Abstract : By analyzing the machining process and principles, this paper examines the influence of various factors on the surface quality of the workpiece during machining.
Keywords : Plastic deformation cutting parameters
1 Introduction
In the machinery manufacturing industry, machining is a crucial production method. Products in this industry are becoming increasingly complex in structure, demanding higher precision and performance. Because the surface quality of machined parts significantly impacts their performance characteristics, such as wear resistance, contact stiffness, fatigue strength, fit properties, corrosion resistance, and precision stability, the surface quality requirements for critical surfaces of machine parts are quite high. However, the use of inappropriate machining methods and suboptimal selection of cutting parameters directly affect the surface quality of machine parts. This article provides a qualitative analysis of one aspect of the factors influencing surface quality during machining.
2. Cutting principle and process
2.1 Cutting Principle
Metal cutting is the process of removing excess material layers from a blank or workpiece using cutting tools. This removal of excess material layers essentially involves disrupting the crystal structure within the material layer using the cutting tool, breaking down the original crystal structure and obtaining a new one. Due to the molecular bonds within the crystal, energy is generated and consumed during metal cutting.
2.2 Cutting process
The cutting process is a plastic deformation, primarily characterized by slippage, that occurs when the cutting layer is compressed by the rake face of the tool. This process is described in Figure 1, which illustrates the chip formation process, i.e., the cutting process.
When the tool's rake face pushes against the cutting layer, a stress field is generated within the cutting layer, with the stress increasing closer to the cutting edge. As shown in Figure 1, the workpiece is squeezed by the tool and begins to undergo elastic deformation (left side of line OA). Subsequently, the internal stress gradually increases. When the shear force reaches the material's yield limit (from line OA to line OB), the metal material begins to slip and undergo plastic deformation (the slip amount is the vertical distance from point 1 to point 2). As the slip deformation proceeds, the shear stress continuously increases. When the deformation reaches the material's maximum allowable value (line OC), the cutting layer is crushed and destroyed (the slip amount from point 3 to point 4), forming chips, thus achieving the purpose of cutting.
Figure 1
Slippage occurs at equilibrium point P, followed by elastic and plastic deformation, generating internal stress within the workpiece. This stress can be roughly divided into three regions, as shown in Figure 2: deformation region I, deformation region II, and deformation region III. These regions are influenced by cutting parameters, as well as tool wear, built-up edge, and the depth and degree of surface hardening on the workpiece. Consequently, they directly or indirectly affect the surface quality of the workpiece.
Figure 2
The following section will elaborate and analyze the factors that affect the deformation zone during the cutting process, as well as the factors that are affected by the deformation zone, and thus the factors that affect the surface quality of the workpiece.
3. Factors affecting the deformation zone and the zone affected by deformation
3.1 Cutting parameters
Cutting parameters refer to the total cutting depth, feed rate, and cutting speed. When machining a workpiece, different cutting parameters result in different degrees of plastic deformation of the cutting layer, which directly affects the stress distribution in the first and second deformation zones during chip formation, and indirectly or directly affects the surface quality of the workpiece after machining.
3.1.1 Depth of Cut
The depth of cut refers to the perpendicular distance between the machined surface and the surface to be machined. The magnitude of the depth of cut determines the magnitude of the cutting force and, more importantly, the degree of plastic deformation of the cutting layer in the first deformation zone. Different cutting processes will have varying degrees of impact on the cutting tool. For example, in machining where the chip form is mainly extruded chips, due to the large cutting thickness, low cutting speed, and small tool rake angle, the tool will vibrate under the influence of the large cutting thickness, creating fluctuating cutting forces. This results in instantaneous contact defects between the cutting edge and the cutting layer, leading to a large surface roughness on the workpiece and affecting the surface quality of the machined workpiece.
3.1.2. Feed rate
The feed rate determines the nominal cross-sectional area of the cutting layer, which in turn determines the nominal thickness and nominal width of the cutting layer. These factors affect the line contact length between the main cutting edge and the cutting layer. When the feed rate is small, the contact area between the main cutting edge and the cutting layer decreases, so only a portion of the main cutting edge participates in the cutting work. Since the cutting force that the cutting edge can withstand per unit area is limited, when the cutting force exceeds the cutting edge's withstand capacity, the cutting edge locally fractures, forming residual machined surfaces, thus causing residual machining defects and affecting the surface quality of the workpiece. When the feed rate is large, the cutting force on the tool increases, and the tool's geometry changes. This change in tool posture can easily generate vibrations, forming feed ripples at the cutting edge, which also affects the surface quality of the workpiece.
