It is essential to carefully select the appropriate end milling blade shapes for hardened steel mold cavities. High-speed cutting has emerged as one of the top ten mold manufacturing technologies in recent years, introducing a new approach to mold processing. Among these technologies, high-speed machining of hardened steel has gained significant attention. Solid carbide tools are commonly used for rough and finish machining of mold cavities, with a focus on improving processing efficiency during rough and semi-finish machining. General-purpose end mills, such as ball-end mills and corner radius end mills, perform well under favorable conditions with small cutting parameters but may suffer from severe wear, chipping, or even breakage of the cutting edge under high feed rates, large cutting widths, and small cutting depths typical in the mold industry. Modern mold production demands that tools achieve maximum metal removal rates under high-speed machining conditions to improve efficiency, with much of the research focusing on cutting technology. This paper discusses the optimization of tool blade design to enhance performance and meet the requirements for high-efficiency machining.

Conventional end mills have the following drawbacks under high-efficiency cutting conditions:

1.To reduce cutting forces and facilitate chip evacuation, the end edges of corner radius end mills typically feature a concave design with a high edge at the tip and a low center. This means that only the tip participates in cutting during face milling, resulting in high stress and a tendency for chipping.

2.The cutting edge of ball-end mills exhibits both negative rake angles and low-speed cutting inefficiencies, leading to a low metal removal rate.

3.Cutting forces are primarily radial, with the main cutting forces directed along the X and Y axes, causing tool chatter under high-efficiency cutting conditions.

To address these issues, the tool’s end edge shape is optimized by incorporating an arc design for edge protection. The concave straight edge is replaced with an arc edge where the tip is lower and the center is higher. The benefits include:

1.Increased cutting edge length, reducing the cutting load per unit, and distributing the cutting allowance and cutting force across the entire edge shape.

2.The bottom arc design, with a larger radius and smaller main cutting angle, reduces cutting forces and cutting-induced vibrations.

End Mill Cutter Shape Design and Analysis

Taking a φ10 four-flute end milling cutter as an example, the optimized arc-shaped milling cutter blade outperforms conventional end mills (corner radius and ball-end mills) in terms of effective cutting edge shape and length Le (black thick line) at the same cutting depth (0.5mm), as shown in Figure 1 and Table 1. The arc-shaped milling cutter has the longest effective cutting edge length, followed by the ball-end milling cutter, with the corner radius end milling cutter having the shortest. To comprehensively evaluate the performance of arc-shaped milling cutters compared to conventional end mills under high-efficiency machining conditions, both cutting simulations and cutting experiments were conducted for comparison.

During the cutting simulation, both the end milling cutter and the workpiece were simplified and precisely configured to ensure accurate calculations. A 10mm section of the end mill was selected as the simulation cutting portion, with detailed settings applied only to the cutting edge area. The total rotation angle of the tool during simulation was set to 190°, ensuring complete data for tool entry and exit points. The cutting parameters were set based on relatively large values commonly used in the mold industry: vc= 120m/min，fz=0. 4mm ／z，ap = 0. 5mm，ａe =10mm. The workpiece material selected was hardened steel (SKD11, with a hardness of HRC58), and the tool material chosen was carbide. The milling method used was climb milling. The cutting simulation model and the cutting conditions are shown in Figure 2.

 

Comparison of Cutting Forces

Cutting force is a crucial indicator of cutting performance. Excessive cutting force significantly impacts tool life. The cutting forces in the X, Y, and Z directions were directly extracted using the AdvantEdge post-processing program, showing how cutting forces fluctuate over time (see Figure 3).

From the figure, it is evident that the corner radius end milling cutter exhibits relatively stable cutting behavior. In contrast, the ball-end mill shows significant fluctuations in cutting force. This instability is attributed to the ball-end mill’s two-flute connection at the center, where long and short teeth alternate during milling. The variation in the number of active cutting edges leads to changes in the effective cutting edge length, resulting in substantial fluctuations in cutting force. The optimized arc-shaped end milling cutter initially encounters a larger cutting allowance when it begins to engage with the workpiece, resulting in higher cutting forces. As the cutting progresses deeper into the workpiece, the cutting allowance is uniformly removed radially along the tool, causing the cutting force to decrease and stabilize.

