US3831415A

US3831415A - Self tapping fastener and method and dies for making same

Info

Publication number: US3831415A
Application number: US00398317A
Authority: US
Inventors: E Skierski
Original assignee: USM Corp
Current assignee: Emhart Industries Inc
Priority date: 1972-05-24
Filing date: 1973-09-18
Publication date: 1974-08-27
Anticipated expiration: 1991-08-27

Abstract

A self tapping threaded fastener, the method of making same and the rolling dies therefor wherein said fastener is adapted for assembly with a member in which a standard internal thread is formed by the fastener and in which a zero running torque or controlled prevailing torque is established between said fastener and member. A plurality of thread forming lobes are disposed on an otherwise standard, substantially fully formed thread on the tapered work entering end, wherein predetermined lobes on said tapered end proximate the shank portion are rolled such that their ultimate projection is within the envelope of the imaginary projection of the convolution of the shank thread toward the tip of said fastener. The method of making said fastener includes rolling a substantially continuous thread from said shank portion throughout said tapered work entering portion and simultaneously forming spaced apart thread forming lobes superimposed upon said thread in said work entering portion, then subsequently rerolling said threaded shank portion and said predetermined lobes on the work entering portion to the cylindrical thread convolution of said shank portion, as projected. The dies for rolling said fastener include a first, and second rolling dies, said dies having a fastener entrance end and a fastener exit end, and disposed intermediate said ends thread forming surfaces including a plurality of parallel thread crest forming grooves and thread root forming ridges having a shank forming flat portion being bounded along one edge by an upwardly inclined tip forming surface, said first rolling die being the primary rolling die and having a shank and tip forming section including a lobe forming section wherein a plurality of regularly spaced lobe forming pockets are disposed in said thread forming grooves and ridges in said tip forming surface, a reroll support section wherein said thread forming surfaces match said shank forming flat portion and a release section, said reroll support section of a predetermined rolling length so as to reroll a predetermined number of said thread forming lobes, and a said second rolling die having shank and tip forming sections, and a reroll section so disposed as to reroll a predetermined number of said thread forming lobes.

Description

Uited' States Patent [191 Skierski Aug. 27, 1974 SELF TAPPING FASTENER AND METHOD AND DIES FOR MAKING SAME [7.61 Inventor; lsfitlaa s aiiettes tceana [73] Assignee: QSM Corporation, Boston, Mass. [22] Filed: Sept. 18, 1973 [21] Appl. No.: 398,317

Related US. Application Data [62] Division of Ser. No. 256,370, May 24, 1972.

[52] US. Cl. 72/88, 72/469 [51] Int. Cl B21b 3/06 [58] Field of Search 72/88, 90, 469; 10/10 R [56] References Cited UNITED STATES PATENTS 2,293,930 8/l942 Braendel 72/88 X 3,186,464 6/1965 Baumle 151/22 3,308,645 3/1967 Hampton 72/469 3,426,642 2/1969 Phipard,Jr 85/46 Primary Examiner-Milton S. Mehr Attorney, Agent, or Firm-Aubrey C. Brine; Vincent A. White; Richard B. Megley ABSTRACT member. A plurality of thread forming lobes are disposed on an otherwise standard, substantially fully formed thread on the tapered work entering end, wherein predetermined lobes on said tapered end proximate the shank portion are rolled such that their ultimate projection is within the'envelope of the imaginary projection of the convolution of the shank thread toward the tip of said fastener. The method of making said fastener includes rolling a substantially continuous thread from said shank portion throughout said tapered work entering portion and simultaneously forming spaced apart thread forming lobes superimposed upon said thread in said work entering portion, then subsequently rerolling said threaded shank portion and said predetermined lobes on the work entering portion to the cylindrical thread convolution of said shank portion, as projected.

rolling die being the primary rolling die and having a shank and tip forming section including a lobe forming section wherein a plurality of regularly spacedlobe fomiing pockets are disposed in said thread forming grooves and ridges in said tip forming surface, a reroll support section wherein said thread forming surfacesmatch said shank forming flat portion and a release section, said reroll support section of a predetermined rolling length so as to reroll a predetermined number of said thread forming lobes, and a said second rolling die having shank and tip forming sections, and a reroll section so disposed as to reroll a predetermined number of said thread forming lobes.

