NO750695L - - Google Patents

Description

Self-tapping fastening device and method _and forms for manufacturing the same

* ■ This invention relates to a self-threading threaded fastening device which, in accordance with preferred embodiments, produces a set ldp or a controlled dominant torque during the insertion of the fastening device into the receiving element.

A large number of different self-threading threaded fasteners are available today. Most of these fasteners fall into one of two main groups of thread forming function: Cutting or drop forging. Although both types are effective when it comes to forming an internal thread in a receiving element, drop forging is now more common. One of the more typical fasteners of this type is shown in U.S. Pat. patent 3,426,642 of 11 February 1969. The thread-forming means of this type of countersunk screw is a tab construction (21 in Figs. 1, 2 and 3 of the said patent), which is superimposed on an essentially continuous threading from the tip throughout the shaft.

Although the fastening device described above is effective

in its capacity as a thread former with a low torque, it is burdened with properties that are closely related to previously known methods of mass production of the same. Currently, these fastening devices are rolled between two thread rolling matrices, where each individual matrix is provided with a number of parallel thread-foot-forming elevations, thread-forming grooves and tab-forming pockets, as during the rolling between these matrices with strong pressure the material on the screw shank is removed (sink hammer) to form the thread windings. When designing self-tapping screws that have thread forming tabs,

the thread forming grooves in the matrices are provided with a number of pockets which are incisions or bulls. These pockets provide space for the moving plastic metal and form a built-up part which is essentially superimposed on the thread. Since the tab-forming parts of the die are actually holes in the thread-forming grooves, the envelope curve for the tabs will naturally extend beyond the envelope curve for the thread turns in the area near the connection point of the shaft and the conical portions of the threading device where the diameter of the conical part approaches that of the shaft ( see Fig. 3 in the above-mentioned patent). When the fastener is made with a thread turn having a substantially uniform cross-section (a common shape), the thread-forming tabs near this connection point will countersink an internal thread in the element receiving the fastener, larger than the actual threads on the shaft portion of the fastener. Thus, when the fastening device is fed into the receiving element, there is no prevailing torque between the counter-forged internal thread in the receiving element and the shaft thread on the fastening device. Rather, there is something of a loose fit between the fastening device and the receiving element. It will be quickly recognized that such a feature can in some cases be considered a defect which limits the universal application of the fastening device.

This shortcoming has been recognised, and an attempt has been made to remedy the described disadvantage. Figures 24 to 26 in the above-mentioned patent document show an attempt to provide a self-tapping screw which also provides a controlled prevailing torque (zero clearance - or a positive engagement between the fastening device and the designed internal thread). As shown, the known fastening device comprises a threaded part for penetration into the workpiece and with a conical thread design with decreasing main and pitch diameters, but a constant minimum diameter. Although the shown fastener can produce a dominant torque because the height and thickness of the thread-forming tabs decrease toward the end penetrating the workpiece, from a value less than that of the thread form of the shaft portion, its overall effectiveness and serviceability as a self-tapping screw is impaired.

The efficiency of a self-tapping screw is great

draw in the thread design in the part that enters the workpiece. The thread design on this must not only sink the internal threads for the subsequent shaft part, but also advance the fastening device into the receiving element. Unless the thread form "bites" deep enough into the side material of the receiving element to pull inward, the fastener will not move forward and engage the desired thread. On the contrary, it will "rip out" and leave the receiving element without any threads or with an enlarged hole.

The fastening device according to the present invention eliminates both of the above-mentioned problem areas by producing a fully formed thread on the part that penetrates into the workpiece mainly down to the tip of this and a flap construction that sinks the internal thread in the receiving element with a minimum of torque and still within the dimensional requirements for the shaft thread.

As already mentioned, each of the two thread-rolling matrices for producing the above-described conventional fastening device is provided with tab-like pockets. As a result, these matrices must be carefully assembled in automatic rolling machines and finally adjusted precisely in position, so that successive tabs on said thread turns are placed alternately and regularly. Despite the care taken during such assembly of these matrices, they will quickly lose their original alignment. Likewise, often during the actual rolling, e.g. during the initial insertion the fastener blank may shift in one of the conventional arrays and cause the arrays to lose their relative alignment and the regular spacing between the tabs to be destroyed. As can be expected, irregular spacing between the tabs will interfere with the thread-forming function. In direct contrast to this, the matrices according to the present invention will ensure regular placement of the tabs•regardless of the relative matrix adjustment or displacement of the fastening device blank.