3.1.3 Cutting speed
Cutting speed can be simply understood as the frequency with which the tool passes through a unit length of the cutting layer per unit time. Once the depth of cut and feed rate are determined, the cutting speed affects the internal stress (friction temperature) in the second and third deformation zones. Generally, a high cutting speed increases the number of times the tool passes through a unit length of the cutting layer per unit time, resulting in zero-feed cutting in certain areas, i.e., fine grinding, which is beneficial for shaping the machined surface quality of the workpiece. Conversely, a high cutting speed with a low feed rate will affect the internal stress in the deformation zone, influencing other factors and thus affecting the surface quality of the machined workpiece.
3.2 Plump
At medium or low cutting speeds, the friction between the chips and the tool generates high cutting temperatures in the second deformation zone. This causes some of the chip particles in contact with the tool in the second deformation zone to harden and detach. As the chips flow along the tool's rake face, these detached chip particles adhere to the rake face and become trapped. Under the influence of the high cutting temperature in the second deformation zone, they form cold welds, which accumulate to form a built-up edge. As the built-up edge gradually increases in size, it stops growing when the contact conditions and stress on the chip and tool rake face change. During the cutting process, when the workpiece or tool is subjected to impact, vibration, or changes in cutting force, the built-up edge may partially fracture or completely detach.
The effects of built-up edge on the surface quality of machined surfaces include:
1) Because the front end of the built-up edge extends beyond the cutting edge, the nominal thickness of the cutting layer increases. Moreover, the generation, growth, and shedding of the built-up edge have a certain periodicity. Therefore, the nominal thickness of the cutting layer is variable, and the cutting force acting on the tool is also variable. This causes the tool to vibrate, resulting in fluctuating cutting force. This causes instantaneous contact defects between the cutting edge and the cutting layer, affecting the surface roughness of the workpiece and thus the surface quality of the workpiece machining.
2) When built-up edge breaks, in addition to some of the fragments being carried away by the chips, some will flow into the contact area between the tool and the workpiece, forming "grooves" on the workpiece surface. Furthermore, the fragments may embed into the workpiece surface, forming hard points, causing tool damage, increasing the surface roughness of the workpiece, and affecting the surface quality of the workpiece machining.
3.3 Surface Hardening
During metal cutting, due to the cutting force, the machined surface of the workpiece is subjected to friction from the flank face of the tool, causing the metal in the third deformation zone of the workpiece surface to undergo large plastic deformation. This causes the metal lattice to twist, the grains to elongate and break, and further deformation of the metal to be hindered, thus strengthening the material and increasing its hardness. At the same time, the cutting temperature will weaken the material, and higher temperatures will cause phase transformation.
The squeezing and friction between the tool's flank and the workpiece causes a sudden increase in temperature at the contact layer, resulting in surface hardening of the machined layer and the generation of residual stress. This phenomenon affects the workpiece surface quality in several ways:
The instantaneous high temperature during cutting causes localized changes in the surface structure of the workpiece, resulting in oxidation in certain areas. This intense heat can burn the workpiece surface, reducing its wear resistance, corrosion resistance, and fatigue strength. Severe burning or residual stress concentration can also lead to cracks on the workpiece surface. Furthermore, surface hardening increases the wear on the tool's flank face, affecting the surface quality of the machined workpiece.
4. Other factors affecting surface quality
4.1 Cutting Method
Cutting methods include free and non-free, right-angle and oblique-angle cutting. Different cutting methods will affect the plastic deformation and internal stress of the workpiece surface, thus affecting the surface quality of the workpiece.
When a cutting tool engages in cutting with only one straight cutting edge, this is called free cutting. Its characteristics include: chips flowing out in a generally consistent direction; deformation of the cut metal occurring within a linear unit; and the plastic deformation tendency and internal stress direction of the cut layer being consistent, resulting in minimal impact on the workpiece surface quality.
