There are two main reasons: before the workpiece is deformed, the external force-clamping force, which is caused by improper selection of the position and size of clamping force. The clamping point must be in the real place (not hanging below), and the clamping force should not be too large, as long as it can be clamped. After the workpiece is deformed, it is caused by processing stress, that is, the internal stress generated by the tool on the workpiece during processing is invisible. 1. Temper at low temperature to eliminate internal stress and restore the workpiece to its original state. 2. Under the premise of not affecting the surface quality, the concave part shall be sandblasted (but the granularity, position and distance of the spray gun shall be properly selected) so that the concave part can be stretched, inspected and leveled while spraying. 3. Using rubber hammer to correct the workpiece (that is, using the extension method) requires workers to have certain experience. When knocking, you can't knock the high part, but extend the short side to relieve the long side, so as to achieve the purpose of correction.