SCREW WITH MILLING SECTION EMBEDDED IN THE THREAD

Description

[0001] The present invention relates to a screw designed for fastening metal components such as trapezoidal or corrugated sheet metal to wooden substructures, both across the surface and at the overlapping joints of metal sheets. It is used on facades and roofs. BACKGROUND

[0002] Metal sheets, especially corrugated or trapezoidal sheet metal, are often used to cover facades and roofs. It is common practice to attach such cover sheets or coverings to a wooden substructure, such as beams, battens, or rafters, using screws. For economic reasons, it is desirable that these metal sheets (made of aluminum, galvanized and / or painted sheet steel, or similar) can be attached directly to the substrate without pre-drilling. Therefore, a screw designed for this purpose must be able to penetrate these sheets and then anchor itself securely in the wooden substructure.

[0003] This can be achieved by using a self-drilling screw with a molded drill bit. However, a known accompanying problem is the metallic drilling chips created when penetrating the sheet metal panels, as these must be removed to prevent corrosion.

[0004] It is common practice to use a hardened threaded tip instead of a molded drill bit, which has a reduced tendency to form chips. Bimetallic screws made of stainless steel with a welded-on tip made of hardened carbon steel are also frequently used. A threaded tip refers to the thread extending from the shaft to the tip and continuing more or less to the tip. When the screw is inserted, the metal of the sheet metal is not removed by machining, but rather displaced and deformed. The thread on the cone of the threaded tip also cuts a thread into the displaced metal, thus assisting the screw's advance. STATE OF THE ART

[0005] EP 3 617 533 A1 discloses a screw for fastening metal panels to wood. It has a thread that extends from the screw tip over the cone to the shaft and is designed as a double thread in the cone area. The thread on the cone has wider flank angles than on the shaft.

[0006] Furthermore, DE 10 2012 215 645 discloses a bimetallic screw with a thread extending from the thread tip to the shaft. It is designed as a sheet metal thread in the conical area and as a wood thread in the shaft area.

[0007] Furthermore, German Patent Application DE 27 32 695 teaches that for a self-tapping and thread-forming fastener with a threaded tip, it is advantageous if the thread height gradually decreases from the transition between the shaft and tip to the end. The thread is multi-start, and one thread pitch tapers to the tip of the thread-forming screw.

[0008] Many individual screw design elements are known in the prior art—bimetallic design, threaded point, double thread—which have been used in varying combinations and size ratios for the described application. The object of the invention is to propose a screw that is inexpensive to manufacture, high-performance, and, above all, simple and safe to handle. DESCRIPTION OF THE INVENTION

[0009] This object is achieved by a particularly effective, user-optimized design of a generic screw, as described by the features of independent claim 1. The subclaims describe further variants and embodiments.

[0010] A generic screw comprises the following functional sections, which merge into one another or adjoin one another, described from the screw tip to the screw head: A conical thread tip which carries a double thread, whereby this double thread comprises a main thread and a secondary thread. Adjacent to this is an essentially cylindrical milling section. Its properties are due to a large number of milling ribs, which are designed as steep threads. Their outer diameter DF is measured across the ribs, similar to a thread. Subsequently, along the screw's longitudinal axis, there is an essentially cylindrical shank section which carries only the main thread and in which the thread outer diameter is DN. Further on, there is an essentially cylindrical, smooth, thread-free shank section. The screw terminates in a head section with a force application point.

[0011] The term "merging into or adjacent to one another" expresses that the described areas can be identified structurally and / or functionally, but from a technical point of view or for technical reasons, they do not have to have a hard boundary.

[0012] For example, threaded sections at the transition between two sections can be assigned to both. There may also be smooth transitions, but this does not affect the differentiation or identification of the functional sections. The shank section (threaded, threadless), longitudinal section, and milling section refer to those areas of a screw that characterize a functional section.

[0013] The use of scraping edges, milling ribs, or scraping grooves, as used in the functional milling section of the invention, is known. They can be oriented in various ways, e.g., arranged approximately parallel to the screw axis on the shaft. As an alternative to the axis-parallel arrangement, a high-helix thread can also be implemented as the milling section. The high-helix thread can be clockwise (like the main thread, but with a significantly higher pitch) or counterclockwise.

[0014] Both concepts reduce the splitting effect of the screw when penetrating wood because the milled section locally disrupts the wood's fiber structure. This means that the stresses exerted by the cone on the wood are (needed to be) dissipated less deeply into the material. Furthermore, the milled material can be more easily compacted into the recessed sections between the thread turns, which in turn helps reduce the torque required when driving the screw.

