I. Principles and Influencing Factors of Climb Milling and Conventional Milling in CNC Milling Machines
(A) Principles and Related Influences of Climb Milling
During the machining process of a CNC milling machine, climb milling is a specific milling method. When the rotation direction of the part where the milling cutter contacts the workpiece is the same as the feed direction of the workpiece, it is called climb milling. This milling method is closely related to the mechanical structure characteristics of the milling machine, especially the clearance between the nut and the screw. In the case of climb milling, since the horizontal milling component force will change and there is a clearance between the screw and the nut, this will cause the worktable and the screw to move left and right. This periodic movement is an important problem faced by climb milling, which makes the movement of the worktable extremely unstable. The damage to the cutting tool caused by this unstable movement is obvious and it is easy to cause damage to the teeth of the cutting tool.
During the machining process of a CNC milling machine, climb milling is a specific milling method. When the rotation direction of the part where the milling cutter contacts the workpiece is the same as the feed direction of the workpiece, it is called climb milling. This milling method is closely related to the mechanical structure characteristics of the milling machine, especially the clearance between the nut and the screw. In the case of climb milling, since the horizontal milling component force will change and there is a clearance between the screw and the nut, this will cause the worktable and the screw to move left and right. This periodic movement is an important problem faced by climb milling, which makes the movement of the worktable extremely unstable. The damage to the cutting tool caused by this unstable movement is obvious and it is easy to cause damage to the teeth of the cutting tool.
However, climb milling also has its unique advantages. The direction of the vertical milling component force during climb milling is to press the workpiece onto the worktable. In this case, the sliding and friction phenomena between the teeth of the cutting tool and the machined surface are relatively small. This is of great significance for the machining process. Firstly, it is beneficial to reduce the wear of the teeth of the cutting tool. Reducing the wear of the teeth of the cutting tool means that the service life of the cutting tool can be extended, reducing the machining cost. Secondly, this relatively small friction can reduce the work hardening phenomenon. Work hardening will increase the hardness of the workpiece material, which is not conducive to the subsequent machining processes. Reducing work hardening helps to ensure the machining quality of the workpiece. In addition, climb milling can also reduce the surface roughness, making the surface of the machined workpiece smoother, which is very advantageous for machining workpieces with high requirements for surface quality.
It should be noted that the application of climb milling has certain conditional limitations. When the clearance between the screw and the nut of the worktable can be adjusted to less than 0.03 mm, the advantages of climb milling can be better exerted because the movement problem can be effectively controlled at this time. In addition, when milling thin and long workpieces, climb milling is also a better choice. Thin and long workpieces require more stable machining conditions during the machining process. The vertical component force of climb milling helps to fix the workpiece and reduce problems such as deformation during the machining process.
(B) Principles and Related Influences of Conventional Milling
Conventional milling is the opposite of climb milling. When the rotation direction of the part where the milling cutter contacts the workpiece is different from the feed 方向 of the workpiece, it is called conventional milling. During conventional milling, the direction of the vertical milling component force is to lift the workpiece, which will cause the sliding distance between the teeth of the cutting tool and the machined surface to increase and the friction to increase. This relatively large friction will bring a series of problems, such as increasing the wear of the cutting tool and making the work hardening phenomenon of the machined surface more serious. The work hardening of the machined surface will increase the surface hardness, reduce the toughness of the material, and may affect the accuracy and surface quality of the subsequent machining processes.
Conventional milling is the opposite of climb milling. When the rotation direction of the part where the milling cutter contacts the workpiece is different from the feed 方向 of the workpiece, it is called conventional milling. During conventional milling, the direction of the vertical milling component force is to lift the workpiece, which will cause the sliding distance between the teeth of the cutting tool and the machined surface to increase and the friction to increase. This relatively large friction will bring a series of problems, such as increasing the wear of the cutting tool and making the work hardening phenomenon of the machined surface more serious. The work hardening of the machined surface will increase the surface hardness, reduce the toughness of the material, and may affect the accuracy and surface quality of the subsequent machining processes.
However, conventional milling also has its own advantages. The direction of the horizontal milling component force during conventional milling is opposite to the feed movement direction of the workpiece. This characteristic helps the screw and the nut to fit tightly. In this case, the movement of the worktable is relatively stable. When milling workpieces with uneven hardness such as castings and forgings, where there may be hard skins on the surface and other complex situations, the stability of conventional milling can reduce the wear of the teeth of the cutting tool. Because when machining such workpieces, the cutting tool needs to withstand relatively large cutting forces and complex cutting conditions. If the movement of the worktable is unstable, it will aggravate the damage to the cutting tool, and conventional milling can relieve this situation to a certain extent.
