【正文】 e is provided by a lead screw. Whereas motion of the carriage when driven by the feedrod mechanism takes place through a friction clutch in which slippage is possible， motion through the lead screw is by a direct， mechanical connection between the apron and the lead screw. This is achieved by a split nut. By means of a clamping lever on the front of the apron，the split nut can be closed around the lead screw. With the split nut closed， the carriage is moved along the lead screw by direct drive without possibility of slippage. Modern lathes have a quickchange gear box. The input end of this gearbox is driven from the lathe spindle by means of suitable gearing. The out put end of the gear box is connected to the feed rod and lead screw. Thus， through this gear train，leading from the spindle to the quickchange gearbox， thence to the lead screw and feed rod， and then to the carriage， the cutting tool can be made to move a specific distance， either longitudinally or transversely， for each revolution of the spindle. A typical lathe provides， through the feed rod， fortyeight feeds ranging from inch inch per revolution of the spindle， and， through the lead screw， leads for cutting fortyeight different threads from to some older and some cheaper lathes， one or two gears in the gear train between the spindle and the change gear box must be changed in order to obtain a full range of threads and feeds. Milling is a basic machining process in which the surface is generated by the progressive formation and removal of chips of material from the workpiece as it is fed to a rotating cutter in a direction perpendicular to the axis of the cutter. .In some cases the workpiece is stationary and the cutter is fed to the work. In most instances a multipletooth cutter is used so that the metal removal rate is high， and frequently the desired surface is obtained in a single pass of the work. The tool used in milling is known as a milling cutter. It usually consists of a cylindrical body which rotates on its axis and contains equally spaced peripheral teeth that intermittently engage and cut the workpiece. In some cases the teeth extend part way across one or both ends of the cylinder. Because the milling principle provides rapid metal removal and can produce good surface finish， it is particularly wellsuited for massproduction work， and excellent milling machines have been developed for this purpose. However， very accurate and versatile milling machines of a generalpurpose nature also have been developed that are widely used in jobshop and tool and die work. A shop that is equipped with a milling machine and an engine lathe can machine almost any type of product of suitable size. Types of Milling Operations. Milling operations can be classified into two broad categories， each of which has several variations： peripheral milling a surface is generated by teeth located in the periphery of the cutter body； the surface is parallel with the axis of rotation of the cutter. Both flat and formed surfaces can be produced by this method. The cross section of the resulting surface corresponds to the axial contour of the cutter. This procedure often is called slab milling. 1. In face milling the generated flat surface is at right angles to the cutter axis and is the bined result of the actions of the portions of the teeth located on both the periphery and the with the face portions providing a finishing action. The basic concepts of peripheral and face milling are illustrated in Fig. Peripheral milling operations usually are performed on machines having horizontal spindles， whereas face milling is done on both horizontaland verticalspindle machines. Surface Generation in Milling. Surfaces can be generated in milling by two distinctly different methods depicted in Fig. Note that in up milling the cutter rotates against the direction of feed the workpiece， whereas in down milling the rotation is in the same direction as the feed .As shown in Fig., the method of chip formation is quite different in