3 Claims, 10 Drawing Figures SELF TAPPING FASTENER AND METHOD AND DIES FOR MAKING SAME This is a division of application Ser. No. 256,370, filed May 24, 1972.

BACKGROUND OF THE INVENTION This invention relates -to a self tapping threaded fastener which, according to preferred embodiments, provides a zero running or a controlled prevailing torque during insertion of the fastener into the receiving member.

A wide variety of self tapping threaded fasteners are available in the market place today. Most of these various fasteners fall into one or the other of two principal categories of thread forming action: cutting or swaging. While both types of fasteners are effective in forming an internal thread in a receiving member, the swaging variety is now more common. One of the fasteners illustrative of the swaging types is disclosed in US. Pat. No. 3,426,642, issued Feb. 1 l, 1969. The thread forming vehicle in this type of swaging screw is a lobe structure (21 in FIGS. 1, 2 and 3 of the patent above referred to) which is superimposed over a substantially continuous thread convolution, from tip through shank.

While the above described fastener is remarkably effective in its capacity as a low torque thread former, it suffers from characteristics inherent in the previously known methods of mass manufacture. Presently, these fasteners are rolled between two thread rolling dies, each provided with a plurality of parallel thread root forming ridges and thread crest forming grooves and each adapted with lobe forming pockets, whereby in rolling between said dies under heavy pressure the material of the screw shank is moved (swaged) during a rolling action to form the thread convolution. In forming self tapping screws having thread forming lobes, the thread crest forming grooves of the dies are provided with a series of pockets which are indentations or punched holes. These pockets accommodate the moving plastic metal, forming a built-up portion which is essentially superimposed on the thread formed. since the lobe forming portions of the die are in reality holes in the thread crest forming grooves, the envelope of the lobes naturally extends beyond the envelope of the thread convolution in the region proximate the juncture of the shank and tapered portions of the fastener where the diameter of tapered portion approaches that of the shank (see FIG. 3, of the aforementioned patent). Where the fastener is formed with a thread convolution that is of substantially uniform cross section (a common form) the thread forming lobes proximate that juncture swage an internal thread in the fastener receiving member larger than the actual thread on the shank portion of said fastener. Thus, as the fastener is inserted into the receiving member, no prevailing torque exists between the swaged internal thread of the receiving member and the shank thread of the fastener. Rather, there is somewhat of a loose fit between the fastener and the receiving member. It should be quickly recognized that such a characteristic may in some instances be considered a deficiency which restricts the universal utilization of the fastener.

This shortcoming has been recognized and an attempt made to overcome the described deficiency. FIGS. 24 through 26 of the aforementioned patent illustrate an attempt to provide a self tapping screw which also provides a controlled prevailing torque (zero clearance or a positive interference between the fastener and the formed internal thread). As illustrated, the prior art fastener contains a threaded work entering portion having a tapered thread formation with decreasing major and pitch diameters but a constant minor diameter. While the illustrated fastener might provide a prevailing torque since the height and thickness of the thread forming lobes diminish toward the work entering end from a value less than that of the thread formation on the shank portion, its overall effciency and desirability as a self tapping screw are impaired.

The effectiveness of a self tapping screw is in large measure concentrated in the thread formation in the work entering portion. The thread formation thereon must not only swage an internal thread for the approaching shank portion but also advance the fastener into the receiving member. Unless the thread form bites deeply enough into the side material of the receiving member to auger in, the fastener will not advance and swage the desired thread. Rather, it will strip out, leaving the receiving member with no threads or an enlarged hole.

The fastener of the present invention overcomes both of the above problem areas by providing a fully formed thread on the work entering portion substantially down to the tip thereof, and a lobular construction which swages the internal thread in the receiving member with a minimum of torque, yet within the dimensional requirements of the shank thread.

