How to Overcome Deformation in CNC Turning of Thin-Walled Parts?

When cutting, thin-walled parts are prone to deformation due to the action of cutting force, resulting in an ellipse or small in the middle and large at both ends of the ”waist” phenomenon. In addition, the thin-walled metal processing heat dissipation is poor, easy to occur thermal deformation, the processing quality of the workpiece is difficult to guarantee. The parts shown in the following figure are not only inconvenient to clamp but also difficult to process the processing parts, so special thin-walled cylinders and shaft guards need to be designed.

1. Process analysis

According to the technical requirements specified in the drawings, the inner and outer wall surface roughness of the seamless steel pipe can reach about 1.6 microns, which can be achieved through turning, and the inner and outer holes are 0.03mm, which requires higher requirements for thin-walled parts. When mass production, the process flow is rough: blanking – front–end face – car outer circle – inside the hole – quality inspection.

The ”inner hole machining” process is essential for quality control. Throw out the outer circle, a point on the thin wall, a point on the inner hole, it is difficult to guarantee 0.03mm.

2. Key technology of car hole

The rigidity and cutting performance of the inner hole turning tool is the key technology of turning holes. Increase the rigidity of turning tool hole, take the following measures:

Increase the cutting area of the inner turning tool as much as possible. Usually, the tip of the inner turning tool is located above the handle. In this way, the less depression of the handle, the smaller the cutting area is, as shown in the figure below. If the tip of the inner hole turning tool is placed on the centerline of the shank, the shank volume in the hole can be greatly increased, as shown in the figure below.

The extension length of the tool shank should be as long as possible to reach the length of 5-8mm after processing, in order to increase the rigidity of the tool shank and reduce the vibration during cutting.

3. Deal with chip problems.

A rough turning tool mainly controls the cutting direction, and the cutting direction is the surface to be machined (front row chip). For this reason, as shown in the figure below, the inner hole turning tool with positive edge inclination is used.

When finishing turning, it is required that the chip flow toward the front chip tilted toward the center (hole center chip discharge), so we should pay attention to the direction of sharpening the knife, and use the forward inclined circular arc method. The fine turning tool alloy shown in the following figure uses YA6, the current M type, which has better bending strength, viscosity resistance, impact toughness, and temperature with the steel.

When grinding the edge, the front Angle is ground in A circle with an arc Angle of 10°-15°, the back Angle is 0.5-0.8mm from the wall according to the machining arc (the radian of the tool bottom line), the cutting edge Angle K direction of C is §0.5-1, the trimming edge is R1-1.5 along with the chip edge point B, the secondary back Angle is ground into 7°-8°, and the A-A point of the inner edge of E is ground into A circle to discharge chip outward.

4. Treatment method.

(1) A shaft guard must be made before processing. The main purpose of shaft protection: to cover the thin-wall inner hole of the car according to the original size, fix it with the front and rear top, so that it does not deform when processing the outer circle, and maintain the quality and precision of the outer circle processing. Therefore, the processing of shaft protection is the key process in thin-walled steel wire processing.

Shaft guard hair embryos were fabricated with 45# carbon structure round steel. The end face of the front, open two B-shaped pointed holes, the outside of the car thick round, leaving 1mm. By quenching and tempering, and then finishing the car to leave 0.2 mm allowance for grinding. Remake the fire surface with HRC50 hardness, and then grind it into the shape as shown below with a cylindrical grinder. The accuracy meets the requirements. Use it after completion.

(2) in order to make the workpiece a machining completion, the hair embryo leaves the clamp and cut off the allowance.

(3) Hair embryos should be conditioned and tempered first, and the hardness should be HRC28-30(hardness in the machinable range).

C620 is used for turning tools. First, the front tip is fixed on the fixed long cone surface. In order to prevent the deformation of the long cone surface when clamping the thin-walled sleeve, an open-ring thick sleeve is added, as shown in the figure below.

In order to ensure mass production, one end of the outer circle of the thin-walled cylinder is processed into a uniform size, and the T ruler is the axial clamping position. After the thin-walled cylinder body is pressed, the quality of the hole in the car can be improved and the size can be maintained. Since a certain amount of heat is generated, it is difficult to know the size of the expansion. The cutting fluid should be fully poured to reduce thermal deformation.

(5) with automatic centering three-jaw chuck clamping front end, thick inner ring. The fine car with a margin of 0.1-0.2mm should be processed to the requirements of the full matching and roughness of the shaft that requires a margin.Unload the inner hole turning tool, insert the guard shaft to the front tip, clamp it with the tail tip according to the length requirement, replace the outer turning tool, rough the outer turning, and finish the turning to meet the requirements of the drawing. After passing the inspection, use the cutting knife to cut off according to the required length. In order to make the broken shaft smooth when disconnected, the blade should be oblique grinding, so that the end face of the workpiece is smooth; The shaft guard is grinding less when the shaft is broken, so as to leave a gap on the broken shaft; In order to reduce the deformation of the broken shaft, prevent vibration, and the cause of the fall and collision of the broken shaft, the shaft guard is grinding less when the broken shaft is disconnected.

 5. Summary.

Using the above method to process thin-walled parts, the problem of deformation, size error, and shape error can not meet the requirements is solved. The practice has proved that the processing efficiency is high, the operation is simple, and it is suitable for the thin-walled parts with long processing time, easy to master size, one-time completion, and batch production.

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