Up-milling vs Down-milling, Which One to Choose?

Milling is a machining process using rotary cutters to remove material from the workpiece. It has high machining productivity, it is used for a variety of complicated cutting operations, such as slot cutting, threading, drilling, and so on.The machining accuracy can reach to IT9-IT7 generally, and the surface roughness Ra value can be 12.5-1.6um normally.

In the milling machining process, based on the difference between the milling cutter rotation direction and the workpiece feed direction, is generally defined as down-milling and up-milling.

Down-milling, also known as climb milling, is a machining process, when the cutter cuts into the workpiece, the direction of cutting speed is the same as the feed direction of the workpiece. 

Up-milling, also known as conventional milling, stands for another milling machining process, when the milling cutter cuts into the workpiece, the cutter rotation direction at the point of engagement is opposite to the workpiece feed direction.

Down milling and up milling process

What is the difference between up-milling and down-milling? Here to show you the details.

1)Differences on chip thickness, heat generated, rubbing effect, and work-hardening:

  In down-milling, the chip thickness gradually decreases from the maximum at the start of the cut to zero, the cutting tool is easy to cut into the workpiece. Avoid the cutting edge rubbing against the surface before engaging in the cut. Thus less heat was generated, and minimal work-hardening.

  In up-milling, the uncut chip thickness gradually increases from zero to maximum at the point of disengagement. The cutting edge will rub with the machined surface before starting a real cutting process, it will lead to high temperature and work hardening on the machined surface. Also, the thick chips at the end of the cut will cause high tensile stresses which will result in rapid cutting-edge failure and shorter tool life. Also, the chips may stick or be welded to the cutting edge, which will be carried to the start of the next cut. 

2). Differences on cutting force, backlash, and machining stablility.

  In down milling, the vertical cutting force is downward, which will help to press the workpiece tighter, and the horizontal cutting force is in the same direction as the workpiece feed direction, it tends to pull the workpiece into the cutter, and there is the clearance between the lead screws and nuts. If the machine is without a backlash eliminator, the feed rate is increased inadvertently, the working table will vibrate, and it will lead to excessive chip thickness and cutting edge broken.

  In up milling, the vertical cutting force is upward, it tends to lift off the workpiece, in order to mount the workpiece firmly, a more rigid fixture is needed. And the horizontal cutting force is the opposite of the workpiece feed direction, it tends to push the cutter away from the workpiece, and ensure better engagement between the lead screws and nuts, so it is not required a backlash eliminator in this milling process. 

3). Difference on power consumption.

  In down milling, the average cutting thickness is bigger, and cutting deformation is smaller, so it consumes less power compared to up milling process, for example, if cutting the carbon steel, the power consumption of down milling is 5% lower than that of the up milling, but for the difficult-to-machine material, the saver can be up to 14%. 

How to choose the right milling method? Up-milling or Down- milling.

  If the following conditions can be met, it is better to use down milling.

1. The machine has a backlash eliminator, which can reliable eliminate the clearance between the lead screws and nuts.

2. No superficial hardened layer on the workpiece. 

   Down milling is also used for fine machining and cutting thin-wall parts as well as plastic and nylon parts. 

  Especially for milling the difficult-to-machine materials, using down milling can not only reduce cutting deformation but also reduce cutting force, as well as power consumption.

   However, up-milling will be considered for the below cases. 

1. If the milling machine doesn’t have the backlash eliminator.

2. The workpiece is with superficial hardened layer, like casting parts or forging parts, it is better to adopt up-milling because, in the down-milling process, the cutting edge starts the cutting from the workpiece surface which has a superficial hardened layer, it will easy to damage the cutting edge.

3. When large variations in working allowance, it is better to adopt up milling.

4. When using ceramic inserts in the heat-resistant alloy, up-milling is recommended, because the impact force may damage the ceramics cutter when cutting into the workpiece.

5. When using a solid carbide end mill for side milling (finishing), especially in hardened materials, up-milling is preferred. This makes it easier to obtain wall straightness with smallertolerances and better 90-degree angles.