GB1214460A - Method and tool for driving self-tapping screws - Google Patents

Abstract

1,214,460. Power actuated screwdriver. R. T. ALLSOP. 17 June, 1968 [23 June, 1967 (2); 7 Oct., 1967], Nos. 28981/67, 28982/67 and 45977/67. Heading B3N. A method of driving a self-tapping screw 37 having a piercing point 38 into a sheet metal workpiece comprises the steps of (a) engaging the workpiece with the screw, the screw head with a power-operated rotary bit 36 which is axially reciprocable; (b) applying simultaneously to said bit rotary torque and axial vibratory impacting to cause the formation of a pilot hole and (c) maintaining the vibratory impacting at least until the piercing point 38 has completely entered the workpiece to its full depth. Rotary torque is applied throughout the screwdriving while the depth of impacting varies from the minimum depth referred to above with thin sheet to prevent buckling to the full depth of driving with thick sheet as the resistance to screwing is reduced by the vibration. A suitable tool, Fig. 1, has an air motor 21 driving through a gear-box 22 and 24, gearing (25) (Fig. 2, not shown) 26, portion 40 of the bit, which portion rotates with, but is slidable within, the gear 26. An impact piston 45, controlled by a flutter valve device 50, reciprocates within a cylinder 44 of larger diameter than but coaxial with the bore 42 in which the end 41 of the bit slides. A sleeve 52 threadedly engages both a sleeve 55 surrounding the end 36 of the bit and an end cap 31 of the tool, and the axial position of the sleeve 55 is adjusted so that the piston 45, although still reciprocating, ceases to impact the bit which is still rotated when the screw has been driven to a chosen depth. Alternatively a pneumatic time delay (Fig. 5, not shown) may be disposed in the line between the air valve 18 and the reciprocating piston 45 to determine the period of vibration. In a further alternative (Fig. 7) the bit is rotated through a pair of partly relatively rotatable clutch members 98, 99, with cam surfaces 107 on their opposing faces. As the resistance torque increases as the screw head reaches the workpiece the clutch member 99 moves to the left against the spring 104, due to the action face cam surfaces 107, and engages the flange 106 on the bit to move it away from the path of the piston 103. A modified tool (Fig. 7A, not shown) has a motor driving the bit through bevel gears (121), (122) and the screwdriver bit comprises two parts which are held apart by a spring (132) to prevent impact on a screw unless the tool is pressed against a workpiece to compress the spring.