Self-tapping screw, method and dies for making the same and method for joining thin workpieces

ES 2 269 752 T3

DESCRIPTION

Self-tapping screw, method and dies for its manufacture and method for joining thin pieces.

Field of the invention

The present invention relates to self-tapping screws and, more specifically, to self-tapping screws, blanks for self-tapping screws, and to methods for forming self-tapping screws using appropriate roll forming dies.

Such self-tapping screws and methods for the manufacture thereof are disclosed in GB-A-1 022 355.

Background of the invention

It is recognized by those skilled in the art that when conventional self-tapping screw elements are used to assemble thin-metal sheet components, they are of limited efficiency due to problems related to the need to keep the screw tightening torque at low relative values. This is necessary to minimize the potential for tightening the anchor during assembly and thereby causing the screw to rotate. Anchor material is that part of the bonded assembly that is withdrawn farthest from the bottom of the screw head. Tightening and rotation will cause a loss in the clamping load of the assembly and a consequent deterioration of the assembly.

Figure 1 shows a disadvantage commonly encountered with a self-tapping screw 102, which features a conventional single-entry thread for placing thin sheet metal components 104 and 106 in a bonded assembly. The screw contains a conventional circular cross section.

When the anchoring material of the screw 106 (also called "nut element"), which is the material furthest from the head of the screw, has a width 108 equal to or less than the axial pitch 110 of the screw (here generally defined as part " fine ”), the lead-in face 112 of the coil or thread usually deflects the anchor material 106 so that the anchor material follows the gap between adjacent coils, coils, or threads. This type of anchoring and assembly material may not produce the most effective assembly holding load. Also, improper thread joint contact occurs.

In order to avoid certain disadvantages of the single-entry thread design when it is used to join thin materials, a real circular cross-section screw has been used lately, which has multi-entry threads that are generated around a part. brute with head. The use of a multi-entry thread tends to better engage the anchor material, preventing the anchor material from becoming entrapped between threads by the more distributed arrangement of multiple posts on the perimeter of the anchor material pilot hole.

However, even the use of multi-entry threads is not a complete solution, when only used. Figure 2 remarkably shows a continuous setback associated with conventional screws for joining thin pieces, presenting threads of one or more entries (circular or non-circular in cross section). The example screw 200 maintains a parallel core diameter 202 located as close as possible to the bottom 204 of the screw head 206. This application can cause a reduction in assembly performance. Manufacturing restrictions generally create incomplete filling of the screw thread ridges adjacent to the bottom 204 of the screw head 206, thereby creating a non-specific and non-adjustable inverted thread taper 208. In this way, the thread ridges located closer to the entry point of the screw are of a greater diametrical magnitude than the thread ridges located closer to the head. The inverted thread taper 208 has the disadvantage that it produces a gap 210 between the external and internal mating threads of the assembly. Said gap 210 causes the joint thread contact to be reduced in the essential area of ​​the assembled structure and causes a breakage of the assembly with a lower application torque than expected.

Hitherto the use of a screw of circular cross section (as described above) has been preferred. Although certain advantages with respect to thread forming can be achieved by using a non-circular cross-sectional screw, the use of a non-circular cross-section, such as the commercially available roll-forming self-tapping multilobular screw, has been considered. generally as detrimental to assembly. Such non-circular cross sections are considered not to have the necessary resisting force relative to the applied torque when attached to the assembly.

Consequently, an object of the present invention is to provide a self-tapping screw and an associated method for forming such a screw through the use of a non-circular cross-sectional blank, thus producing a primarily multilobular screw with thread-forming properties. advantageous, and a desirable multi-lead thread. However, such a screw should exhibit a high degree of resistance to vibrational loosening, as the material loosens between lobes, despite the use of a generally non-circular cross-section in the "bonding zone" of the bonded assembly.

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Summary of the invention

This invention provides a solution to the disadvantages present in the state of the art by means of a method of manufacturing a self-tapping screw from a head blank, with which a multilobular self-tapping screw is obtained for joining thin parts, the which exhibits desirable thread-forming characteristics in the tapered tip and body parts, as well as increased resistance to vibrational loosening near the head. The resulting screw, obtained from the aforementioned blank and method, incorporates a part of the tapered root threaded between the screw head and the body part, which presents a cross section that goes from being almost circular in area adjacent to the bottom of the head to be a maximum oval (or lobular) cross section at the intersection of the tapered root part and the screw body. This shape, combined with a thread profile outer diameter that remains largely constant from the body part through the tapered root part, and a multiple helical coil shape (lead-in thread), ensures a more secure fixation. of the screw in a fine anchor material with the anchor material extruded axially back and forth around the root part.

