Thread forming nut with locking portion
BACKGROUND OF THE INVENTION 
     This invention relates to fasteners for mounting a work piece on a support member, and further relates to improved fastening nuts which provide locking action for securement upon an unthreaded stud. 
     In the assembly or erection of structures in which an unthreaded stud is employed to mount a work piece upon a support member, a threaded nut has been conventionally used to cut its own mating thread into the stud as it is driven onto the stud to secure the assemble joint. For example, automotive bumpers have been provided with a protective, resilient &#34;facia strip&#34; which typically includes an elongate stud passing through a hole in the bumper so that a typical metallic hex nut is driven onto the projecting end of the stud to secure the protective strip on the bumper. Typically, the stud is integrally molded with the protective strip from resilient engineering resin, for example, resins commercially available under the Registered Trademark Bexloy from E. I. DuPont. The conventional metal hex nut has a tendency to bind as it is driven onto a plastic stud often resulting in distortion and twisting detachment of the stud so that there is a high frequency of joint failures. 
     These deficiencies are eliminated by the improved fastening nut and improved joint provided by the present invention. 
     SUMMARY OF THE INVENTION 
     The improved nut is secured to an unthreaded stud which is inserted through a central bore in the nut. The bore includes a guide means which provides alignment guidance during sliding insertion of the stud into the bore. The guide means preferably has an annular configuration with an inner diameter slightly larger than the diameter of the stud so that the initial insertion of the stud does not produce significant frictional resistance. The bore of the nut further includes a helical thread, preferably a single thread turn, which is disposed in the bore axially distinct from the guide means and terminates within the bore adjacent the guide means. The helical thread has a minor diameter smaller than the stud diameter and also projects radially inwardly into the bore further than the guide means. 
     The stud is initially inserted into the bore and slides past the guide formation until it engages the lead of the thread. At this point, torque is applied on the nut and the thread cold-forms a mating thread impression into the periphery of the stud. The resulting thread impression on the stud includes radially outwardly displaced, cold-formed stud material which obstructs any vibrational retraction of the guide means from the stud. The nut is preferably molded from a harder material than the stud which yields to the cold-forming thread of the nut. 
     In a preferred embodiment of an improved joint employing the nut, the joint structure secures a resilient, protective fascia strip to an automotive bumper. The stud is integrally molded with the fascia strip and is inserted through an aperture in the bumper to expose the projecting end of the stud. The stud end is inserted through the guide means within the bore of the nut. The guide means is defined by an annular pattern of axially aligned teeth with a tooth-to-tooth diameter slightly larger than the peripheral outer diameter of the stud enabling the teeth to provide aligning guidance in sliding the stud into the bore. The nut is molded from a harder plastic than the stud so that the helical thread cold-forms a mating thread impression which obstructs retraction of the teeth to maintain securement of the joint. The major diameter of the nut thread is dimensioned so that the major diameter of the formed stud thread impression is not excessively larger than the inner diameter of the annular tooth pattern so that the nut can be backed off the stud by application of sufficient torque to drive the teeth through the thread impression without stripping or severely deforming the thread impression. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded, perspective view of an embodiment of a joint and fastening nut of the invention; 
     FIG. 2 is an end view of the assembled joint of FIG. 1; 
     FIG. 3 is a sectional view in which the unthreaded stud shown in FIG. 1 is partially inserted into the bore of the nut to a point where the end of the stud abuts the nut thread; 
     FIG. 4 is a sectional view taken along the line 4--4 in FIG. 2, viewed in the indicated direction; 
     FIG. 5 is a sectional view taken through the nut shown in FIGS. 1-4; 
     FIG. 6 is a sectional view taken along line 6--6 in FIG. 4, viewed in the indicated direction; and 
     FIG. 7 is a sectional view of a modified nut in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1-4, an embodiment of an assembled joint according to the present invention is generally designated by referenced character 10. In the illustrated joint 10 a resilient, protective fascia strip 12 molded from engineering resin is mounted on a typical automotive bumper 14. A generally cylindrical unthreaded stud 16 integrally moulded and projecting from the strip 12 is inserted through and projects from a mounting aperture 14a in the bumper 14 as best shown in FIG. 4. A locking nut 18 is then driven onto the stud 16 to secure the joint 10 and the assembly of the strip 12 on the bumper 14. 
     Referring again to FIGS. 1 and 2, the nut 18 preferably has a peripheral profile in a TORX pattern for the advantages in driving the nut. Preferably, the nut 18 also includes a slightly concave flange 20 forming the bearing surface at the pilot or entrance end of the nut 18. The nut 18 has a central, pilot bore 22 which opens at the pilot end adjacent the flange 20 and extends rearwardly through the nut, but terminates within the nut 18 at a medial point between the opposite ends of the nut. As best illustrated in FIGS. 5 and 6, the peripheral wall of the pilot bore 22 is formed by an annular pattern of axially elongate teeth 24 whose apexes are directed radially inwardly. The annular pattern of teeth apexes have an apex-to-apex, or minor diameter designated A which is at least equal to and preferably slightly greater than the diameter B of the stud 16; these relative dimensions of the diameters A and B enable the teeth 24 to provide alignment guidance so that the nut 16 can be inserted through the pilot end of the nut which is slipped over the stud in generally straight relation as shown in FIG. 3, with no significant frictional resistance in the initial operation to mount the nut. 