The main purpose of the present invention includes the production of a self-threading fastening device, a method of manufacturing the same and matrices for this purpose, which fastening device has the ability to countersink an internal thread in a receiving element, where the fastening device in accordance with the preferred embodiment exhibits zero running torque or a controlled prevailing moment at the introduction into the receiving element.

For this purpose and in accordance with certain features of the invention, the self-threading fastening device according to the present invention comprises a cylindrical shaft part and a conical insertion part that ends in a tip. A cylindrical screw thread is provided on both the shaft part and the insert part. Superimposed on the continuous helical thread on the insertion part are thread-forming tabs which in the preferred embodiments have a uniform size and cross-section and corresponding in cross-section to the cross-section of the thread on which they are placed. In the area of connection of the shaft and the insertion part of the miter device are certain tabs such that they will be within the imaginary enveloping curve of the cylindrical screw thread on the shaft as it is projected axially outside the connection point. This is achieved by re-rolling to reduce the flaps that otherwise protrude outside the shaft thread's enveloping curve.

The method of manufacturing the fastener in accordance with the invention includes rolling a continuous thread from the shank portion through the conical insert portion and forming mutually spaced thread-forming tabs superimposed on the thread turns of the insert portion, then successively re-rolling the threaded shank portion and some of the tabs on the insert portion to the cylindrical thread of the shaft part as it is projected towards the tip.

The matrices for rolling fastening devices in accordance with the invention include a first and a second matrix, the matrices having an input end and an outlet for the fastening device and placed between these ends thread-forming surfaces including a number of parallel thread-forming grooves and thread-foot-forming elevations. Each of the thread forming surfaces has a planar portion for shaft forming which is bounded along one edge by an upwardly sloping tip forming surface. Said first roll die is the primary roll die and includes a tab-forming section on said tip-forming surface, where a number of evenly spaced tab-like pockets are placed in the thread-forming grooves. This first rolling die also includes a bare section for re-rolling, where the thread-forming surface fits the shaft-forming planar part. and a release section, the carrying section for re-rolling carried a pre-determined yalse length to roll certain of the aforementioned thread-forming tabs which would otherwise project outside the enveloping curve of the shank thread turn when this is projected towards the tip. Said second rolling die has sections for forming the shaft and tip and a section for re-rolling which is so arranged as to re-roll said certain thread-forming tabs back to the enveloping curve for the shaft thread winding.

The invention will be better understood from the following detailed description with reference to the drawings, where fig. 1 is a side view of a fastening device comprising the invention, fig. 2 is a partial section of the fastening device in fig. 1 for the second roller stage, fig. 2a is an end view of the fastening device in fig. 2, fig. 3 is a partial section of the fastening device in fig. 2 after the second rolling step, fig. 3a is an end view of the fastening device in fig. 3, fig. 4 is a perspective view of the matrices for manufacturing the fastening device in accordance with the invention, fig. 5 is a partial section of the matrices in fig. 4 but during the rolling operation, fig. 6 is a sequence view of the arrays of FIG. 4 during the rolling, fig. 7 is a partial cross-sectional view showing the second rolling sequence of the dies, and FIG. 8 is a partially cross-sectional sequence diagram of an alternative embodiment of the invention in fig. 6.

With reference to the drawings and in particular fig. 1, the reference number 10 denotes a self-tapping screw as one embodiment of the invention. The screw 10 comprises a cylindrical shaft part 12 with a head 14 at one end and a conical part 16 penetrating the work piece which ends in a tip 18 at the other end of the shaft. The tip 18 may end in a sharp point or it may be blunt as shown. The head 14 is appropriately fitted with means, such as a transverse groove 20

for driving intervention with a tool (not shown), such as a conventional screwdriver. The head 14 does not necessarily need to be extended.

The screw 10 has a thread turn 22 which begins at the tip 18 and extends as a helical thread 24 on the conical part 16 penetrating the workpiece and extends at least partially up the shaft 12 as a cylindrical screw thread 26. Viewed in cross-section (see fig. 2a) the thread 26 is of cylindrical shape over its entire extent on the shaft 12. Each of the thread shapes includes a foot r, flanks f and a top c, which are characterized in connection with the thread's reference number, e.g. 22r, 24f, 26c.

The thread turn 2-4 (Fig. 1) on the part 16 which penetrates into the workpiece comprises thread-forming tabs 30 which extend from 2 Lyr along the flanks 2 Ly f to the top 24c and appear as elevations superimposed on the thread 2Ly. Tabs 30 have an enveloping curve 32 (Fig. 2) in cross-section which runs mainly parallel to the flanks 24-f and the top 24c and, except as described later, in the area of the connection point 25 between the shaft 12 and the part 16 penetrating the workpiece, has a tab top 30c that extends beyond the top 24c of the bunch 24.

