referring to fig3 - 5 , there is illustrated a driven member in the form of a fastener 10 , which in the illustrated embodiment is a socket - head fastener , such as a screw or bolt . the fastener 10 has an enlarged head 11 provided with an axial recess 12 in the end surface 13 thereof . the recess 12 is of uniform depth throughout its entire cross - sectional area and has a flat bottom surface 14 ( see fig5 ). the recess 12 is provided with six radially outwardly extending lobes 15 equiangularly spaced apart around the longitudinal axis of rotation of the fastener 10 , the lobes 15 all being identically shaped and extending the full depth of the recess 12 . each of the lobes 15 includes an arcuate tip portion 16 and arcuate side wall portions 17 ( fig3 and 4 ), the arcuate side wall portions 17 of adjacent lobes 15 forming parts of a common part - cylindrical surface which extends between the two lobes . referring also to fig1 and 2 , there is provided a rotatable driving tool , generally designated by the numeral 20 , which includes a shank 21 provided at its distal end with a bit 22 . the bit 22 tapers to a circularly cylindrical tip 23 , which has a longitudinal axis of rotation x and terminates in a flat end face 24 disposed perpendicular to the axis x . formed in the cylindrical surface of the tip 23 are six equiangularly spaced - apart and identically constructed concave depressions or grooves 25 , each of which defines a part - cylindrical surface 26 which has an axis inclined with respect to the axis x and which extends longitudinally of the tip 23 from the end face 24 to an inner end 28 . the grooves 25 cooperate to define six equiangularly spaced - apart and identically constructed driving portions or blades 30 , each having a part - cylindrical outer peripheral surface 31 which forms a part of the cylindrical surface of the tip 23 and is bounded by parallel side edges 32 and 33 each of the blades 30 is substantially circularly helical , having a predetermined helix angle a ( see fig1 and 5 ). the helix angle a is illustrated in fig1 as the angle between a side edge 32 of one of the blades 30 and the projection of the axis x onto the surface of the cylindrical tip 23 . thus , it will be appreciated that the cylindrical tip 23 of the tool 20 is generally in the shape of a helical gear . referring in particular to fig1 it can be seen that each of the side edges 32 extends from an inner end point 34 , at which it joins the adjacent one of the ends 28 , to an outer end point 35 at the end face 24 . similarly , each of the side edges 33 extends from an inner end point 36 to an outer end point 37 . as indicated above , the end points 34 and 36 are spaced apart the same distance w 1 as are the end points 35 and 37 , this distance being the width of the blade 30 . this width is less than the width w 2 of the arcuate tip portion 16 of each of the lobes 15 of the recess 12 by a clearance distance c ( fig5 ). the longitudinal extent of the blades 30 and the magnitude of the helix angle a are such that the end points of either one of the side edges 32 and 33 of a blade 30 are offset from each other in directions substantially normal to the axis x a distance o ( fig2 ). which is greater than the clearance distance c . accordingly , when the bit 22 of the tool 20 is inserted in the recess 12 of the fastener 10 , the blades 30 will wedge into an interference fit in the lobes 15 before the bit 22 bottoms out in the recess 12 . more specifically , referring to fig4 and 5 , the trailing side edges 33 of the blades 30 may slide along engagement points 40 at the outer ends of the lobes 15 , until the end points 35 engage the opposite sides of the lobes 15 at engagement points 41 , spaced a predetermined distance d from the bottom surface 14 of the recess 12 . thus , it will be appreciated that each blade 30 wedges into interference engagement with the opposite side walls of the associated lobe 15 at the engagement points 40 and 41 , and the application of an axial force on the tool 20 will wedge the bit 22 into firm engagement with the fastener 10 , securely to retain the fastener 10 on the tool 20 . preferably , the parts are arranged so that the distance d is small enough to permit a significant depth of insertion of the bit 22 into the recess 12 to insure sufficient longitudinal spacing between the engagement points 40 and 41 to attain an efficient transfer of the torque from the driving tool 20 to the fastener 10 . at the same time , the distance d must be large enough to prevent bottoming out of the bit 22 in the recess 12 even after wear of the parts resulting from repeated usage . in a preferred embodiment of the invention the distance d may be in the range of from about 15 % to about 25 % of the overall depth of the recess 12 . it will further be appreciated that the total wedging force tending to retain the fastener 10 on the tool 20 is related to the axial force applied to the tool 20 . also , up to a point , the conversion of axial force to lateral wedging force is inversely proportional to the magnitude of the helix angle a . it has been determined that if the helix angle a becomes too large there is no longer an effective wedging action . in the case of a six - bladed tool 20 , it has been found that if the helix angle a exceeds six degrees , effective wedging action may be lost . in a constructional model of the six - bladed embodiment of the invention disclosed in the drawings , it has been found that optimum results are attained with a helix angle of about 2 . 5 degrees , but in general it is believed that various helix angles in the range of from about one degree to about six degrees will work satisfactorily , depending upon the configuration of the fastener 10 and tool bit 22 . the operation of the present invention to securely retain the fastener 10 on the tool 20 will be effective irrespective of the direction of rotation of the tool 20 in driving the fastener 10 . however , the retention effected by the invention may be greater in one direction of rotation than another , depending upon the direction of inclination of the blades 30 with respect to the axis of rotation x . thus , for example , in the embodiment illustrated in the drawings , rotation in a counterclockwise direction may have a tendency to &# 34 ; screw &# 34 ; the tool bit 20 into the recess 12 , thereby tending to enhance the wedging action . in a constructional model of the invention , the depth of the recess 12 and the longitudinal extent of the cylindrical tip 23 of the tool 20 are typically less than one - quarter inch , although it will be appreciated that this may vary depending upon the size of the fastener 10 . the tool bit 22 is preferably formed of a suitable steel , while the fastener 10 may be formed of any desired material . when the fastener 10 is formed of a relatively soft material , such as brass or plastic , an increased angle a may be desirable to increase the amount of fastener deformation and , thus , the amount of wedging available to retain the fastener 10 on the tool 20 . while the present invention has been described above in connection with a six - bladed tool 20 for mating with a six - lobed fastener recess 12 , it will be appreciated that other fastener and tool configurations could be used . accordingly , any number of driving portions or blades 30 greater than two could be utilized . furthermore , the driving portions need not necessarily be in the form of laterally projecting blades . thus , for example , a generally flat - sided polygonal configuration , such as a hexagonal configuration , could be used . if fewer than six driving portions or blades are used , a larger helix angle a may be desirable . from the foregoing , it can be seen that there has been provided an improved rotatable driving tool and an improved assembly of tool and cooperating fastener which effectively retains the fastener on the tool , while still providing an efficient transfer of torque from the tool to the fastener , irrespective of the material of the fastener .