Each time a milling edge enters a cut, it is subjected to a shock load. The right type of contact between edge and material at the entry and also at the exit of a cut must be considered for successful milling.
In down-milling (climb milling), the cutting tool is fed with the direction of rotation.
- 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.
Note: When using ceramic inserts in heat resistant alloys, up-milling is recommended, because ceramics are sensitive to impact at workpiece entry.
Feed direction of the tool places different demands on the workpiece fixture. During up-milling, it should resist lifting forces. During down-milling, it should resist pulling forces.