Hollow bolt or screw head - may be worked hot or cold and has central cavity with external wall with segments

摘要

The screw head has a central cavity (3) which is bounded by a wall (19). The wall may be made up of circular segments. The segments have a height which is lower than the main head (1). The wall may also have another profile. The head may have a square section (2). The shapes may be formed by hot or cold working, and may be formed in a head or in a central cavity within a block.