Neodymium Block Magnet with Countersunk Hole 20 x 20 x 3 mm N48. 
 
A neodymium magnet (NdFeB) is very strong. It is magnetised in a strong magnetic field during manufacture, providing high magnetic attraction in a compact size. Compared with ferrite, neodymium provides much greater magnetic attraction. A protective surface coating helps improve durability. N48 indicates the magnetic grade. NdFeB grades commonly range from N35 to N52, with higher numbers generally indicating stronger magnets. The with Hole design for mechanical attachment, allowing screw mounting without adhesive.
 
The 20×20×3 mm size provides a strong hold for practical tasks, including quick assemblies, attachments and technical projects. For good results, use a flat, clean metal surface. Any gap caused by thick paint or an uneven surface reduces holding force. Do not overtighten the screw during installation. Neodymium is hard but brittle and can chip or crack under excessive pressure. A washer can help.
 
Warning: keep away from children. Strong attraction can cause pinching injuries, and the magnets may break. Store them with spacers. Keep them away from bank cards, sensitive electronics and medical devices.