How to Overcome Deformation? Tips for CNC Turning Thin-walled Parts

During the cutting process, thin walls are easily deformed by the cutting force, resulting in an elliptical or "waist-shaped" phenomenon with a small middle and large ends. In addition, due to the poor heat dissipation during processing, thin-walled sleeves are very easy to produce thermal deformation, and it is not easy to ensure the processing quality of parts. The parts in the figure below are not only inconvenient to clamp, but also difficult to process the processing parts. It is necessary to design a special thin-walled sleeve and shaft guard.

Process analysis
According to the technical requirements provided by the drawings, the workpiece is processed with seamless steel pipes. The surface roughness of the inner hole and the outer wall is Ra1.6μm, which can be achieved by turning, but the cylindricality of the inner hole is 0.03mm, which is relatively high for thin-walled parts. In mass production, the process route is roughly: cutting-heat treatment-turning end face-turning outer circle-turning inner hole-quality inspection.

The "inner hole processing" process is the key to quality control. If we put aside the outer circle and thin-walled sleeves and cut the inner hole, it is difficult to ensure a 0.03mm cylinder.

Key technology for turning holes
The key technology for turning holes is to solve the rigidity and chip removal problems of the inner hole turning tool. To increase the rigidity of the internal hole turning tool, take the following measures:

(1) Try to increase the cross-sectional area of ​​the tool holder. Usually, the tip of the internal hole turning tool is located on the top of the tool holder, so the cross-sectional area of ​​the tool holder is small, less than 1/4 of the cross-sectional area of ​​the hole, as shown in the left figure below. If the tip of the internal hole turning tool is located on the center line of the tool holder, the cross-sectional area of ​​the tool holder in the hole can be greatly increased, as shown in the right figure below.

(2) The tool holder extension length should be 5-8mm longer than the length of the workpiece to increase the rigidity of the tool holder and reduce the vibration during the cutting process.

Solve the chip removal problem
Mainly control the direction of the cutting outflow. The rough turning tool requires the chips to flow to the surface to be processed (front chip removal). For this purpose, an internal hole turning tool with a positive edge inclination angle is used, as shown in the figure below.

When finishing, the chip flow direction is required to be centripetal and forward chip removal (hole center chip removal). Therefore, when sharpening the tool, attention should be paid to the grinding direction of the cutting edge. The chip removal method of the arc should be forward. As shown in the figure below, the alloy of the finishing tool is YA6, the current M type, which has good bending strength, wear resistance, impact toughness, and anti-sticking and temperature with steel.

When sharpening, the front angle is ground to a circular arc angle of 10-15°, the back angle is 0.5-0.8mm away from the wall according to the processing arc (the arc of the tool bottom line), the c cutting edge angle k is §0.5-1 along the chip edge B point finishing edge is R1-1.5, the secondary back angle is ground to 7-8°, and the A-A point of the E inner edge is ground to a circle to remove chips outward.

Processing method
Before processing, a shaft guard must be made. The main purpose of the shaft guard is to cover the inner hole of the machined thin-walled sleeve with the original size, and fix it with the front and rear centers so that it can process the outer circle without deformation, and maintain the quality and accuracy of the outer circle processing. Therefore, the processing of the shaft guard is a key link in the process of processing thin-walled sleeves.

45﹟ carbon structural round steel is used to process the shaft guard blank; the end face is turned, the B-type center holes at both ends are opened, and the outer circle is roughly turned, leaving a 1mm allowance. After heat treatment, tempering and shaping, fine turning and grinding with a 0.2mm allowance. Re-heat treat the broken fire surface, the hardness is HRC50, and then it is ground by the external cylindrical grinder as shown in the figure below, the accuracy meets the requirements, and it is ready for use after completion.

In order to complete the workpiece processing in one go, the blank leaves a clamping position and cutting allowance.
First, the blank is heat treated, tempered and shaped, and the hardness is HRC28-30 (hardness within the processable range).
The turning tool uses C620. First, the front center is placed in the spindle taper to fix it. In order to prevent the workpiece from deforming when clamping the thin-walled sleeve, an open-loop thick sleeve is added, as shown in the figure below.
In order to maintain mass production, one end of the outer circle of the thin-walled sleeve is processed to a uniform size d, and the size of t is the axial clamping position. The thin-walled sleeve is pressed tightly to improve the quality of turning the inner hole and maintain the size. Considering the generation of cutting heat, the expansion size of the workpiece is difficult to grasp. It is necessary to pour sufficient cutting fluid to reduce the thermal deformation of the workpiece.

Use an automatic centering three-jaw chuck to clamp the workpiece, turn the end face, and rough turn the inner circle. Leave a margin of 0.1-0.2mm for fine turning, and replace the fine turning tool to process the cutting margin to the shaft guard to meet the requirements of overfit and roughness. Remove the inner hole turning tool, insert the shaft guard to the front center, clamp it with the tailstock center according to the length requirements, change the outer circle turning tool to rough turn the outer circle, and then fine turn to meet the requirements of the drawing. After passing the inspection, use a cutting tool to cut according to the length requirements. In order to make the cut smooth when the workpiece is cut, the blade edge should be grinded obliquely to make the end face of the workpiece smooth; the shaft guard is ground smaller to leave a gap for cutting, and the shaft guard is to reduce the deformation of the workpiece, prevent vibration, and prevent it from falling and being damaged when cutting.

Conclusion
The above method for processing thin-walled sleeves solves the problem of deformation or size error and shape error that cannot meet the requirements. Practice has proved that the processing efficiency is high, easy to operate, and suitable for processing longer thin-walled parts. The size is easy to grasp, one-time completion, and batch production is also more practical.