The sheared material that flows along with the cutting tool surface in the form of small pieces during metal cutting is called a chip. Chip actually has two surfaces such as a shiny surface (which has contact with the rake surface of the tool) and other is a rough surface (which has no contact with any solid body). The basic types of chips produced in orthogonal cutting are
- Continuous chip
- Discontinuous chip
- Continuous chip with built-up edge
- Serrated chip
These types of chips are formed during metal cutting. The chip formation during metal cutting is based on various factors such as,
- Mechanical properties of a workpiece
- Depth of cut
- Tool rake angle
- Cutting velocity
- Feed rake
- Cutting fluid used while machining
- Cutting region temperature
- Required surface finishes
- Friction coefficient
Based on the above-listed factors, the chip is formed during metal cutting.
Now let’s briefly learn about the types of chips in metal cutting.
Continuous chips are in the form of a ribbon-like structure (long coil) with the same thickness throughout the length. This kind of chip is formed while cutting the ductile material (such as low carbon steel, copper, brass, and aluminum alloys) at high cutting velocities and pressure of the tool cutting edge in compression and shear. This makes the material ahead of the tool cutting edge. Sharp cutting edge and low friction on the tool-chip interface also plays a major part in this chip formation.
The deformation zone ahead of the cutting edge is called a primary deformation zone. Due to the friction at the tool chip interface, the work material gets additional deformation and this zone is called the secondary deformation zone. The thickness of this zone is directly proportional to the friction.
While machining the soft metals at low speed and low rake angles, distortion occurs on the machined surface. So it gives unfavorable results while machining the soft metals.
Overall, the formation of a continuous chip results in a good surface finish and smooth cutting. It also increases the tool life and decreases power consumption.
Chip breakers are equipped along with the cutting tool, to avoid the chip tangling about the tool. This usually happens in the turning process.
While machining the brittle materials (such as cast iron, bronze, and high carbon steel) at low cutting velocities the chips are formed in discontinuous segments. These segments are attached loosely to each other. During deformation, the material lacks its ductility and it attains fracture simultaneously periodic ruptures occur on the chip flow. During this chip formation, the cutting forces vary frequently.
This type of chip is produced mainly due to high friction in tool-chip interface, large feed, and depth of cut.
When machining ductile materials, discontinuous chips results in poor surface finish and reduce the tool life.
Stiffness of the cutting tool and holding devices plays a major role in discontinuous chip formation because improper stiffness may cause vibration, dimensional inaccuracy, poor surface finish, and even damage the cutting tool.
While comparing continuous chip, discontinuous chips are easy to handle because of its short length. Also, it can be easily disposed off.
Continuous chip with built-up edge
.During the cutting process, the friction between the tool-chip interfaces are high and it makes the chip material weld itself to the tool rake surface near the tooltip which is called a build-up edge (BUE). It may be severe when the chip continuously rubs the tool surface. It acts as a cutting edge of the tool. The formation of the build-up edge is temporary and unstable.
During machining, some of its own parts are removed. After machining, it may partially stick on the chip and the machined surface. This results in poor surface finishing. However, it gives a poor surface finish it also reduces the tool wear and increases the tool life because it acts as an additional layer over the tool cutting edge
The formation o Build up edge (BUE) can be reduced by following factors such as,
- By using a sharp tool
- Increasing the cutting velocities
- Reducing the chip thickness
- Increasing the rake angle
- Using capable cutting fluid.
Serrated chip or segmented chip
The other names of serrated chips are serrated chips and non-homogeneous chips. This chip has a saw tooth appearance due to an alternating large zone of high shear strain followed by a small zone of low shear strain. Due to this cyclic chip formation, it becomes semi-continuous. This type of chip formation occurs while machining the difficult-to-machine materials (such as titanium alloys, nickel-based superalloys, and austenitic stainless steel) at high cutting velocities.
What are the conditions required for chip formation?
Now let’s see about the conditions that promote the formation of these types of chip formation.
Conditions of continuous chip formation
The conditions that promote the formation of continuous chip are stated here.
- Using ductile materials ( such as low carbon steel, copper, etc)
- Small chip thickness
- Small feed
- High cutting velocity
- Large rake angle of tool
- Sharp cutting tool
- Using proper cutting fluid for lubrication while machining
- Less friction between the tool-chip interfaces.
These conditions encourage the formation of this chip type.
Conditions for discontinuous chip formation
The conditions that promote the formation of a discontinuous chip is stated here.
- Using brittle materials (such as high carbon steel, gray cast iron, bronze, etc.)
- Using rough and uneven workpiece
- High chip thickness
- Low cutting velocity
- Using inadequate cutting fluid while machining
- Cutting at large feeds
- High friction between tool-chip interface
The above conditions encourage the formation of discontinuous chip.
Conditions for continuous chip with built-up edge (BUE) formation
The following conditions promote the formation of continuous chip with a built-up edge.
- Using ductile materials for machining
- Low and medium cutting velocity
- Small rake angle of tool
- Large chip thickness
- Rough feed
- High friction between tool-chip interface
- Using ineffective cutting fluid while machining
- Large uncut thickness
These conditions result in the formation of continuous chip with build-up edge.
Conditions for serrated chip (or) segmented chip formation
The following condition results in the formation of serrated chip (or) segmented chip.
- Using difficult-to-machine materials (such as titanium alloys, austenitic stainless steels etc)
- High cutting velocities