The tabs 30 are preferably separated from each other around the circumference along the thread turn and arranged in axial rows (see fig.

1, 2 and 3) which extends from the tip 18 over the part 16 penetrating the workpiece to but not into the shaft part 12.

Furthermore, the tabs 30 are shown with front and rear surfaces 30a and 30t (Fig. 2) which are gradually curved out from the flanks 24f so that they are rounded. It should be understood, however, that the thread-forming tabs can assume very different forms in addition to those shown. but there are three main requirements when it comes to the shape. The first requirement is that the tab profile must exceed the thread profile, on which it is arranged to carry the screw in that area, so that a reduction of the friction between the screw and the receiving element is effected when the internal thread is lowered. The other requirement for the shape of the flap is that it should gradually rise above the flanks 24-f. and the top 24c so that it does not cause cutting of the material in the receiving element. The third requirement for the construction of the tab is that successive tabs on the part penetrating the workpiece project progressively further outwards from the screw axis with the last tab against the shank present on the part penetrating the workpiece and the projection of certain predetermined tabs is re-rolled . to be within the imaginary projection of the cylindrical shaft thread in the direction of the tip. As will be explained in the following, this new construction of the present invention is made possible by means of new matrices and a new method for the production of self-threading fastening devices.

The one in fig. 1, 2 and 3 shown self-threading fastening device is preferably produced by rolling the shaft threads, the thread penetrating into the workpiece and tabs on a conventional cylindrical screw blank using an apparatus as shown in fig. 4 through 8. The roll dies include a primary roll die 42 and a secondary roll die 44-These dies each include a planar shank roll section 46 extending longitudinally from the fastener inlet ends 43 and 43a toward the fastener outlet ends 45 and 45a-Shaft roll section 46 includes grooves 4-8 for rolling the threaded top and elevations 50 for rolling the threaded base. Each die is likewise provided with an inclined conical roll section 52 which extends substantially in the same direction as, and forms a boundary for, the shank roll section 46. The aforementioned conical roller section also includes grooves 48 and elevations 50. The primary matrix 42 has, particularly on the inclined conical roller section 46, depressions or pockets 54 placed in the grooves 48 to form tabs shown in fig. 2 and 2a. These recesses are preferably arranged in rows (A, B and C in fig. 4-6 and 8).

The screw blank 40 is inserted in a conventional manner between the thread roll matrices 42 and 44 as shown. on fig. 4. In the shaft rolling section 46, the grooves 48 for rolling the top of the thread and the elevations 50 for rolling the base of the thread will produce a cylindrical screw thread 26 on the shaft part 12 as shown in fig. 1 - 3. The same matrix elements are preferably provided with conical rolling sections 52 with elevations 50 and grooves 48. Thus, the conical part 16 penetrating into the workpiece is rolled on, the fastening device simultaneously with the shaft and the fastening device 10 in this way provided with helical thread 24

which extends mainly to the tip 18. The dies 4-2 and 44 and the rolling method described so far are commonly known.

The tabs 30a and 30' can be formed initially at the same time as the rolling of the thread 24 by providing the top-forming grooves 48 with pockets 54 which have a complementary shape to the tabs 30, more specifically 30a and 30' in fig. 2 and 2a. During the rolling process on the screw blank 4-0, the movement of metal on the blank 40 is forced down into the pockets 54) so that the tabs 30a and 30' appear on the thread 24 and appear to be superimposed on the flanks 24f and the top 24c of this thread. As already mentioned, the above-described initial design of the tabs 30a and 30' as shown in fig. 2. As will be noted with the described construction of the roll die and method, the recesses 54 extend to a depth greater than the depth of the thread forming grooves 48 in which they are located. The actual result of the method thus far described is thus to produce an incipient flap 30 and 30' (Figs. 2a, 5, 6 and 8) on the upper region of the penetration part 16 which extends radially outside the enveloping curve 27 of the cylindrical screw thread 26 as it is projected, towards the tip. Such protrusions of the tabs 30' outside the shaft tg the boy's_envelopment curve deprive the self-threading fastening device of the desired minimal clearance or controlled movement moment with the internal

threaded receiving element.