As previously mentioned, each of the two thread rolling dies for forming the above described conventional fasteners is adapted with lobe forming pockets. In practice the dies must be carefully mounted in automatic rolling machines, being finally shimmed into position so that successive lobes on said thread convolutions are alternately and regularly placed. Experience dictates that said conventional rolling dies do not long retain their initial adjustment. Also, often during actual rolling, e.g., in initial insertion, the fastener blank slips on one of the dies, causing the dies to lose their relative adjustment and the regular spacing of the lobes is destroyed. As may be expected, irregular spacing of the lobes interferes with the thread forming function. The dies of the present invention insure regular placement of the lobes irrespective of relative die adjustment or blank slippage.

SUMMARY OF THE INVENTION The overall objects of the invention include providing a self tapping fastener, a method of making same and dies therefor which fastener also possesses the capacity to swage an internal thread in a receiving member, wherein the fastener exhibits, according to the preferred embodiment, zero running torque or a controlled prevailing torque upon insertion into the receiving member.

To these ends and in accordance with certain features of the invention, the fastener includes a cylindrical shank portion having a tapered work entering portion terminating in a tip. A cylindrical helical thread is rolled on the shank portion and continued in a spiral helical thread in the work entering portion. Superimposed upon the continuing spiral helical thread on the work entering portion are thread forming lobes, which are, in the preferred embodiments, of uniform size and cross section, and similar in cross section to the thread upon which they are disposed. In the area of the juncture of the shank and work entering portion of said fastener, certain lobes are rerolled, being those which otherwise project beyond said shank thread envelope. Said rerolling is subsequent to initial forming, to cause said lobes to be within the imaginary envelope of the cylindrical helical thread of the shank as projected axially beyond said juncture.

The method of making said fastener includes rolling a continuous thread from said shank portion throughout said tapered work entering portion and forming spaced apart thread forming lobes superimposed upon the thread convolutions of said work entering portion, then subsequently rerolling said threaded shank portion and certain of said lobes on the work entering portion, to the cylindrical thread convolution of said shank portion, as projected tipwardly.

The dies for rolling said fastener include a first and a second rolling die, said dies having a fastener entrance end and a fastener exit end, and disposed intermediate said ends, thread forming surfaces including a plurality of parallel thread crest forming grooves and thread root forming ridges. Each of said thread forming surfaces has a shank forming flat portion which is bounded along one edge by an upwardly inclined tip forming surface. Said first rolling die is the primary rolling die including a lobe forming section on said tip forming surface wherein a plurality of regularly spaced lobe forming pockets are disposed in said thread forming grooves. Said first rolling die also includes a reroll support section wherein said thread forming surface matches said shank forming flat portion, and a release section, said reroll support section being of a predetermined rolling length so as to reroll certain of said thread forming lobes which otherwise project beyond the envelope of said shank thread convolution as projected tipwardly. Said second rolling die has shank and tip forming sections and a reroll section so disposed as to reroll said certain thread forming lobes back to the envelope of said shank thread convolution.

DESCRIPTION OF THE DRAWINGS FIG. I is a pictorial view of a fastener embodying the invention.

FIG. 2 is a partial sectional view of the fastener of FIG. 1 prior to the reroll step.

FIG. 2a is an end view of the fastener of FIG. 2.

FIG. 3 is a partial sectional view of the fastener of FIG. 2 but later in rolling sequence.

FIG. 3a is an end view of the fastener of FIG. 3.

FIG. 4 is a pictorial view of the dies for making a fastener according to the invention.

FIG. 5 is a partial sectional view of the dies of FIG. 4, but in rolling sequence.

FIG. 6 is a sequential view of the dies of FIG. 4 during rolling.

FIG. 7 is a sectional elevation showing the reroll sequence of said dies.

FIG. 8 is a sectional, sequential view of an alternative embodiment of the invention of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and FIG. I in particular, reference number 10 indicates a self tapping screw illustrating one embodiment of the invention. Screw l0 includes a cylindrical shank portion 12 having a head 14 at one end thereof and a tapered work entering portion 16 terminating in a tip 18 at the other end thereof. Tip 18 may terminate in a sharp point or may be blunted, as illustrated. Head 14 is conveniently adapted with means such as a transverse kerp 20 to be drivingly engaged by a tool (not shown) such as a conventional screw driver. Head 14 is not necessarily enlarged.

