In CNC machining, the rotation direction of the milling cutter is generally unchanged, but the feed direction is changed. There are two common phenomena in milling: down milling and up milling.

The cutting edge of the milling cutter is subjected to an impact load every time it cuts. In order to successfully mill, you must consider the correct contact between the cutting edge and the material when cutting in and cutting out. In the milling process, the workpiece is fed in the same or opposite direction as the rotation direction of the milling cutter, which will affect the cut-in, cut-out of milling, and whether to use down milling or up-cut milling.

01 The golden rule of milling-when milling from thick to thin, the formation of chips must be considered. The decisive factor for chip formation is the position of the milling cutter. We must strive to form thick chips when the blade cuts in, and thin chips when the blade cuts out to ensure a stable milling process. Remember the golden rule of milling "from thick to thin" to ensure that the chip thickness is as small as possible when the blade is cut out. 02 Down milling In down milling, the cutting tool feeds in the direction of rotation. As long as the machine tool, fixture and workpiece permit, down milling is always the preferred method. In edge down milling, the chip thickness will gradually decrease from the beginning of the cutting, and eventually reach zero at the end of the cutting. This prevents the cutting edge from scratching and rubbing against the surface of the part before participating in the cutting.

A large chip thickness is advantageous, as the cutting force tends to pull the workpiece into the milling cutter, keeping the cutting edge cutting. However, because the milling cutter is easily pulled into the workpiece, the machine tool needs to deal with the table feed gap by eliminating backlash. If the milling cutter is pulled into the workpiece, the feed will increase unexpectedly, which may result in excessive chip thickness and fracture of the cutting edge. In these cases, consider using up milling. 03 Up-milling In up-milling, the feed direction of the cutting tool is opposite to the direction of rotation. The chip thickness gradually increases from zero until the end of the cutting. The cutting edge must be forcibly cut in to produce a scratching or polishing effect due to friction, high temperature, and frequent contact with the work-hardened surface caused by the front cutting edge. All this will shorten the tool life.

The thick chips and higher temperature generated when the cutting edge cuts out will cause high tensile stress, which will shorten the tool life, and the cutting edge will usually be damaged quickly because of this. It may also cause the chips to stick or weld to the cutting edge, which will then carry it to the starting position of the next cut, or cause the cutting edge to break instantaneously.

The cutting force tends to push the milling cutter and the workpiece away from each other, and the radial force tends to lift the workpiece from the table. When the machining allowance changes significantly, up-milling may be more advantageous. When using ceramic inserts to process high-temperature alloys, it is also recommended to use up-cut milling, because ceramics are more sensitive to the impact generated when cutting into the workpiece. 04 The feed direction of the workpiece fixture tool puts forward different requirements on the workpiece fixture. In the up-milling process, it should be able to resist the lifting force. During down milling, it should be able to resist downforce.

05 Comparison table of down milling and up milling