How to Choose the Down Milling and Up Milling in CNC Milling?

In CNC milling, how to choose the down milling and up milling? What's the difference between the two types? Let's take a look at it.

What are down milling and up milling

The proper selection of down milling and up milling can effectively improve the surface processing quality. Only by distinguishing the down milling and up milling can we reasonably select the right and left tool radius compensation commands G41 and G42.

Down milling and up milling are related to the rotation direction and feed direction of the milling cutter. When the rotation direction of the milling cutter is the same as the feed direction of the workpiece, it is called down milling. When the rotation direction of the milling cutter is opposite to the feed direction of the workpiece, it is called up milling. In the practical learning and application, most people regard down milling and up milling as the forward and reverse rotation of the spindle on the panel of CNC milling machine, that is, down milling is realized by forward rotation of the spindle, and up milling is realized by reverse rotation of the spindle.

In fact, the forward and reverse rotation of the spindle are affected by the tool rotation direction (not directly related to down milling and up milling). In order to fully understand down milling and up milling, it is necessary to know the rotation direction and feed direction first.

1. Rotation direction of milling cutter

The forward and reverse rotation of the spindle can be realized by commands or buttons on the CNC milling machine. The forward and reverse rotation of the spindle are limited by the direction of rotation of the milling cutter.

For general end mills, most of them are right-handed, so the spindle can only rotate forward; for left-handed thread cutters and right-handed thread cutters, the spindle can rotate backward and forward.

2. Feed direction of milling cutter

Because the milling cutter is rotating, the feed direction must be defined in clockwise and counterclockwise. If the tool is on the left side of the work piece, the feed direction is called clockwise. Conversely, when the tool is on the right side of the work piece, the feed direction is defined as counterclockwise.

For the same tool, the same path, the milling method may be different. If 20 right-handed end mill is used to process a profile with length, width and height of 80605 and a groove with length, width and depth of 80605, the milling method is different when its feed direction is the same and cutter compensation direction is different. When machining the profile, the cutter is on the left side of the workpiece, the feed direction is clockwise, and the spindle of the right-hand end mill can only rotate forward, so it is called down milling. When machining the groove, the cutter is on the right side of the workpiece, the feed direction is counterclockwise, and the rotation direction of the milling cutter is opposite to the feed direction, so it is called up milling.

Difference between and selection of down milling and up milling 

The down milling workpiece is under pressure, which is of good finish; while the up milling workpiece is under tension, which is easy overcut.

There is a tendency to let the cutter pass in down milling and a tendency to gnaw deeply in up milling. If there is a corner inside, it is better to use down milling.

There are different CNC milling applications. When you need to process some soft materials, such as red copper, it is best to use the up milling because the finish of up milling will be much better than that of the down milling. 

Because there is a gap between the feed screw threads of the worktable, the cutting force of the milling cutter will make the workpiece move forward in down milling. However, compared with up milling, down milling can reduce the tool wear, improve the surface finish and ensure the dimensional accuracy.

When the worktable is hydraulically driven, there is no hard skin on the surface of the workpiece blank, and the process system should have enough rigidity. In order to reduce the cutting deformation, cutting force and power consumption, it is necessary to use the milling along the cutting line as far as possible, especially for the materials difficult to be processed. On the contrary, when forging blank there is a hard layer and slag accumulation on the cutting surface, and an uneven surface of the workpiece, up milling should be used.

Relationship between milling mode and cutter compensation instruction in CNC Milling

For ordinary milling machine, due to the poor self-locking of the lead screw, the clearance between the lead screw and the nut can not be eliminated. The down milling will cause gnawing or even striking, so it is suitable to use the up milling instead of the down milling. While the CNC milling machine has good self-locking, it can use the down milling, and the finish of the down milling is good.

CNC milling machine processing generally based on ordinary milling machine processing, can make precision CNC milling and will not encounter the phenomenon of hard surface of workpiece. In the CNC milling machine processing, it generally adopts the way of down milling.

For example, if you select different cutter compensation instructions to program rectangular groove, the programming origin is the geometric center of the surface of the rectangular groove, using G41 and G40 to program, that is to carry out down milling.

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