[0015] The term "cone" as a form specification for the thread crest refers to a conical, particularly pointed-conical, basic shape of the thread crest, without taking into account the thread attached to it. Furthermore, the "cone" does not refer to the exact geometric-mathematical shape, but rather to the technically feasible shape or the shape realized and identifiable as a cone in the screw.

[0016] As is the case with the prior art, the screw is driven via one of the many known force application points in the head section. External hexagon heads are widely used for this application, but hex lobe, internal hexagon, or similar force application points can also be used. The screw head can offer a flat stop surface on its underside facing the shaft, e.g., for use with a washer, a sealing washer, or both. Depending on the application, other designs are also possible.

[0017] The present invention is characterized, on the one hand, in that the main thread runs uninterrupted and with a constant pitch from the tip section of the cone (thread crest) over the milled section to the head-side end of the thread-bearing shaft section. The use of the term tip section instead of tip of the cone is intended to express that the tip of such a screw can vary during manufacturing (e.g. be deformed). Screws are generally manufactured as mass products, which means that the pointed end of the cone in a screw cannot be manufactured mathematically perfectly and its shape can be changed or impaired by subsequent process steps. This does not, however, detract from the functionality. In summary, the tip section means the technical end area of ​​the thread crest, which is a few mm in length.

[0018] Furthermore, the invention is characterized in that the (milling) ribs of the milling section are countersunk into the thread root of the continuous main thread in such a way that the thread crests of the continuous main thread with nominal outer diameter DB project beyond the milling ribs. Thus, DF < DB . Here, the arrangement of the milling section on the cylindrical shaft instead of on the cone results in a functional separation between the penetration and expansion sections, i.e., the cone and the milling section. It is clear from the application profile that the cone is intended to penetrate the metal sheets with as little chipping as possible and cut a thread, while the milling section is intended to perform its task in the wood of the substructure. This also reduces the risk of the milling section being used undesirably in the metal.

[0019] As mentioned, the main thread runs continuously from the cone through the milling section into the threaded, cylindrical shaft section. This has the advantage that the thread, which has been grooved into the metal of the sheet metal by the thread crest, remains effective throughout, allowing the thread to be guided seamlessly from the cone to the threaded shaft section. This also contributes to the prevention of metal chips.

[0020] An alternative, complementary design of this screw provides for a retaining thread section between the unthreaded shaft section and the head section, followed by a short (in the sense of a few millimeters) unthreaded sub-head section. The retaining thread section is designed as a short retaining thread with 1 to 3 turns. The retaining thread is preferably designed as a double thread. The pitch of the retaining thread is greater than that of the main thread; in this case, it is preferably 1.6 to 1.9 times the pitch of the main thread. The flank geometry of the retaining thread can also be selected asymmetrically, with the flank facing the head being steeper than the flank facing the thread crest. This improves the support of the drilled metal plate.

[0021] This addition ensures that when a drilled metal plate reaches the under-head threaded section, it is pulled towards the head faster than the screw as a whole sinks into the substructure due to the higher pitch of the under-head thread. Furthermore, the thread-free under-head section creates a receiving zone for the drilled metal plate without the thread milling or tearing out the metal plate as it continues to turn. Apart from sealing problems, unwanted chip formation could otherwise occur. Thus, the metal plate is ultimately held or clamped between the head-side under-head thread runout. The double thread design improves the quality of the support on the then twice present thread runout.

[0022] The screw is preferably designed so that the length of the unthreaded lower head section and the retaining thread section are each between 2 mm and 5 mm. As mentioned, this screw can also be used to accommodate a washer / sealing washer in the unthreaded lower head section. The specialist will select the length of the section according to the application profile.

[0023] For both screw variants described, the main thread is preferably designed symmetrically with a flank angle of 60° ± 3°. This symmetrical flank angle results in good pull-out values ​​in wood and is also robust enough to ensure thread forming in metal.

[0024] Furthermore, the screw according to the invention is preferably designed such that the secondary thread begins in the tip section of the cone and its thread height continuously increases. After half the length of the cone (essentially viewed in the axial direction), the thread height decreases again and tapers off in the transition area between the cone and the milling section. Taper off refers to a non-abrupt end of the thread, which is achieved by a decrease in the thread height towards zero.

