During the cutting process, thin-walled parts are prone to deformation under cutting forces, resulting in elliptical or waist-shaped shapes that are smaller in the middle and larger at both ends. Furthermore, thin-walled sleeves have poor heat dissipation during machining, making them highly susceptible to thermal deformation and compromising part quality. The part shown in the image is not only inconvenient to clamp, but also difficult to machine, requiring the design of a dedicated thin-walled sleeve and shaft protector.
I. Process Analysis According to the technical requirements provided in the drawings, the workpiece is machined from seamless steel tubing. The surface roughness of the inner hole and outer wall is Ra 1.6μm, achievable by turning. However, the cylindricity of the inner hole is 0.03mm, a high requirement for thin-walled parts. In mass production, the process route is roughly: blanking-heat treatment-face turning-outer diameter turning-inner hole turning-quality inspection. The "inner hole machining" process is crucial for quality control. Ignoring the outer diameter and the thin-walled sleeve, it's difficult to guarantee a 0.03mm cylindricity during inner hole cutting. II. Key Technologies for Hole Turning The key technologies for hole turning are solving the rigidity and chip removal problems of the inner hole turning tool. To increase the rigidity of internal turning tools, the following measures can be taken: 1. Maximize the cross-sectional area of the tool shank. Typically, the tool tip of an internal turning tool is located on top of the tool shank, resulting in a relatively small cross-sectional area, less than 1/4 of the hole's cross-sectional area, as shown in the left figure below. If the tool tip is located on the centerline of the tool shank, the cross-sectional area of the tool shank in the hole can be significantly increased, as shown in the right figure below. 2. The tool shank extension length should be 5-8mm longer than the length of the workpiece to increase the rigidity of the tool shank and reduce vibration during cutting. III. Solving chip removal problems mainly involves controlling the direction of chip flow. Roughing tools require chips to flow towards the machined surface (front chip removal). Therefore, internal turning tools with a positive rake angle are used, as shown in the figure below. During finish turning, the chip flow should be centripetal and forward-leaning (chip removal from the center of the hole). Therefore, when sharpening the tool, pay attention to the grinding direction of the cutting edge and use a forward-leaning arc chip removal method, as shown in the figure below. The finish turning tool uses YA6 alloy, currently of type M, which has good bending strength, wear resistance, impact toughness, and resistance to adhesion and temperature with steel. When sharpening, the rake angle should be ground to a circular arc angle of 10-15°, the clearance angle should be 0.5-0.8mm away from the wall according to the machining arc (the bottom line of the tool follows the arc), the cutting edge angle c should be §0.5-1, and the finishing edge along the cutting edge B point should be R1-1.5. The secondary clearance angle should be ground to 7-8°. The inner edge A-A point should be ground into a circle for outward chip removal. IV. Machining Method 1. A retainer must be made before machining. The main purpose of the retainer is to fit the machined thin-walled sleeve inner hole to its original size, and fix it with front and rear centers so that the outer circle can be machined without deformation, maintaining the machining quality and accuracy of the outer circle. Therefore, the machining of the retaining shaft is a crucial step in the machining of the thin-walled sleeve. The retaining shaft blank is machined using 45# carbon structural round steel; the end face is machined, and two B-type center holes are drilled at both ends. The outer diameter is rough-machined, leaving a 1mm allowance. After heat treatment and tempering, it is then finish-machined, leaving a 0.2mm allowance, and ground. The surface is then re-heat-treated to a hardness of HRC50, and then ground on an external cylindrical grinder to the shape shown in the diagram below, achieving the required precision. It is then ready for use. 2. To ensure the workpiece can be machined in one pass, the blank is left with clamping and cutting allowances. 3. The blank is first heat-treated and tempered to a hardness of HRC28-30 (within the machinable range). 4. A C620 cutting tool is used. First, the front center is placed into the spindle taper position and fixed. To prevent workpiece deformation when clamping the thin-walled sleeve, an open-ring thick sleeve is added, as shown in the diagram below. To maintain mass production, one end of the thin-walled sleeve's outer diameter is machined to a uniform size d. The t is used for axial clamping, and the thin-walled sleeves are pressed together to improve quality during internal bore machining and maintain dimensional stability. Considering the heat generated during cutting, the workpiece's expansion dimensions are difficult to control. Sufficient cutting fluid needs to be poured in to reduce thermal deformation of the workpiece. 5. Clamp the workpiece firmly with an automatic centering three-jaw chuck, machine the end face, and rough machine the inner diameter. Leave a 0.1-0.2mm allowance for finish machining. Change to a finish machining tool to machine the required allowance to meet the requirements of the wear shaft's overfit and surface roughness. Remove the internal bore machining tool, insert the wear shaft to the front center, clamp it with the tailstock center according to the length requirements, change to an external bore machining tool for rough machining, and then finish machine to meet the drawing requirements. After inspection and approval, cut off to the required length using a cut-off tool. To ensure a clean cut when the workpiece is broken, the cutting edge should be beveled to make the workpiece end face flat. The section of the protective shaft that is ground smaller is to leave a gap during cutting. The protective shaft is also ground to reduce workpiece deformation, prevent vibration, and prevent damage from falling during cutting. V. Conclusion The above method for machining thin-walled sleeves solves the problem of deformation or dimensional and shape errors that prevent the workpiece from meeting requirements. Practice has proven that the machining efficiency is high, it is easy to operate, and it is suitable for machining long thin-walled parts. The dimensions are easy to control, and it can be completed in one go, making it practical for mass production.
