Performance Verification of Solid Carbide Close-Tooth Cutting Tools

To tackle problems like tool wear, chipping, and inadequate blade precision during cutting, a specialized tool design for milling gas turbine blades was created. Key design considerations for a solid carbide fine-pitch cutter were outlined, and the cutting performance of this tool was validated through field testing.

 

01 Introduction

Solid carbide milling cutters are commonly utilized in the machining of blades within the energy and aviation sectors. These blades are often constructed from challenging materials like stainless steel, titanium alloys, or high-temperature alloys. The machining of blades in these industries demands high precision in shape and surface quality. When working with difficult-to-machine materials, tools face several challenges, including chipping, wear, excessive cutting heat, tool hardening, high vibrations, and poor surface quality of the workpiece. Since the tool engages directly with the workpiece during the production of blades, the design of the tool significantly influences both the quality and efficiency of the machining process.

This paper focuses on the development of a solid carbide fine-pitch cutter specifically designed for the machining of energy blades, with a primary emphasis on the profile of gas turbine blades. The workpiece material used in this study is B50A789F stainless steel. The design of the tool and the studies on its cutting performance demonstrate the reliability of the proposed tool design. The shape of the tool blade is illustrated in Figure 1.

 

02 Tool Design

The surface machining process for gas turbine blades typically involves the following steps: casting, milling, and polishing. Ensuring high standards for both the blade shape and surface quality is crucial during this process. Depending on specific user requirements, the surface roughness during milling is generally controlled to Ra ≤ 3.2μm, while during polishing, it is controlled to Ra ≤ 0.8μm.

Rough surfaces on gas turbine blades can increase heat transfer between the pressure and suction sides, leading to greater friction between the fluid and the blade surface. This heightened interaction enhances convective heat transfer, resulting in an increase in the average metal temperature of the blades, which can significantly affect their lifespan. Additionally, a rough blade surface increases the overall surface area, which provides more heat transfer area and raises the risk of corrosion and wear, further shortening the lifespan of the blades.

When designing a fine-pitch cutter for blade surface machining, it is essential to consider not only the accuracy of the blade’s surface and ensuring that the blade profile meets required dimensions, but also to control surface roughness within Ra ≤ 3.2μm. Furthermore, the cutter design must prevent overheating during machining, as this can adversely affect both tool life and machining efficiency. Therefore, the design of a solid carbide fine-pitch cutter focuses on four key aspects: tool material, shape, manufacturing optimization, and surface coating design.

 

2.1 Tool Material

The blade is made from B50A789F steel, a commonly used material for gas turbine compressor blades and guide vanes. This steel shares compositional characteristics with 04Cr15Ni7Cu2MoNb alloy structural steel and is classified as a precipitation-hardening martensitic stainless steel. Its primary strengthening mechanism relies on the aging hardening of the copper-rich phase, as well as the precipitation hardening of molybdenum and niobium.

Given the cutting properties of B50A789F steel, the manufacturing and cutting tools must meet the following requirements: 1) High hardness to withstand plastic deformation of the substrate during manufacturing and cutting; 2) High strength to manage high-speed cutting and the heavy loads encountered in practical applications; 3) High wear resistance to maintain a sharp cutting edge throughout the cutting process; and 4) A minimized tool edge radius.

To fulfill these requirements, tool materials should exhibit small grain size, toughness, and good thermal conductivity. This helps reduce heat accumulation in the cutting edge area due to effective heat conduction, thereby lowering cutting temperatures and enhancing tool wear resistance. As a result, this minimizes tool wear and chipping during blade machining, leading to superior surface quality.

Solid carbide is an appropriate material for machining stainless steel, with submicron-grained materials exhibiting a grain size of 0.6 μm being particularly ideal for milling processes. The metallographic structure of a solid carbide close-pitch tool is illustrated in Figure 2, and the composition and physical properties of the tool material are outlined in Table 1.

 

2.2 Tool shape

(1) Number of tool blades
In the design of tools for the milling process of gas turbine blade surfaces, a dense tooth structure is utilized. This design feature provides several advantages: it helps distribute the cutting force during operation. With more teeth on the tool, each individual tooth experiences a reduced cutting force, leading to improved surface quality on the blade.

Typically, an ordinary flat-head end mill is designed with 3 to 4 blades. In contrast, a solid carbide dense tooth cutter is generally selected based on its outer diameter, featuring between 10 to 12 blades. However, if a tool has too many blades—especially in the case of small-diameter tools—it can negatively impact the cutting edge strength and the space available for chip removal.

Considering all these factors, selecting 10 to 12 blades for a solid carbide dense tooth cutter is most reasonable. This choice helps ensure that the tool experiences lower cutting forces while maintaining the strength of the cutting edges and providing adequate space for chip removal.

 

(2) Tip radius

The integral carbide dense-tooth cutter is specifically designed for processing B50A789F steel, a material known for its high plasticity and toughness. During cutting, chips often adhere to the tooth edge surface, which can negatively impact sharpness and cause rapid wear. When the edge wears down quickly, the tool may vibrate during cutting, leading to vibration marks on the blade and scratches caused by the worn edge. This results in the blade profile accuracy and surface quality exceeding tolerance levels.

To address these issues, the integral carbide dense-tooth cutter incorporates a rounded corner design at the tip. This design effectively protects the tip, reduces tool wear during cutting, and contributes to a smooth processed surface with fewer burrs. However, it is important to note that an excessively large tip radius can dull the tool and shorten its lifespan. Therefore, a carefully considered rounded corner design is implemented to balance protection and performance.

