Machining of double eccentric sleeves
Double eccentric sleeves are sleeve-type components with two different eccentricities. They are widely used in transmission mechanisms in precision machinery, textile machinery, printing machinery, and other equipment. The coordination of the two eccentric structures enables complex motion transmission, such as in eccentric cam mechanisms and reciprocating motion mechanisms. Key parameters for double eccentric sleeves include the eccentricities of the two eccentric holes (e1 and e2), the outer diameter and length of the sleeve, and the angular relationship between the two eccentric holes (e.g., 90°, 180°, etc.). The difficulty in machining double eccentric sleeves lies in ensuring the accuracy of the two eccentricities, the parallelism of the two eccentric holes, and the positional accuracy relative to the outer circle. Therefore, a rational design of the machining process and the selection of appropriate clamping and measurement methods are required.
The material selection and blank preparation for double eccentric sleeves must be determined based on the operating conditions. Common materials include 45 steel, 40Cr, and QT450-10. 45 steel is suitable for double eccentric sleeves under normal loads. After quenching and tempering, it reaches a hardness of 220-250HB and exhibits excellent overall mechanical properties. 40Cr is suitable for double eccentric sleeves under medium loads. After quenching and tempering, it reaches a hardness of 240-280HB and exhibits high strength and toughness. QT450-10 ductile iron is suitable for double eccentric sleeves subjected to impact loads and offers high wear resistance and shock absorption. Blank type is selected based on production batch size. For small batches of single pieces, round steel blanks are used; for large batches, die forgings or castings are used to improve material utilization and mechanical properties. The dimensional tolerance of the blank needs to be determined according to the machining allowance. Generally, a machining allowance of 5-8mm is reserved for the outer diameter and length, and a machining allowance of 3-5mm is reserved for the inner hole. The straightness error of the blank should be ≤0.5mm/m to avoid affecting the subsequent machining accuracy.
The machining process for double eccentric bushings must adhere to the principle of “datum first, step-by-step processing.” A typical process is: blanking → forging/casting → annealing → rough turning the outer diameter and end faces → quenching and tempering → semi-finishing the outer diameter and end faces (determining the datum) → marking the first eccentric hole → drilling the first eccentric hole bottom hole → rough turning the first eccentric hole → marking the second eccentric hole line → drilling the second eccentric hole bottom hole → rough turning the second eccentric hole → aging → semi-finishing both eccentric holes → finish turning the outer diameter and end faces → finish turning both eccentric holes → final inspection. The primary task of the rough turning stage is to remove the majority of the stock, laying the foundation for subsequent machining. After rough turning, a 2-3mm semi-finishing stock is reserved for the outer diameter and inner hole, and a 1-2mm stock is reserved for the end faces. Quenching and tempering improves the material’s cutting and mechanical properties and reduces deformation during subsequent machining. When semi-finishing the outer circle and end face, the accuracy of the reference surface must be guaranteed. The roundness of the reference outer circle is ≤0.02mm, and the perpendicularity between the end face and the reference axis is ≤0.01mm/100mm, providing a reliable positioning reference for the processing of eccentric holes.
The clamping method for double eccentric sleeves is crucial for ensuring eccentricity accuracy. The appropriate clamping method must be selected based on the angular relationship between the two eccentric holes. When machining the first eccentric hole, a four-jaw single-action chuck can be used for alignment. The runout of the outer diameter can be measured with a dial indicator, and the jaw position can be adjusted to achieve the required first eccentricity. Alignment accuracy can reach 0.005mm. Alternatively, a dedicated eccentric clamp can be used, with locating pins mating with the workpiece’s reference holes to ensure consistent eccentricity. When machining the second eccentric hole, indexing is required based on the angular relationship between the two eccentric holes (e.g., 90°). This can be done using an indexing head or by marking on a four-jaw single-action chuck to ensure an angular error of ≤±5° between the two eccentric holes. For mass production of double eccentric sleeves, a dedicated indexing fixture is used, enabling automated indexing and clamping, improving production efficiency and maintaining an angular error of ≤±2°. Workpiece rigidity should be considered during clamping. For long, slender double eccentric sleeves with an aspect ratio greater than 5, a steady rest or toolholder is required to prevent bending and deformation during machining.
The turning process of double eccentric sleeves requires a phased approach to gradually improve precision. For rough turning of eccentric holes, use a YT15 carbide boring tool at a cutting speed of 80-100 m/min, a feed rate of 0.2-0.3 mm/r, a cutting depth of 2-3 mm, and a semi-finishing allowance of 0.5-1 mm. During semi- finishing, correction for eccentricity and roundness errors is necessary. A trial cut can be used to measure the eccentric hole dimensions and eccentricity, adjust the tool position, and allow a finishing allowance of 0.1-0.3 mm after semi-finishing. For finishing, use a W18Cr4V high-speed steel boring tool or a TiAlN-coated carbide boring tool at a cutting speed of 60-80 m/min, a feed rate of 0.1-0.15 mm/r, and a cutting depth of 0.1-0.2 mm. Use emulsion cooling to ensure a surface roughness Ra ≤ 1.6 μm. When finishing the outer circle, the axis of the two eccentric holes must be used as the reference to ensure that the coaxiality error between the outer circle and the eccentric hole is ≤0.02mm.
The precision inspection of double eccentric sleeves requires parameters such as dimensional accuracy, form and position tolerances, and eccentricity. Dimensional accuracy is inspected using inside and outside micrometers to measure the diameters of the inner and outer holes, with tolerances generally controlled to IT7-IT8. Form and position tolerances are inspected using a dial indicator and V-block to measure the roundness (≤0.01mm), cylindricity (≤0.015mm), and parallelism (≤0.01mm/100mm) of the eccentric hole. Eccentricity is measured using a dial indicator or eccentricity gauge. The dial indicator method places the outer diameter of the double eccentric sleeve on a V-block and rotates the workpiece. The eccentricity is determined by taking half the difference between the maximum and minimum readings on the dial indicator, with a measurement accuracy of up to 0.005mm. The eccentricity gauge method offers even higher accuracy, reaching 0.001mm, and is suitable for inspecting high-precision double eccentric sleeves. The angular relationship between the two eccentric holes can be measured using a universal angle ruler or a three-dimensional coordinate measuring machine. The angular error should be ≤±5′. For mass-produced double eccentric bushings, first-article inspection and sampling inspection are required. Mass production can only begin after the first-article inspection passes. The sampling ratio is generally 5%-10% to ensure the stability of product quality.
