What causes built-up edge?

Built-up edge generally refers to the triangular cross-section that appears at the tip of the tool when processing medium carbon steel. The small piece of metal drill appendage has a hardness two to three times that of the workpiece material. There are many factors that affect the built-up edge, mainly the workpiece material, cutting speed, etc. Most people are not particularly clear about the causes of built-up edge. So what are the causes of built-up edge? The following are the causes of chip accumulation.

During the cutting process, the main factors affecting built-up edge are as follows:

Because built-up edge is produced under conditions of great pressure, strong friction and severe metal deformation. Therefore, cutting conditions will inevitably affect the generation, growth and disappearance of built-up edge through these effects.

1. Workpiece material. When the hardness of the workpiece material is low and the plasticity is high, the metal deformation during the cutting process is large, and the friction coefficient (greater than 1) and the contact zone length between the chip and the front cutting edge are relatively large. Under such conditions, built-up edge is likely to occur. When the workpiece has low plasticity and high hardness, the possibility of built-up edge and the height of built-up edge also decrease, such as hardened steel. Cutting brittle materials is less likely to produce built-up edge.

2. Tool rake angle. Increasing the tool rake angle can reduce chip deformation, friction between the chip and the front cutting edge, cutting force and cutting heat, and can inhibit the formation of built-up edge or reduce the height of built-up edge. According to relevant information, when the tool rake angle γ0 ≥ 40, the possibility of built-up edge is small.

3. Cutting speed. Cutting speed mainly affects built-up edge through cutting temperature and friction coefficient. When the tool does not have a negative chamfer, no built-up edge will be produced under extremely low cutting speed conditions. Taking medium carbon steel as an example, when the cutting speed Vc<2m/min, no built-up edge will be generated. When Vc>2m/min～30m/min, the built-up edge grows to its maximum from production. That is to say, when the cutting temperature is around 300°C, the friction coefficient between the chip and the tool is the largest, and the built-up edge reaches the highest height. As the cutting speed and corresponding cutting temperature increase, the height of the built-up edge gradually decreases. During high-speed cutting (Vc> 120m/min), due to the high cutting temperature (above 800℃), the sliding resistance and friction coefficient of the chip bottom layer are significantly reduced, and the built-up edge will also be eliminated. Therefore, in our daily finishing, in order to achieve a lower machined surface roughness, we use high-speed cutting within the allowable range of tool heat resistance, or low-speed cutting (Vc<5m/min) to prevent the formation of built-up edge and improve the quality of the machined surface.

4. Cutting thickness. When cutting plastic materials, the cutting force and the length of the contact area between the chip and the front cutting edge will increase with the increase of cutting thickness, which will increase the possibility of forming built-up edge. Therefore, during finishing, in addition to selecting a larger tool rake angle and cutting within the cutting speed range that avoids the formation of built-up edge, the feed rate or tool main deflection angle should be reduced to reduce the cutting thickness.