The cutting edge of the milling cutter is subjected to impact loads every time it cuts in. For successful milling, the correct contact between the cutting edge and the material during the cut must be considered. During the milling operation, the workpiece is fed in the same or opposite direction as the milling cutter's rotation, which affects the milling cut-in, cut-out, and whether the milling method is down milling or up milling.
1. The golden rule of milling - from thick to thin
When
milling, always consider chip formation. The determining factor for chip formation is the position of the milling cutter, and always aim for thick chips when the cutter cuts in and thin chips when the cutter cuts out to ensure a stable milling process. Remember the golden rule of milling, "thick to thin", to ensure that the chip thickness is as small as possible when the cutter cuts out.
2. Down milling
In climb milling, the cutting tool is fed in the direction of rotation. Whenever the machine tool, fixture and workpiece allow, climb milling is always the preferred method.
In edge milling, the chip thickness gradually decreases from the beginning of the cut to zero at the end of the cut. This prevents the cutting edge from scratching and rubbing the part surface before it engages in cutting.
Large chip thickness is advantageous, as cutting forces tend to pull the workpiece into the cutter, keeping the cutting edge in the cut. However, because the cutter tends to be pulled into the workpiece, the machine tool needs to handle table feed clearance by eliminating backlash. If the cutter is pulled into the workpiece, the feed will increase unexpectedly, which can lead to excessive chip thickness and broken cutting edges. In these cases, consider reverse milling.
3. Up milling
In up-cut milling, the cutting tool is fed in the opposite direction to its rotation.
Chip thickness starts at zero and gradually increases toward the end of the cut. The cutting edge must be forced into the cut, creating a scratching or polishing effect due to friction, high temperatures, and constant contact with the work-hardened surface created by the previous cutting edge. All of this shortens tool life.
Thick chips and high temperatures when the cutting edge cuts out will cause high tensile stresses, which will shorten tool life and the cutting edge will usually fail quickly. It may also cause the chip to stick or weld to the cutting edge, which will then carry it to the start of the next cut, or cause the cutting edge to break instantly. Cutting forces tend to push the milling cutter and workpiece away from each other, while radial forces tend to lift the workpiece from the worktable.
When there are large changes in machining allowance, up-cut milling may be beneficial. Up-cut milling is also recommended when CNC machining high-temperature alloys with ceramic inserts, as ceramics are sensitive to the impact of cutting into the workpiece.
4. Workpiece fixture
The feed direction of the tool places different demands on the workpiece fixture. During up-milling, it should be able to resist the lifting force. During down-milling, it should be able to resist the down force.
