In CNC machining, the direction of rotation of the milling cutter is generally unchanged, but the feed direction is changed. This leads to two common phenomena in milling: down milling and down milling.
The cutting edge of the milling cutter is subjected to impact loads every time it cuts in. For successful milling, it is necessary to consider the correct contact between the cutting edge and the material when the cutting edge cuts in and out in one cut. In the milling process, the workpiece is fed in the same or opposite direction as the direction of rotation of the milling cutter, which affects the milling cut-in, cut-out, and whether down milling or down milling is used.
01 The golden rule of milling – from thick to thin
When milling, always consider chip formation. The determining factor of chip formation is the position of the milling cutter. Always 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. Keep in mind the golden rule of milling “from thick to thin” to ensure that the chip thickness is as small as possible when the cutting edge cuts out.
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, fixture, and workpiece allow.
In edge Climb Milling, the chip thickness decreases from the beginning of the cut to zero at the end of the cut. This prevents the cutting edge from scraping and rubbing against the part surface before it engages in the cut.
Large chip thickness is advantageous, as the cutting forces tend to pull the workpiece into the cutter, keeping the cutting edge in the cut. However, since the cutter tends to be pulled into the workpiece, the machine tool needs to handle the table feed clearance by eliminating backlash. If the cutter is pulled into the workpiece, the feed will increase unexpectedly, which may result in excessive chip thickness and broken cutting edges. In these cases, consider reverse milling.
03 Climb Milling
In reverse milling, the cutting tool is fed in the opposite direction of its rotation.
The chip thickness starts at zero and increases gradually until the end of the cut. The cutting edge must be forced into the cut, which causes a scratching or polishing effect due to friction, high temperatures and constant contact with the work-hardened surface caused by the previous cutting edge. All of this shortens tool life.
The thick chips and high temperatures generated when the cutting edge cuts out will cause high tensile stresses, which will shorten tool life and the cutting edge will usually be damaged quickly. It may 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 break instantly.
Cutting forces tend to push the milling cutter and workpiece away from each other, while radial forces tend to lift the workpiece from the worktable.
When there are large changes in machining allowance, reverse milling may be beneficial. When machining high-temperature alloys with ceramic inserts, reverse milling is also recommended, because ceramics are sensitive to the impact generated when cutting into the workpiece.
04 Workpiece clamping
The feed direction of the tool places different requirements on the workpiece clamping. During reverse milling, it should be able to resist lifting forces. During down milling, it should be able to resist downward forces.