According to one embodiment, the head blank is formed by introducing a wire or rod of generally circular or lobular cross section into a die cavity of a head die. At the same time that the wire or rod is driven into the head cavity, it is plastically deformed to the desired four-part blank shape, the head, the conical input part, the body part and the sharp part. The die cavity has a suitable cross-section, so that the body part and the tapered entry point part of the finished blank are formed with a multilobular cross-section, while the tapered part, adjacent to the head, has a essentially circular cross section.

To form the threaded lobe screw, the finished blank is fed through laterally moving roll forming dies, which apply sufficient pressure to cause plastic deformation of the blank surface. The dies are held at an equal distance, which, due to the lobular cross-section of the blank, causes an oscillating rolling rotation in the blank during lateral movement of one of the dies relative to the other. This rolling-forming process, in the area of ​​the tapered root part, creates a thread pattern that maintains an approximately constant outside diameter with respect to the body part, but the inside (root) diameter (the cavities of each threads) is continuously tapering out towards the head. Also, the tapered root portion is thus provided with the desired maximum oval near the body portion and with an almost circular cross-section adjacent to the lower portion of the head. This variation in oval along the tapered root portion results from the reduced forming pressure present in the larger diameter area near the head.

A screw formed in accordance with the present invention provides a new tapered root portion, adjacent to the head, having a variable cross section that creates mechanical resistance against loosening caused by the effects of vibration or other external forces as the material fixed by the screw is loosened between the lobes.

Brief description of the drawings

The aforementioned, as well as other objectives and advantages of the invention will be explained more clearly through the following detailed description of the drawings, in which:

Figure 1, already described, is a cross section of a part of one of the sides, showing an example of deviation of the anchor material when a conventional single entry helical thread is used on a screw;

Figure 2, already described, is a cross section of a part of one of the sides, which shows the creation of a sharp inverted thread shape associated with manufacturing restrictions, and which presents a diameter of the core of the thread root parallel screw along the entire length of a screw;

Figure 3 is a side view of a self-tapping screw with an example of a sharpened root portion to obtain higher vibration resistance and clamping force on thin materials;

Figure 4 is a cross section of a portion of one of the sides, showing the formation of a head blank according to an embodiment of this invention;

Figure 5 is a side view of a head blank formed in accordance with the forming embodiment of the blank set forth in Figure 4;

Figure 6 is a cross-sectional view of the conical pointed portion of the head blank taken along line 6-6 of Figure 5;

Figure 7 is a cross-sectional view of the body portion of the head blank taken along line 7-7 of Figure 5;

Figure 8 is a cross-sectional view of the conical portion of the head blank taken along line 8-8 of Figure 5;

ES 2 269 752 T3 Figure 9 is a cross section of a part of one of the sides of the formed screw obtained, from the head blank of Figure 5, introduced by thread forming dies, according to a shape of embodiment of this invention;

Figure 10 is a cross section of the head blank in the forming dies, at different points along a rolling line, taken generally along the portion of the body located in the vicinity of line 11-11 of figure 9;

Figure 11 is a cross-sectional view of the part of the formed screw body, obtained with the roll forming process, taken along line 11-11 of Figure 9;

Figure 12 is a cross-sectional view of the tapered root portion of the screw formed at a location adjacent to the body portion, taken along line 12-12 of Figure 9;

Figure 13 is a cross-sectional view of the tapered root portion of the screw formed at a location adjacent to the head, taken along line 13-13 of Figure 9;

Figure 14 is a cross section of a part of one of the sides of a finished screw, formed according to Figure 9, for fixing a pair of thin sheets of material; and Figure 15 is a cross-sectional view of the stress patterns generated by the screw in the anchoring material, taken along line 15-15 of Figure 14.

Detailed Description of Illustrative Embodiments

I. General principles

Through other background, Figure 3 shows an exemplary screw 300 that generally prevents certain disadvantages associated with a sharp inverted thread shape adjacent to the head and therefore increases clamping force and strength. vibrational loosening. Briefly, the screw 300 includes a head 302, a sharpened root portion 304 (which tapers radially outward in a direction along the body toward the head 302), a central body portion 306, a of the conical entry 308 and an exemplary entry point 310 (such as a self-piercing point).

