Flow drilling screw, arrangement having a flow drilling screw, and method for connecting at least two workpieces

Abstract

The invention relates to a flow drilling screw having a shank and having a screw head that has a drive formation, wherein: the shank is equipped in parts with a tapping thread and with a thread-free drilling tip; the drilling tip has a polygonal cross-section at least in parts; the polygonal cross-section has a plurality of convex first peripheral portions having a large radius of curvature and a plurality of convex second peripheral portions having a small radius of curvature; each second peripheral portion having a small radius of curvature is situated between two first peripheral portions having a large radius of curvature; and a cross-sectional enlargement of the tip proceeding from the free end of the tip is more pronounced in a first length region extending from the free end of the tip as far as 0.2 times to 0.4 times the length, in particular as far as one third of the length, than over the further course of the tip as far as the tapping thread.

Description

[0001] Flow-hole forming screw, arrangement with a flow-hole forming screw and method for connecting at least two workpieces

[0002] The invention relates to a flow-hole forming screw, an arrangement with a flow-hole forming screw and a method for connecting at least two workpieces.

[0003] Flow hole forming screws are used to join several components, at least one of which is not pre-drilled, for example two sheets of metal. The tip of the flow hole forming screw is placed on one of the workpieces, pressed down, and rotated. The two workpieces are heated by the resulting frictional heat, and the tip of the flow hole forming screw penetrates both workpieces, thereby forming a through-hole in both workpieces. The flow hole forming screw is provided with a self-tapping thread so that the screw cuts its own thread in the through-hole created by the hole-forming tip. Joining high-strength and / or thick workpieces with flow hole forming screws places high demands on the design of the flow hole forming screws.

[0004] The invention is intended to improve a flow-hole forming screw, an arrangement with a flow-hole forming screw and a method for connecting at least two workpieces.

[0005] The object underlying the invention is achieved by a flow-hole forming screw having the features of claim 1, an assembly having the features of claim 19, and a method for joining two workpieces having the features of claim 20. Advantageous developments of the invention are specified in the respective subclaims.

[0006] A flow-hole-forming screw according to the invention has a shaft and a screw head with a drive formation, wherein the shaft is provided in sections with a self-tapping thread and with a hole-forming, thread-free tip. The hole-forming tip has at least in sections a polygonal cross-section, wherein the polygonal cross-section has several convex first circumferential sections with a large radius of curvature and several convex second circumferential sections with a small

[0007] radius of curvature. A second circumferential section with a small

[0008] The radius of curvature is arranged between two first circumferential sections with a large radius of curvature. The circumferential sections are designed and merge into one another in such a way that there is no edge between the first circumferential sections and the second circumferential sections. In particular, the first circumferential sections and the second circumferential sections merge tangentially into one another. An increase in cross-section of the tip in a first region of the length, which runs from the free end of the tip up to 0.2 times to 0.4 times the length, in particular up to one third of the length, is greater than in the further course of the tip up to the self-tapping thread. An increase in cross-section of the hole-forming tip is therefore initially large, starting from the free end of the tip, and then decreases towards the end of the hole-forming tip.It has been shown that with this type of hole-forming tip design, even very thick workpieces can be reliably joined with the hole-forming screw. The so-called K dimension, defined by the difference between the diameters of a circumcircle and an incircle of the polygonal cross-section, can remain constant or vary over the length of the hole-forming tip.

[0009] In a further development of the invention, an increase in the cross-section of the tip in a second region of the length of the tip, which adjoins the first region and extends up to 0.4 times to 0.7 times the length of the tip, in particular taking up a second third of the length of the tip, is greater than an increase in the cross-section in a third region of the length of the tip, which adjoins the second region and extends up to the beginning of the thread, in particular taking up a third third of the length of the tip.

[0010] As a result, the outer contour of the hole-forming tip becomes flatter from the free end toward the thread. This allows even thick material pairings to be reliably penetrated with the hole-forming tip.

[0011] In a further development of the invention, a difference between the diameters of a circumcircle and an incircle of the polygon-like cross-section changes over the length of the hole-forming tip.