[0025] In a preferred variant, the screw described here is designed such that the nominal diameter or outer diameter DN of the main thread in the (thread-bearing, cylindrical) shank section is larger than the outer diameter DB of the main thread in the milled section. In a preferred embodiment, the difference is six-tenths of a millimeter in diameter. This also facilitates the forming of the screw.

[0026] Furthermore, it is preferred that, in the screw described here, the flank height of the main thread at the transition between the milling section and the cone, starting from the outer diameter DB, decreases steadily across the tapered cone and tapers off at the tip section of the cone. Reducing the thread height (with otherwise identical thread geometry) or increasing the thread height starting from the tip section reduces the resistance of the screw when penetrating / forming the metal plate and simplifies thread forming.

[0027] If you want to illustrate this flank height behavior geometrically, you can do so as follows: Construct a cone formed by the tangents at the thread crests of the main thread on the cone and their common intersection with the screw's longitudinal axis. The tangents thus form an envelope over the thread crests in the shape of a cone. The common intersection lies in front of the technical tip of the screw, for the manufacturing reasons mentioned above. The tip angle of the conical envelope is preferably 35° ± 5°.

[0028] To better describe the geometry of the milling section, the (geometric) projection of a thread crest of a milling rib onto the screw's longitudinal axis is best used. This projection preferably includes a cutting angle of 30° ± 10°. The pitch of this high-pitch thread is preferably negative, i.e., opposite to the main thread. Figuratively speaking, the milling ribs describe a high-pitch left-hand thread, in contrast to the helix of the main thread or the double thread on the shaft and cone.

[0029] In another preferred embodiment, the screw in question is designed as a bimetallic screw, i.e., with a stainless steel head and shaft and a welded carbon steel tip. This would make the cylindrical shaft sections B, C, D (and, if present, E, F) and the head section G of stainless steel, and the cone of the thread tip of carbon steel. Technically, before rolling or rolling the thread, a carbon section is welded to a stainless steel wire blank of the appropriate length, and the screw is then manufactured as a whole in its final shape.

[0030] As is well known to experts, such a screw can be provided with decorative or protective coatings if necessary or depending on the application. These include corrosion-inhibiting layers of zinc, zinc-nickel, or other metals, as well as paints, waxes, oils, and the like. It is also conceivable to apply such coatings only to certain areas of the screw, for example, a decorative head coating tailored to the sheet metal profile being screwed into. DESCRIPTION OF THE CHARACTERS

[0031] Figure 1 shows a side view of a screw according to the invention in basic design Figure 2 shows a side view of a screw according to the invention extended by additional shaft sections Figure 3 shows an enlarged detail of section "Z" from Figure 1 , namely the thread crest with subsequent milling section. Figure 4 shows the detail Y from Figure 1 with a cross-section of the main thread.

[0032] In Figure 1A first embodiment of a screw 100 is shown, with the basic components G, D, C, B, A. G represents the head section with force application 130. Here, in side view, the force application 130 is shown as a hexagon, which has a stop surface or stop underside 135 at its end facing the shaft. This is followed, to the right in the image or towards the tip, by a smooth, thread-free shaft section D, which is cylindrical. The length of this section as well as the following shaft section C with the main thread is determined by the intended use and application and is therefore designed and manufactured as required. The thread-bearing shaft section C extends over the milling section B and this in turn into the cone A. Both B and A are Figure 3 explained in more detail.

[0033] Figure 2shows a variant 200 of the screw according to the invention, in which two additional functional areas are inserted between the head section G and the thread-free shaft section D. Adjacent to the head section G is a short thread-free lower head section F, followed towards the tip by a support thread section or holding thread section E. The latter advantageously deviates from the thread geometry of the main thread.

[0034] A realization of the screw according to the invention as in Figure 2shown has a nominal diameter DN of 6.4mm. It is manufactured in various lengths. In a typical example, the screw has an effective length (sections A up to and including F) of 150mm, of which the stainless steel portion is 132mm. Sections D, E and F together have a length of approximately 74mm. For all screws of the same diameter, the length of the carbon steel section is approximately 18mm. For all screws of this diameter, the length of section A is approximately 7mm, and of section B approximately 4mm. This means that an approximately 7mm long section of the shank section C with the main thread is also made of carbon steel before it continues in stainless steel. For the screw in question, a main thread pitch of p=2.5mm is planned, which applies to all sections AC. The outside diameter in section BDN is approximately 5.8mm.