The screw body 306 has arranged along its outer periphery a plurality of continuous helical coils, spirals, or threads 312. The screw thread (helical coils) has an outer diameter D. According to one embodiment, the magnitude of D it can be between 1.6 and 10 mm. However, this and other values ​​specified herein are only examples of a typical screw application of the present invention. The principles described herein can be applied to screws, as well as corresponding materials, of any type and / or size.

According to one embodiment, there are two separate continuous coils, spirals, or helical threads (also called "multi-lead threads") arranged along the body 306, such that the screw is double-lead or double-lead, but it is explicitly provided that other separate thread numbers may be used. In this embodiment, the use of a plurality of threads also serves to prevent deformation of the anchor sheet. It is important to mention that the term "thread", as used herein, can refer to the general continuous formation or formations, to the helical winding along the axis or cylindrical body, or to the individual profile formations of cavities and parts. over a cross section of the screw. The context in which the term is used should help the reader to differentiate between the two uses of the general term.

In an illustrative embodiment, the inventive tapered root portion 304 of the screw, which is located axially between the head 302 and the body 306 of the screw, has an axial length W of at least two, and preferably no more than about 3 5 times the axial pitch P. The sharpened root portion 304 has a maximum diameter R adjacent to the head 302, and is tapered downward to a diameter equal to the root diameter B of the screw thread. Angle φ is the included angle that the sharpened root portion 304 makes with respect to body 306. Angle φ should measure between about 6 ° and 15 °, preferably between 8 ° and 10 °.

The threads 312 are arranged on the tapered root portion 304 so that the outer diameter D of the screw and the threads is approximately the same magnitude along the tapered root portion 304 and the body of the screw 306. . Although it is sometimes preferable that all threads between the head (including the tapered root part) and the entry point area have approximately the same outside diameter, It is envisaged that in alternate embodiments a different diameter and thread profile may be used in an area closer to the entry point and further away from the tapered root part to achieve certain advantageous effects with respect to thickness and materials. of specific manufactured parts. Consequently, the threads are envisaged to have approximately the same diameter in the sharp root portion and in a screw body portion directly adjacent to the sharp root portion (the adjacent portion extends from the sharp root portion in at least a distance that is four times the thread pitch towards the entry point).

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II. Lobular blank formation

The principles described above (eg, a tapered root portion and a multi-lead thread) can generally be applied to a screw having a multi-lobed thread-forming cross-section. Such a multilobal cross section generally consists of an odd number of lobes (eg 3, 5, 7, etc.) defining an oval (non-circular) perimeter. In general, when a lobe screw is inserted into an appropriately sized pilot hole, the lobes plastically deform the material to create the roll formed threads. These threads maintain a certain degree of springback upon formation, thereby exerting clamping pressure on the screw, which advantageously reduces vibrational loosening. An example of a multilobular cross section, featuring three such lobes, is the Trilobular ™ line of self-tapping screws, developed by, and available through Research Engineering and Manufacturing Inc. in Middletown, RI and Conti Fasteners AG in Switzerland.

The formation of a cooling head blank, which is used to finish forming a finished screw in connection with one embodiment of this invention, is generally shown in Figure 4. This blank is normally a three-lobed cross-sectional blank, although the use of other numbers of lobes is expressly provided. A hard head die 402 is shown, which exhibits a forming die cavity 404. The forming process usually begins when a cut piece of wire or rod of generally lobular or circular cross-section of predetermined size is placed in the cavity of the die. Often, three or more different die cavities are used in a line, which represent different formation phases, where the blank is progressively moved from one die cavity to another. In the illustrated example, the final formation matrix 402 is shown. Die cavity 404 is a dimensioned hole corresponding to the desired finished blank shape. The die cavity 404 has side walls of sufficient thickness and hardness to ensure that no deformation of the same occurs when the unfinished blank is inserted inside it to be plastically deformed until the desired finished blank 405 is obtained, as shown. The upside down cross section of die cavity 404 (not shown), of course, defines the desired blank cross section (multilobular or circular perimeter, as described in more detail below).