[0012] By changing the so-called K dimension, the penetration properties of the hole-forming tip can be influenced. In particular, the contact surface between the tip and the workpiece during positioning, turning, and hole forming can be influenced to ensure reliable hole formation and penetration of the workpiece.

[0013] In a further development of the invention, the difference between the diameters of the circumscribed circle and the inscribed circle of the polygonal cross-section is greater than zero, in particular greater than 0.1 mm, over the entire length of the hole-forming tip. Especially for workpiece pairings with a large thickness and a long hole-forming tip of the flow-hole-forming screw, a polygonal design of the cross-section of the hole-forming tip over its entire length is extremely advantageous. The difference between the diameters of the circumscribed circle and the inscribed circle of the polygonal cross-section (K dimension) can be constant over the entire length of the hole-forming tip.

[0014] In a further development of the invention, the difference between the diameters of the circumcircle and the incircle of the polygon-like cross-section (K dimension) initially increases starting from the free end of the hole-forming tip and then decreases again until the transition to the thread.

[0015] In a further development of the invention, the difference between the diameters of the circumcircle and the incircle of the polygon-like cross-section (K dimension) increases starting from the free end of the hole-forming tip in the first region of the length of the hole-forming tip and decreases again until the transition into the thread.

[0016] It has been found that an increase in the so-called K-dimension up to approximately the first third of the length is advantageous for a reliable and process-safe formation of the through hole by means of the hole-forming tip.

[0017] In a further development of the invention, the difference between the diameter of the circumcircle and the incircle of the polygon-like cross-section (K dimension) increases from the free end of the hole-forming tip from the initial value to 2 times to 3 times the initial value, wherein in particular the initial value is measured from the free end of the tip at 1 / 20 to 1 / 10 of the length of the hole-forming tip.

[0018] Such a significant increase in the initial value, up to 2 to 3 times the initial value, enables reliable and process-safe formation of the through hole using the hole-forming tip. Especially when the free end of the tip is rounded, the initial value is measured at 1 / 20 to 1 / 10 of the length of the hole-forming tip, i.e., after the tip has been rounded off.

[0019] In a further development of the invention, the initial value of the difference between the diameters of the circumcircle and the incircle of the polygon-like cross-section (K dimension) is between 0.1 mm and 0.2 mm, in particular between 0.1 mm and 0.15 mm. In a further development of the invention, the difference between the diameters of the circumcircle and the incircle of the polygon-like cross-section (K dimension) initially increases, starting from the free end of the hole-forming tip, and then decreases again up to the transition into the self-tapping thread to the initial value at the free end of the tip, in particular at 1 / 20 to 1 / 10 of the length of the tip, or decreases to 1.6 times to 1.3 times the initial value.

[0020] In a further development of the invention, an angular position of the first circumferential sections and the second circumferential sections changes around a central longitudinal axis of the shaft, viewed over the length of the hole-forming tip.

[0021] In particular, the polygonal cross-section exhibits a twist along the length of the hole-forming tip. Providing a twist to the polygonal cross-section at the hole-forming tip facilitates the reliable and process-safe formation of the through hole using the hole-forming tip. The twist reduces the force required during hole forming. Thus, the hole can be formed with less force, or the hole forming time can be reduced with the same force.

[0022] In a further development of the invention, the angular position of the first circumferential sections and the second circumferential sections changes over the entire length of the tip between 0° and 50°, in particular between 20° and 45°.

[0023] In a further development of the invention, the angular position of the first circumferential sections and the second circumferential sections changes more significantly in the first region of the hole-forming tip's length than in the second region and the third region of the tip's length, with the second region adjoining the first region and the third region extending from the end of the second region to the transition into the thread. A twist can be present over the entire length of the tip. The twist can vary over the entire length of the tip.

[0024] A stronger twist of the polygonal cross-section in a length range that adjoins the free end of the tip facilitates the penetration of the workpieces and can thus ensure a reliable and process-safe formation of the through hole by means of the hole-forming tip.

[0025] In a further development of the invention, the angular position of the first circumferential sections and the second circumferential sections in the first region of the length of the tip varies between 0° and 50°, in particular between 20° and 45°. In a further development of the invention, a radius of curvature at the free end of the tip is between 0.4 mm and 0.6 mm.