Screw 10 has a thread convolution 22 commencing at tip 18 and extending as a spiral helical thread 24 on the tapered work entering portion 16, and extending at least part way up shank 12 as a cylindrical helical thread 26. As viewed in cross section (see FIG. 2a) thread 26 is of cylindrical shape throughout its full extent on said shank 12. Each of the thread forms includes a root r, flanks f and a crest c, which are identitied in conjunction with the thread reference number, e.g., 22r, 24f, 260.

Thread convolution 24 (FIG. 1) in work entering portion 16 contains thread forming lobes 30 extending from Mr along flanks 24f to crest 24c, appearing as bulges superimposed upon thread 24. Lobes 30 exhibit an envelope 32 in cross section which generally parallels flanks 24f and crest 24c and, except as later defined in the region of the juncture 25 of shank 12 and work entering portion 16, exhibits a lobe crest 300 which extends beyond the crest 24c of thread 24.

Lobes 30 are preferably circumferentially spaced apart along the thread convolution, being arranged in axial rows (see FIGS. 1, 2 and 3) extending from tip 18 through work entering portion 16 to, but not on, shank portion 12.

Further, the lobes 30 are illustrated as having leading and trailing surfaces 300 and 30!, (FIG. 2) respectively, gradually curving out of flanks 24f so as to be rounded. However, it is to be understood that the thread forming lobes may take a wide variety of shapes in addition to those shown, however, there are three principal requirements upon shape. The first requirement is that the lobe profile exceed the thread profile upon which disposed in order to support the screw in that region so as to effect a reduction of friction between the screw and the receiving member as the internal thread is being swaged. The second requirement upon lobe shape is that it rise gradually above the flanks 24f and crest 240 so as to not cause cutting of the material of the receiving member. The third lobe structural requirement is that successive lobes on the work entering portion extend progressively further outward from the screw axis with the last lobe shankwardly being on said work entering portion and the projection of certain predetermined lobes being rerolled to be within the imaginary projection of the cylindrical shank thread convolution tipwardly. As will be subsequently disclosed, the novel dies and method of manufacture of the self tapping fastener enables the novel structure of the present invention.

The self tapping fastener illustrated in FIGS. 1, 2 and 3 is made preferably by rolling the shank thread, work entering thread and lobes on a conventional cylindrical screw blank by apparatus illustrated in FIGS. 4 through 8. The rolling dies include a primary rolling die 42 and a secondary rolling die 44. These dies each include a flat shank rolling section 46 extending longitudinally from fastener entrance ends 43 and 43a toward fastener exit ends 45 and 45a. Shank rolling section 46 includes thread crest rolling grooves 48 and thread root forming ridges 50. Each die is also adapted with an inclined taper rolling section 52 generally coextensive with said shank rolling section 46 and forming a boundary thereof. Said taper rolling section also includes grooves 48 and ridges 50. Primary die 42 includes recesses or pockets 54 disposed in said grooves for forming lobes of FIGS. 1 3. Preferably, said recesses are arranged in rows (A, B and C).

Screw blank 40 is introduced in a conventional manner between thread rolling dies 42 and 44, as illustrated in FIG. 4. ln shank rolling section 46, thread crest rolling grooves 48 and thread root forming ridges 50 roll a cylindrical helical thread 24 on shank portion 12 (as illustrated in F168. 1 3). Preferably the same die members are adapted with taper rolling sections 52 having ridges 50 and grooves 48. Thus, the tapered work entering portion 16 is rolled on the fastener si- .multaneously with the shank and the fastener thus provided with spiral helical thread 26 extending substantially to tip 18. Dies 42 and 44 and the rolling method described thus far are generally known.

Lobes 30' may be initially formed simultaneously with the rolling of thread 24 by providing crest forming grooves 48 with pockets 54 complementarily shaped to the lobes 30 of FIGS. 1 and 2. During the rolling process upon screw blank 40 the movement of metal of blank 40 is forced into pockets 54 such that the lobes are expressed upon thread 24 appearing to be superimposed upon the flanks 24f and crest 240 of that thread. As previously mentioned, the above described is the initial forming of lobes as is illustrated at 30 in FIG. 2. As may be appreciated with the rolling die structure and method thus described, recesses 54 extend to a depth greater than the depth of thread crest forming grooves in which they are disposed. The net result of the method described thus far is to provide an initial lobe 30' (FIGS. 2, 5 and 6) on the upper reaches of work entering portion 16 which extends radially beyond the envelope 27 of the cylindrical helical thread 26 as projected tipwardly. Such projections of lobes 30' beyond the shank thread envelope deprive the self tapping fastener of the desired zero clearance or controlled running torque with the internally threaded receiving member.

