Fixing screw with polygon shaped recess in head for key

摘要

The screw (11) has an internal recess (19) of polygon shape in its head. The matching end (25) of a key (12) can be made to engage the recess to turn the screw. The polygon in the head is undercut to a diameter (M) and a depth (TE). The key (25) is undercut to a diameter (P) at a height (HW). The diameter of the undercut clears the sides of the polygon in the screw head when the head of the key is pushed right down into the undercut in the head. The diameter (M) of the undercut in the head is such that it allows the key to be rotated in it so that the corners of the polygon of the key can be brought into line with the sides of the polygon in the head.