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.
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 length of the screw is measured from the tip to the underhead.