[0026] In a further development of the invention, a length of the hole-forming tip is 40% to 60%, in particular 50%, of the length of the screw shaft.

[0027] In a further development of the invention, the screw is made of an alloyed or unalloyed tool steel.

[0028] The high material requirements for flow-drilling screws can be met by using tool steel. The tool steel can be unalloyed, such as carbon steel, especially C62 steel, or alloyed, such as chromium-vanadium steel.

[0029] In a further development of the invention, the tool steel is hardened.

[0030] Within the scope of the invention, only a portion of the flow-hole forming screw, for example, only the flow-hole forming tip, can be hardened. Different portions of the screw can have different hardnesses.

[0031] In a further development of the invention, the screw is provided with a corrosion protection coating and a seal.

[0032] For example, the flow-drilling screw is coated with a ZnNi coating for corrosion protection and additionally coated with an organic transparent coating for sealing. A sealant can have a positive effect on thread forming.

[0033] The object underlying the invention is also achieved by an arrangement with a flow-hole forming screw according to the invention and at least two workpieces which are connected to one another by means of the flow-hole forming screw, wherein a length of the tip is equal to or greater than a length of a through-opening produced in the two workpieces by means of the screw.

[0034] In this way, the through hole can first be formed using the hole-forming tip and only then does the thread engage in the produced

[0035] Through hole. This allows the self-tapping thread to be formed reliably, since the advance of the flow-hole forming screw takes place between the formation of the

[0036] A distinction is made between a through hole created by the hole-forming tip and the grooving of a thread. The length of the through hole is the length of the through hole created in the entire assembly with which the screw tip is in contact, i.e., in particular, the thickness of the components to be joined and the length of the through hole created during hole forming.

[0037] The object underlying the invention is also achieved by a method for connecting at least two workpieces with a flow-hole forming screw according to the invention, in which the production of a through-hole by means of the hole-forming tip of the screw is provided and, after the production of the through-hole, the grooving of a thread in the through-hole by means of the grooving thread of the screw is provided.

[0038] Further features and advantages of the invention will become apparent from the claims and the following description of a preferred embodiment of the invention in conjunction with the drawings. In the drawings:

[0039] Fig. 1 is a side view of a flow-hole forming screw according to the invention,

[0040] Fig. 2 is a plan view of the flow hole forming screw of Fig. 1,

[0041] Fig. 3 is a partially sectioned, schematic view of an arrangement with the flow-hole forming screw of Fig. 1 and two workpieces,

[0042] Fig. 4 is a partially sectioned side view of the screw of Fig. 1,

[0043] Fig. 5 is a representation of the percentage cross-sectional increase of the hole-forming tip of the screw of Fig. 1,

[0044] Fig. 6 two schematic views of the hole-forming tip of the screw of Fig. 1, showing the twist, and

[0045] Fig. 7 is a representation of the K-dimension of the polygonal cross-section of the hole-forming tip of the screw of Fig. 1 seen over the length of the hole-forming tip.

[0046] Fig. 1 shows a hole-forming screw 10 according to the invention, which has a shaft 12 and a screw head 14. The screw head 14 is provided with a drive formation, which is more clearly visible in the plan view of Fig. 2. By means of the drive formation, the screw 10 can be rotated.

[0047] In addition to the drive structure, the screw head 14 is provided with a cylindrical portion 16 in which, see Fig. 3, a circumferential groove 18 is formed. After the screw 10 has been fully screwed into a workpiece or two workpieces, the groove 18 accommodates any material rising from the top side of the workpieces when forming a through hole in the workpieces and when forming a self-tapping thread, so that an annular portion 20 of the underside of the screw head 14, located outside the groove 18, can rest flatly on the top side of a workpiece.

