Methods for preventing and reducing deformation of thin-walled workpieces
Thin-walled workpieces, defined as those with a small ratio of wall thickness to diameter or length, are susceptible to deformation during machining due to their poor rigidity and strength, influenced by factors such as cutting forces, clamping forces, and cutting heat. This can lead to reduced machining accuracy and even scrap. Therefore, research on methods to prevent and reduce deformation in thin-walled workpieces is crucial for ensuring machining quality and improving production efficiency. These methods involve multiple aspects, including clamping method, tool selection, cutting parameters, and machining technology, requiring a comprehensive approach to achieve optimal results.
Choosing the right clamping method is the primary measure to prevent deformation of thin-walled workpieces. Traditional hard clamping with a three-jaw self-centering chuck can easily cause workpiece deformation, so an improved clamping method is needed. For thin-walled sleeve-like workpieces, open sleeves or sector-shaped soft jaws can be used for clamping. Open sleeves increase the contact area with the workpiece and reduce the clamping force per unit area. Sector-shaped soft jaws can be bored to fit perfectly with the workpiece’s outer diameter, preventing excessive localized force. For thin-walled plate-like workpieces, vacuum suction cups or magnetic worktables can be used for clamping, utilizing uniform suction or magnetic force to secure the workpiece and reduce clamping deformation. Alternatively, multi-point clamping with pressure plates can be employed, increasing the number of pressure plates and the contact area between the pressure plate and the workpiece to evenly distribute the clamping force. Furthermore, for complex, thin-walled workpieces, specialized fixtures can be designed. Through proper positioning and distribution of clamping points, these fixtures ensure uniform force during machining and minimize deformation.
Tool selection and optimization of geometric parameters are crucial for minimizing deformation in thin-walled workpieces. Suitable tools can reduce cutting forces and minimize deformation. Tool materials should be carbide or ceramic tools with good wear resistance and high cutting efficiency to reduce tool wear and ensure stable cutting forces. Tool geometry should be optimized to minimize cutting forces. The rake angle should be large (15°–25°) to sharpen the blade and minimize deformation. The back angle should be 8°–12° to reduce friction between the back face and the workpiece. A large lead angle (75°–90°) should be used to reduce radial cutting forces, which are one of the main causes of deformation in thin-walled workpieces. Furthermore, the tool’s nose radius should be appropriately reduced, generally 0.2–0.5 mm, to reduce the impact of cutting forces on the workpiece. The tool’s extended length should be kept as short as possible to increase tool rigidity and avoid vibration.
Optimizing cutting parameters can effectively reduce machining deformation in thin-walled workpieces. A reasonable combination of cutting speed, feed rate, and depth of cut can reduce cutting forces and heat. The cutting speed should be high. For thin-walled steel workpieces, carbide cutting tools can use a cutting speed of 100-200 m/min. Higher cutting speeds reduce cutting forces, facilitate rapid chip removal, and reduce friction with the workpiece. However, excessively high cutting speeds can increase cutting temperatures, leading to thermal deformation of the workpiece. Therefore, the feed rate should be appropriately determined based on the workpiece and tool materials. A low feed rate (0.05-0.15 mm/r) can reduce cutting forces and prevent significant elastic deformation in the workpiece. However, a low feed rate can reduce machining efficiency and easily cause tool slippage. Therefore, efficiency must be balanced with quality. The depth of cut should be layered, with each cut limited to a minimum depth (0.1-0.5 mm). Multiple passes should be used to remove excess stock, evenly distribute cutting forces, and minimize workpiece deformation.
Controlling deformation caused by cutting heat is crucial for preventing deformation in thin-walled workpieces. Cutting heat causes thermal expansion in the workpiece, which in turn causes contraction and deformation upon cooling, affecting dimensional accuracy. To reduce the generation and impact of cutting heat, cutting fluid should be used extensively. This fluid can dissipate significant cutting heat and lower workpiece temperature. Emulsions can be used for machining steel, while kerosene or specialized cutting fluids can be used for machining aluminum alloys. The cutting fluid should be sprayed directly onto the cutting surface via a high-pressure jet to enhance cooling. Intermittent cutting can also be employed, with appropriate pauses during machining to allow sufficient time for the workpiece to dissipate heat and prevent heat accumulation. For thin-walled workpieces requiring the utmost precision, machining can be performed under constant temperature to minimize the impact of ambient temperature fluctuations on workpiece deformation.
A rational machining process can fundamentally reduce deformation in thin-walled workpieces. The principles of “separate roughing and finishing, inner and outer surfaces first, and symmetrical machining” should be adhered to. During roughing, most of the stock is removed, resulting in better workpiece rigidity and minimal deformation. After roughing, aging treatment should be performed to eliminate internal stresses and prevent deformation during subsequent machining. Aging can be either natural or artificial, and multiple aging treatments may be required for workpieces requiring high precision. Finishing should be performed after roughing and aging, as only minimal stock removal is required, resulting in lower cutting forces and minimal deformation. The machining sequence should prioritize inner surfaces over outer surfaces, as these are more rigid and less deformed after inner machining. For thin-walled workpieces with symmetrical structures, a symmetrical machining approach should be adopted. This involves starting from the center of symmetry and evenly removing stock to both sides to ensure uniform stress distribution and minimize deformation. Furthermore, during finishing, multiple surfaces should be machined in a single setup, minimizing the number of setups and preventing deformation caused by clamping errors.
