Milling cutter cutting edges are subjected to shock loads with each plunge. For successful milling, consideration must be given to the correct contact pattern between the cutting edge and the material during entry and exit of a cut. In a milling operation, the workpiece is fed in the same or opposite direction to the milling cutter's rotation, which affects the milling entry, exit, and whether the milling method is climb or up milling.
01 The golden rule of milling - from thick to thin
When milling, it is important to consider chip formation. The determining factor for chip formation is the position of the milling cutter and it is important to aim for thick chips on entry and thin chips on exit to ensure a stable milling process. Remember the golden rule of milling "thick to thin" to ensure that the edge cuts out with the smallest possible chip thickness.
02 Climb milling
In climb milling, the cutting tool is fed in the direction of rotation. Climb milling is always the preferred method whenever the machine tool, fixture and workpiece allow it.
In edge climb milling, the chip thickness will gradually decrease from the beginning of the cut and eventually reach zero at the end of the cut. This prevents the cutting edge from scratching and rubbing against the surface of the part before engaging in cutting.
Large chip thicknesses are advantageous, and cutting forces tend to pull the workpiece into the milling cutter, keeping the cutting edge in cut. However, since the milling cutter is prone to being pulled into the workpiece, the machine tool needs to handle table feed clearance by eliminating backlash. If the milling cutter is pulled into the workpiece, the feed will increase unexpectedly, potentially causing excessive chip thickness and cutting edge breakage. In these cases, consider up milling.
03 Up milling
In up milling, the cutting tool is fed in the opposite direction of its rotation.
Chip thickness increases gradually from zero until the end of the cut. The cutting edge must be forced in, creating a scratch or polish effect due to friction, high temperature, and frequent contact with the work-hardened surface created by the leading cutting edge. All of this reduces tool life.
The thick chips and higher temperatures produced when the cutting edge cuts out will result in high tensile stresses, which will shorten tool life and often result in rapid failure of the cutting edge. It can also cause the chip to stick or weld to the cutting edge, which then carries it to the start of the next cut, or cause the cutting edge to chip away momentarily.
Cutting forces tend to push the cutter and workpiece away from each other, while radial forces tend to lift the workpiece from the table.
Up milling can be beneficial when machining allowances vary widely. Up milling is also recommended when machining superalloys with ceramic inserts, as ceramics are more sensitive to impacts when cutting into the workpiece.
04 Work Fixture
The feed direction of the tool places different demands on the workpiece holder. It should be able to resist the lifting force during up milling. During climb milling it should be able to resist downforce.
