Lathes have given rise to planers, now built of great strength and in highly complicated designs. In a lathe the object turns upon centers against a tool; a planer carries its tool in a revolving cylinder, the work being fed in a straight line. A shaper, with much the same essential construction, moves along its work, the wood or metal operated on remaining stationary. With a planer or a shaper the size and uniformity of the work depend upon the skill of the operator. The planer has led to the invention of a machine which dispenses with this skill. Bramah, in 1811, employed a revolving cutter to plane iron, adapting to metal the familiar mechanism for planing wood. This was the beginning of the milling machine, now so remarkably developed and improved. A skilled mechanic sets the machine and the chucks which hold the work; an unskilled hand can continue the operations, his products being uniformly of the dimensions and forms desired. Intricate shapes are easily executed, quite impracticable on any other machine. At first the revolving mechanism and its cutters were a single piece of metal; to-day cutters of costly quality are inserted in cheap metal; these inserted cutters when worn out are easily replaced. [Illustration: Iron planer; a, b, c, d, fixed cutting tools; M, moving bed. Niles-Bement-Pond Co., New York.] [Illustration: Iron shaper: a, b, fixed cutting tools. K, M, traveling bars. Niles-Bement-Pond Co., New York.] [Illustration: Milling machine, R. K. Le Blond Machine Tool Co., Cincinnati. A, table; B, overhanging arm; C, cutters; D, spindle; E, feed box.] In many cases the milling machine ousts the planer as much more economical. At the shops of the Taylor Signal Company, Buffalo, a miller of the Cincinnati Milling Machine Company does nine-fold as much work as a planer. It takes a first cut 1/8 inch deep across a full width of 12 inches, makes 60 revolutions per minute, feeds .075 inch per turn, giving a table travel of 4-1/2 inches per minute, with an accuracy limit of .001 inch. [Illustration: Milling cutters with inserted teeth. Cincinnati Milling Machine Co.]