II. Detailed Analysis of the Characteristics of Climb Milling and Conventional Milling in CNC Milling Machines
(A) In-depth Analysis of the Characteristics of Climb Milling
- Changes in Cutting Thickness and the Cutting Process
During climb milling, the cutting thickness of each tooth of the cutting tool shows a pattern of gradually increasing from small to large. When the tooth of the cutting tool just contacts the workpiece, the cutting thickness is zero. This means that the tooth of the cutting tool slides on the cutting surface left by the previous tooth of the cutting tool in the initial stage. Only when the tooth of the cutting tool slides a certain distance on this cutting surface and the cutting thickness reaches a certain value, does the tooth of the cutting tool really start cutting. This way of changing the cutting thickness is significantly different from that of conventional milling. Under the same cutting conditions, this unique starting method of cutting has an important impact on the wear of the cutting tool. Since the tooth of the cutting tool has a sliding process before starting cutting, the impact on the cutting edge of the cutting tool is relatively small, which is beneficial to protecting the cutting tool. - Cutting Path and Tool Wear
Compared with conventional milling, the path that the teeth of the cutting tool travel on the workpiece during climb milling is shorter. This is because the cutting method of climb milling makes the contact path between the cutting tool and the workpiece more direct. Under such circumstances, under the same cutting conditions, the wear of the cutting tool when using climb milling is relatively small. However, it should be noted that climb milling is not suitable for all workpieces. Since the teeth of the cutting tool start cutting from the surface of the workpiece each time, if there is a hard skin on the surface of the workpiece, such as some workpieces after casting or forging without treatment, climb milling is not appropriate. Because the hardness of the hard skin is relatively high, it will have a relatively large impact on the teeth of the cutting tool, accelerate the wear of the cutting tool, and even may damage the cutting tool. - Cutting Deformation and Power Consumption
The average cutting thickness during climb milling is large, which makes the cutting deformation relatively small. Small cutting deformation means that the stress and strain distribution of the workpiece material during the cutting process is more uniform, reducing the machining problems caused by local stress concentration. At the same time, compared with conventional milling, the power consumption of climb milling is less. This is because the distribution of the cutting force between the cutting tool and the workpiece during climb milling is more reasonable, reducing unnecessary energy losses and improving the machining efficiency. In large-scale production or machining environments with requirements for energy consumption, this characteristic of climb milling has important economic significance.
(B) In-depth Analysis of the Characteristics of Conventional Milling
- Stability of Worktable Movement
During conventional milling, since the direction of the horizontal cutting force exerted by the milling cutter on the workpiece is opposite to the feed movement direction of the workpiece, the screw and the nut of the worktable can always keep one side of the thread in close contact. This characteristic ensures the relative stability of the movement of the worktable. During the machining process, stable movement of the worktable is one of the key factors ensuring machining accuracy. Compared with climb milling, during climb milling, since the direction of the horizontal milling force is the same as the feed movement direction of the workpiece, when the force exerted by the teeth of the cutting tool on the workpiece is relatively large, due to the existence of the clearance between the screw and the nut of the worktable, the worktable will move up and down. This movement not only disrupts the stability of the cutting process, affects the machining quality of the workpiece, but also may damage the cutting tool seriously. Therefore, in some machining scenarios with high requirements for machining accuracy and strict requirements for tool protection, the stability advantage of conventional milling makes it a more appropriate choice. - Quality of Machined Surface
During conventional milling, the friction between the teeth of the cutting tool and the workpiece is relatively large, which is a prominent characteristic of conventional milling. The relatively large friction will cause the work hardening phenomenon of the machined surface to be more serious. The work hardening of the machaged surface will increase the surface hardness, reduce the toughness of the material, and may affect the accuracy and surface quality of the subsequent machining processes. For example, in some workpiece machining that requires subsequent grinding or high-precision assembly, the cold-hard surface after conventional milling may require additional treatment processes to eliminate the cold-hard layer to meet the machining requirements. However, in some specific cases, such as when there is a certain requirement for the surface hardness of the workpiece or the subsequent machining process is not sensitive to the surface cold-hard layer, this characteristic of conventional milling can also be utilized.
III. Selection Strategies of Climb Milling and Conventional Milling in Actual Machining
In actual CNC milling machine machining, the selection of climb milling or conventional milling needs to consider multiple factors comprehensively. Firstly, the material characteristics of the workpiece need to be considered. If the hardness of the workpiece material is relatively high and there is a hard skin on the surface, such as some castings and forgings, conventional milling may be a better choice because conventional milling can reduce the wear of the cutting tool to a certain extent and ensure the stability of the machining process. However, if the hardness of the workpiece material is uniform and there is a high requirement for surface quality, such as in the machining of some precision mechanical parts, climb milling has more advantages. It can effectively reduce the surface roughness and improve the surface quality of the workpiece.
In actual CNC milling machine machining, the selection of climb milling or conventional milling needs to consider multiple factors comprehensively. Firstly, the material characteristics of the workpiece need to be considered. If the hardness of the workpiece material is relatively high and there is a hard skin on the surface, such as some castings and forgings, conventional milling may be a better choice because conventional milling can reduce the wear of the cutting tool to a certain extent and ensure the stability of the machining process. However, if the hardness of the workpiece material is uniform and there is a high requirement for surface quality, such as in the machining of some precision mechanical parts, climb milling has more advantages. It can effectively reduce the surface roughness and improve the surface quality of the workpiece.
The shape and size of the workpiece are also important considerations. For thin and long workpieces, climb milling helps to reduce the deformation of the workpiece during the machining process because the vertical component force of climb milling can press the workpiece better onto the worktable. For some workpieces with complex shapes and large sizes, it is necessary to comprehensively consider the stability of the worktable movement and the wear of the cutting tool. If the requirement for the stability of the worktable movement during the machining process is relatively high, conventional milling may be a more appropriate choice; if more attention is paid to reducing the wear of the cutting tool and improving the machining efficiency, and under the conditions that meet the machining requirements, climb milling can be considered.
In addition, the mechanical performance of the milling machine itself will also affect the selection of climb milling and conventional milling. If the clearance between the screw and the nut of the milling machine can be accurately adjusted to a relatively small value, such as less than 0.03 mm, then the advantages of climb milling can be better exerted. However, if the mechanical precision of the milling machine is limited and the clearance problem cannot be effectively controlled, conventional milling may be a more secure choice to avoid machining quality problems and tool damage caused by the movement of the worktable. In conclusion, in CNC milling machine machining, the appropriate milling method of climb milling or conventional milling should be reasonably selected according to the specific machining requirements and equipment conditions to achieve the best machining effect.