The formation of the finished blank 405 in the die cavity 404 is typically accomplished by several pulses applied by a piston or punch 406. The example punch 406 is typically driven by a powerful mechanical actuator (not shown) that pushes the blank underneath. significant pressure (see arrow 408) to simultaneously shape the internal shape of the finished blank and the shape of the head of the blank. Like progressive die cavities, different shaped punches can be used during blank 405 formation, each with a shape that progressively more closely matches the shape of the blank 410 head. In this example , the blank head 410 includes a Phillips drive array 412 (shown in exposed cross section). However, said transmission formation may take on any desired shape, and may alternatively comprise (for example) a standard hollow transverse groove, a six-lobe transmission cavity, or a hexagonal socket cavity. On the other hand, the blank can be provided with any internal or external thread wrench for coupling the shape of the transmission head, or it can contain any other system that allows the torque to be transmitted to the screw through an appropriate system. and convenient. It should be noted that a movable ejector pin 414 can be used to stop the tip of the blank and eject the finished blank (see arrow 416) into a channel or other conduit (not shown), to target a thread forming die (described below).

The die cavity 404 generates at least four clearly formed parts of the blank, in accordance with the various embodiments of this invention. Referring now to FIG. 5, these finished cooling head blank portions 405 are shown and described in more detail. According to an illustrative embodiment, said parts include a conical pointed section (or "inlet") 502, a body section 504 and a conical part 506 adjacent to the head 410, as well as the head itself.

The general cross sections present in the conical pointed section 502, the body section 504, and the tapered portion 506 are respectively described in Figures 6, 7, and 8. In general, the screw blank defines a continuously tapered shape from the tip. 510 to 410 head. The three-lobed cross-sectional shape that characterizes the example blank is most clearly shown in the cross-sectional views of Figures 6 and 7 (see lobes 602 and 702, respectively). Lobes 602 and 702 are centered around axis 512, and remain within respective coaxial circles 606 and 706 (shown in the illustration). The "cavity" areas between lobes 602 and 702 are separated by an oval gap K1 and K2 (respectively). As described, the larger the "oval", the higher the K value. This can also be defined as the degree of "lobularity" in accordance with the teachings of this invention.

Similarly, Figure 8 shows a cross section taken transversely through the approximate axial midpoint of the tapered portion 506. This cross section, centered about the axis 512 is basically circular, and representative of the circular profile along this portion. . Referring also to Figure 5, the tapered portion 506 tapers outwardly as it approaches the bottom 514 of the head 410 from the body section 504. The degree of increase in taper is usually not less than twice the axial pitch of the screw thread (reference is made to pitch AP of Figure 14) when formed and is preferably not longer in the axial direction than a WB value, which is three and a half times the axial pitch (AP) of the thread formed.

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III. Thread formation

Figures 9 and 10 generally show threading on blank 405 to create the complete threaded screw (indicated herein by 902). In this embodiment, screw 902 is engaged by flat roll forming dies 908 and 910 along the axis of the blank just below head 410. These dies apply sufficient pressure (arrows 912) to cause plastic deformation (and material flow) of the blank surface in each of the three sections (502, 504, and 506) when one of the die plates 908 is moved laterally (arrow 1002) relative to the other die plate 910. During the movement of the die plate, the spacing between the die plates remains constant, as shown by the equidistant center line 1004 between the facing die-forming surfaces. Since the dies engage an eccentric cross-sectional blank, movement of die 908 involves rolling rotation (arrows 1006) in the oscillating blank. Specifically, the center axis 512 of the rolling bolt / blank traces a path 1008 that alternately extends above and below the center line 1004. Although the distance between the dies is constant, a cross-section measurement using a flat-mouth micrometer will show constant dimensions over the entire circumference of the final threaded screw.

Referring in particular to FIG. 9, dies 908 and 910 are provided in the region of the formed tapered root portion 906 with a characteristic outward taper 920. This outward taper, acting on the sharp part of the blank, creates a resulting thread pattern with threaded profile tops that maintain a relatively constant outside diameter (dashed lines 914) with respect to the thread tops on the body part 904, but the inside diameter (the cavities of each thread profile), as indicated by the dashed line 916, exhibits a characteristic outward continuous taper directed toward head 410. The continuous sharp shape is generated largely by the action of dies 908 and 910 moving over the rolled blank. In addition, the material flow of the blank, based on the shapes of the dies, produces a more continuous appearance, as shown in Figure 9 (and Figure 15 described below). This outwardly tapered shape is in direct contrast to the unwanted incomplete padding near the head, which often appears in the state of the art.

