Continuous Chip Formation and Built-up Edge

In metal cutting operations like turning, milling, and drilling, the material being removed from the workpiece forms chips. The shape and characteristics of these chips depend on various factors, including the material properties, cutting conditions (speed, feed, depth of cut), tool geometry (rake angle, relief angle), and lubrication.

A continuous chip is typically formed when ductile materials are machined at high cutting speeds and relatively low feed rates. The material shears continuously ahead of the tool's cutting edge.

A built-up edge (BUE) is a common phenomenon during machining, especially at lower cutting speeds and higher feed rates. It is an unstable wedge of workpiece material that adheres to the rake face of the cutting tool near the cutting edge. This material gets welded onto the tool face due to the high pressure and heat generated during the cutting process, combined with friction between the chip and the tool.

Reasons for Built-up Edge Formation

Several factors contribute to the formation of a built-up edge. Let's analyze the given options:

High Cutting Speed

High cutting speeds generally increase the temperature at the tool-chip interface. However, high speeds also tend to reduce the contact time between the chip element and the tool face, allowing less time for welding to occur. Higher speeds often lead to smoother chip flow and can prevent or reduce the formation of a significant built-up edge, favoring a continuous chip without BUE.

Large Rake Angle with Low Feed Rate

A large positive rake angle reduces the cutting force and the shear angle, leading to thinner chips and less deformation. It also reduces the friction between the chip and the tool rake face. A low feed rate also contributes to thinner chips. These conditions typically promote smooth cutting and can result in continuous chips with minimal or no built-up edge.

High Friction between Tool & Chip

High friction at the tool-chip interface is a primary cause of built-up edge formation. High friction leads to:

• Increased heat generation: More energy is converted into heat due to sliding friction.
• Higher pressure: The contact area between the chip and the tool rake face is under intense pressure.
• Adhesion and Welding: The combination of high heat, high pressure, and close contact causes the workpiece material to adhere to the tool face and weld onto it, layer by layer, forming the built-up edge.

This built-up edge grows and breaks off cyclically, leading to variations in cutting forces and poor surface finish.

Sharp Cutting Edges

Sharp cutting edges reduce the initial cutting force and friction compared to dull edges. A sharp edge promotes smooth shearing of the material and reduces the tendency for the workpiece material to stick to the tool face. Therefore, sharp cutting edges typically minimize the formation of a built-up edge.

Conclusion on Built-up Edge

Based on the analysis, high friction between the tool and the chip is the most significant factor responsible for the formation of a built-up edge when machining ductile materials, which can occur even when forming a continuous chip under certain conditions (e.g., moderate speeds, high friction). The other options (high speed, large rake angle, sharp edge) tend to reduce friction and discourage BUE formation.