylinder and other actu ating machinery is placed. By the use of swages, fullers and flatters, hot metal is ham mered into forms, often by the use of a pro gressive series of dies: The largest hammer of this kind was erected in 1891 at Bethlehem (Pa.) Steel Works. Its general dimensions and weight are as follows: Height, 90 feet; width, 42 feet; weight of anvil and foundation, 1,800 tons; weight of housings, steam chests, pressure cylinders and piston, about 1,000 tons. Ram 19 feet long, 10 feet wide and 4 feet thick, weighing 100 tons. Lighter forma of drop ham mers are actuated by compressed air and are called pneumatic hammers. They are used principally as cornice bending machines.