The tip radius of the cutter ranges from R0.5 mm to R1 mm, determined by a comprehensive evaluation of the tool diameter and the number of cutting edges. A visual representation of the tool structure is provided in Figure 3.

 

2.3 Tool Manufacturing Optimization

Tool manufacturing technology consists of two primary areas: tool preparation programming and simulation, and grinding process optimization. A demonstration of tool grinding programming is illustrated in Figure 4. For this paper, the programming of a solid carbide fine-pitch cutter began by importing the tool design drawing into preparation parametric software. A tool preparation CAM (Computer-Aided Manufacturing) model was then created based on the design parameters, followed by a grinding simulation using specialized simulation software.

The designed tool was subsequently ground on a five-axis tool grinder, employing dedicated tool grinding CAM software for both modeling and simulation. Given the stringent dimensional accuracy and surface quality requirements for solid carbide fine-pitch cutters, existing tool grinding processes were thoroughly examined. Key factors such as the grinding wheel section shape, mounting angle, grit count, and the number of grinding passes were optimized to ensure strict compliance with process requirements during precision machinery, thereby guaranteeing both dimensional accuracy and surface quality.

A 3D demonstration of the tool’s edge profile is shown in Figure 5. Profile scanning of the tool edge after grinding revealed a smooth and continuous edge, free from burrs and micro-chipping. Additionally, the inspection of the tool tip radius, as depicted in Figure 6, confirmed a smooth transition. This analysis indicates that the manufacturing process for solid carbide dense-tooth cutters successfully meets the design requirements.

 

 

2.4 Tool Surface Coating

To extend the life of cutting tools, increase surface hardness, and enhance resistance to high temperatures and wear, a solid carbide fine-pitch cutter designed for machining B50A789F stainless steel was treated with a surface coating. The coating applied is AlTiN (aluminum-rich), which serves as a general-purpose coating. This AlTiN coating has a relatively high aluminum content, and further increasing this content improves its oxidation resistance and mechanical properties at elevated temperatures while maintaining its crystal structure.

Figure 7 displays the micromechanical probe hardness curve of the AlTiN coating under load. At a maximum indentation load of 20 mN, the AlTiN coating maintains a high-hardness plateau, registering a hardness of 38.3 GPa and an elastic modulus of 305 GPa. Additionally, Figure 8 presents a scanning electron microscope (SEM) image of the cross-section of the AlTiN coating, which reveals fine grains, a smooth fracture surface, and the absence of columnar grains, indicating a nanocrystalline structure. The performance parameters of the AlTiN coating are summarized in Table 2.

 

 

03 Study on the Cutting Performance of Solid Carbide Close-Pitch Cutters

The study on the cutting performance of solid carbide close-pitch cutters was primarily conducted at the user’s machining center. A D20mm cutter with 12 cutting edges and a 1mm corner radius was selected for the tests. The cutting conditions and parameters are detailed in Table 3 and Table 4, respectively.

 

The blade being machined on-site was a gas turbine blade made from B50A789F steel. Before milling, the blade was in its forged state. Figure 9 shows the clamping of the blade at the customer’s site. A solid carbide fine-pitch cutter was primarily used for milling the blade profile. During the CNC manufacturing process, the blade was relatively stable, with no noticeable machine vibration.

After milling, the surface profile of the blade was measured using three-dimensional coordinates (CMM). The measured data showed a deviation of 0.023 to 0.038 mm, which met the customer’s dimensional accuracy requirements. The CMM report for the blade profile is illustrated in Figure 10.

Additionally, the surface roughness of the blade was measured to be Ra ≤ 3.2 μm, and there were no burrs, scratches, or other defects that could affect the surface quality. The milled surface of the blade is shown in Figure 11.

 

 

The solid carbide fine-pitch cutter developed in this project successfully machines two blades continuously while meeting user requirements for both dimensional accuracy and surface quality. After machining two blades, the tool’s cutting edge showed no defects such as coating flaking or chipping, which could affect its lifespan. Therefore, the development of this solid carbide fine-pitch cutter for gas turbine blade surface machining is considered a success.

 

04 Conclusion

On-site user testing confirmed that the solid carbide fine-pitch cutter operates smoothly without vibrations and shows moderate tool edge wear within the effective machining range. The dimensional accuracy and surface quality of the machined blades meet the required cutting specifications. This demonstrates that the solid carbide fine-pitch cutter—composed of submicron-grained carbide material and coated with AlTiN—can withstand high temperatures and resist surface wear during the machining of blades, effectively reducing surface friction.

The tool features a dense-pitch, multi-edge design with rounded tips, which helps to minimize cutting resistance and maintain a smooth cutting action. During the manufacturing process, specialized CAM software was utilized for modeling and simulation, along with grinding process optimization, to ensure the tool met all design requirements.

This solid carbide dense-tooth cutter is capable of meeting the cutting needs for gas turbine blades made from B50A789F steel, making it an ideal tool for gas turbine blade processing.

 

 

If you want to know more or inquiry, please feel free to contact info@anebon.com

At Anebon, we believe in the values of innovation, excellence, and reliability. These principles are the foundation of our success as a mid-sized business that provides customized CNC components, turning parts, and die casting zinc alloy parts for various industries such as non-standard devices, medical, electronics, auto accessories, and camera lenses.