Plastic turning tools
Plastic turning tools are specialized cutting tools designed for the specific properties of plastic materials. Due to the low hardness (60-120 Shore A), high plasticity, and low thermal conductivity of plastics, turning operations differ significantly from metal machining. These tools require sharp cutting edges, low friction, and excellent chip evacuation to prevent defects such as plastic melting, surface tearing, and burrs. The material, geometry, and edge preparation of plastic turning tools directly impact machining quality and efficiency, requiring specific design based on the plastic type (e.g., thermoplastic, thermoset) and machining requirements (e.g., surface roughness, dimensional accuracy).
The material selection of plastic turning tools must meet the requirements of sharpness and wear resistance. Commonly used materials include high-speed steel, cemented carbide, diamond and cubic boron nitride (CBN). Among them, high-speed steel tools (such as W18Cr4V) have sharp cutting edges and are inexpensive. They are suitable for processing general thermoplastics (such as polyethylene and polypropylene), but have poor wear resistance and short tool life; cemented carbide tools (such as YG) have better wear resistance than high-speed steel and are suitable for processing reinforced plastics containing glass fiber or mineral fillers. After fine grinding, the cutting edge can obtain a surface roughness of less than Ra0.8μm; diamond tools (natural diamond or polycrystalline diamond) have extremely high hardness (HV10000) and low friction coefficient (0.05-0.1), and are suitable for high-precision and high-finish plastic processing (such as organic glass and polyoxymethylene). The surface roughness of the processed surface can reach Ra0.02μm, and the tool life is 50-100 times that of cemented carbide; The tool is suitable for processing high-temperature resistant plastics (such as polyimide) and can withstand high cutting temperatures without causing bonding.
The geometric design of plastic turning tools requires a focus on optimizing the rake angle, clearance angle, and cutting edge radius to suit the cutting characteristics of plastics. The rake angle is typically 10°-30°, much larger than that of metalworking tools. A larger rake angle reduces cutting forces and heat, preventing melting of the plastic due to elevated temperatures. When machining soft plastics (such as polyethylene), the rake angle is 20°-30°, while when machining hard plastics (such as polycarbonate), the rake angle is 10°-20°. The clearance angle is 8°-15° to reduce friction between the tool flank and the workpiece surface, preventing scratches on the plastic surface. For thin-walled plastic parts, the clearance angle can be increased to 12°-15° to reduce radial forces and prevent workpiece deformation. The cutting edge radius is a critical parameter for plastic tools and must be controlled within 0.01-0.05mm. A sharp cutting edge reduces plastic deformation and prevents surface tearing. Excessively large cutting edge radius can easily cause plastic flow during cutting, resulting in burrs or wrinkles. Fine grinding with a diamond wheel can achieve a smaller cutting edge radius.
Cutting parameters for plastic turning must be determined based on the plastic type and tool material. The selection of cutting speed, feed rate, and depth of cut must balance machining efficiency and surface quality. Cutting speed significantly impacts plastic machining quality. Too low a speed results in prolonged contact between the tool and the plastic, generating excessive heat and causing the plastic to melt. Too high a speed can cause vibration and affect surface roughness. When machining thermoplastics, the cutting speed for high-speed steel tools is 50-100 m/min, for carbide tools 100-300 m/min, and for diamond tools 300-1000 m/min. When machining thermosets, the cutting speed can be reduced by 20%-30%, as thermosets are brittle and prone to cracking at excessive speeds. The feed rate is typically 0.05-0.2 mm/r. Lower values (0.05-0.1 mm/r) are used for fine machining to achieve low surface roughness, while higher values (0.1-0.2 mm/r) are used for rough machining to improve efficiency. The cutting depth is determined according to the machining allowance, 0.5-2mm for rough machining and 0.1-0.5mm for fine machining to avoid deformation of plastic parts due to excessive cutting depth.
Cooling, lubrication, and chip removal measures in plastic turning have a significant impact on machining quality. Due to the poor thermal conductivity of plastics (approximately 1/100-1/1000 of that of metal), cutting heat easily accumulates in the cutting area, causing the plastic to soften or melt. Therefore, effective cooling methods are necessary. When machining general plastics, compressed air cooling can be used to prevent cutting fluid from contaminating the workpiece and to blow chips away from the machining area. When machining easily meltable plastics (such as nylon and polyvinyl chloride), a cutting fluid with better cooling properties (such as kerosene or specialized plastic cutting fluid) should be used. The cutting fluid temperature should be controlled between 10-20°C to absorb cutting heat and prevent the plastic from softening. Poor chip removal can cause chips to rub against the machined surface, resulting in scratches or burrs. Therefore, the tool must have a suitable chip groove. The width and depth of the chip groove should be larger than the chip size. For plastics with curling chips (such as polyethylene), a chip groove can be ground into the rake face of the tool to curl the chips into a tight ball for discharge. In addition, the workpiece needs to be clamped with soft jaws or special fixtures to increase the contact area, reduce the clamping force, and prevent the plastic parts from being deformed or indented due to excessive force. For thin-walled parts, vacuum adsorption or magnetic adsorption clamping methods can be used to further reduce clamping deformation.
The sharpening and maintenance of plastic turning tools directly impact their cutting performance. During sharpening, ensure the surface roughness of both the rake and flank faces is ≤ Ra0.02μm. Fine grinding with a diamond grinding wheel (grit size W1-W5) at a grinding speed of 10-15m/s is recommended to achieve a sharp cutting edge. After sharpening, inspect the cutting edge with a magnifying glass for defects such as chipping and nicks. If any defects are present, resharpen them to avoid scratches on the machined surface. Regularly inspect the tool for wear during use. Any minor wear (approximately 0.02-0.05mm) requires prompt resharpening to prevent further wear and loss of plastic machining quality. Store the tool in a dedicated tool box to prevent the cutting edge from colliding with other objects. Diamond tools should be kept away from magnetic materials to prevent magnetization and the attraction of iron chips. Proper sharpening and maintenance can extend the life of plastic turning tools and ensure stable and consistent plastic machining.