[0048] The screw shaft 12 is provided with a hole-forming tip 22 starting from its free end. The hole-forming tip 22 has a polygonal cross-section comprising a plurality of first circumferential sections with a small convex curvature and a large radius of curvature and a plurality of second circumferential sections with a large convex curvature and a small radius of curvature, wherein a second circumferential section with a large curvature and a small radius of curvature is arranged between two first circumferential sections with a small curvature and a large radius of curvature. Transitions between the individual circumferential sections are rounded so that no edge occurs at the transition between a first circumferential section and a second circumferential section. The polygonal cross-section of the hole-forming tip 22 is schematically indicated in Fig. 4. The polygonal cross-section has a circumscribed circle and an inscribed circle.The difference between the circumscribed circle and the inscribed circle defines the so-called K dimension, which is a measure of the deviation of the polygonal cross-section from a circular shape. The hole-forming tip 22 is followed by a self-tapping thread 24. Starting at the end of the hole-forming tip 22, the self-tapping thread 24 initially has an increasing outer diameter. As the self-tapping thread 24 continues, it then has a constant outer diameter.

[0049] Between the end of the thread 24 and the underside of the screw head 14 there is a short, thread-free section 26, which is also referred to as the thread run-out underhead.

[0050] It can be seen from Fig. 1 that an increase in cross-section starting from the free end 28 of the hole-forming tip 22 up to the self-tapping thread 24 does not occur uniformly, but decreases with increasing distance from the free end 28 of the hole-forming tip 22. In particular, it can be seen that the free end 28 of the hole-forming tip 22 is rounded. In a first region 30 of the length of the hole-forming tip 22, wherein the first region 30 in the illustrated embodiment corresponds to a first third of the length of the tip 22, an increase in cross-section is noticeably greater than in the further course of the hole-forming tip 22. In a second region 32 of the length of the hole-forming tip 22, in Fig. 1 corresponding to a second third of the length, the increase in cross-section is less than in the first region 30, but still greater than in a third region 34 of the length of the hole-forming tip 22, in Fig. 1 corresponding to a third third of the length.

[0051] The percentage increase in the cross-section of the hole-forming tip 22 is plotted in Fig. 5. In deviation from the illustration in Fig. 5, an initial value is not measured at 0 mm, i.e. not at the free end 28 of the hole-forming tip 22, but at approximately 1 / 10 to 1 / 20 of the length of the hole-forming tip. In Fig. 1, a line 36 is drawn which lies at approximately 1 / 20 of the length of the hole-forming tip 22, whereby the line 36 represents an imaginary line and is used to measure the initial value for a percentage increase in cross-section. In the first region 30, in the illustration in Fig. 5 i.e. from 0 mm to 4 mm, the cross-section increases sharply and is 4 mm, or 69% of the final cross-section at the end of the hole-forming tip 22 at 12 mm. Starting from the initial value of 18%, the cross-section in the first area 30 increases by 51%.

[0052] In the second area 32, the cross-section increases in percentage from 69% to 92% of the final cross-section at 12 mm at the end of the hole-forming tip 22. As a result, the cross-section increase in the second area 32 is 23% and is thus significantly lower than in the first area 30.

[0053] In the third region 34, the cross-section of the hole-forming tip 22 increases from 92% to the final value of 100%. As a result, the cross-section of the hole-forming tip 22 increases by only 8% over the course of the third region 34, i.e., significantly less than in the second region 32 and significantly less than in the first region 30.

[0054] Fig. 5 and Fig. 1 also show that the cross-section of the hole-forming tip 22 increases over the entire length of the hole-forming tip 22 up to the transition into the thread 24, i.e., in the first region 30, the second region 32, and the third region 34. Consequently, the hole-forming tip 22 has no cylindrical section.

[0055] In the illustrated embodiment, the different regions 30, 32, 34 of the hole-forming tip 32 each have a length corresponding to one-third of the length of the hole-forming tip 22. However, within the scope of the invention, the length of the regions 30, 32, 34 can also be between 0.2 and 0.4 times the length of the tip, and in particular, the regions 30, 32, 34 can also be of different lengths.

