During CNC machining, when steel, aluminum, or other ductile metals are cut at low or medium cutting speeds, a small mass of metal may adhere to the rake face of the cutting tool. This adhered metal is known as a built-up edge (BUE). But do you know how a built-up edge forms? And what effects does it have on the machining process?
Causes of Built-Up Edge (BUE)
When CNC machining ductile materials such as steel under conditions of relatively low cutting speed, where continuous chips are formed, a triangular wedge-shaped metal mass often adheres to the rake face near the cutting edge. This adhered metal mass is known as a built-up edge (BUE), as illustrated in the figure. The hardness of the BUE is extremely high, typically 2–3 times that of the workpiece material. When the BUE reaches a stable state, it can temporarily replace the cutting edge in the cutting action. The size of the BUE is commonly represented by its height, denoted as ( H_b ).
During cutting, severe friction occurs between the underside of the chip and the rake face of the tool. When the contact interface reaches a certain temperature under high pressure, the workpiece material adheres to the rake face through adhesion, also known as “cold welding.” As the continuously flowing chip passes over the adhered metal layer, if the temperature and pressure remain suitable, material at the bottom of the chip is also hindered. It adheres to the previously cold-welded layer on the rake face. As a result, the adhered layer gradually grows and eventually forms a built-up edge.
The formation and growth of BUE are closely related to the properties of the workpiece material, as well as the temperature and pressure distribution within the cutting zone. Materials with a stronger work-hardening tendency are more likely to generate BUE. When the cutting-zone temperature and pressure are too low, BUE does not form. Conversely, when the temperature is excessively high, the material softens and BUE formation is also suppressed. For carbon steel, the BUE height reaches its maximum when the cutting-zone temperature is approximately 300–350°C, while the BUE disappears automatically once the temperature exceeds 500°C.
When the depth of cut ( a_p ) and feed rate ( f ) remain constant, the BUE height ( H_b ) is closely related to the cutting speed ( v_c ), because the heat generated during cutting increases with increasing cutting speed. As shown in the figure:
- Region I: Low-speed zone where no BUE forms
- Region II: BUE height increases as ( v_c ) increases
- Region III: BUE height decreases as ( v_c ) increases
- Region IV: No BUE forms
Influences of Built-Up Edge
(1) Increase in Effective Rake Angle
The BUE adhered to the rake face effectively increases the actual rake angle of the tool (see Fig. ), thereby reducing cutting forces.
(2) Variation in Cutting Thickness
The front portion of the BUE extends beyond the cutting edge, increasing the actual cutting thickness by an increment of ( Δ h_D ). As the BUE grows, ( Δ h_D ) gradually increases. Once the BUE breaks away or detaches from the rake face, ( Δ h_D ) decreases rapidly. Since the formation, growth, and detachment of BUE occur cyclically, fluctuations in cutting thickness inevitably lead to variations in cutting force.
(3) Increase in Surface Roughness
The portion of the BUE extending beyond the cutting edge is irregular in both shape and height, which increases the surface roughness of the machined surface. In addition, detached fragments of the BUE may become embedded in the machined surface, further degrading surface quality.
(4) Influence on Tool Life
A stable BUE adhered to the rake face can temporarily replace the cutting edge during machining, thereby reducing direct tool wear and extending tool life. However, if the BUE repeatedly breaks away from the rake face, it may pull away particles of tool material from the rake face itself—particularly in carbide tools—resulting in accelerated wear and reduced tool life.
Measures to Prevent Built-Up Edge
The influence of BUE on the cutting process has both beneficial and detrimental aspects. During rough machining, where dimensional accuracy and surface finish requirements are relatively low, BUE may assist the cutting process by acting as a temporary cutting edge. However, during finish machining, BUE must be minimized or eliminated to ensure dimensional accuracy and surface integrity.
The following measures can be adopted to suppress BUE formation:
- Properly select the cutting speed to avoid the medium-speed range where BUE is most likely to form.
- Use cutting fluids with excellent lubricating performance to reduce friction between the underside of the chip and the tool rake face.
- Increase the tool rake angle to reduce the pressure between the rake face and the chip.
- Appropriately increase the hardness of the workpiece material to reduce its work-hardening tendency.
