Cutter position

• Down-milling is always preferred wherever the machine tool, fixture and workpiece will allow.
• In peripheral down-milling, the chip thickness will decrease from the start of cut, gradually reaching zero at the end of cut. This prevents the edge from rubbing and burnishing against the surface before engaging in the cut.
• The large chip thickness is advantageous, and the cutting forces tend to pull the workpiece into the cutter, holding the cutting edge in the cut.

Exceptions, when up-milling is preferred:

• However, as the cutter tends to be pulled into the workpiece, the machine needs to handle the table-feed play using back-lash elimination.
• If the tool pulls into the workpiece, feed is unintentionally increased which can lead to excessive chip thickness and edge breaking.
• Up-milling may be advantageous when large variations in working allowance occur.

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

• The chip thickness starts at zero and increases toward the end of the cut. Cutting forces tend to push the cutter and workpiece away from each other.
• High tensile stresses, caused when the edge is leaving the workpiece, will often result in rapid edge failure.
• The cutting edge has to be forced into the cut, creating a rubbing or burnishing effect due to friction, high temperatures and, often times, contact with a work-hardened surface caused by the preceding edge. All this reduces the tool life.
• Forces, mainly radial, will tend to lift the workpiece from the table.
• Thick chips at the exit from the cut will reduce tool life.
• The large thickness and higher temperature at the exit will sometimes cause chips to stick or weld to the cutting edge, which will then carry them around to the start of the next cut, or cause momentary edge frittering.