the inward collapsing of the sides of such u shaped members as a result of the controlled collapsing the u shaped members of the blunt end 21 of the penetrating portion 13 assume configurations as illustrated in fig8 and engage spaced bulbous masses of metal when the fastener is driven so that any rotation or swivel action of the fastener relative to the sheet metal is resisted or prevented in the operation of the device the sheet metal 15 is located in a position contiguous to a backing member or anvil 27 the insulating material 14 is positioned on top of the sheet metal and a fastener 10 is driven through the insulating material into the sheet metal by an axial driving force from a hammer or pneumatic gun not shown normally the head of a driving tool is magnetized so that the fastener 10 intimately engages the same and such tool is operated to cause an axial driving force to be applied so that the fastener penetrates entirely through the insulating material and strikes the sheet metal 15 with substantial impact the axial impact force causes the penetrating portion 13 of the fastener to partially penetrate the sheet metal and extrude a pair of bulbous masses 28 into the cavity 22 until the lower surfaces of the enlargements 24 engage the upper surface of the sheet metal to limit further penetration during the penetration of the end portion of the fastener the weaker sides of the u shaped members collapse or curl inwardly toward each other due to the inwardly inclined side walls of the cavity 22 as well as the reduced thickness thereof adjacent to the.