Some of the tabs 30' are initially formed (protruding outside the shaft thread enveloping curve 27) essentially simultaneously with the formation of the screw thread 24 (see Figs. 2 and 2a). These tabs 30a including tabs 30' are substantially uniform (in the preferred embodiment) from the tip 18 to the shaft 12. After the initial shaping, the initially rolled tabs 30a and especially the tabs 30' are re-rolled into finally formed tabs 30 (see Figs. 3, 4 and 6) to provide an accurately dimensioned thread-forming surface, particularly in the vicinity of the transition zone 25 between the penetrating portion 16 and the shaft 12. Description of the construction of the die to achieve this secondary rolling follows later in this review. Thus, tabs 30' which would otherwise project outside the enveloping curve of the thread turn on the shaft, are rolled back to the precisely desired enveloping curve. With this renewed rolling, the tabs 30 are formed in such a way that the envelope curve for each of them falls within the imaginary envelope curve for the cylindrical screw thread on the shaft, as this is projected outside the transition area 25 on the shaft 12 and the penetration part 16 in the direction towards the tip, see fig. 3 and 3a. It must further be noted that this advanced rolling takes place simultaneously on the tops 30c and the sides 30s of the tabs 30', so that the entire cross-sectional envelope of the tab 30 lies within the correct dimensions of the gage mold 22 on the shaft, which - in the present embodiment - gives a screw 10 with a controlled running torque when the fastening device or screw 10 is fed into the receiving element after the internal thread has been drop-forged.

In the output journey in fig. 1 and 3, where all rows (A, B and C) of "tabs 30 have been re-rolled (fig. 3), very precise thread forming occurs in the final tabs. This is partly due to the controlled dimensioning that is carried out during the repeated rolling during the design of these final tabs and partly the lower loads that are encountered when forging these tabs during the repeated rolling because their function now has the character of final small adjustments of the thread designed in advance during the initial rolling and the first tabs. The overall effect of some rerolled tabs during the thread machining operation is to equalize the moment load on the thread-forming tabs. With equalized loads on the tabs, adverse stresses are avoided in the screw as it produces the internal thread, which in turn greatly increases the ratio of operating and tear-out. Fasteners manufactured in accordance with conventional methods exhibit a ratio of approximately 3 to 1, while fasteners nger according to the present invention produces ratios of up to 10 to 1. • <

Another significant advantage in threading properties is achieved with a screw that is made as described in the preceding paragraph. It is known that a large number of different materials, in which these self-tapping screws are inserted, have "memory".

That is that the material has a certain elasticity and after being subjected to countersinking of the internal thread by means of the attachment device to the desired shape of the shaft thread, the internal thread exhibits a slight tendency to return to its original or "filled-in" shape. Thus, there will be some disturbance or engagement between the internal countersunk thread and the shaft thread 26 when the screw is fast into the threaded hole. This engagement ensures "zero clearance" and in reality a locking function. The degree of locking or engagement can be directly controlled by the degree of rolling away that takes place at each tab row in the secondary roll section 60 of the die 44-The more of the top 30c and the sides 30f that are rolled away, the smaller the internal thread will be and thus the greater the engagement or the prevailing torque on the screw.

The dies 42 and 44 which are intended to produce the above-described novel fasteners of the invention have a section 60 for repeated rolling or adjustment, where the thread-forming surface 62 is formed to the desired final shape and dimension of the tabs 30. Similarly, the die has 42 a section 6.1 for secondary rolling with a thread-forming surface 63. In the shown designs, these forming surfaces are provided with elevations 50 and grooves 48 which can be compared to those in the shank thread-forming section 46, where the cylindrical screw thread is rolled. These secondary rolling sections 60 and 6.1 are arranged on dies 42 and 44 in sequence after said shaft rolling sections 46 and conical rolling sections 52. In the preferred embodiment, the secondary rolling sections 60 and 61 are only a short additional section on the dies 42 and 44 near the exit ends 45 and 45a. The length of the sections in the rolling direction is approximately equal to the circumference of the fastening device 10. This ensures a secondary rolling over the entire circumference of the fastening device 10 and thus of all tab rows (especially of the tabs 30' in rows A, B and C which otherwise project outside the enveloping curve 27 for the shaft thread when this is projected in the direction of the tip).

In one embodiment of the dies, the secondary roll section 61 of the die 44 may have a roll length which is substantially shorter than the circumference of the fastening device. In such embodiments, a new rolling of fewer than all tabs 30' can be carried out. In the matrices shown in fig. 8, the tabs 30' formed with the recesses 54 for rows A and B will be rolled back to the predetermined shape (cylindrical shaft thread) and the tabs 30' in row C will remain once rolled. This is so because the thread forming surface 62 in this embodiment has a limited length so that it does not support and process the fastening device beyond the first two thirds of the secondary rolling. Naturally, the primary die 42 supports the screw during this secondary rolling and the amount of secondary rolling can also be controlled by the length of the supporting section 63.

