In CNC machining, there are various milling cutters, such as End Mill, Roughing End Mill, Finishing End Mill, Ball End Mill, and so on.The rotation direction of the milling cutter is generally constant, but the feed direction is variable. There are two common phenomena in milling processing: forward milling and backward milling.
The cutting edge of the milling cutter is subjected to impact loads every time it cuts in. In order to achieve successful milling, it is necessary to consider the correct contact between the cutting edge and the material during the cutting process and during the cutting process. In the milling process, the workpiece is fed in the same or opposite direction to the direction of rotation of the milling cutter, which affects the cutting in and out of the milling process, as well as whether to use forward or backward milling methods.
1. The Golden Rule of Milling – From Thick to Thin
When milling, it is important to consider the formation of chips. The decisive factor for chip formation is the position of the milling cutter, and it is important to 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 thickness of the chips when the cutting edge is cut is as small as possible.
2. Forward milling
In forward milling, the cutting tool is fed in the direction of rotation. As long as the machine tool, fixture, and workpiece allow, forward milling is always the preferred method.
In edge milling, the chip thickness will gradually decrease from the beginning of cutting to zero at the end of cutting. This can prevent the cutting edge from scratching and rubbing the surface of the part before participating in 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, due to the ease of the milling cutter being pulled into the workpiece, the machine tool needs to handle the feed gap of the workbench by eliminating backlash. If the milling cutter is pulled into the workpiece, the feed will unexpectedly increase, which may lead to excessive chip thickness and cutting edge fracture. In these cases, consider using reverse milling.
3. Reverse milling
In reverse milling, the feed direction of the cutting tool is opposite to its rotation direction.
The chip thickness gradually increases from zero until the end of cutting. The cutting edge must be forced to cut in, so as to produce scratching or polishing effect due to friction, high temperature and frequent contact with the work hardening surface caused by the front cutting edge. All of this will shorten the tool life.
The thick chips and high temperatures generated during the cutting of the cutting edge will lead to high tensile stress, which will shorten the tool life and usually result in rapid damage to the cutting edge. It may also cause chips to stick or weld to the cutting edge, which will then carry them to the starting position of the next cutting, or cause the cutting edge to instantly shatter.
The cutting force tends to push the milling cutter away from the workpiece, while the radial force tends to lift the workpiece off the workbench.
When there is a significant change in machining allowance, reverse milling may be more advantageous. When using ceramic inserts to process superalloys, it is also recommended to use reverse milling, because ceramics are sensitive to the impact generated when cutting into the workpiece.
4. Workpiece fixture
The feed direction of the cutting tool has different requirements for the workpiece fixture. During the reverse milling process, it should be able to resist lifting forces. During the milling process, it should be able to resist downward pressure.
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Post time: Jun-08-2023