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Should CNC machining choose down milling or up milling?

Posted by sere22 on August 7th, 2020

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: down milling and up milling.

The cutting edge of the milling cutter is subjected to an impact load every time it cuts. In order to successfully mill, you must consider the correct contact between the cutting edge and the material when cutting in and cutting out. In the milling process, the workpiece is fed in the same or opposite direction as the rotation direction of the milling cutter, which will affect the cutting in and out of milling, and whether to use down milling or up milling.

01 The golden rule of milling-from thick to thin

When milling, always consider the formation of chips. The decisive factor for chip formation is the position of the milling cutter. We must strive to form thick chips when the cutting edge cuts in and thin chips when the cutting edge cuts out to ensure a stable milling process. Remember the golden rule of milling "from thick to thin" to ensure that the chip thickness is as small as possible when the blade is cut.

02 Down milling

In down milling, the cutting tool feeds in the direction of rotation. As long as the machine tool, fixture and workpiece permit, 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 scratching and rubbing against the surface of the part before participating in the 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, since the milling cutter is easily pulled into the workpiece, the machine tool needs to deal with the table feed gap by eliminating backlash. If the milling cutter is pulled into the workpiece, the feed will increase unexpectedly, which may cause excessive chip thickness and cracking of the cutting edge. In these cases, consider using up milling.

03 Up milling

In up-milling, the feed direction of the cutting tool is opposite to the direction of rotation.

The chip thickness gradually increases from zero until the end of cutting. The cutting edge must be forcibly cut in to produce scratching or polishing effects due to friction, high temperature, and frequent contact with the work hardened surface caused by the front cutting edge. All this will shorten the tool life.

The thick chips and higher temperature generated when the blade is cut out will cause high tensile stress, which will shorten the tool life, and the cutting edge will usually be damaged quickly. It can also cause the chips to stick or weld to the cutting edge, which will then carry it to the starting position of the next cut, or cause the cutting edge to break 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 milling may be more advantageous. When using ceramic inserts to process high-temperature alloys, it is also recommended to use up milling, because ceramics are more sensitive to the impact generated when cutting into the workpiece.

Also See: Up Milling, Cutting Edge, Milling Cutter, Down Milling, Up, Milling, Edge

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