The underhead knurling has two distinct functions. Once the screw has been tightened, the indentation causes the unscrewing torque used to be greater, making the fixing more stable in all those applications which are subjected to movements and / or vibrations during their use. In the electric field, however, the indentation scrapes the surface of the metal, effectively creating connectivity with the material where grounding is required. However, it must be taken into account that the serration can never replace the tightening of a screw with the serrated washer, which is much more performing in itself.
Self-tapping screws without a tip find particular application in pass-through positions, where the screw protrudes from the fixed plates and you want to avoid the risk of accidental injuries caused by the tip coming out. They are therefore accident-prevention in that they reduce the possibility of accidental cuts with the tip even during various maintenance operations.
The large head favors a wider sealing surface as well as the possible covering of through holes in case of sandwich coupling between two sheets, while the low profile and without edges makes them particularly suitable in "visible" applications. This screw has wide fields of application ranging from the mechanical sector to the household appliance, from the auto motive to the aluminum window frame.
The washer that seems pre-assembled adds a further aesthetic value.
The length of the screw is measured from the tip to the underhead.