[0056] Fig. 4 shows a side view of the screw 10 of Fig. 1. In Fig. 4, a first longitudinal section 40 of the shaft 12 of the hole-forming screw 10 is shown, which extends from the free end 28 of the shaft into the self-tapping thread 24. In this area 40, the shaft 12 has a polygonal cross-section. As already explained, the polygonal cross-section has an incircle and a circumcircle, with a difference between the circumcircle and the incircle determining the so-called K dimension of the shaft 12. In a further section 42, which extends over part of the self-tapping thread 24 to the underside of the screw head 14, the cross-section of the shaft 12 is circular. Between the first section 40 and the second section 42 there is a transition section 44 in which the cross section of the shaft 12 changes from the polygonal cross section in the region 40 to the circular cross section of the region 42.The transition section 44 lies in the area of ​​the self-tapping thread 24. A self-tapping section 43 lies partly in the area 40 and partly in the transition section 44. In the self-tapping section 43, the outer diameter of the self-tapping thread 24 increases. The flank height of the self-tapping thread 24 increases from zero to the maximum final value in the self-tapping section 43. The self-tapping section 24 performs a large part of the material deformation when forming a thread into one or more components.

[0057] Within the scope of the invention, the difference between the circumcircle and the incircle, i.e. the so-called K-dimension, changes over the length of the hole-forming tip 22.

[0058] In addition, an angular position of the first circumferential sections with a small convex curvature and a large radius of curvature and the second circumferential sections with a large convex curvature and a small radius of curvature changes over a length of the hole-forming tip 22. In the screw according to Fig. 1, this creates a twist in the area of ​​the hole-forming tip 22. The two schematic representations of the hole-forming tip 22 in Fig. 6 illustrate the twist in the area of ​​the hole-forming tip 22. The two representations in Fig. 6 are each formed by cuts through the shaft 12 placed closely adjacent to one another. The solid lines 50A, 50B, 50C in Fig. 6 each mark the center of a respective second circumferential section with a large convex curvature and a small radius of curvature.It can be seen that an angular position of the first circumferential sections and the second circumferential sections changes around a central longitudinal axis of the shaft 12 along the hole-forming tip 22, corresponding to a twist of the polygonal cross-section. It can also be seen from Fig. 6 that this angular position of the first circumferential sections and the second circumferential sections does not change linearly but unevenly over the length of the hole-forming tip 22. The upper illustration in Fig. 6 shows that the angular position initially changes significantly and then becomes smaller. In other words, the hole-forming tip 22 has a stronger twist in a first length range than in a second length range, which adjoins the first length range and extends to the screw head-side end of the tip.

[0059] Fig. 7 shows a diagram from which the so-called K dimension can be seen. A change in the angular position of the first circumferential sections and the second circumferential sections around a central longitudinal axis of the shaft can also be seen, since in the illustrated embodiment, the twist—in other words, an angular position of the first circumferential sections and the second circumferential sections—changes in parallel with the so-called K dimension, i.e., the difference between the circumcircle and the incircle of the polygonal cross-section. In other words, the twist is strongly pronounced in an area where the K dimension is also large.

[0060] Fig. 7 shows that the K dimension changes in the first region 30 of the length of the hole-forming tip 22, i.e., from 0 to 4 mm in the illustrated embodiment, such that the K dimension increases sharply. In the illustrated embodiment, the K dimension increases from a value of 0.1 mm to a value of approximately 0.25 mm at the end of the first region 30.

[0061] In the second region 32 of the length of the hole-forming tip 22, the K dimension drops again, in the illustrated embodiment from a value of approximately 0.25 mm to a value of approximately 0.2 mm. In the third region 34 of the length of the hole-forming tip 34, the K dimension drops further, but less sharply than in the second region 32. In the illustrated embodiment, the K dimension in the third region 34 drops from a value of 0.2 mm to approximately 0.15 mm.

[0062] It can be seen from Fig. 7 that the K dimension does not drop to the initial value at the beginning of the hole-forming tip 22. It can also be seen that the K dimension does not drop to zero over the entire length of the hole-forming tip 22; in other words, the hole-forming tip 22 thus has a polygonal cross-section over its entire length.

[0063] As already explained, a twist of the hole-forming tip 22 is stronger in the first region 30 than in the second region 32 and in the third region 34. In the second region 32, the twist is even slightly stronger than in the third region 34. In other words, in the first region 30 of the length of the hole-forming tip 22, an angular position of the first circumferential sections and the second circumferential sections about a central longitudinal axis of the shaft changes more strongly per unit length than in the second region 32 and in the third region 34.

