The main causes of workpiece deformation are as follows:
Thermal deformation
1. Thermal deformation of tools
The cutting heat will heat the blade and body, deform and extend the cutter head, and change the size of the workpiece. The elongation of the cutter head is related to the length of the depth of the cutter head, the size of the section, the thickness of the blade, and the sharpness of the blade. The greater the depth of the cutter head, the greater the elongation; The section of the cutter bar is inversely proportional to the elongation; The thicker the blade, the smaller the elongation.
2. Thermal deformation of machine tools
Due to the heat generated by the friction between cutting heat and machine tool parts, some parts of the machine tool will be heated and deformed. For example, the deformation of the lathe spindle box will increase the height of the spindle center and displace in the horizontal direction.
3. Thermal deformation of workpiece
Because the cutting heat will make the workpiece hot, the temperature will rise. There are two kinds of workpiece heating: balanced heating and unbalanced heating. Equilibrium heating will change the size of the workpiece, while the shape will remain unchanged; When uneven heating occurs, not only the size but also the shape of the workpiece will change.
Deformation caused by internal stress
When there is stress in the part without any external compliance, the internal stress is balanced with each other, so there is no change in the external surface. The internal stress sometimes almost reaches the failure limit, but it is no different from the parts without internal stress in appearance.
The causes of internal stress in production include the following aspects:
1. Internal stress of castings
After the metal liquid is poured into the mold, the volume will shrink during solidification and cooling. During shrinkage, it will be blocked by the mold, or due to the temperature difference in each part of the casting during cooling, which will make each part elongate or compress and produce internal stress.
2. Internal stress of forgings and heat treated parts
The internal stress of forgings and heat-treated parts is mainly caused by uneven cooling during hot processing. The root of internal stress in hot working is the transformation of material from self plastic state to elastic state, and various reasons for temperature difference.
3. Internal stress in cold working
During cold working, the surface of the workpiece is hardened and internal stress appears in the metal of the surface layer. After the stress layer is cut off, the stress is redistributed, causing the bar, sheet and disc to deform.