Metal Cutting Mechanics: Principles of Chip Formation

Understanding Metal Cutting Mechanics

Metal cutting is the controlled separation of a material to create a new shape. This process creates two new surfaces and, unlike splitting wood or slate, the surfaces cannot be rejoined. It involves significant plastic deformation, characterized by relatively low forces but very high stresses due to the force being applied over tiny areas.

Tool Geometry and Angles

The relationship between the wedge and clearance angles determines the cutting performance:

• Positive Rake: Can be independent of the wedge and clearance angle.
• Negative Rake: Not independent of the wedge and clearance angle.

Key tool components include:

• Cutting edge: Separates a chip from the parent work material.
• Rake face: Directs the flow of newly formed chips.
• Rake angle: Measured relative to a plane perpendicular to the work surface.
• Relief angle: The flank of the tool that provides clearance between the tool and the newly generated work surface.

Cutting Zones and Deformation

Cutting action involves shear deformation of the work material to form a chip, exposing a new surface:

• Primary shear zone: The zone where deformation is formed by shear.
• Secondary shear zone: A zone of shear deformation in the chip resulting from friction between the chip and the tool rake face. Its effect increases with higher friction.

Orthogonal vs. Oblique Cutting

• Orthogonal cutting: The cutting edge is straight and perpendicular to the direction of primary motion. This represents two-dimensional mechanics.
• Oblique cutting: The cutting edge is set at an angle (the tool cutting edge inclination λs), representing three-dimensional mechanics.

Note: Cutting edges of complex tools can be modeled as oblique or orthogonal elements.

Chip Formation and Velocity

Chip formation is defined by the following parameters:

• Shear plane: The plane along which a chip is formed by shear deformation.
• Shear angle: The angle between the shear plane and the work surface.
• Shear strain: Increases with a decreasing shear angle or rake angle.

Velocity in cutting zones includes:

• Cutting speed (Vc): The traveling speed of the tool relative to the workpiece.
• Velocity of chip (Vf): The flowing speed of the chip over the tool rake face.
• Shearing speed (Vs): The velocity at which shear occurs in the shear plane.