[0064] In the second region 32, an angular position of the first circumferential sections and the second circumferential sections about a central longitudinal axis of the shaft per unit length changes even more than in the third region 34.

[0065] As already explained, in the illustrated embodiment, the length of the regions 30, 32, 34 is each one-third of the length of the hole-forming tip 22. Within the scope of the invention, the length of the regions 30, 32, 34 can be between 0.2 and 0.4 of the length of the hole-forming tip 22. In particular, the regions 30, 32, 34 can also be of different lengths.

[0066] Fig. 3 shows an arrangement 60 according to the invention with a screw 10 according to the invention and two workpieces 62, 64 which are to be connected with the screw 10 according to the invention.

[0067] It can be seen from Fig. 3 that the length of the hole-forming tip 22 is greater than the sum of the thicknesses of the two workpieces 62, 64. The length of the hole-forming tip 22 is even greater than the sum of the thicknesses of the workpieces 62, 64 and the thickness of a passage 66 which is formed starting from the underside of the second workpiece 64 when a through-opening is formed through the two workpieces 62, 64 by means of the hole-forming tip 22. The length of the through-opening is the length of the through-opening produced in the entire assembly of the components 62, 64 and the passage 66. Using the example in Fig. 3, the length of the through-opening is equal to the sum of the thicknesses of the components 62, 64 and the maximum thickness of the passage 66. The length of the through-opening is designated by the letter L in Fig. 3.

[0068] The length of the hole-forming tip 22 is thus dimensioned such that the through-hole in the two workpieces 62, 64 and the forming passage 66 is already completely created before the beginning of the self-tapping thread 24 engages the through-hole. This is also advantageous because the feed rate differs between the formation of the through-hole using the hole-forming tip 22 and the forging of a thread using the self-tapping thread 24. When forging a thread, the feed rate is predetermined by the pitch of the self-tapping thread 24, whereas the feed rate when forming the through-hole using the hole-forming tip 22 depends on the heating of the two workpieces 62, 64 and the contact pressure of the screw 10.

[0069] When a thread is formed into the through hole, i.e., following the state shown in Fig. 3, material is thrown up from the top side of the upper workpiece 62. This thrown-up or upwardly displaced material of the upper workpiece 62 can then be received in the circumferential groove 18 in the underside of the screw head 14. In the final screwed state, the annular region 20 on the underside of the screw head 14 rests on the top side of the workpiece 62.

Claims

Patent claims 1. Flow-hole forming screw (10) with a shaft (12) and a screw head (14) with a drive formation, wherein the shaft (12) is provided in sections with a self-tapping thread (24) and with a hole-forming, thread-free tip (22), wherein the hole-forming tip (22) has at least in sections a polygonal cross-section, wherein the polygonal cross-section has a plurality of convex first circumferential sections with a large radius of curvature and a plurality of convex second circumferential sections with a small radius of curvature, wherein in each case a second circumferential section with a small radius of curvature is arranged between two first circumferential sections with a large radius of curvature, characterized in that a cross-sectional increase of the tip (22) starting from the free end of the tip (22) in a first region (30) of the length, which goes from the free end of the tip (22) to 0.2 times to 0.4 times the length, in particular up to one third of the length, is greater than in the further course of the tip (22) up to the groove thread (24).

2. Flow hole forming screw according to claim 1, characterized in that a cross-sectional increase of the tip (22) in a second region (32) of the length of the tip (22), which adjoins the first region (30) and extends up to 0.4 times to 0.7 times the length of the tip (22), in particular takes up a second third of the length of the tip (22), is greater than a cross-sectional increase in a third region (34) of the length of the tip (22), which adjoins the second region (32) and extends up to the beginning of the self-tapping thread (24), in particular takes up a third third of the length of the tip (22).

3. Flow-hole forming screw according to claim 1 or 2, characterized in that a difference between the diameters of a circumscribed circle (C dimension) and an inscribed circle (E dimension) of the polygon-like cross section (K dimension) changes over the length of the hole-forming tip.

4. Flow-hole forming screw according to claim 3, characterized in that the difference between the diameters of the circumcircle and the incircle of the polygon-like cross-section (K dimension) over the entire length of the hole-forming tip is greater than zero, in particular greater than / equal to 0.1 mm.

