CNC Lathe Machining Slender Shaft Research
In the transmission structure and components of large machines, the application of slender shaft is more extensive. Because of its poor rigidity, processing by gravity, cutting force, cutting heat and other impact characteristics, resulting in the processing of such parts is more difficult. The article analyzes the deformation, accuracy, surface roughness and other issues that are likely to occur in the machining of slender shafts on CNC lathes, and proposes the use of appropriate clamping and machining methods and other measures to further ensure the quality and efficiency of the machining of slender shafts.
1. Reasons for Errors in Machining Slender Shaft
1.1 Analysis of slender shaft
A slender shaft is defined as a shaft with a length-to-diameter ratio greater than 25 (L/D > 25). This is a challenging turning process. Single-piece or small-batch production can be done on CNC lathes.
In the past, high-speed steel turning tools were used at low speeds. Due to the poor rigidity of the slender shaft, the tool generates large radial forces. This causes heating and vibration in the workpiece. Over time, these factors lead to bending or even breakage, which doesn’t meet processing requirements. Clearly, this inefficient method needs improvement.
To avoid bending and deformation during machining, use carbide turning tools. Additionally, analyze the entire machining process in detail. Consider fixtures, machine tools, process arrangement, tool parameters, cutting dosage, and methods. These improvements will enhance the quality of the slender shaft.
1.2 Error caused by force deformation and thermal deformation
Internal stress, also called residual stress, remains inside the workpiece even without external forces. This stress keeps the material temporarily stable. Over time, residual stress fades, and the workpiece returns to a stress-free state. This leads to errors in the parts, causing them to fail to meet processing requirements.
When a slender shaft blank is clamped on the chuck, it sags slightly due to gravity. The weak rigidity of the shaft makes it vulnerable to bending or even breaking. If this happens during the initial processing, it affects the rest of the turning. This not only wastes resources but also prevents successful completion of the task. Reducing forces on the shaft is crucial for proper processing.
During processing, the heat is generally uniform, causing the shaft’s temperature and diameter to rise slowly. This heat causes slight expansion. After cooling, the diameter, size, and cylindricity of the part may have errors. For example, when clamped at both ends, the heat causes the slender shaft to elongate slightly. This results in bending, which leads to cylindricity errors.
2. Measures to Improve the Quality of Slender Shaft Machining
2.1 Appropriate clamping method
Rough machining generally adopts the method of one clamp and one top. When the top of the top is too tight, the slender shaft in the turning due to heat and can not be extended, and eventually bending deformation. Therefore, when conditions permit, the elastic top can be used. In this way, in the face of the elongation of the part, the elastic top can be expanded and contracted in the direction of the axis, minimizing the bending of the workpiece.
Two centers are generally used for finishing. This method can more accurately position the long axis to ensure coaxiality. However, due to the poor rigidity of the parts easily deformed vibration reasons, this method is only suitable for clamping the ratio of length to diameter, finish turning allowance is small and coaxiality requirements of higher parts, and processing needs to be filled with sufficient coolant to take away the cutting heat.
2.2 Reasonable turning method
To reduce the impact from forces and heat, use auxiliary tools like heel tool holders and center holders. These tools support the slender shaft’s surface to reduce radial deformation. This improves part rigidity and stability. However, ensure the support surface aligns with the workpiece surface or applies the right pressure to the elongated shaft for effective support.
Normally, the tool moves in a positive direction, from the tailstock to the chuck. If needed, the tool can move in the opposite direction, from the chuck to the tailstock. This directs axial force to the tailstock’s elastic center, compensating for part elongation.
For fine turning of slender shafts, high speed may not meet surface roughness requirements. White steel knives offer good toughness and impact resistance. To improve roughness, use a hand-ground white steel knife with a 3mm blade width. Turn at low speed with a small backfeed, and add hydraulic lubricant for better dimensional accuracy and surface finish.
2.3 Selection of tool angle and cutting three elements
Tool Geometry Parameters Selection
Select reasonable tool geometry parameters to reduce cutting force in the diameter direction. The front angle is the angle between the front blade surface and the base surface. A sharper blade with a larger front angle is weaker. Therefore, increase the front angle to reduce radial force, while maintaining tool strength. It is recommended that the front angle be between 15° and 25°.
The main deflection angle is the angle between the main cutting edge and the tool feed direction. Reducing the main deflection angle increases the depth of the cutting edge transition. This reduces the cutting edge’s bearing load, increases heat dissipation, and improves tool life. The recommended range for the main deflection angle is 75°–85°.
Tool Inclination and Cutting Speed
The inclination angle controls chip discharge direction. The angle can be positive, negative, or zero. To direct chips to the surface being machined, choose an inclination angle between 3° and 6°. Grind each carbide tool angle in place with a grinding wheel. Additionally, grind roughing and finishing tools separately for improved cutting efficiency.
Cutting speed affects the machined part. A higher rotational speed generates more cutting heat and increases centrifugal force. Feed rate plays a key role in the surface quality of the slender shaft. Larger feed rates cause more wear on the tool back face. Increases in backstroke also increase cutting force. To improve long shaft quality, use lower cutting speeds, smaller feed amounts, and backstroke during finishing.
3. Ensure the Surface Roughness of Long Shaft
To ensure a stable and proper cutting process, we must protect the long shaft’s surface. For example, when machining a single long shaft in a short time, use a clamping system with one chuck and one top. However, if the jaws clamp directly onto the machined surface, they may leave marks.
To avoid this, place a 1mm thick copper protection ring on the surface. The jaws will clamp the outer circle by deforming the copper ring. This protects the surface roughness of the long shaft.
For batch production with coaxiality requirements, the copper ring may deform or rupture with frequent clamping. In this case, use a thin-walled protection ring made from 45 steel, about 2.5mm thick. This protects the surface during batch production.
It is best to use a four-jaw chuck with single-action. Then, use a percentage table to adjust the jaws on the outer circle. This ensures centering accuracy for the long shaft.
4. Conclusion
Turning of long and thin shaft parts has always been an important part of the operation of the turner, because of its thin and long structural characteristics, resulting in the processing of such parts has become more difficult. However, through the use of the center frame and follow the tool holder, the use of reverse tool travel, the selection of appropriate tool geometry and cutting parameters, cutting methods and other improvement measures, to avoid the occurrence of the phenomenon of slender shaft vibration, bending and other phenomena, so that the entire machining process is simplified, and ultimately improve the machining efficiency of the long shaft.