【正文】 the two cases. In up milling the c hip is very thin at the beginning, where the tooth first contacts the work， and increases in thickness, being a maximum where the tooth leaves the work. The cutter tends to push the work along and lift it upward from the table. This action tends to eliminate any effect of looseness in the feed screw and nut of the milling machine table and results in a smooth cut. However, the action also tends to loosen the work from the clamping device so that greater clamping forcers must be employed. In addition, the smoothness of the generated surface depends greatly on the sharpness of the cutting edges. In down milling， maximum chip thickness occurs close to the point at which the tooth contacts the work. Because the relative motion tends to pull the workpiece into the cutter， all possibility of looseness in the table feed screw must be eliminated if down milling is to be used. It should never be attempted on machines that are not designed for this type of milling. In as mush as the material yields in approximately a tangential direction at the end of the tooth engagement， there is much less tendency for the machined surface to show tooth marks than when up milling is used. Another consider able advantage of down milling is that the cutting force tends to hold the work against the machine table， permitting lower clamping force to be employed. This is particularly advantageous when milling thin workpiece or when taking heavy cuts. Sometimes a disadvantage of down milling is that the cutter teeth strike against the surface of the work at the beginning of each chip. When the workpiece has a hard surface， such as castings do， this may cause the teeth to dull rapidly. Milling Cutters. Milling cutters can be classified several ways. One method is to group them into two broad classes， based on tooth relief， as follows： 1. Profilecutters have relief provided on each tooth by grinding a small land back of the cutting edge. The cutting edge may be straight or curved. form or camrelieved cutters the cross section of each tooth is an eccentric curve behind the cutting edge， thus providing relief. All sections of the eccentric relief，parallel with the cutting edge， must have the same contour as the cutting edge. Cutters of this type are sharpened by grinding only the face of the teeth， with the contour of the cutting edge thus remaining unchanged. Another useful method of classification is according to the method of mounting the cutter. Arbor cutters are those that have a center hole so they can be mounted on an arbor. Shank cutters have either tapered or straight integral shank. Those with tapered shanks can be mounted directly in the milling machine spindle， whereas straightshank cutters are held in a chuck. Facing cutters usually are bolted to the end of a stub of Milling Cutters. Plain milling cutters are cylindrical or diskshaped， having straight or helical teeth on the periphery. They are used for milling flat surfaces. This type of operation is called plain or slab milling. Each tooth in a helical cutter engages the work gradually， and usually more than one tooth cuts at a given time. This reduces shock and chattering tendencies and promotes a smoother surface. Consequently， this type of cutter usually is preferred over one with straight teeth. Side milling cutters are similar to plain milling cutters except that the teeth extend radially part way across one or both ends of the cylinder toward the center. The teeth may be either straight or helical. Frequently these cutters are relatively narrow， being disklike in shape. Two or more side milling cutters often are spaced on an arbor to make simultaneous， parallel cuts， in an operation called straddle milling. Interlocking slotting cutters consist of two cutters similar to side mills， but made to operate as a unit for milling slots. The two cutters are adjusted to the desired width by inserting shims between them. Staggeredtooth milling cutters are narrow cylindrical cutters having staggered teeth