Comprehensive Comparison

The average values of the simulated cutting forces and cutting temperatures were calculated (see Table 1). It can be observed that the ball-end mill operates at a lower cutting temperature but experiences greater fluctuations in cutting force. The corner radius end milling cutter, with its shorter effective cutting edge, generates smaller cutting forces. The optimized arc-shaped end milling cutter produces higher cutting forces, predominantly in the axial direction.

Since the cutting edges involved in the process are concentrated on the end edges, they can be considered as simultaneously engaged in cutting. The resultant cutting force ftotalf_{total}ftotal? and the cutting force in the axial plane fxyf_{xy}fxy? per unit length of the cutting edge are shown in Table 2. The corner radius end mill exhibits the highest cutting force per unit length, indicating that under these conditions, the tool’s cutting edge experiences a higher cutting load, making it more prone to chipping and cutting vibrations. The optimized arc-shaped end milling cutter has the lowest cutting force per unit length, suggesting a more reasonable distribution of the cutting load.

Cutting Experiment Analysis

To comprehensively evaluate the cutting performance of the tools, the experiment was conducted to verify both cutting force and tool durability.

The experimental material, SKD11 with a hardness of HRC61, was the same as that used in the cutting simulation. The machining was performed on a MIKRON UCP1000 machining center using climb milling and dry cutting. The cutting process and parameters were consistent with those used in the cutting simulation.

Cutting Force Experiment

The cutting force sensor used was a Swiss Kistler 9265B three-component piezoelectric dynamometer, along with a charge amplifier and a corresponding data acquisition and processing system. After filtering, the cutting force values are shown in Table 3. The results indicate that the optimized arc-shaped end milling cutter generates a larger overall cutting force, but the cutting load per unit length is the smallest, consistent with the conclusions drawn from the cutting force simulation.

Cutting Performance Experiment for End Milling Cutter

Cutting hardened steel typically results in significant tool wear and short tool life, especially under high-efficiency cutting conditions, where differences in tool performance become more apparent. As shown in Figure 4, after machining a single groove (cutting length of 130mm), the optimized arc-shaped end mill exhibited normal wear, while the corner radius end mill and ball-end milling cutter both experienced edge chipping, leading to tool failure. The optimized arc-shaped end mill only showed edge chipping and failure after a cutting distance of 10 meters, which is more than ten times the tool life of conventional corner radius end mills and ball-end mills.

Analysis of Experimental Results

The experimental results indicate that cutting hardened steel leads to significant tool wear. Under high-efficiency cutting conditions, tools face extreme situations where only those that meet the machining demands can be used; otherwise, they are unsuitable. The optimized arc-shaped end milling cutter, with its modified blade shape, redistributes the cutting forces, reduces the cutting load per unit length on the effective cutting edge, and improves the tool’s cutting performance, thereby meeting the demands of high-efficiency cutting.

Conclusion

This study focused on modifying the tool blade shape to meet the high-efficiency cutting requirements for hardened steel. Through general cutting simulations and cutting experiments, the following conclusions were drawn:

1.Cutting simulations and cutting force experiments revealed that the optimized arc-shaped end mill generates greater cutting forces, particularly in the axial direction, compared to conventional corner radius and ball-end mills. However, it has a lower cutting force per unit length, with cutting temperatures comparable to those of the corner radius end mill.

2.Cutting experiments demonstrated that due to the modified blade shape, the cutting load per unit length of the cutting edge is reduced. As a result, the optimized arc-shaped end milling cutter outperforms conventional corner radius and ball-end mills under high-efficiency and heavy-load cutting conditions in mold manufacturing.