Enhancing Efficiency in High-Speed Milling Through Rectangular Grid Path Optimization

The processing characteristics of high-speed milling have led to the optimization of the rectangular grid processing path. This optimization involves simulating and analyzing the force conditions of different trajectories. Physical cutting tests were conducted to record spindle vibrations for various processing paths. The results indicate that the cutting process using the spiral processing path is the most stable and best suited for high-speed milling.

 

# 01 Introduction

Five-axis high-speed CNC machining is characterized by high efficiency, high precision, and superior surface quality. It offers significant advantages in surface machining and has become a primary method for enhancing machining efficiency and quality while reducing costs. High-speed machining imposes stringent requirements on machine tools, fixtures, and tools, as well as on the tool motion paths. If the tool path is not designed appropriately, sudden changes in the material removal rate can adversely affect both machining efficiency and quality. Therefore, generating a smooth and continuous tool path is crucial for successful high-speed cutting. Notably, rectangular grids in large shell segments represent approximately 80% of all machining features. Programmers must adapt the grid processing strategy to create a safe, efficient, and accurate machining path in order to achieve optimal high-speed machining results.

 

# 02 Requirements for tool paths in high-speed machining

The requirements for tool paths in high-speed machining are as follows:

-Avoid Rapid Reversing:
Tool paths should feature smooth transitions at corners. Rapid reversing not only slows down machining speeds but also leads to a quick increase in the cutting angle at corners, which increases the force on the tool and compromises cutting stability. Therefore, it’s essential to avoid rapid reversing.

 

-Maintain a Constant Load:
The amount of metal removed during machining should remain constant to ensure a stable cutting process. This consistency enhances heat transfer and improves overall machining quality.

 

-Ensure a Smooth Tool Path:
It’s important to have a smooth transition in the tool path during cutting. Minimizing the number of times the tool cuts in and out during the machining process results in a more stable cutting path. The tool should enter the cut smoothly, and a straight-cutting method should be avoided to achieve better results.

 

# 03 Rectangular grid traditional processing path

Due to the limitations of previous equipment capabilities and processing plans, the typical processing method employs a low speed, large cutting depth, and low feed rate. Tool path planning is focused on accommodating the maximum tool diameter. The processing path illustrated in Figure 1 is commonly utilized.

 

Using a shell mesh as an example, since the mesh fillet has a radius of R10mm, a φ20mm milling cutter is employed for processing the mesh. The mesh fillet is naturally formed by the tool’s fillet. The processing path is shown in Figure 2.

The cutting of the fillet position is illustrated in Figure 3. During the processing of the fillet, the cutting angle increases sharply by 90°, resulting in higher forces on the tool. This often leads to issues such as overcutting and chattering at the corners, which severely affects cutting stability. To achieve an effective cutting process, the only viable solution is to reduce both the speed and feed rate, but this significantly impacts processing efficiency.

# 04 High-speed milling machining path planning

4.1 Reciprocating machining path

The optimized reciprocating machining path is illustrated in Figure 4a. During the machining process, the machine tool frequently changes direction, leading to rapid acceleration and deceleration. This frequent and abrupt reversal can cause a sharp increase in cutting force, which ultimately results in lower machining efficiency and compromised product quality. To address this issue, the machining trajectory has been optimized. The new trajectory, depicted in Figure 4b, incorporates rounded corner transitions at the reversal points to enhance performance.

 

 

4.2 U-shaped processing path

The U-shaped processing path before optimization is illustrated in Figure 5a. During the processing, frequent reversals are necessary, leading to regular acceleration and deceleration of the machine tool. When approaching the fillet position, the cutting angle increases rapidly, often causing overcutting and chattering at the corners. This results in poor cutting stability and, consequently, a decline in product quality.

To adapt to high-speed processing, the trajectory has been optimized, as depicted in Figure 5b. A fillet transition has been added at the corners, and the straight-line cuts between the trajectories have increased the fillet. This new processing method features a trajectory that moves from the inside to the outside, utilizing a spiral feed approach. This effectively addresses the issues related to the poor control of the feed trajectory in narrow grids.

4.3 Spiral machining path
The spiral cutting method illustrated in Figure 6 is utilized for this process. In this method, a spiral feed is employed for each layer. The machining path follows a circular arc with continuous curvature, ensuring that the cutting amount remains consistent at various points along the path. This approach results in uniform milling and allows for smooth changes in the speed of each axis, significantly enhancing both tool life and machining efficiency. Additionally, the spiral trajectory eliminates the need for the tool to shift between rows, requiring only a single cut-in and cut-out to complete the machining process.

 

# 05 Virtual simulation

The VERICUT 3D virtual simulation software is utilized to simulate and analyze various optimized cutting trajectories. Its Force physical optimization module specifically simulates and evaluates the forces acting on the tool during the cutting process for different paths. By using the optimization module, users can select a cutting material parameter library, input tool information, and run the program to obtain the forces exerted on the tool during cutting.

The tool forces for the optimized reciprocating processing path, U-shaped processing path, and spiral processing path are illustrated in Figures 7 to 9, respectively. The simulation results indicate that during the cutting process, both the reciprocating and U-shaped paths exhibit unstable tool forces. There are sharp variations in force during the corners and the tool’s movement, leading to poor cutting stability. In contrast, when employing the spiral processing path, the tool force remains relatively stable throughout the cutting process, indicating better cutting stability.

 

# 06 Cutting verification

The cutting test was performed on three optimized trajectories. The cutting speed was set to n=12000 r/min, and the spindle vibration was recorded during the CNC machining process.

 

6.1 Reciprocating trajectory spindle vibration curve
Figure 10 displays the spindle vibration curve for the reciprocating machining path. It is evident that the cutting process lacks stability, with significant fluctuations in cutting vibration, which are not conducive to high-speed machining. Additionally, due to the constraints of the machining trajectory, the feed method can only proceed with an oblique feed along the shape, with the oblique feed angle generally limited to 5° or less. Consequently, the feed trajectory needs to be extended, which is especially impractical for narrow grids.

 

6.2 Spindle vibration curve of U-shaped trajectory

Figure 11 illustrates the spindle vibration curve for the U-shaped machining path. It is evident that this cutting method provides greater stability compared to reciprocating machining. However, there is some tool shifting between the machining trajectories. During the tool-shifting process, the cutting amount increases, leading to an increase in cutting vibration.

6.3 Spiral trajectory spindle vibration curve

Figure 12 illustrates the spindle vibration curve for the spiral machining path. This machining method employs a spiral feed for each layer, creating a circular arc with continuous curvature. As a result, the spindle vibration is smoother compared to the other two trajectories, leading to the most stable CNC metal cutting process.

 

# 07 Conclusion

The reciprocating machining path exhibits significant fluctuations during the cutting process, resulting in instability. This method is also constrained by the feed technique, making it unsuitable for processing small-sized grids. In contrast, the U-shaped machining path offers more stability than the reciprocating method; however, it experiences tool shifting between trajectories. This tool shifting can increase both the cutting amount and vibration.

The spiral machining path, characterized by continuous curvature in a circular arc, provides a more uniform milling experience. In this method, the tool force remains consistent without sharp changes. Compared to the other two machining trajectories, the cutting process with the spiral path is the most stable. This trajectory effectively reduces fluctuations in tool load and minimizes the number of tool shifts, resulting in improved machining outcomes.

To achieve high-speed machining, it is essential to maintain a uniform cutting load that does not vary dramatically. Arc transitions should be incorporated at the corners of the tool path to avoid sharp angles, and all feed and retract transitions should be as smooth as possible.

 

 

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