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Should CNC milling or down milling be selected?
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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 processing: down milling and up milling.The cutting edge of the milling cutter is subjected to an impact load every time it is cut. In order to successfully mill, it is necessary to consider the correct contact between the cutting edge and the material when the cutting edge is cut in one cut and when it is cut out. In the milling process, the workpiece is fed in the same or opposite direction as the direction of rotation of the milling cutter, which affects the milling cut-in and cut-out and whether the method of down-milling or up-milling is used.
One. The golden rule of milling-from thick to thin
When milling, be sure to consider the formation of chips. The decisive factor for chip formation is the position of the milling cutter. It is important to strive to form thick chips when the blade is cut in and thin chips when the blade is cut out to ensure a stable milling process. Remember the golden rule of milling "from thick to thin" to ensure that the thickness of the chip when the blade is cut out is as small as possible.
Second, down milling
In down milling, the cutting tool feeds in the direction of rotation. As long as machine tools, fixtures and workpieces allow, down milling is always the preferred method. In edge 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 rubbing and rubbing the part surface before participating in the cutting.
Large chip thickness is advantageous, and the cutting force tends to pull the workpiece into the milling cutter to keep the cutting edge cutting. However, because the milling cutter is easily pulled into the workpiece, the machine tool needs to handle the table feed clearance by eliminating backlash. If the cutter is pulled into the workpiece, the feed will increase unexpectedly, which may result in excessive chip thickness and cutting edge rupture. In these cases, consider using up milling.
Three, reverse milling
In reverse milling, the cutting tool feed direction is opposite to its rotation direction.
The chip thickness gradually increases from zero until the end of cutting. The cutting edge must be forcibly cut in, resulting in rubbing or polishing effects due to friction, high temperature, and frequent contact with the work hardened surface caused by the preceding cutting edge. All this will shorten the tool life.
The thick chips and high temperature generated when the blade is cut will result in high tensile stress, which will shorten the tool life, and the cutting edge will usually be damaged quickly. It may also cause the chips to stick to or weld to the cutting edge, which will then carry it to the beginning of the next cut, or cause the cutting edge to collapse 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-cutting may be more advantageous. When machining high-temperature alloys with ceramic inserts, up-milling is also recommended because ceramics are more sensitive to the impact of cutting into the workpiece.
Four. Workpiece fixture
The feed direction of the tool puts different requirements on the workpiece fixture. During the up-milling process, it should be able to resist lifting forces. During down milling, it should be able to resist downforce.
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