Reference will now be made to the cross sections for coiled screw 902, shown in more detail in Figures 11 to 13. Referring first to Figure 11, the body portion 904 shows the largest oval value K3 between the lobes. 1102 and the intervention "cavities".

The cross section of Figure 12, in which a section of the portion of the tapered root now threaded 906 adjacent to the body 904 is observed, shows a reduced oval value K4 between the lobes 1202 (but which is still a significant oval value) . It is generally understood that the bearing die applies reduced pressure in proportion to the increase in taper (as the taper approaches the bottom 514 of the head). Consequently, a relatively small oval value K5 is reported for the cross section of Figure 13, relatively close to the bottom of the head. This small (minimal) oval cross section will be defined as a "nearly circular cross section" for the purposes of this description. In some embodiments, the quasi-circular cross section can be essentially circular. So the term should include this alternative.

Clearly, the oscillatory rolling motion of the blank described above, under the influence of the rolling-forming dies, tends to influence the final formation of the sharp circular cross-section adjacent to the bottom of the head. It is also important to mention that the combination of thread-forming dies and blanks employed herein produces a screw that exhibits the advantageous thread-forming characteristics of a multi-lobe screw with increased resistance to vibrational loosening associated with a cross-sectional screw. circular in the region of the clamping area of ​​the assembly. This quasi-circular cross section is generated, in part, as a result of the tapered root portion advantageously reducing the formation pressure in said region.

IV. Union assembly

As shown in Figure 14, the wound and finished screw 902 is attached to an assembly consisting of a first thin piece 1402 and a second piece of anchor material 1404. The head 410 is firmly compressed against the first piece 1402. As As has already been shown, two or more separate helical input coils, spirals, or threads 1406 and 1408 have been formed on screw 902. These are represented by diametrically opposed thread profiles 1406 and 1408, located in the same axial position along the axis of the screw. As mentioned above, the finished screw has any lobular cross-sectional area (lobes 3, 5, 9, 11, etc). It should be noted that the tapered root portion 906 couples and extrudes (see extrusion points 1405) the anchor material 1404 back and forth, which provides additional thread engagement between the anchor material 1404 and the anchor material 1404. 1406 threads, etc. Extrusion occurs, in part, because the thread cavities in the tapered root portion taper outward toward the head, while the upper portions of the threads largely maintain the same diameter as those in the al minus the adjacent part of the body section (and they become totally more circular in cross section near the head). In this manner, when the screw is tightened into the fine anchor material 1404, the material is increasingly compressed against the tapered root threaded cavities to the axially backward and forward directed yield point, as shown.

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By way of repetition, the use of two or more opposing lead coils or threads indicates that various points along a given cross section of the anchor material are secured. This is done in the area of ​​the tapered root portion that extends between the nearly circular cross section and the more oval cross section, thus increasing resistance against loosening caused by vibration and other external forces. On the finished screw, threads 1406, 1408, etc. define an axial pitch AP, which is the distance between adjacent thread crests. As already mentioned above with reference to FIG. 5, the length of the sharpened root part WB is at least twice the length of the axial passage AP according to one embodiment. In another exemplified embodiment, the length of the tapered root portion WB is not more than 3.5 times the axial pitch AP.

Figure 15 shows in more detail the effects of force distribution through a cross section of anchor material 1404. Specifically, the tapered root portion of screw 906 is in anchor material 1404 to generate a model. of tension classified by a series of tension lines 1501 that reveal a concentration of the tension gradient in the vicinity of each of the lobes 1502. The three lobes 1502 are shown in this example.

Alternating relaxed stress regions and concentrated stress motifs increase the mechanical strength of the closure to loosen from the material based on vibration or other external forces.

The foregoing is a detailed description of certain embodiments of the invention. Things can be changed or added, for example, the design of the self-tapping thread can be changed and an alternate thread-forming profile can be substituted in an alternate embodiment. In the same way, the number of continuous helical coils or threads, and other similar aspects can vary greatly. In addition, the materials and hardness of the screw, as well as the material of the underlying anchor sheet, can be changed. Consequently, screw materials are treated where and when necessary to resist (in any case) applied torque when coupled with specific anchor materials of a predetermined type. Such treatments may include surface hardening and / or induction hardening. Therefore, this description is to be understood by way of example only.