CNC Machining: How to Solve the Deformation of CNC Thin-Walled Workpiece?

Turning and milling thin-walled parts (aluminum, aluminum alloys, pure titanium, copper, magnesium alloys) are prone to deformation during CNC machining. The oval or "waist shape" has a small, medium and large end, which makes it difficult to guarantee the quality of the part. Its clamping design tends to be the most discussed point. Let's take a look at two examples of the design of thin-walled fixtures on turned and milled parts, and how they solve the deformation problem.

In the process of CNC turning, some thin-walled parts are often processed. When machining thin-walled workpieces, due to the poor stiffness of the workpiece, the deformation of thin-walled workpieces on CNC lathes during the turning process is common.

1. Due to the thin wall of the workpiece, it is easy to deform under the action of clamping pressure. Thus affecting the dimensional accuracy and shape accuracy of the workpiece. When the three-jaw chuck shown in Figure 1 is used to clamp the workpiece to machine the inner hole, it will slightly become a triangle under the action of the clamping force, but a cylindrical hole will be obtained after turning the hole. When the lower jaw is released and the workpiece is removed, due to elastic recovery, the outer circle returns to a cylindrical shape, while the inner hole becomes an arc-shaped triangle. When measured with an internal micrometer, the diameters D in all directions are equal.

2. Under the action of cutting force (especially radial cutting force), vibration and deformation are easy to occur, which affects the dimensional accuracy, shape, position accuracy and surface roughness of the workpiece.

3. Because the workpiece is thin, the cutting heat will cause thermal deformation of the workpiece, and it is difficult to control the size of the workpiece. For thin-walled metal workpieces with a large linear expansion coefficient, such as continuous semi-finished turning and finishing in one CNC machining, the thermal deformation of the workpiece caused by the cutting heat will have a great impact on its dimensional accuracy, and sometimes even cause the workpiece to be stuck on the fixture.
 

We know how CNC lathes process deformed thin-walled workpieces, so how to deal with the deformation of thin-walled workpieces on CNC lathes? Several solutions are described below.

1. The workpiece is divided into rough parts. In the rough machining process in the finishing stage, due to the large cutting allowance, the clamping force increases slightly, and the deformation also increases accordingly; in the finishing machining process, the clamping force can be slightly reduced, on the one hand clamping deformed. On the other hand, it can also eliminate deformation caused by excessive cutting forces during roughing.

2. When using geometric parameters reasonably for precision machining of thin-walled workpieces, the rigidity requirements are high, the wiper blade is not easy to be too long (usually 0.2~0.3mm), and the blade should be sharp.

3. Increase the clamping contact surface, use a slit sleeve or some special soft jaws. Increase the contact surface, so that the clamping force is evenly distributed on the workpiece, so that the workpiece is not easily deformed during the clamping process.

4. Fully perfuse cutting fluid. By fully filling the cutting fluid, the cutting temperature is reduced and the thermal deformation of the workpiece is reduced.

5. Increase CNC machining ribs. Some thin-walled workpieces are specially manufactured with several machining ribs at the clamping position, thereby improving the rigidity of the workpiece, so that the clamping force acts on the machining ribs and reduces the deformation of the workpiece. After machining is complete, the machining ribs are removed. 

6. During axial clamping, the fixing device should be used to rotate thin-walled workpieces. Radial clamping should not be used as much as possible. It is best to use the axial clamping method shown in Figure 4. The workpiece is axially clamped by the end face of the axial clamping sleeve (threaded sleeve). Since the clamping force F is distributed along the axial direction of the workpiece, the axial rigidity of the workpiece is relatively large, and clamping deformation is not easy to occur.

According to the rotational symmetry function of the taper, when turning a thin-walled workpiece with a large aspect ratio, use the No. 5 Morse cone to install the taper shank of the self-centering part into the spindle hole of the lathe; after the nut 3 is installed in place, it is locked and positioned by the nut 2 ; The end face of the workpiece to be processed is close to the shoulder of the conical block, the nut 8 is rotated, and the outer conical surfaces of the conical block 4 and the conical block 7 force the spring flaps of the spring chuck to open evenly outward, and the workpiece is automatically and evenly fixed.

Center locking; top head, nut 10, screw, thrust ball bearing, tail cone, top shaft, needle roller bearing, screw plug, sealing ring 17 and 19 and tapered bearing The top part is composed of the tail cone using No. 4 Morse The cone and the Morse taper hole of the machine tool tailstock are automatically centered and locked, and the outer cone surface of the top head is pressed against the other end hole of the workpiece, so that both the self-centering component and the top component are in the rotation center of the lathe spindle; the workpiece is turned after finishing, loosening The center assembly, the counter-rotating nut 8 and the spring belt taper block 7, the spring collet can be loosened to remove the workpiece. When the precision of the machine tool is low or the rigidity is poor, the cutting speed should be appropriately reduced to ensure the quality of the workpiece.

The main working part of the self-centering device for clamping large length-diameter ratio thin-walled workpieces is the taper fit, which gives full play to the rotational symmetry function of the cone. ;The combined design of roller bearing, thrust bearing and needle roller bearing of the top component ensures that the workpiece to be processed is at the center of rotation of the main shaft of the lathe, which solves the deformation problem of thin-walled workpieces with large aspect ratios.