CNC Machining of chilled cast iron
Chilled cast iron is a composite material formed by rapid cooling, resulting in a hard white cast iron layer on the surface while retaining a gray cast iron core. It features high surface hardness (up to 400-600 HBW), excellent wear resistance, and high core toughness. It is widely used in the manufacture of friction- and impact-bearing components such as machine tool guides, rollers, and brake drums. However, turning chilled cast iron is challenging. The high hardness and brittleness of the surface hard layer can lead to severe tool wear, poor surface quality, and even chipping. Therefore, optimal turning processes and tool selection are crucial to ensure both efficiency and quality.
The turning characteristics of chilled cast iron are primarily due to the microstructure and properties of its surface hard layer. A thorough understanding of these characteristics is crucial for developing appropriate process strategies. The surface white layer of chilled cast iron, composed of cementite and pearlite, is dense, hard, and brittle. This layer exhibits minimal plastic deformation during cutting, leading to chip breakage. Cutting forces are concentrated at the tool edge, increasing tool wear. The wear rates on the rake and flank faces are significantly higher than those of conventional gray cast iron. Furthermore, the presence of hard particles (such as carbides) within the white layer can cause intense abrasive wear on the tool during cutting, further shortening tool life. The gray cast iron core, while relatively soft, exhibits a nonuniform hardness at the transition zone to the surface hard layer. This leads to significant fluctuations in cutting forces during turning, which can easily cause vibration and compromise surface quality. Furthermore, chilled cast iron exhibits poor thermal conductivity, making it difficult to dissipate cutting heat. This can lead to high temperatures at the tool edge, causing thermal wear and cracking. Therefore, effective cooling measures are essential during turning.
The selection of tool materials is crucial for turning chilled cast iron. Tool material must meet the requirements of high hardness, high wear resistance, high heat resistance, and a certain level of toughness. Common tool materials include cemented carbide, ceramics, and cubic boron nitride ( CBN ). Among cemented carbide tools, tungsten-cobalt-titanium ( YT ) alloys, due to their titanium content, offer excellent wear and heat resistance, but relatively low toughness, making them suitable for turning the soft core and transition zone of chilled cast iron. Tungsten-cobalt ( YG ) alloys offer higher toughness but slightly lower wear resistance, making them suitable for turning chilled cast iron with a thin surface hard layer. Ultrafine-grained cemented carbides (such as YG10H and YT05 ) offer even higher hardness and wear resistance and are suitable for turning the surface layer of medium-hard chilled cast iron. Ceramic cutting tools (such as Al₂O₃ -based and Si₃N₄ -based ceramics) have a hardness of up to 90-95 HRA and superior wear and heat resistance to cemented carbide. They are suitable for turning high-hardness, chilled cast iron surface layers, but they are relatively brittle and should be used without impact loads. Cubic boron nitride (CBN) cutting tools have a hardness of up to 90-95 HRC and excellent wear and heat resistance, making them the ideal tool material for turning chilled cast iron. They are particularly suitable for finishing high-hardness surface layers, achieving high CNC machining accuracy and surface quality. However, they are more expensive and are therefore suitable for mass production or high-precision CNC machining.
The rational design of tool geometry significantly impacts the turning performance of chilled cast iron and needs to be optimized based on the tool material and CNC machining requirements. The selection of the rake angle must take into account both cutting force and tool strength. Due to the brittleness of the hard layer on the surface of chilled cast iron, the rake angle is generally negative (-5°–10°) to enhance tool edge strength and reduce the risk of chipping. Since the core of gray cast iron is softer, the rake angle can be appropriately increased to 0°-5° to reduce cutting force. The back angle is generally 6°-10°. Although a too large back angle can reduce friction between the back face and the workpiece, it will reduce tool strength. A too small back angle will increase friction, leading to accelerated tool wear. The lead angle is usually 45°-75°. A larger lead angle can reduce radial cutting force and vibration, and is suitable for workpieces with poor rigidity. A smaller lead angle can increase tool tip strength and is suitable for turning workpieces with thicker surface hard layers. The tool tip radius is generally 0.5-1.5mm. A larger radius can increase tool tip strength and improve heat dissipation, but it also increases radial cutting forces and can easily cause vibration. Therefore, a reasonable selection should be made based on workpiece rigidity and CNC machining accuracy. Furthermore, the tool’s cutting edge should be properly chamfered (0.1-0.3mm width, -10°-15° angle) to enhance edge strength and extend tool life.
Cutting parameters should be selected based on a comprehensive consideration of CNC machining efficiency, tool life, and CNC machining quality. While ensuring minimal tool wear, maximize cutting efficiency. Cutting speed has the greatest impact on tool life. When turning the surface hard layer of chilled cast iron, the cutting speed for carbide tools is generally 30-60 m/min, for ceramic tools 80-120 m/min, and for CBN tools 100-200 m/min. When turning the core of gray cast iron, the cutting speed can be increased to 80-120 m/min (carbide) or 150-200 m/min (ceramic). The feed rate should be determined based on CNC machining accuracy and surface quality requirements. For rough turning, use a feed of 0.2-0.5 mm/r, and for finish turning, use 0.1-0.2 mm/r. Excessive feed increases cutting forces, causing vibration and tool chipping. Excessive feed increases friction between the tool and the workpiece, exacerbating tool wear. The cutting depth should be determined based on the thickness of the surface hard layer. During rough turning, the hard layer is removed in one go, with a cutting depth greater than the hard layer thickness (generally 3-5mm) to prevent the tool from repeatedly cutting between the hard and soft layers. During finish turning, the cutting depth should be 0.5-1mm to ensure surface quality. Furthermore, when turning chilled cast iron, sufficient cutting fluid must be used. Extreme pressure emulsions or sulfurized cutting oils are generally used. This fluid is sprayed directly into the cutting area through a high-pressure cooling system to lower cutting temperatures, reduce tool wear, and improve surface quality.