     Referring to FIGS. 3-5, the teeth 24 extend the full length of the pilot ore 22 and terminate therewith at a medial depth within the nut at a generally annular location designated C which also corresponds to the terminal, remote end of a single, helical, internal thread turn generally designated 26. The thread turn 26 is generally coaxial with the pilot bore 22 and begins just inside the trailing nut aperture 28 formed by an abbreviated conical counter-bore. 
     Referring particularly to FIG. 3, the minor diameter D of the thread turn 26 is smaller than the stud diameter B so that when the inserted stud end 16a reaches the medial end C of the teeth 24, the nut 18 is then rotatably driven. The thread 26 interferes with and displaces the peripheral material of the stud to form a mating, external thread impression generally designated 17 into the previously unthreaded stud 16 as shown in FIG. 4. In order to promote the thread forming on the plastic stud with outward displacement of stud material to form the thread impression 17, in contrast to thread cutting and consequent removal of stud material, thread turn 26 can preferrably have a tapered or rounded lead portion 30. For this reason, the stud 18 is fabricated, preferably by molding a harder resin, for example polyoxymethylene such as Delrin R (E. I. DuPont), relative to the plastic composition of the stud which yields to the thread 26. The stud material which is displaced outwardly in forming the thread impression 17 results in a major diameter E which is not only larger than the original stud diameter B, but is also larger than the minor diameter A of the annular pattern of teeth 24 on the nut 18; as a result, the outwardly displaced stud material of impression 17 obstructs the path of retraction of the teeth 24. This obstruction promotes secured retention of the joint 10 by preventing withdrawal of the nut 18 until sufficient torque is deliberately applied to retact the teeth 24 through the obstructing stud impression 17 by a tooled operation to back off the nut. Preferably, the minor diameter D should not be excessively smaller than the minor diameter A of the annular pattern of teeth 24 in order to prevent excessive resistance to retraction of the teeth 24 which could produce stripping or deforming the stud thread impression 17 when the nut 18 is deliberately backed off for removal of the nut from the stud. 
     The teeth 24 serve dual functions in providing alignment guidance in sliding the nut 18 onto the stud 24 and preventing the nut from becoming misaligned or cocked as the single thread 26 is driven into the stud to form the impression 17; in addition, the teeth 24 also serve as a locking structure which is obstructed from withdrawal in the fully assembled joint 10. This locking feature allows the joint to be maintained without nut retraction, but with low clamping of the nut flange 20 against the bumper 14 to allow tolerance for differences in thermal expansion of the plastic fascia strip 12 (and stud 16) in relation to the typical metallic bumper 14, as well as the hard plastic nut 18. 
     The nut 18 can be molded by employing separate molding core pins which meet end wise within the mold to respectively form the threaded portion and the toothed, guide bore portion of the central passageway. 
     Referring to FIG. 7, a modified embodiment of the nut designated 18&#39; includes features similar to those of the nut 18, with the exception that the guide bore 22 prime has a generally smooth, peripheral wall which terminates in an annular ring 32 projecting inwardly into the bore; the ring provides alignment guidance and locking-withdrawal prevention similar to the teeth 24 of the nut 18. Accordingly, the inner diameter of A&#39; of the ring 32 is equal to or slightly greater than the stud diameter B. The length of the ring is governed by provision for sufficient guidance to prevent misalignment in slipping the nut 18&#39; over the stud and maintaining the alignment in driving the thread 26&#39; onto the stud. While the ring 32 must provide sufficient purchase and interference with the stud thread impression to prevent vibrational retraction of the nut 18&#39; and loosening of the joint, the ring can be chamferred at 32a to enable the ring to pass over the stud thread impression 17 with little distortion thereof when the nut 18&#39; is to be removed. To facilitate such removal, the ring 32 can include spaced interruptions governed by the necessary balance between the required locking interference with the stud thread impression relative to the imposed force necessary to back off and remove the nut 18&#39; when desired. Similar modifications of the guide and locking formation can be designed in accordance with the described objectives of the nut and joint. 
     While particular embodiments of the nut and joint of the invention have been shown and described in detail, it will be obvious to those skilled in the art that changes and modifications of the present invention, in its various aspects, may be made without departing from the invention in its broader aspects, some of which changes and modifications being matters of routine engineering or design, and others being apparent only after study. As such, the scope of the invention should not be limited by the particular embodiment and specific construction described herein but should be defined by the appended claims and equivalents thereof. Accordingly, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.