Orthogonal Optimization of Geometric Parameters of Spherical End Mills for Titanium Alloy Slotted
**I. Introduction**
Fan wheels and diagonal wheels are critical components of an engine, and their machining quality directly influences the thrust and efficiency of the engine. These complex surfaces can be machined using multi-axis CNC machining centers with 3D CAD/CAM technology. However, the workpiece material, TC6 titanium alloy, is notoriously difficult to machine due to its poor thermal conductivity and high strength. As a result, the performance of the CNC milling cutter plays a vital role in determining the geometric accuracy, surface finish, and overall processing efficiency. To address these challenges, five-axis CNC grinding technology is employed to customize ball-end milling cutters for titanium alloy surfaces. By applying the orthogonal experimental design method, the geometric parameters of the milling cutter can be optimized, significantly improving machining efficiency and reducing production costs.
**II. Characteristics of Titanium Alloy Surface CNC Milling**
1. **Surface Forming Methods in CNC Milling**
The CNC milling process involves two main surface forming methods: wire-forming and surface-forming. In surface-forming, all the side edges of the milling cutter act as cutting edges, making it ideal for machining curved surfaces such as inclined, cylindrical, conical, and toroidal surfaces. Wire-forming, on the other hand, treats the surface as a set of profile lines, with each point on the tool’s edge creating a single line during machining. This method is particularly suitable for complex freeform surfaces. Key features include:
- Ball-end mills are commonly used for complex surface shaping, sometimes replaced by cone-end ball-end mills for better rigidity.
- Surface roughness tends to be relatively poor in surface-forming methods.
- In curved surface machining, the contact between the tool and the workpiece changes continuously due to multi-axis linkage and uneven chip thickness, affecting cutting stability.
2. **Cutting Characteristics of TC6 Titanium Alloy**
TC6 (Ti-6Al-2.5Mo-2.0Cr-0.3Si-0.5Fe) is a heat-resistant titanium alloy with a composition that includes 15–20% stabilizing elements, operating effectively at temperatures between 350°C and 450°C. Its mechanical properties at room and high temperatures are summarized in Table 1.
| Property | Value |
|----------|-------|
| δb (MPa) | 931 |
| δ (%) | 10 |
| ψ (%) | 23 |
| αK (MJ/m²) | 0.3 |
| Temperature (°C) | 450 |
| δb (MPa) | 588 |
| δ100 (MPa) | 539 |
The cutting characteristics of TC6 titanium alloy include:
- High cutting edge load due to small chip deformation coefficient, leading to localized stress near the cutting edge and potential chipping.
- Elevated cutting temperatures caused by poor thermal conductivity and short chip contact length, resulting in temperatures about twice as high as those when cutting 45 steel.
- Rapid tool wear from both high temperature and chemical reactions with oxygen and nitrogen in the air, forming brittle layers that increase abrasive wear. Additionally, strong chemical affinity between titanium and tool materials leads to adhesion and accelerated wear.
**III. Orthogonal Optimization of Ball-End Mill Geometry Parameters**
To meet the specific requirements of fan wheel and diagonal flow wheel machining, five-axis CNC grinding centers are used to manufacture custom ball-end milling cutters for titanium alloy surfaces. An orthogonal test method was applied to optimize several sets of geometric parameters for ball-end mills used in this application. The tool geometry parameter combinations are listed in Table 2.
| Tool No. | Rake Angle γ₀ (°) | Relief Angle α₀ (°) | Blade Width br₠(mm) | Helix Angle β (°) |
|----------|------------------|--------------------|----------------------|-------------------|
| 1 | 8 | 8 | 0.2 | 36 |
| 2 | 8 | 10 | 0.3 | 38 |
| 3 | 8 | 12 | 0.4 | 40 |
| 4 | 10 | 8 | 0.3 | 40 |
| 5 | 10 | 10 | 0.4 | 36 |
| 6 | 10 | 12 | 0.2 | 38 |
| 7 | 12 | 8 | 0.4 | 38 |
| 8 | 12 | 10 | 0.2 | 40 |
| 9 | 12 | 12 | 0.3 | 36 |
Test conditions included a spindle speed of 540 rpm, feed rate of 140 mm/min, and a depth of cut of 4 mm. The test results were evaluated based on flank wear: 0 points if no wear after 20 minutes, 1 point if 0.12 mm wear after 30 minutes, and 3 points if more than 0.05 mm wear. The results and analysis are presented in Tables 3 and 4.
From the test results, it was observed that the best wear resistance was achieved with the following parameters: rake angle γ₀ = 12°, relief angle α₀ = 8°, blade width br₠= 0.4 mm, and helix angle β = 38°. This combination proved to be the optimal configuration for ball-end mills used in machining fan wheel surfaces. Additionally, tools needed to be re-sharpened after 30 minutes of use to maintain efficiency and prevent excessive wear.
**IV. Conclusion**
By employing whole CNC grinding technology and the orthogonal test method, customized milling cutters can be developed for titanium alloy surfaces, tailored to specific workpiece materials and machining needs. Even with standard tool materials, optimizing the geometric parameters significantly improves cutting performance. The tests demonstrated that the life of the orthogonal-optimized slotted ball-end mill exceeded twice that of similar-sized tools from Swiss Mikron, achieving international standards in terms of cutting quality and productivity. This approach not only enhances efficiency but also reduces manufacturing costs, making it a valuable strategy in advanced machining applications.
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