In CNC machining, the rotation direction of the milling cutter is generally constant, but the feed direction is changed. There are two common phenomena in milling: down milling and up milling.
The milling cutter’s cutting edge is subjected to an impact load each time it is plunged. In order to successfully mill, you must consider the correct way of contact between the cutting edge and the material during cutting in one cut.In the milling process, the workpiece is fed in the same or opposite direction as the rotation direction of the milling cutter, which affects the cutting in and out of the milling and whether the method is down milling or up milling.
When milling, be sure to consider chip formation. The decisive factor for chip formation is the position of the milling cutter. Be sure to strive to form thick chips when the cutting edge is cut in and thin chips when the cutting edge is cut out to ensure a stable milling process. Keep in mind that the golden rule of milling is “thick to thin” to ensure that the chip thickness is as small as possible when cutting out.
In down milling, the cutting tool is fed in the direction of rotation. As long as machine tools, fixtures and workpieces allow it, down milling is always the preferred method.
In edge down milling, the chip thickness will gradually decrease from the beginning of cutting and eventually reach zero at the end of cutting. This prevents the cutting edge from rubbing against the surface of the part before participating in cutting.
Large chip thicknesses are advantageous, as cutting forces tend 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 needs to handle the table feed gap by eliminating backlash. If the milling cutter is pulled into the workpiece, the feed will unexpectedly increase, which may result in excessive chip thickness and chipping of the cutting edge. In these cases, consider using up milling.
In up milling, the cutting tool feeds in the opposite direction to its direction of rotation.
The chip thickness gradually increases from zero until the end of the cutting. The cutting edge must be forcibly cut in, resulting in scratching or polishing effects due to friction, high temperatures, and frequent contact with the work-hardened surface caused by the front cutting edge. All this will shorten tool life.
Thick chips and higher temperatures generated during cutting out of the cutting edge will cause high tensile stresses, which will shorten the tool life and the cutting edge will usually be damaged quickly. It can also cause chips to stick or weld to the cutting edge, which can then carry it to the starting position for the next cut, or cause the cutting edge to momentarily chip.
Cutting forces tend to push the cutter and the workpiece away from each other, and radial forces tend 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, up-milling is also recommended, because ceramics are more sensitive to the impact generated when cutting into the workpiece.
Feed direction of the tool places different demands on the workpiece fixture. During up milling, it should resist lifting forces. During down milling, it should resist pulling forces.