Vibration in turning and its reduction measures
Vibration during the turning process is a significant factor affecting machining quality and production efficiency. At the very least, it can cause chatter marks and increased roughness on the workpiece surface. At the worst, it can lead to tool chipping, workpiece failure, and even damage to the machine tool. Turning vibration can be divided into forced vibration and self-excited vibration based on the cause. Forced vibration is caused by external periodic interference forces, such as spindle imbalance and gear meshing impact. Self-excited vibration is generated by the dynamic interaction between the tool and the workpiece during the cutting process and is closely related to factors such as cutting force, tool geometry, and workpiece material. Reducing turning vibration requires multiple approaches, including vibration source identification, process parameter optimization, and equipment status improvement, with systematic measures needed to achieve stable cutting.
Reducing forced vibration begins with eliminating external interference sources. Machine tool spindle imbalance is a common source of vibration, especially at high speeds. Unbalanced mass on the spindle generates centrifugal force, resulting in periodic interference. Solutions include dynamically balancing the spindle to ensure the imbalance is within 5g / mm . Regularly check the spindle bearing clearance and replace bearings promptly if the radial clearance exceeds 0.01mm to ensure spindle rotation accuracy. Gear transmission system meshing shock is also a significant source of forced vibration. This shock can be reduced by improving gear machining accuracy (e.g., from grade 8 to grade 7 ) and increasing gear lubrication (using extreme-pressure gear oil). For belt-driven machine tools, uniform belt tension is essential to prevent periodic vibration caused by belt slippage. Tension is generally maintained between 15-25N and can be measured and adjusted using a belt tension meter.
Reducing self-excited vibration requires optimizing cutting process parameters. Cutting speed is a key factor influencing self-excited vibration. Medium-speed cutting (60-100 m/min) is most likely to cause self-excited vibration. Therefore, adjusting the cutting speed to avoid the resonance zone can effectively suppress vibration. Low-speed cutting (30-50 m/min) or high-speed cutting (120-150 m/min) can effectively suppress vibration. For example, when turning 45 steel, increasing the cutting speed from 80 m/min to 120 m/min can reduce the amplitude of self-excited vibration by over 50%. Optimizing tool geometry can also reduce self-excited vibration. Increasing the lead angle (from 75° to 90°) can reduce radial cutting forces, while maintaining a rake angle of 15°-20° ensures a sharper cutting edge and reduces cutting resistance. Furthermore, adding a damping device to the tool shank, such as affixing a high-damping material (such as rubber or lead), can absorb vibration energy and reduce amplitude.
Improving workpiece and tool rigidity is a fundamental measure for reducing vibration. Insufficient workpiece rigidity is the primary cause of vibration when turning slender shafts and thin-walled parts. Auxiliary support devices, such as steady rests and steady rests, can be used to enhance rigidity. The contact area between the steady rest and the workpiece should be made of wear-resistant material (such as cast iron), and the support force should be adjusted to an appropriate range (generally 5-10N) to suppress vibration without damaging the workpiece surface. For slender shafts with an aspect ratio exceeding 30, a double steady rest configuration can be used, with one at each end of the workpiece, to further improve rigidity. Insufficient tool rigidity can amplify vibration, so it is important to select a thicker toolholder (with a shank diameter no less than 1.5 times the cutting diameter of the tool) and minimize the toolholder extension (no more than three times the shank diameter). If necessary, use carbide toolholders to leverage their high elastic modulus to reduce deformation.
Overall optimization of the process system is crucial for long-term stable cutting. The stability of the machine tool foundation directly affects vibration transmission. The machine tool should be installed on a dedicated foundation. Vibration-damping pads (such as rubber pads or spring pads) 50-100mm thick should be placed between the foundation and the ground. This can reduce vibration transmission by over 40%. Regularly check the lubrication status of the machine tool guideways. Use a centralized lubrication system to ensure a uniform oil film on the guideway surface and reduce vibration caused by guideway friction. For CNC lathes, servo system parameters should be regularly calibrated, such as adjusting the position loop gain and velocity loop proportional coefficient, to ensure smooth and impact-free feed motion. Furthermore, the use of cutting fluid can help reduce vibration. High-pressure cooling (pressure ≥5MPa) reduces tool-chip adhesion, reduces cutting force fluctuations, and cools the cutting zone to reduce thermal deformation, indirectly suppressing vibration. By combining these measures, turning vibration amplitude can be controlled within 0.01mm, ensuring a workpiece surface roughness of Ra1.6μm or less.