Certain of lobes 30' are initially formed, (projecting beyond shank thread envelope 27) substantially simultaneously with the forming of spiral helical thread 24 (see FIG. 2). These lobes 30 are substantially uniform (in the preferred embodiments) from tip 18 to shank 12. Subsequent to initial formation finally formed lobes 30 are rerolled (See FIG. 6 at 30) to provide an accurately gauged thread swaging surface, particularly in the vicinity of the junction of work entering portion 16 with shank 12. Description of the die structure to accomplish this reroll appears later in this specification. Thus, lobes 30', which otherwise would project beyond the envelope of the thread convolution on the shank, are rolled back to that exact desired envelope. By this reroll the lobes are formed such that the envelope of each falls within the imaginary envelope of the cylindrically helical shank thread, as projected beyond the junction of the shank 12 and work entering portion 16 tipwardly. Further, it should be noted that this reroll occurs concurrently on crests c and sides 30s of lobe 30, so that the entire cross-sectional envelope of the lobe 30 is within the proper dimensions of the thread form 22 of the shank, which, in the present embodiment, provides a screw 10 with a controlled running torque, as the fastener is inserted into the receiving member, after the internal thread has been swaged.

In the embodiment of FIGS. 1 and 3, where all rows (A, B and C) of lobes 30 are rerolled (FIG. 3), very precise thread forming occurs in the final lobes. This is due in part to the controlled gauging of these final forming lobes and, in part, to the lower swaging loads encountered by these lobes, since their function is now in the nature of a final minor adjustment of thethread previously swaged by the earlier lobes. The overall effect of certain rerolled lobes during the thread swaging function is to equalize the torque load upon thread forming lobes. With equalized loads upon the lobes, adverse stresses are avoided in screw as it swages the intemal thread, which, in turn, dramatically increases the drive vs. strip ratio. Fasteners made according to conventional methods demonstrate a ratio of approximately 3 to 1 whereas the fasteners of the present invention provide up to a 10 to 1 ratio.

Another significant advantage in thread operating characteristics is gained from a screw formed per the preceding paragraph. It is known that a wide variety of materials into which these self tapping screws are inserted have memories. That is, the material has some resilience and, subsequent to subjection to the internal thread swaging by the fastener to the exact form of the shank thread, the internal thread exhibits a slight tendency to return to its original or filled in" form. Thus, some interference between the internal swaged thread and the shank thread 26 exists when the screw is inserted into the threaded hole. This interference insures zero clearance and, in fact, a locking function. The degree of lock, or the amount of interference, may be directly controlled by the amount of roll-off performed upon each lobe row in reroll section 60 of die 44. The more of crest 30c and side 30s that is rolled off, the smaller the internal thread will be and thus the greater the interference or prevailing torque upon the screw.

Dies 42 and 44 which are adapted to produce the above described novel fastener of the invention have a reroll or sizing section 60 wherein the thread forming surface 62 is shaped to the desired final shape and size of the lobes. Likewise, die 42 has a reroll support section 61 with a thread forming surface 63. In the illustrated embodiments, these forming surfaces are provided with ridges 50 and grooves 48 comparable to those in the shank thread forming section 46 wherein the cylindrical helical thread is rolled. These reroll

sections

60 and 61 are disposed on dies 42 and 44 subsequent to said shank rolling sections 46 and taper rolling sections 52. ln the preferred embodiment, reroll

sections

60 and 61 are merely a short extra section on dies 42 and 44 adjacent the exit ends 45 and 45a. The length of the sections in the rolling direction is approximately equal to the circumference of the fastener 10. This insures a reroll over the entire circumference of the fastener 10, and thus of all lobe rows (specifically, of those lobes 30' of rows A, B and C which otherwise project beyond the envelope 27 of the shank thread, as projected tipwardly).

