The determining moment of any mechanical machining process for chip removal is represented by the point where the cutting edge of the tool comes into contact with the workpiece.
In this area, complex phenomena occur which, if properly controlled, allow for high-quality processing.
In this article we will analyze the main factors that influence the correct formation of the chip and the ways in which they can be managed effectively.
Keep reading.
The mechanics of chip formation are determined by a series of physical phenomena, among which the plastic deformation of the material and the subsequent detachment of the surface layer are of particular importance.
As the tool penetrates the workpiece, high pressure and temperature conditions are generated in the cutting zone. These conditions induce the material to undergo significant plastic deformation, a prelude to the definitive detachment of the chip from the body of the piece.
This set of phenomena is localized in the so-called primary deformation zone, characterized by the presence of plastic sliding planes along which the material flows. In this region, the mechanical energy accumulated during deformation is largely converted into heat, mainly due to friction and the high stresses present.
A crucial aspect of the material removal process is the control of the plastic flow angle, a parameter that determines the direction along which material deformation occurs and which directly affects the chip formation mode. This angle is influenced by numerous factors, including cutting speed, chip thickness, tool geometry, and the forces involved in the process.
Optimal management of these parameters is essential to ensure the quality of the machining process and to prevent unwanted phenomena, such as overheating of the cutting zone and premature tool wear.
The cutting speed exerts a direct influence on the plastic sliding angle and, consequently, on the behavior of the chip during the machining process.
As the cutting speed increases, the energy transferred to the material increases, resulting in an increase in the temperature in the cutting zone and an intensification of plastic deformation phenomena.
These conditions must be appropriately managed in order to ensure optimal parameters, both in relation to the material of the workpiece and the constituent material of the cutting edge.
Excessively high cutting speeds generate very high temperatures, promoting accelerated wear phenomena and reducing the useful life of the tool.
On the contrary, cutting speeds that are too low keep the temperature below the values necessary for correct chip sliding on the tool chest, promoting the formation of temporary material adhesions on the cutting edge. This phenomenon, known as the carry cutting edge, leads to a worsening of the surface roughness of the workpiece, an increase in wear and a consequent reduction in tool life.
It is also important to remember that the hard metal used in the making of tools requires specific thermal conditions to ensure optimal performance and avoid fracture risks.
The geometry of the tool is a further determining element in chip formation. In particular, the upper rake angle — that is, the angle formed between the cutting surface of the tool and the surface of the workpiece — plays a fundamental role.
A positive rake angle reduces cutting strength, promoting the generation of continuous chips and decreasing the effort required for machining. However, excessively high values of this angle can compromise the robustness of the tool, especially in the presence of particularly hard materials.
On the contrary, a negative rake angle increases the mechanical strength of the cutting edge, making it more suitable for processing hard or brittle materials, but simultaneously increases the compression of the material during cutting. This condition promotes chip fragmentation and the formation of discontinuous chips. While this may be advantageous in terms of cooling and wear reduction, it can at the same time compromise the quality of the surface finish.
The recording angle γ — defined as the angle between the longitudinal axis of the tool and the surface of the workpiece — also significantly affects the behavior of the chip.
Higher values (around 10°) favor the formation of thinner chips, while lower values tend to generate thicker chips and increase radial forces, with the risk of inducing vibrations during processing.
Simplifying the analysis of the phenomenon, the cutting forces interact with the material being processed by determining the plastic sliding angle.
If the cutting speed is the main factor responsible for the thermal increase, the chip thickness — adjusted via the feed parameters — and the upper rake angle of the tool control the pressures that directly affect the plastic sliding angle.
Increasing this angle causes an increase in pressures and, consequently, an increase in temperature in the cutting area.
When machining particularly hard materials, it is necessary to employ very small, or even negative, rake angles in order to ensure the robustness of the tool and its ability to withstand the stresses of machining.
In such conditions, it is desirable to reduce both the cutting speed and the feed rate, so as to keep the inclination and temperature of the plastic creep within optimum values.
The friction generated on the chest and side of the tool also contributes significantly to the increase in temperature.
The use of coolant, when possible, represents a valid tool to reduce friction and effectively control thermal conditions in the cutting zone.
Chip formation is strongly influenced by a combination of factors, including cutting speed, tool geometry, chip thickness, and operating forces during the process.
The accurate control of the plastic flow angle and chip thickness is essential to govern the material's behavior during processing.
Optimizing these parameters allows not only to improve the quality of the machined surface, but also to extend the useful life of the tool, preventing unwanted phenomena such as excessive wear or the formation of irregular chips.
To learn more about chip removal and how Sau can improve your manufacturing processes, contact us today.
Sau Team
SAU - Quality Tools Engineering since 1982.