Using an eccentric chuck to turn an eccentric workpiece
Using an eccentric chuck to turn eccentric workpieces is an efficient and precise machining method suitable for mass-produced parts with one or more eccentric journals, such as eccentric shafts, crankshafts, and cams. By adjusting the eccentricity, an eccentric chuck can quickly clamp eccentric workpieces of varying specifications, eliminating the tedious, multiple alignment steps required with traditional four-jaw chucks. This method offers machining efficiency 3-5 times higher than a four-jaw chuck, and boasts an eccentricity accuracy of 0.01-0.03mm. It is widely used in the automotive, engine, and compressor manufacturing industries. Mastering the eccentric chuck’s structural types, adjustment methods, clamping techniques, and turning processes is crucial for ensuring high-quality machining of eccentric workpieces.
Eccentric chucks can be categorized by function into single, double, and multiple eccentric chucks, each suitable for machining different eccentric workpieces. A single eccentric chuck consists of a base, an eccentric body, and a three-jaw chuck. The eccentric body rotates about the center of the base, and the eccentricity is adjusted using a dial (ranging from 0 to 50 mm). This makes it suitable for machining single-eccentric workpieces, such as eccentric shafts and eccentric sleeves. A double eccentric chuck, based on the single eccentric chuck, incorporates an independently adjustable eccentric mechanism, allowing for simultaneous adjustment of two different eccentricities. This makes it suitable for machining double-eccentric workpieces, such as crankshaft connecting rod journals. Multiple eccentric chucks, with multiple eccentric adjustment units, can process complex workpieces with three or more eccentric components. While offering high adjustment accuracy, they also come in a complex structure and are primarily used in precision manufacturing. Eccentric chucks typically have a centering accuracy of 0.01 mm and a repeatability of ≤ 0.005 mm. Chuck diameters are available in sizes such as 200 mm, 320 mm, and 400 mm, depending on the workpiece size.
Adjusting the eccentric chuck is a key step in ensuring eccentricity accuracy, primarily involving eccentricity setting, centering, and balancing. Setting the eccentricity requires following the workpiece drawing requirements. Loosen the eccentric body’s retaining bolts and rotate the eccentric body until the dial indicates the desired eccentricity (e). For example, to machine an eccentric shaft with e = 5mm, adjust the dial to 5mm and tighten the retaining bolts. The dial accuracy is typically 0.01mm. After adjustment, recheck with a dial indicator. Measure the eccentric body’s outer diameter. Rotate the chuck one full revolution. The actual eccentricity is 1/2 the difference in dial indicator readings. This deviation must be ≤ 0.01mm. Centering ensures that the workpiece’s reference journal is concentric with the chuck. Install the workpiece’s reference journal into the three-jaw chuck and gently clamp it. Use a dial indicator to align the reference journal’s radial runout to ≤ 0.01mm, then tighten the chuck. For long eccentric workpieces, a centering tip is required at the tailstock end. The centering tip axis must coincide with the eccentric axis of the eccentric chuck. This can be achieved by adjusting the tailstock offset, with a deviation of ≤0.01mm. Balancing is used to reduce centrifugal forces during high-speed rotation. A balancing weight is installed in the balancing groove of the eccentric body. The weight and position of the balancing weight are calculated based on the eccentricity and workpiece weight to ensure that the vibration speed during chuck rotation is ≤0.1mm/s.
The skill of clamping eccentric workpieces with eccentric chucks directly affects machining stability and accuracy, and the appropriate clamping method needs to be selected according to the workpiece structure. For eccentric shafts with center holes at both ends, a “one clamp and one push” clamping method is adopted. The chuck end clamps the workpiece reference journal, and the tailstock center pushes the eccentric end center hole to ensure accurate axial positioning of the workpiece and axial movement ≤0.01mm. For eccentric sleeves without center holes, a soft-jaw chuck can be used to clamp the outer circle. The soft jaws need to be pre-machined according to the outer diameter size of the workpiece, with a contact area of ≥80%. The clamping force is controlled at 5-10kN to avoid workpiece deformation. For slender eccentric workpieces with an aspect ratio exceeding 5, additional tool holder support is required. The tool holder is supported on the workpiece reference journal, with a contact pressure of 0.2-0.3MPa with the workpiece to prevent bending deformation during machining. When clamping, pay attention to the center of gravity of the workpiece and try to make the center of gravity coincide with the rotation center of the chuck to reduce the influence of centrifugal force. For large eccentric workpieces weighing more than 50kg, auxiliary supports are required to prevent the chuck from being overloaded and damaged.
When turning eccentric workpieces with an eccentric chuck, the cutting parameters must be adjusted according to the eccentricity, workpiece material, and machining stage, with a focus on controlling cutting forces and vibration. During rough turning, the primary focus is stock removal. A cutting speed of 30-60 m/min (steel) or 20-40 m/min (cast iron), a feed rate of 0.2-0.3 mm/r, and a depth of cut of 1-3 mm are recommended. A turning tool with a larger entering angle (90°) should be used to reduce radial cutting forces. During finish turning, dimensional accuracy and surface quality must be maintained. A cutting speed of 60-100 m/min (steel) or 40-70 m/min (cast iron), a feed rate of 0.1-0.15 mm/r, and a depth of cut of 0.1-0.3 mm should be used. A carbide tool (such as YT15) should be used, with a rake angle of 10°-15° and a relief angle of 6°-8°. Adequate cooling is essential during cutting. High-pressure cutting fluid (1-2 MPa) should be sprayed directly onto the cutting zone to reduce cutting temperatures and prevent tool wear. Due to the centrifugal force when the eccentric workpiece rotates, the speed should not be too high, usually ≤800r/min. The larger the eccentricity, the lower the speed should be. For example, when e=10mm, the speed should be ≤500r/min to avoid excessive vibration affecting the processing quality.
The quality inspection and error control of turning eccentric workpieces with eccentric chucks is the final step to ensure qualified processing. The main inspection items include eccentricity, roundness, cylindricity and surface roughness. The eccentricity inspection adopts the dial indicator method. The workpiece is mounted on the yaw meter. The dial indicator probe touches the eccentric shaft neck. The workpiece is rotated one circle. The 1/2 of the difference between the maximum and minimum values of the dial indicator reading is the actual eccentricity. The error must be controlled within the range required by the drawing (usually ±0.02mm). The roundness and cylindricity inspection uses a roundness meter. The roundness error of the eccentric shaft neck is ≤0.005mm, and the cylindricity error is ≤