The machine is fully automatic inspection, mainly magnetic particle inspection. Magnetic particle inspection is based on the interaction between the leakage magnetic field at the bolt defect and the magnetic powder, aiming at the difference between the magnetic permeability of the bolts (such as cracks, slag inclusions, mixing, etc.) and the magnetic permeability of steel. These materials are discontinuous after magnetization. The magnetic field at the location will be turbulent, forming part of the magnetic flux leakage, and a leakage magnetic field will be generated on the surface of the workpiece, thereby attracting the accumulation of magnetic powder at the defect where the magnetic powder is formed. The accumulation of these magnetic powders is observed and explained, and the purpose of rejecting defective products has been achieved.