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vertical movements are made only by hand. Lead screws are used for operating all of the table movements in many of the smaller sizes and all of the larger machines, but in some of the smaller ones a rack and pinion are employed for the longitudinal movement. The smallest sizes of machines have no power feeds at all, and are called hand milling machines. (See illustration on page 46.) In these, the table and knee are moved by means of racks and pinions operated by levers. They are convenient for manufacturing purposes on some classes of small work, as they can be operated very rapidly.
It is the practice in the classes of work to which the medium and larger sizes of plain milling machines are adapted to take heavy cuts at fast speeds and coarse feeds. The rigid construction of the machine enables this to be successfully done, and it is in this ability that the chief value of the plain machine is found.
Universal Milling Machine. The Universal milling machine is justly regarded by many to be the most important machine tool employed today; for with it much of the work of the planer and shaper—heretofore considered indispensable machines in every shop—can be done with an appreciable saving of time. Spur, bevel and spiral gears, twist drills, and all kinds of straight and taper milling can also be economically produced.
It was first patented February 21st, 1865, by Mr. J. R. Brown, of the firm of J. R. Brown & Sharpe, who designed it for the purpose of milling the grooves in twist drills, but adopted it shortly after for producing small spirals used in the manufacture of sewing machiens. (An illustration of the original universal milling machine is shown on page 6.)
The cuts on pages 10 and 44 are representative of modern universal milling machines. This style of machine is essentially the same in construction as the plain milling machine, and the table has the same movements. But, in addition, the table swivels upon the saddle and can be set at an angle to the spindle in a horizontal plane. Also, it is fitted with a mechanism known as a spiral head, for use in spiral milling and indexing to obtain any required spacing on the periphery of work. The introduction of the swivel renders the table a little less stable than that of the plain machine, though in common practice heavy cuts are taken. It is apparent, however, that the offices of the two machines are in a way distinc. A universal machine is the better for general shop purposes, but where continuous heavy milling of straight cuts is to be done the plain machine is preferable.
