When the materials are fixed, the reasons from the following major aspects:
1. Clamping: whether the tooling fixture is used appropriately, including selection type, layout position, clamping strength, and reference plane selection.
2. Chips: including tool selection, tool edge, feed speed, chip direction, cutting depth, and heat dissipation conditions.
3. Material: material size, shape, thickness, whether there is internal stress.

Because the flatness is poor, it is mainly caused by deformation. The reason for material deformation is due to the action of force, that is, internal stress and external force. The internal stress before processing may be due to the previous welding and large-scale cutting. The subsequent processing started before it could be released. During the subsequent processing, it was released while processing, so the end was uneven. The external force is due to excessive force during clamping and the material has been deformed (elastically elastic deformation), it is flat during processing. After loosening, the elastic deformation recovers and it becomes uneven.

For the processing of threaded holes, you can refer to the selected inserts and materials used, and whether the corresponding program is suitable for turning. This should be confirmed during debugging. Usually such large threads are usually processed by turning.