CNC Machining of eccentric shafts
Eccentric shafts are crucial components in mechanical transmissions. Their axis is offset from the reference axis, enabling specific motion or force transmission through eccentric rotation. Examples include crankshafts in internal combustion engines and eccentric shafts in stamping presses. CNC Machining eccentric shafts presents significant challenges, primarily due to the precise control of eccentricity, workpiece clamping stability, and vibration suppression during CNC machining. The appropriate turning method and process parameters must be selected based on the eccentric shaft’s structural dimensions, eccentricity, and required precision to ensure optimal CNC machining quality and production efficiency.
Process analysis and preparation before turning an eccentric shaft are fundamental to ensuring CNC machining quality. This process begins with analyzing the part drawing, selecting the blank, and determining the clamping plan. First, the key parameters of the eccentric shaft must be determined, including the base shaft diameter, eccentric shaft diameter, eccentricity, length of each shaft segment, and precision requirements (such as eccentricity tolerance, roundness, and cylindricity). For example, an eccentric shaft has a base shaft diameter of 50mm, an eccentric shaft diameter of 30mm, an eccentricity of 10mm, an eccentricity tolerance of ±0.03mm, and a surface roughness Ra of 1.6μm. The blank is typically made of forgings or round steel. For large eccentric shafts, forgings can improve the material’s internal structure and mechanical properties; for small eccentric shafts, round steel can reduce costs. The blank diameter should be determined based on the maximum shaft segment diameter and CNC machining allowance, typically allowing 3-5mm of CNC machining allowance. The clamping scheme needs to be selected according to the size of the eccentricity and the precision requirements. When the eccentricity is small and the precision requirements are not high, the three-jaw chuck clamping method can be used. When the eccentricity is large and the precision requirements are high, the four-jaw chuck or two-center clamping method should be used.
The clamping method chosen for eccentric shaft turning directly impacts CNC machining accuracy and efficiency and requires flexible application based on specific circumstances. For small parts with a single eccentric shaft and an eccentricity e ≤ 5mm, a three-jaw chuck with a shim can be used. By calculating the shim thickness (x = 1.5e + k) and adjusting it through trial cuts, eccentric positioning can be quickly achieved, making it suitable for single-piece, small-batch production. For eccentric shafts with an eccentricity e = 5-15mm and requiring higher precision, a four-jaw chuck is more suitable. Correction is achieved by marking the eccentric axis to coincide with the spindle axis, with an accuracy of up to 0.02mm. However, this requires the operator to possess proficient correction skills. For slender eccentric shafts with an aspect ratio greater than 5, a two-center clamping method should be used to avoid bending deformation during CNC machining. Eccentric center holes are machined at both ends of the workpiece, with center supports ensuring workpiece rigidity. Used in conjunction with a toolholder, processing stability can be further improved. For mass-produced eccentric shafts, eccentric chucks or special fixture clamping methods are the best choice to improve efficiency, which can achieve rapid positioning and clamping and ensure the consistency of eccentricity.
When turning eccentric shafts, the cutting parameters should consider the centrifugal force generated by eccentric rotation to reduce vibration and tool wear. The cutting speed should be lower than that of ordinary shaft parts. When turning steel eccentric shafts with high-speed steel tools, the cutting speed should be 30-60 m/min, while carbide tools can use 80-120 m/min. Lower cutting speeds can reduce vibration caused by centrifugal force. The feed rate should be determined according to the CNC machining stage. For rough turning, the feed rate should be 0.2-0.4 mm/r to quickly remove excess stock; for finish turning, the feed rate should be 0.1-0.15 mm/r to ensure surface quality. The depth of cut should be moderate, with a single cut of no more than 3 mm during rough turning to avoid excessive radial forces that may cause workpiece bending; for finish turning, the depth of cut should be 0.5-1 mm to correct for form errors. Due to the unbalanced forces generated by the rotation of the eccentric shaft, the cutting parameters should be appropriately reduced during cutting. If necessary, balance blocks should be installed in non-CNC machining areas to reduce the impact of vibration on CNC machining accuracy. Extreme pressure emulsions should be used as the cutting fluid, using high-pressure spraying to cool and lubricate the cutting area, reducing cutting temperatures and tool wear.
Precision control in eccentric shaft turning requires a focus on eccentricity, form, and position errors. Eccentricity can be measured using a dial indicator. Mount the workpiece between two centers, contact the outer diameter of the eccentric shaft with the dial indicator probe, and rotate the workpiece one full revolution. The actual eccentricity is half the difference between the maximum and minimum readings on the dial indicator, with a measurement accuracy of up to 0.01mm. For eccentric shafts requiring higher precision (e.g., an eccentricity tolerance of ±0.01mm), specialized measuring tools or a coordinate measuring machine (CMM) are required. Form errors primarily include the roundness and cylindricity of the eccentric shaft, which can be reduced by controlling cutting parameters and tool geometry. During finish turning, use a smaller feed rate and a sharper tool edge to avoid chatter marks on the workpiece surface. Position error primarily refers to the parallelism of the eccentric shaft with the reference axis. During clamping, the parallelism error between the two axes must not exceed 0.02mm/m. This can be achieved by correcting the radial runout of the workpiece reference axis. For crankshaft parts with multiple eccentric sections, it is also necessary to control the phase angle error between each eccentric shaft. Generally, an indexing fixture or the indexing function of a CNC lathe is used to ensure phase accuracy.
Common problems and solutions in eccentric shaft turning are important guarantees for smooth CNC machining. If excessive vibration occurs during CNC machining, it may be due to loose workpiece clamping, excessive eccentricity, or excessively high cutting parameters. This can be solved by increasing the clamping force, adding a balancing block, or reducing the cutting speed. If the eccentricity error is out of tolerance, it is necessary to check the clamping correction accuracy or whether the shim thickness calculation is correct. Recalibrate or adjust the shim thickness before conducting a trial cut. If ripples or scratches appear on the workpiece surface, it may be caused by tool wear, insufficient cutting fluid, or excessive spindle clearance. The tool needs to be replaced, the cutting fluid needs to be replenished, or the spindle clearance needs to be adjusted in a timely manner. For bending deformation of slender eccentric shafts, the reverse cutting method (the tool is fed from the head to the foot of the bed) can be used to use the cutting force to offset part of the bending torque, or aging treatment can be used to eliminate CNC machining stress. By taking appropriate solutions to different problems, the turning quality of the eccentric shaft can be effectively improved and the performance of the parts can be guaranteed.
With the increasing popularity of CNC lathes, the turning of eccentric shafts has developed towards automation and high precision. Through programming, CNC lathes can precisely control spindle rotation and tool feed, enabling automatic turning of eccentric shafts. For complex parts with multiple eccentricities or variable eccentricity, CNC CNC machining can significantly improve CNC machining accuracy and efficiency. In CNC turning, the G75 eccentricity cycle instruction or macro programming can be used to automatically calculate the eccentricity trajectory and complete CNC machining, reducing human error. Furthermore, the automatic feeding device and online measurement system equipped on CNC lathes enable continuous batch processing of eccentric shafts and real-time quality monitoring, further improving production efficiency and quality stability. In the future, with the development of intelligent manufacturing technology, the turning of eccentric shafts will achieve digital control of the entire process, from design to finished product, providing the machinery manufacturing industry with higher-quality eccentric shaft parts.
