Up-milling and down-milling are two common milling phenomena in CNC machining. Many people don't understand the difference between them. Today's article will discuss the difference between up-milling and down-milling.
The cutting edge of the milling cutter is subjected to a shock load every time it makes a cut. For successful milling, consideration must be given to the correct contact pattern between the cutting edge and the material as it plunges in and cuts off in one cut. During milling, the workpiece is fed in the same or opposite direction along the direction of milling cutter rotation, which will affect the feed of the cut, cut and whether it is run out or down milled
The Golden Rule of Grinding – From Thick to Thin
When milling, the formation of the cut must be considered. The decisive factor for cutting forming is the position of the milling cutter. It is required to form thick chips when the cutting edge cuts in, and form thin chips when the cutting edge cuts in, so as to ensure the stability of the milling process. Always remember the golden rule of milling "thick to thin" to ensure that the blade cuts with as little chip thickness as possible.
In upward milling, the tool feeds in the direction of rotation. Up-milling is always the preferred method whenever the machine tool, fixture and workpiece allow. When milling on the upper edge, the chip thickness gradually decreases from the beginning of the cut and finally reaches zero at the end of the cut. In this way, the cutting edge can avoid scratching and rubbing the surface of the part before participating in the cutting.
A large chip thickness is advantageous, and cutting forces tend to pull the workpiece into the milling cutter to keep the cutting edge in cut. However, since the milling cutter is easily pulled into the workpiece, the CNC machine needs to eliminate the backlash to deal with the feed clearance of the table. If the milling cutter is pulled into the workpiece, the feed rate increases unexpectedly, which can lead to excessive chip thickness and edge breakage. Back milling should be considered at this time.
When milling down, the feed direction of the tool is opposite to the direction of rotation of the tool. Chip thickness increases gradually until the end of the cut. The cutting edge must cut hard, scratch or polish due to friction caused by the front cutting edge, high temperature and frequent contact with the work-hardened surface. It will shorten the service life of CNC tools.
Thick chips and high temperatures generated by the cutting edge cause high tensile stress, shorten tool life and often quickly damage the cutting edge. It can also cause splinters to stick or weld to the cut edge, which then carries them to the start of the next cut, or cause the cut edge to collapse momentarily.
Cutting forces tend to push the cutter and workpiece away from each other, while radial forces tend to lift the workpiece off the table. When the machining allowance changes greatly, down milling is better. Down milling is also used when machining superalloys with ceramic inserts, as ceramics are sensitive to impact when cutting the workpiece.
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