5. Flow-hole forming screw according to claim 3 or 4, characterized in that the difference between the diameters of the circumcircle and the incircle of the polygonal cross-section (K dimension) initially increases starting from the free end of the hole-forming tip and then decreases again until the transition to the self-tapping thread.

6. Flow hole forming screw according to claim 5, characterized in that the difference between the diameters of the circumcircle and the incircle of the polygonal cross section (K dimension) increases starting from the free end of the hole forming tip in the first region (30) of the length of the hole forming tip (22) and decreases again until the transition into the self-tapping thread (24).

7. Flow hole forming screw according to claim 5 or 6, characterized in that the difference between the diameters of the circumcircle and the incircle of the polygonal cross section (K dimension) starting from the free end of the hole forming tip increases from the initial value to 2 times to 3 times the initial value, wherein in particular the initial value starting from the free end of the tip is measured at 1 / 10 to 1 / 20 of the length of the hole forming tip.

8. Flow-hole forming screw according to claim 7, characterized in that the initial value of the difference between the diameters of the circumcircle and the incircle of the polygon-like cross-section (K dimension) is between 0.1 mm and 0.2 mm, in particular between 0.1 mm and 0.15 mm.

9. Flow-hole forming screw according to one of the preceding claims, characterized in that the difference between the diameters of the circumscribed circle and the incircle of the polygonal cross-section (K dimension), starting from the free end of the hole-forming tip (22), increases in the first region of the length of the hole-forming tip (22) and then decreases again up to the transition into the self-tapping thread (24) to the initial value at the free end of the tip, in particular at 1 / 20 to 1 / 10 of the length of the tip, or decreases to 1.6 times to 1.3 times the initial value.

0. Flow-hole forming screw according to one of the preceding claims, characterized in that, viewed over the length of the hole-forming tip, an angular position of the first circumferential sections and the second circumferential sections changes around a central longitudinal axis of the shaft.

11. Flow hole forming screw according to claim 10, characterized in that the angular position of the first circumferential sections and the second circumferential sections changes over the entire length of the tip between more than 0 degrees to 50 degrees, in particular between 20 degrees and 45 degrees.

12. Flow hole forming screw according to claim 10 or 11, characterized in that the angular position of the first circumferential sections and the second circumferential sections begins in the first region (30) of the length of the hole forming tip (22) and changes more significantly than in the second region (32) and the third region (34) of the length of the tip (22), wherein the second region (32) adjoins the first region (32) and the third region (34) continues up to the transition into the self-tapping thread (24).

13. Flow hole forming screw according to claim 11 or 12, characterized in that the angular position of the first circumferential sections and the second circumferential sections in the first region (30) of the length of the tip (22) changes between more than 0 degrees to 50 degrees, in particular between 20 degrees and 45 degrees.

14. Flow hole forming screw according to at least one of the preceding claims, characterized in that a radius of curvature of the free end of the tip (22) is between 0.4 mm and 0.6 mm.

15. Flow-hole forming screw according to at least one of the preceding claims, characterized in that a length of the hole-forming tip (22) is 40% to 60%, in particular 50%, of the length of the screw shaft (12).

16. Flow-hole forming screw according to one of the preceding claims, characterized in that the screw (10) is made of an alloyed or unalloyed tool steel.

17. Flow hole forming screw according to claim 16, characterized in that the tool steel is hardened.

18. Flow-hole forming screw according to claim 16 or 17, characterized in that the screw (10) is provided with a corrosion protection coating and a seal.

19. An arrangement comprising a flow-hole forming screw (10) according to one of the preceding claims and at least two workpieces (62, 64) which are connected to one another by means of the flow-hole forming screw (10), wherein a length of the tip (22) is equal to or greater than a length of a through-hole produced in the two workpieces (62, 64) by means of the screw (10).

0. A method for connecting at least two workpieces (62, 64) with a flow-hole forming screw (10) according to at least one of claims 1 to 18, characterized by producing a through-hole by means of the hole-forming tip (22) of the screw (10) and, after producing the through-hole, forming a thread in the through-hole by means of the self-tapping thread (24) of the screw (10).

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