Processing characteristics of slender shafts
Slender shafts, defined as shaft parts with a length-to-diameter ratio greater than 20, are widely used in mechanical manufacturing, such as lead screws in machine tools and connecting rods in internal combustion engines. Their machining characteristics primarily include poor rigidity, easy deformation, and difficulty ensuring precision, presenting numerous challenges. Due to their small diameter and long length, slender shafts are susceptible to bending and deformation under the influence of cutting forces, gravity, and thermal stresses. This can lead to problems such as straightness errors and cylindricity errors in the machined shafts. These problems can also cause tool vibration, impacting surface finish. Therefore, understanding the machining characteristics of slender shafts and implementing appropriate process measures are crucial to ensuring machining quality.
The primary difficulty in machining slender shafts is insufficient rigidity, a characteristic that is particularly pronounced during the cutting process. During turning, the workpiece is in a cantilevered state between the chuck and the center. When subjected to radial cutting forces, bending vibrations are easily generated. This vibration not only causes ripples on the workpiece surface, but also increases tool wear and reduces machining accuracy. To improve the rigidity of slender shafts, auxiliary support devices such as a tool rest and a center rest are usually used during machining. The tool rest can move with the tool, supporting the workpiece from the side, effectively offsetting radial cutting forces and reducing bending deformation; the center rest is used to support the middle part of the workpiece and is suitable for machining longer slender shafts, but it requires pre-machined support steps on the workpiece, which increases the complexity of the process. In addition, the reasonable selection of cutting parameters can also reduce the impact of insufficient rigidity. For example, using a smaller back cut and feed rate can reduce cutting forces, thereby reducing deformation of the workpiece.
Thermal deformation is another significant characteristic of slender shaft machining. During the cutting process, friction between the tool and the workpiece generates significant heat, causing the shaft to heat up. Due to the shaft’s long length and being fixed at both ends, it cannot freely expand or contract when heated. This results in significant thermal stress, leading to bending and deformation of the shaft. This thermal deformation can severely impact machining accuracy, especially for slender shafts requiring high precision, where thermal deformation can cause dimensional deviations to exceed tolerances. To minimize the effects of thermal deformation, effective cooling measures are required during machining, such as using cutting fluid to forcefully cool the cutting area and reduce workpiece temperature. Reverse cutting can also be effective. During reverse cutting, the workpiece is heated in the opposite direction of the feed direction, facilitating heat dissipation and reducing thermal deformation. Furthermore, by rationally arranging the process steps, aging treatment can be performed after rough machining to eliminate internal stresses before finishing to ensure final machining accuracy.
Machining slender shafts places special demands on both tools and fixtures. Due to the low rigidity of slender shafts, improper tool geometry can increase cutting forces and exacerbate workpiece deformation. Therefore, a tool with a larger lead angle should be selected. Increasing the lead angle reduces radial cutting forces, which helps minimize workpiece bending. The rake and relief angles should also be appropriately increased to enhance sharpness and reduce cutting resistance. Regarding tool material, carbide tools can be used for roughing to improve cutting efficiency, while high-speed steel tools are preferred for finishing to achieve superior surface quality. The choice of fixture is equally important. Traditional three-jaw chucks can easily cause axial movement and radial deformation in the workpiece. Therefore, when machining slender shafts, a chuck is often used to clamp one end and a center support the other. A certain amount of clearance is left on one end of the chuck to prevent workpiece deformation caused by excessive clamping force. For slender shafts requiring extremely high precision, a flexible center can be used to compensate for thermal expansion and minimize the effects of thermal deformation.
Precision control during slender shaft machining is challenging, primarily in ensuring straightness, cylindricity, and surface roughness. Due to workpiece deformation and vibration, achieving the required straightness after machining is often difficult, necessitating a rational machining process. For example, multiple clamping and segmented machining can be employed, with support positions readjusted after each segment to minimize cumulative workpiece deformation. Semi-finishing can be performed before finishing to remove most of the excess stock and prepare for final machining. Cylindricity control requires ensuring the rotational accuracy of the lathe spindle and tool stability. Regular checks should be made for radial runout and axial play, and worn bearings and guideways should be promptly replaced. Surface roughness control depends primarily on the selected cutting parameters and tool condition. During final machining, high spindle speeds and low feed rates should be used, and the tool should be kept sharp and free of wear. Post-machining inspection is also crucial. Instruments such as dial indicators and micrometers are used to rigorously measure the various precision indicators of the slender shaft. Exceeding tolerances should be repaired or reworked to ensure product quality meets design requirements.
