Up milling versus conventional milling: which is better for CNC machining?

In CNC machining, the rotation direction of the cutter is generally constant, but the feed direction is changing.

Therefore, there are two common phenomena in milling: up milling and conventional milling.

The cutting edge of the cutter is subjected to an impact load with each cut.

To successfully complete milling, it is important to consider the correct contact between the cutting edge and the material in a single cut, as well as the cutting edge in the cut.

During the milling process, the part is fed in the same or opposite direction as the rotation of the cutter, which affects the way in and out milling is done and whether climb milling or conventional milling is used.

01 The Golden Rule of Milling – From Coarse to Fine

When milling, it is always necessary to consider chip formation.

The determining factor for chip formation is the position of the cutter, so it is necessary to try to form thick chips when the blade cuts and thin chips when the blade cuts to ensure a stable milling process.

It is necessary to remember the golden rule of milling, “from coarse to fine”, to ensure the smallest possible chip thickness when the cutting edge leaves the cutter.

02 Up Milling

In climb milling, the cutting tool is fed in the direction of rotation.

As long as the machine, fixture and part allow, climb milling is always the preferred method.

In edge up milling, the chip thickness decreases from the beginning of the cut and eventually reaches zero at the end of the cut.

This prevents the cutting edge from scratching and rubbing the surface of the workpiece before participating in cutting.

The high chip thickness is advantageous because the cutting force tends to pull the workpiece into the cutter, keeping the cutting edge in the cut.

However, because the cutter is easily pulled into the workpiece, the machine tool has to deal with the feed play of the table, eliminating the backlash.

If the cutter is pulled into the workpiece, the feed will increase unexpectedly, which may cause excessive chip thickness and cracked cutting edges.

In these cases, conventional milling is considered to be used.

03 Conventional Milling

In conventional milling, the cutting tool is fed in the opposite direction to its rotation.

The chip thickness gradually increases from zero until the end of the cut.

The cutting edge must be forced, resulting in a rubbing or polishing effect due to friction, high temperatures and constant contact with the hardened surface caused by the front cutting edge.

All this will reduce the tool life.

The thicker chips and higher temperatures caused by cutting edge cutting result in high tensile stresses, which shorten tool life and often lead to rapid damage to the cutting edge.

It can also cause chips to stick or weld to the cutting edge, which can carry them to the start of the next cut or cause the cutting edge to collapse instantly.

The cutting force tends to move the cutter and workpiece apart, while the radial force tends to lift the workpiece off the table.

When the machining margin changes significantly, conventional milling may be more advantageous.

Conventional milling is also recommended when machining high-temperature alloys with ceramic inserts, as ceramics are sensitive to the impact of cutting on the workpiece.

04 Workpiece F configuration

The tool feed direction presents different requirements for clamping the part.

During conventional milling, it must be able to resist lifting forces.

In conventional milling, it must be able to resist a downward force.

05 Comparative Table of Climb Milling vs. Conventional Milling