Heavy-duty turning is a critical process in mechanical manufacturing, especially in industries that deal with large-scale machining. The workpieces involved can be extremely massive, weighing up to 60 to 80 tons or even hundreds of tons. The turning equipment used for such operations has enormous dimensions—horizontal lathes can handle diameters up to 6 meters, while vertical lathes may reach 10 meters. Compared to conventional machining, heavy-duty turning involves larger depths of cut, slower cutting speeds, and lower feed rates. The machining allowance on one side can range from 35 to 50 mm, and due to the uneven distribution of material and potential imbalances in the workpiece, vibrations are common. These vibrations significantly increase the time required for manual adjustments and auxiliary operations. To improve productivity and machine utilization, it is essential to increase the cutting depth and feed rate. This requires careful consideration of cutting parameters, tool selection, and tool geometry, along with the strength characteristics of the tool material.
When selecting tool materials for heavy-duty turning, options include high-speed steel, cemented carbide, cubic boron nitride (CBN), and ceramics. Heavy-duty cutting often involves deep cuts of 30 to 50 mm, and the workpiece surface may have a hardened layer. During roughing, wear is typically abrasive, and although the cutting speed is relatively low (15–20 m/min), the temperature at the chip-tool interface can still be high enough to melt the chip, reducing friction and preventing built-up edge formation. Ceramic tools, despite their high hardness, lack sufficient impact resistance and are not suitable for heavy-duty applications. Similarly, CBN also has limitations. Cemented carbide, however, offers a low friction coefficient, which reduces cutting forces and temperatures, enhancing tool life and making it ideal for high-hardness materials and heavy-duty roughing.
Cemented carbide is divided into three main types: tungsten-cobalt (YG), tungsten-titanium (YT), and tungsten carbide (YW). YG grades provide good strength and toughness but perform poorly at high temperatures, making them unsuitable for heavy-duty turning. YT grades, on the other hand, offer excellent hardness, wear resistance, and heat resistance, making them commonly used for steel machining. However, under low-speed conditions, their toughness decreases, leading to chipping. In such cases, YW or fine-grain carbides like 643 are preferred. Fine-grain alloys exhibit better wear resistance and are more effective for chilled cast iron, improving efficiency by over 100% compared to traditional YW tools. Using carbide tools allows for higher cutting speeds, which is crucial for boosting productivity and reducing cycle times. Multiple passes are often required to remove large stock, and carbide tools can significantly increase cutting speeds during these operations.
The selection of tool angles is also important in heavy-duty turning. For roughing, the rake angle is typically between 8° and 12°, while the cutting edge inclination is set between 10° and 18°. A smaller rake angle increases cutting edge strength, though it slightly raises cutting force. The increased wedge angle improves the sharpness and strength of the tool tip. When dealing with heavy workpieces and impact loads, a cutting edge inclination of 10° to 18° provides the best cutting conditions, helping prevent chipping. A 1mm wide negative clearance and an R2mm radius on the tool tip further enhance impact resistance, though the installation angle must be adjusted accordingly.
Tool structure plays a vital role in heavy-duty turning. Due to the large cutting allowance, the tool must be rigid. While overall stiffness is good, the structure may be bulky and difficult to assemble. Machine tools with flexible disassembly and adequate dynamic stiffness are preferred. Clamping mechanisms such as eccentric pins and head compression are not suitable for heavy roughing, as they may loosen due to vibration, causing damage. Chip evacuation is also a concern, as improper structures can block chip flow. High precision in machine tool manufacturing is essential, as even small errors can affect positioning accuracy. Through practical testing, a specific tool design has proven effective for heavy-duty roughing.
This new carbide tool features an adjustable chip former that rolls chips into a spiral shape. The chip former also serves as the blade holder, securing the insert within the tool holder. During cutting, chips come into contact with the chip separator, which helps manage chip flow. A hard metal sheet, 3–5 mm thick, is welded to this part. The front end presses against the carbide insert, while the rear end engages the stopper. Sawtooth patterns on the backing plate and stopper allow for blade extension after wear. Pads made of alloy and hardened tool steel protect the inserts from breakage. The rectangular blade is sharpened using a special cutter bar, with angles checked via a template. The rake angle is 10°, relief angle 8°, lead angle 55° or 45°, and cutting angle 15°. The tool bar is forged from 45# steel with a hardness of HRC 45–48. Experience shows that spiral chips formed during high-speed cutting are ideal, and if the lathe runs smoothly, the carbide blade remains intact, greatly improving efficiency.
Compared to solid carbide blades, this tool has similar life but is more durable and produces larger chips. Under normal conditions, chips are rolled into long or short spirals. If the cutting depth-to-feed ratio is no more than 3:4, chips break into small pieces upon hitting the holder. This tool is suitable for steel parts on lathes and vertical lathes. For inserts with widths of 30mm and 25mm, maximum chip profiles are 20mm² and 15mm², respectively, when the cutting allowance is uniform. If the crust is non-uniform, the chip section should be reduced by 30–40%. In cases of severe impact, this tool structure should not be used.
The protrusion distance of the carbide insert from the backing plate depends on the chip size. Proper protrusion ensures smooth chip evacuation and prevents damage. Blades must be cleaned and replaced promptly when worn or broken. This tool structure allows for grinding to maintain assembly accuracy, and the pressing bolt is positioned on the flank face, reducing the risk of damage. The plate tool holder enhances blade stiffness, minimizing vibration and improving both efficiency and quality.
In terms of cutting parameters, the roughing stage can involve a depth of up to 50mm, with a cutting speed of about 10m/min and a feed rate of 1.5mm/r. Although the surface roughness may be poor (Ra 12.5–6.3), it can be improved through rolling methods to meet subsequent processing requirements.
In conclusion, heavy-duty turning differs from conventional machining in many ways. Current data and processes are often based on ordinary machining, which may not fully apply. Further research and optimization are needed to improve efficiency and performance in heavy-duty turning.
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