Captive thread forming terminal screw

In combination, a self-tapping, self-captivating fastener member and a relatively thin workpiece having a pre-drilled aperture of predetermined size formed therein, said fastener member including;

• a driving head, an elongated shank extending from said driving head, said shank having formed thereon a threaded portion and an abutment portion disposed adjacent said threaded portion, said threaded portion including a thread lead-in portion and a plurality of thread-forming turns of a size and configuration to engage said aperture periphery and form a mating internal thread thereon, said abutment portion being sized such that the maximum diameter thereof is less than the diameter of said aperture but greater than the crest diameter of the internal threads to be formed thereon, whereby upon assembly, said abutment portion will pass through said aperture but subsequent to the formation of said internal thread, said fastener member cannot be disassembled since the internal thread will preclude said abutment portion from passing out of said aperture, the improvement wherein, said thread lead-in portion is tapered and converges in a direction away from said driving head, terminating at its minimum diameter section adjacent said abutment portion, with the thread turns on said lead-in portion being incomplete or underfilled, the spacing of said abutment portion from said lead-in portion and the length of said lead-in portion having a combined axial extent greater than the depth of said aperture, such that when said abutment portion engages said workpiece upon attempted withdrawal of said fastener member, only said incomplete, underfilled thread turns of the lead-in portion will be disposed proximate said internal thread, with none of the thread forming turns being in mating engagement therewith, said lead-in portion thread turns being of insufficient height to effect mating, driving engagement with said internal thread, such that continued rotation of said fastener member will not result in withdrawal of the fastener and damage to said internal thread, as would result from said abutment portion passing through said aperture.

In combination, a self-tapping, self-captivating fastener member, and a relatively thin workpiece having an aperture formed therein to a predetermined size, said fastener member including a driving head and an elongate shank extending from said driving head, said shank having a threaded portion formed thereon, said threaded portion including a plurality of thread-forming turns adapted to form an internal mating thread on said aperture periphery upon engagement of the fastener member with said workpiece, and a thread lead-in portion which includes one or more incomplete or underfilled thread turns, said thread lead-in portion being tapered and converging in a direction away from said driving head, and an enlarged abutment portion formed on said shank adjacent the terminus of said lead-in portion, and having a maximum diameter which is less than that of said workpiece aperture, but greater than the minimum diameter of said thread lead-in portion, and also greater than the crest diameter of the mating internal thread to be formed by said thread-forming turns, whereby upon assembly of said fastener member with said workpiece, said abutment portion may pass through said aperture with said thread forming turns reshaping said aperture periphery to form said internal mating threads the spacing of said abutment portion from said lead-in portion and the length of said lead-in portion having a combined axial extent greater than the depth of said aperture, such that when said abutment portion engages said workpiece upon attempted withdrawal of said fastener member, only said incomplete, underfilled thread turns of the lead-in portion will be disposed proximate said internal thread, with none of the thread forming turns being in mating engagement therewith, said lead-in portion thread turns being of insufficient height to effect mating, driving engagement with said internal thread, such that continued rotation of said fastener member will not result in withdrawal of the fastener and damage to said internal thread, as would result from said abutment portion passing through said aperture.

A self-tapping, self-captivating terminal fastener member for engagement in a pre-drilled aperture of predetermined size formed in a relatively thin workpiece wherein said fastener upon assembly will provide a captive terminal connection, said fastener member including:

• a driving head, an elongate shank extending from said driving head, said shank having thereon a threaded portion and an abutment portion on the work entering, free end of said shank, said threaded portion including a thread lead-in portion and a thread forming portion said thread forming portion comprising a plurality of thread turns of a size and configuration to engage said aperture periphery and form a mating internal thread thereon, said thread lead-in portion including a tapered shank segment which converges in a direction toward said fastener work entering end and terminating at its minimum diameter section adjacent said abutment portion a thread formed on said tapered shank segment of said lead-in portion, the crest diameter of said thread decreasing in height and thereby tapering in the same direction as said shank segment and at least the last thread turn on said thread lead-in portion being incomplete or underfilled and having a crest diameter that is less than the crest diameter of the internal thread to be formed, as determined by the dimensions of said threads on the thread forming portion, said abutment portion being spaced from the terminus of said incomplete or underfilled thread turn on the lead-in portion and sized such that the maximum diameter thereof is less than the diameter of said pre-drilled aperture in the workpiece, but greater than the crest diameter of the internal thread to be formed on said workpiece, as determined by the dimensions of said thread turns on the thread forming portion, such that upon initial assembly, said abutment portion will pass through said pre-drilled aperture, but subsequent to the formation of the internal threads said fastener member cannot be disassembled since the internal threads will have a crest diameter less than the diameter of said abutment portion and will thus preclude said abutment portion from passing out of said aperture, the spacing of said abutment portion from said terminus of the thread turn on said lead-in portion, and the length of said thread on the lead-in portion that is of lesser diameter than the crest diameter of said internal thread being selected to be greater than the normal thickness of workpieces to be employed in conjunction with a captive terminal fastener, such that when said fastener member is retracted and said abutment portion engages said workpiece upon attempted withdrawal of said fastener, only the portion of the terminus of said thread having a crest diameter less than the crest diameter of the internal thread will be disposed proximate said internal thread formed on the workpiece, and therefore none of the thread turns on said fastener will be in mating, driving engagement with said internal thread, such that continued rotation of said fastener member will merely result in spinning of the fastener relative to the workpiece, and will not produce withdrawal of the fastener abutment portion through said internally threaded aperture, which would damage the internal thread previously formed thereon.