However, when the cutting edge of the tool is curved, several cutting edges participate in the cutting work simultaneously. This situation is called non-free cutting. Its characteristics are that the chip outflow directions do not intersect each other during the cutting process, and the deformation unit of the cut metal is a non-linear unit. The plastic deformation trend and stress direction of the cutting layer are not on the same straight line, which will cause the crystal lattice of the cutting layer to be distorted, elongated, and broken, resulting in cracks on the workpiece surface and affecting the surface quality of the workpiece machining.
The principles of right-angle and oblique-angle cutting are basically the same as those of free and non-free cutting, and their effects on the workpiece cutting process are basically the same, so they will not be elaborated here.
4.2 Tool Wear
Tool wear is classified into two types: wear and breakage. Wear refers to the wear on the rake face and flank face of the tool, and the effects of wear on the rake face and flank face on the surface quality of the workpiece are as follows:
1) Rake face wear: During the workpiece cutting process, if a large cutting speed and cutting depth are used, the rake face will be subjected to increased friction from the chips and the cutting temperature will rise. A groove will be worn out near the main cutting edge of the rake face. This groove will directly act on the cutting layer during the cutting process, forming a "peak" and affecting the surface quality of the workpiece machining.
2) Back face wear: Due to the intense friction between the machined surface and the back face, the high temperature generated by the friction will increase the temperature of the cutting layer in the third deformation zone, exacerbating the hardening of its surface layer. This will further aggravate the wear of the tool's back face, causing the tool's back face to become dull and hardened. This will change the contact between the tool's back face and the cutting layer from the original elastic deformation and plastic deformation to hard contact. The dulled back face will then generate intense friction on the machined surface of the workpiece, affecting the surface quality of the workpiece's machining.
Breakage refers to the chipping phenomenon of the main cutting edge and secondary cutting edge of a cutting tool. When the tool cuts the workpiece, residual cutting occurs, which directly affects the surface quality of the workpiece during machining.
4.3 Vibration
Vibrations are mainly classified into forced vibrations and self-excited vibrations. Forced vibrations are vibrations excited under the influence of external disturbance forces. Self-excited vibrations are vibrations generated internally due to the influence of the stiffness of the machining system. Both types of vibrations affect the natural frequency of the cutting tool during the cutting process, causing fluctuations in the tool's movement when cutting the workpiece, resulting in cutting ripples and affecting the surface quality of the machined workpiece.
4.4 Cutting Fluid
Cutting fluid serves to lubricate, cool, clean, and prevent rust. Its effects on workpiece surface quality during the cutting process include:
1) Lubrication: Cutting fluid penetrates between the tool, chips, and machined surface, forming a lubricating or chemically adsorbed film, which reduces friction between the three. Furthermore, the addition of organic compounds such as sulfur, phosphorus, and chlorine to the cutting fluid allows for chemical reactions with the cutting layer at high temperatures, forming a high-temperature, high-pressure resistant chemical lubricating film. This prevents direct contact between the tool and the cutting layer, reduces the coefficient of friction, and thus reduces wear on both the tool and the workpiece.
2) Cooling effect: Cutting fluid can remove a large amount of cutting heat from the cutting zone, thereby reducing the cutting temperature, changing the distribution of residual stress in the cutting layer, and preventing high temperature from burning the workpiece.
3) The cleaning effect of the cutting fluid can wash away the cutting particles in the cutting zone, preventing the cutting particles from directly acting on the workpiece under the cutting force and causing secondary scratches on the workpiece surface.
4) Rust prevention: Rust-preventive additives in cutting fluid are adsorbed onto the surface of the workpiece to form a protective film, which prevents rust and corrosion on the workpiece surface.
By utilizing the action of cutting fluid during the cutting process, the cutting temperature between the tool and the cutting layer is reduced, the friction between the tool and the workpiece is decreased, the internal stress distribution of the cutting layer is altered, and secondary scratches from the chips are prevented, thus directly or indirectly affecting the surface quality of the workpiece during machining.
5. Conclusion
By analyzing the factors affecting the surface quality of workpieces during machining, mastering the mechanisms of each factor, and organically combining them, we can formulate machining processes that meet the specific precision requirements of different workpieces, effectively changing the surface quality of workpieces during machining. This significantly improves the wear resistance, contact stiffness, fatigue strength, fit properties, corrosion resistance, and precision stability of machine parts, thereby enabling the machine to form a stable working system.
