Turning of difficult-to-machine cast iron
Difficult-to-machine cast iron generally refers to cast iron materials with high hardness, high wear resistance, or high brittleness, such as white cast iron, chilled cast iron, and alloy cast iron. Turning these materials faces numerous challenges, including rapid tool wear, low machining efficiency, and difficulty maintaining surface quality. The hardness of these cast irons often exceeds 300HB, with some chilled cast irons even reaching a surface hardness of 500-600HB, far exceeding the hardness range of ordinary gray cast iron. This places enormous cutting and impact forces on the tool during turning. Furthermore, difficult-to-machine cast iron contains a large number of hard particles, such as carbides and phosphorus eutectics. These hard particles severely scratch and wear the tool edges during cutting, significantly shortening tool life. A mining machinery factory, machining chilled cast iron rolls, was using conventional carbide tools to process only one or two products before they needed to be replaced. This not only affected production efficiency but also increased tool costs.
The selection of turning tools for difficult-to-machine cast iron is key to improving machining efficiency and quality. For high-hardness cast iron, cubic boron nitride ( CBN) tools are currently the ideal choice. They can reach hardnesses of 3000-4000 HV, offer far greater wear resistance than carbide, and can withstand high cutting temperatures and impact loads. When machining alloy cast iron with a hardness of 300-500 HB, solid CBN tools or CBN composite cutter tools can be used, with cutting speeds controlled at 80-150 m/min to ensure both machining efficiency and tool life. For chilled cast iron with a hardness exceeding 500 HB, ultrafine-grain CBN tools are required, along with lower cutting speeds (50-80 m/min) and higher feed rates (0.15-0.3 mm/r) to reduce the risk of tool chipping. When a steel company was processing chilled cast iron rolls, it used ultra-fine grain CBN tools, which increased the tool life from 2 pieces to more than 50 pieces and improved the processing efficiency by 10 times.
The turning process parameters for difficult-to-machine cast iron require optimization based on material properties and tool performance. The choice of cutting speed requires a balance between machining efficiency and tool wear. Excessively high cutting speeds can lead to tool overheating and wear, while too low a speed can reduce production efficiency. Generally speaking, when using CBN tools to machine cast iron with a hardness of 300-400HB, the cutting speed can be set at 100-150m/min; when machining cast iron with a hardness of 400-500HB, the cutting speed is reduced to 80-100m/min. The feed rate should be selected based on surface quality and the impact force the tool can withstand. For finish turning, the feed rate is 0.1-0.2mm/r, increasing to 0.2-0.4mm/r for rough turning. The backing depth should be determined based on the surface hardness distribution of the casting. For the white layer of chilled cast iron, the backing depth of the first pass should be greater than the thickness of the white layer (typically 5-10mm) to avoid repeated cutting of the tool in the hard surface layer. When processing high-chromium cast iron parts, an agricultural machinery factory optimized the cutting parameters and increased the depth of cut from 1mm to 3mm, shortening the processing time by 40% and reducing impact damage to the tool.
Cooling and lubrication technology plays an important role in the turning of difficult-to-machine cast iron. Since the cutting temperature of difficult-to-machine cast iron is high and the chips are mostly in a broken state, it is easy to cause tool overheating and burns on the workpiece surface. Therefore, cutting fluids with good cooling performance are required. Extreme pressure emulsions or extreme pressure cutting oils have excellent lubrication and cooling properties. They can form a lubricating film between the tool and the workpiece, reducing friction and heat generation. At the same time, they help with chip removal and prevent chips from scratching the workpiece surface. In a high-pressure cooling system, the injection pressure of the cutting fluid can reach 10-20MPa, which can effectively break through the steam film in the cutting area and improve the cooling effect. When a machine tool factory processed alloy cast iron guide rails, it used high-pressure extreme pressure emulsion cooling to reduce the surface roughness of the workpiece from Ra3.2μm to Ra1.6μm, and extended the tool life by 30%.
Turning difficult-to-machine cast iron requires careful attention to workpiece clamping and vibration control. Due to the brittle nature of difficult-to-machine cast iron, over-clamping, which can cause cracking in the workpiece, must be avoided during clamping. Soft jaws or specialized fixtures can be used to evenly distribute the clamping force. For large or complex-shaped cast iron parts, auxiliary supports or rigid fixtures are required to enhance workpiece stability and prevent vibration during machining. Vibration can cause tool edge chipping and chatter marks on the workpiece surface, impacting machining quality. Therefore, vibration reduction is necessary by adjusting the machine tool spindle speed and optimizing the tool overhang. A heavy machinery plant, machining large ductile iron crankshafts, employed a rigid toolholder and dynamic balancing device to effectively control vibration during turning, keeping the crankshaft’s roundness error within 0.01mm. With the advancement of materials technology, new types of difficult-to-machine cast iron continue to emerge, and turning technology is also undergoing continuous innovation. For example, the application of high-speed dry cutting technology in difficult-to-machine cast iron not only reduces cutting fluid usage but also improves machining efficiency, providing a new solution for green manufacturing.