The product of the matrices described above, i.e. the second embodiment, is a self-tapping screw having thread-forming tabs 30, all of equal dimension, in the tip-facing region 18 of the penetration part 16. As a result, in the region approaching the transition point 25 to the shaft a row of tabs (C in the drawing) oversized or projecting radially outside the thread 26 on the shaft 12. Thus, the tabs 30 in the rows A and B can countersink the internal thread in the receiving element to exactly the S5/cylindrical screw thread 26 on the shaft and the oversized tabs 30' in row C help to countersink a larger internal thread. This screw fulfills a large part of the requirements for reduced torque during threading as for the first embodiment and countersinks an internal thread in the receiving element which gives mainly a zero torque at times or a very low prevailing torque with the receiving element with shank thread 24 and still gives very tight tolerances and uniformity during production from one screw to the other.

In conventional practice, the tabs are rolled with the help of both matrices and require extensive alignment processes when setting the matrices in the rolling machine. As already mentioned earlier, if a screw blank slips slightly in one of the dies during the original roll condition, the circumferential setting of the tabs is destroyed. Very often in conventional practice, an entire row of tabs can be completely obliterated or rolled away from a screw (or severely deformed) due to problems with setting and sliding.

In contrast to previous practice, the flap-forming recesses 54 according to the present invention are placed in one of the thread-forming matrices (see Fig. 4). By placing all the flap-forming recesses 54 in a matrix such as the matrix 42, correct placement on the circumference of the tabs can be ensured, which contributes to accurate selection

sing of the thread-forming lobes 40.

The present arrangement of the tab-forming recesses 54 and the construction of the matrix produces a fastening device with tabs which are necessarily correctly positioned and cannot later and uncontrolled be ironed or rolled away, All tabs 40 are placed on the thread 24 by means of a single matrix and the relative position or changing this of the cooperating matrix has no effect on the placement of the tabs.

With the invention described above it is to be understood

that self-tapping screws having one or other of a number of different controlled torques during operation can be produced in large quantities and with a uniformity that has not been known within the industry until now, Although the above has been described what has been considered to be the preferred embodiment of this invention, it will be clear to those skilled in the art that various changes and modifications can be made without departing from the true scope and spirit of the invention.

Claims (5)

1. Self-threading threaded fastening device comprising a cylindrical shaft part, a head at one end of the shaft part, a conical insert part which ends in a point at the other end of the shaft part, the shaft and insert parts having a continuous screw thread including a foot, flanks and a top extending from the tip of the insertable part into the workpiece into the shaft part towards said head, and a number of thread-forming tabs superimposed on said thread over the entire insertion part of the shaft, characterized in that the tabs (30) have flanks (30s) extending selectively outwards outside the flanks (24f) of said thread (24) and tab tops (30c) which extend selectively outside the top (24c) of the thread (24), the tabs (30) having an enveloping curve, the cross-section of which is symmetrical in relation to the thread (24) and has a uniform size from the aforementioned tip (18) towards the shaft part (12) until it approaches the connection point (25) for the shaft part (12) and the insertion part (16), where they the following envelope curves for the tabs (30) successively decrease in size symmetrically towards their tab tops (30c) and flanks (30s) so that they are reduced to the imaginary envelope curve of the cylindrical screw thread (26) on the shank portion (12) as projected axially outside said connection point (25) towards the tip (18). 2. Device according to claim 1, characterized by a number of thread-forming tabs (40) arranged in a number of rows (A, B, C) and these rows are evenly arranged around the circumference of the part (16) of the fastening device (10) which penetrates into the workpiece . 3. Device according to claim 2, characterized in that said number of rows (A, B, C) are placed mainly axially on the conical penetration part (16) of the fastening device (10). 4- Device according to claim 3, characterized in that the tabs (30) in all rows (A, B, C) near the transition (25) for the shaft part (12) and the penetration part (16) successively decrease in dimension symmetrically towards their top ( 30c) and flanks (30s) so that they are delimited to the imaginary envelope curve for the cylindrical screw thread (22) on the shaft part (12) as this is projected axially outside the transition (25) towards said tip (18). 5. Device according to claim 1, characterized in that at least one of the said rows of tabs (A, B, C) comprises tabs (30) with essentially uniform dimensions and shape from said tip to said transition between shaft and penetration part, the tabs ( 30) near said transition (25) on the penetration part (1.6) projects radially outside the envelope curve for the shaft thread (26) as it is projected outside the transition (25) towards the tip (18).