In a further embodiment of the dies, the reroll support section 61 of die 44 may have a rolling length substantially shorter than the fastener circumference. In such embodiments, a reroll of fewer than all of the lobes 30' may be affected. By the dies illustrated in FIG. 8, lobes 30' formed by recesses 54 of rows A and B will be rolled back to the predetermined form (cylindrical shank thread) and lobes 30 of row C will remain singly rolled. This is so since the thread forming surface 62 of this embodiment is of a limited length so as to not support and work the fastener beyond the first twothirds of the reroll. Naturally, primary die 42 supports the screw during this reroll and the amount of reroll may also be controlled by the length of support section 63.

The product of the above described dies is a self tapping screw which has the usual thread forming lobes 30 in the tipward 18 region of the work entering portion 16; however, in the region approaching the shank junction 2, one lobe row (C in the illustration) is oversized or projects beyond the thread form of the shank 12. Thus, the lobes 30 of rows A and B may swage the internal thread of receiving member to exactly the cylindrical helical shank thread 26, and the oversized lobes 30 of row C tend to swage a larger internal thread. This screw exhibits much of the reduced thread forming torque requirements of the first embodiment and swages an internal thread in the receiving member which provides essentially a zero running torque or a very low prevailing torque with the receiving member with the shank thread 24, yet provides very close tolerances, and uniformity of manufacture, screw to screw.

ln conventional practice, lobes are rolled by both dies, necessitating elaborate alignment procedures when setting the dies in the rolling machine. As previously mentioned, if a screw blank slips slightly on one die during the initial states of rolling, the circumferential alignment of lobes is disturbed. Quite often in conventional practice, a full row of lobes may be completely wiped or rolled off a screw (or seriously deformed) due to alignment or slippage problems.

Contrary to previous practice, the lobe forming recesses 54 of the present invention are disposed in one of the thread forming dies (See FIG. 4). By placing all of the lobe forming recesses 54 on one die, as on die 42, proper circumferential placement of the lobes may be assured which contributes to accurate rolling of thread forming lobes 30.

Present arrangement of the lobe forming recesses 54 and die structure provides a fastener having lobes which are necessarily properly placed and not subsequently, uncontrollably wiped or rolled off. All lobes 30 are placed upon thread 24 by a single die and the relative position or change thereof of the cooperating die has no effect on lobe placement.

By the invention just disclosed, it should be understood that, now, self tapping screws having any of a variety of controlled running torques can be produced, in mass, and with a uniformity not previously known in the industry. While there have been described herein what, at present, are considered to be preferred embodiments of this invention, it will be evident to those skilled in the art that various changes and modifications may be made without departing from the true scope and spirit of the invention.

l claim:

1. The method of making a self tapping, threaded fastener having a substantially cylindrical shank, a head at one end, and a tapered work-entering portion terminating in a tip at the other end, comprising: rolling a continuous cylindrical helical thread having a root, flanks and a crest on said shank portion and said tapered work-entering portion, forming a plurality of lobe-like projections superimposed on the flanks and crest of said thread at predetermined locations on said tapered work-entering portion, said lobe-like projections having an envelope, the cross section of which is substantially symmetrical to said thread; and rerolling both said threaded shank portion and lobe-containing tapered work-entering threaded portion with a continuation of said cylindrical helical thread previously rolled on said shank, thereby rerolling and reforming certain of said formed lobes on said tapered work-entering portion adjacent said shank portion so as to be within the thread envelope being the imaginary projection of the thread convolutions of said shank portion.

2. The method making a self tapping, threaded fastener according to claim 1 including forming said lobelike projections in a plurality of rows and spacing said rows substantially uniformly circumferentially around said work-entering portion.

3. The method of making a self tapping, threaded fastener according to claim 1 including rerolling a predetermined number of said rows of said lobe-like projections, said number being less than the total number of said rows.

Claims

2. The method making a self tapping, threaded fastener according to claim 1 including forming said lobe-like projections in a plurality of rows and spacing said rows substantially uniformly circumferentially around said work-entering portion.