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Should CNC Machining Choose Down Milling or Up Milling?

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.

1. 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.

2. 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 finally reach zero at the end of cutting, which prevents the cutting edge from scratching and rubbing against the surface of the part before participating in the cutting.

3. 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. 

4. Workpiece fixture

The feed direction of the tool puts forward different requirements on the workpiece fixture. During up-milling, it should be able to resist lifting force. During down milling, it should be able to resist downforce.

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