Metal Cutting Process: Chip Formation and Types
Cutting Process
The cutting process is a controlled interaction between the workpiece, tool, and machine. It is influenced by the cutting conditions, cutting fluids, gripping of the tool and the workpiece, and the machine’s rigidity. Key factors in the formation of the chip are the tool (material and geometry), the work material (material, geometry, stiffness), and cutting conditions.
Oblique and Orthogonal Cutting
Oblique cut is a special case in cutting. When the edge of the tool is perpendicular to the direction of relative motion between the workpiece and the tool, it is known as orthogonal cutting.
Key Definitions in Metal Cutting
- Face of incidence or sidewall Tool A (alpha): The plane near the generated surface.
- Heads or surface spalling A (gamma): The plane created by the sliding chip.
- Cutting speed (lavc): The instantaneous speed of movement cut on the piece.
- Thickness of undeformed chip (ac): The depth of each layer of material removed by the tool.
- Deformed chip thickness (a0): The measured chip thickness after cutting.
- Shear angle, effective normal rake angle: The angle between the tool face and a line perpendicular to the new work surface or workpiece.
- Relief angle or effective normal clearance angle: The angle between the sidewall and the surface generated in the workpiece. It can affect tool wear per unit of time.
- Feed: The movement added to cutting that leads to the progressive elimination of material.
- Cut ratio (rc): The ratio of undeformed chip thickness to the deformed chip thickness.
- Chip width (aw): The chip width measured in the direction of the edge.
- Cutting area (Ac): The area perpendicular to the cutting speed that is swept by the tool.
- Feed area (Af): The area perpendicular to the feed rate swept by the tool.
- Chip removal rate (Zw): The volume of workpiece material removed per unit time. It has units of flow.
Observations During Metal Removal
If we analyze phenomena occurring around metal removal, we would observe that:
- The chip is harder and more brittle than the base material.
- a0 > ac, so rc < 1.
- The face of the chip that has been in contact with A (gamma) is smooth and shiny, while the other is dark and rough.
- The chip changes color from the material.
- Large increases in temperature occur in the cutting area.
- The chip form depends on the material.
In ductile materials, metal removal occurs by plastic deformation in a narrow strip called the shear plane.
Types of Chips
Continuous Chip
The continuous chip is the normal procedure of cutting and provides the best surface finish when machining common ductile materials. It consists of the work material shearing and the sliding of the chip on the side of the cutting tool. Chip formation takes place in the area stretching from the edge of the tool to the union between the surfaces of the part. This area is known as the primary deformation zone. Forces are transmitted to the chip in the existing interfaces between it and the tool face, sufficient to deform the lower layers of the chip as it slides over the tool face (secondary deformation zone).
Built-Up Edge Chip
Built-up edge occurs with very ductile materials or at low cutting speeds. When the friction between the chip and the tool is very high, there is a very strong adhesion between the chip material and the tool surface. The chip begins to slide, not directly onto the rake face, but on material attached to it. This screed blade can reach a size at which it dislodges material adhering to the piece or on the chip, leaving a very poor surface finish.
Discontinuous Chip
Discontinuous or broken chips occur when machining brittle materials (cast iron or cast bronze) and ductile materials at very low speeds and large feeds. During chip formation, the material is subjected to large deformation and, if fragile, will break in the primary deformation zone when chip formation is incipient. Under these conditions, the chip is segmented.