Square head bolt inspections are conducted manually and by machine. Manual inspection is the original and most widely used method. To minimize the release of defective products, production staff typically inspect packaged or shipping products visually to exclude defects (which include thread damage, mixed materials, rust, etc.).

Potential defects of square head bolts (such as cracks, inclusions, and mixed materials) can cause discrepancies in magnetic permeability between the material and steel. After magnetization, the discontinuities in these materials will experience magnetic field distortion, forming leakage flux points on the workpiece surface, which attract magnetic particles to accumulate at the defect locations – magnetic traces. Under proper lighting conditions, these traces reveal the defect positions and shapes. Observing and interpreting these magnetic particle accumulations helps to eliminate defective products.