[0035] Figure 3 shows in detail a design of the front section Z (section from Figure 1) a screw according to the invention, for Figure 1 as 2. In the drawing (from left to right), arranged along the longitudinal axis 180, a cut shaft section C with main thread 110 is shown, followed by a milling section B, which merges into a cone A with thread crest 150. The main thread consists of a thread turn with a flank angle of 60° and a pitch p, which is specified as usual in mm per turn. Figure 4 corresponds to the tear Y of Figure 1 and shows a section of the main thread 110 in section C with the indication of the flank angle and the pitch p.

[0036] In Figure 3 As an example, a thread on the shaft section C is shown with a thread root of 140 and a thread flank of 145. The nominal diameter of this screw DN is measured over the thread crests of the main thread 110 and is in Figure 3 marked.

[0037] The shank section C, while maintaining the pitch and flank angle, transitions into the milled section B with a slightly reduced thread height DB of the main thread 110. There are two reasons for this reduction: Since the screw is produced by cold forming from a cylindrical blank, only the locally available base material is available for any thread formation. Although thickenings can be achieved by upsetting, e.g. in the area of ​​the head section G, this would be very complex in the middle of the shank section and would complicate thread rolling. Since the milling ribs 160 are arranged in the thread root in the milled section B, less material is available for the main thread. Although the same thread height as in section C could be achieved by reducing the flank angle of the main thread 110 in section B, this would compromise the stability of the main thread, especially in the milled section.

[0038] One of the inventive features, however, is the continuous passage of the main thread and the resulting seamless guidance of the screw from the displaced, grooved metal into the wood. Therefore, it is advantageous to reduce the thread height in milling section B rather than the flank angle. Since the screw temporarily has a maximum outer (thread) diameter DB during the setting process after passing through cone A through the metal plate(s), this means that the main thread still engages snugly in the self-grooved channel. At the same time, the screw's resistance to forming when milling section B acts in the wood is lower due to the smaller outer diameter DB < DN, when considering the main thread alone.Tests have shown that the screw's guidance in the grooved metal section is not compromised; in fact, the thread crests of the main thread act as spacers that keep the milled ribs away from the hole edges. At the transition from section B to C, the hole or thread diameter grooved in the metal plate is widened to DN—but by then, milled section B has already passed the metal plate(s).

[0039] Figure 3 shows the milling ribs 160 arranged so that the milling ribs essentially form a steep left-hand thread, which enhances the milling effect in the wood. The line with reference symbol 330 marks the projection of the thread crest 320 onto the screw's longitudinal axis 180. The drawing also shows the cutting angle with the screw's longitudinal axis 180.

[0040] In the area of ​​the cone it is shown how the main thread 110, while maintaining the pitch and the flank angle, slowly decreases from the diameter DB at the transition area from section B to A to zero and ends in the crest section 190. Also marked is the secondary thread 120, which, starting in the crest section, continuously increases like the main thread and ends again in the transition area from A to B. This achieves the double thread on the thread crest on the cone A, which is advantageous for forming in the metal. The tangents at the thread crests of the main thread with a common intersection point 310 form the envelope 300, whereby the intersection point 310 lies on the longitudinal axis 180 of the screw 100 or 200.

Claims

1. Screw (100), comprising the following functional sections which merge into one another or adjoin one another: - a threaded tip (150) formed as a cone (A) and carrying a double thread, wherein the double thread comprises a main thread (110) and a secondary thread (120); - a substantially cylindrical milling section (B) having a plurality of milling ribs (160) designed as a steep thread, the outside diameter of which, measured over the ribs, is DF; - a substantially cylindrical shank section (C) carrying only the main thread (110), in which the external thread diameter is DN; - a substantially cylindrical, smooth, thread-free shank section (D); - a head section (G) with a force application (130), wherein - the main thread (110) extends uninterruptedly and with a constant pitch from the tip section (190) of the cone (A) via the milling section (B) to the head end of the thread-bearing shank section (C); and - the ribs of the milling section (B) are arranged countersunk in the thread base (140) of the main thread (110) passing through in such a way that the thread tips of the main thread (110) passing through with nominal external diameter DB project beyond the milling ribs (160) so that DF < DB applies.

2. Screw (200) according to claim 1, characterized in that a retaining thread section (E) is arranged between the thread-free shank section (D) and the head section (G), designed as a short retaining thread with 1 to 3 turns followed by a short thread-free underhead section (F).

3. Screw (200) according to claim 2, characterized in that the retaining thread is designed as a double thread and the pitch of the retaining thread corresponds to 1.6 to 1.9 times the main thread (110).

4. Screw (200) according to claims 1 and 2, characterized in that the length of the thread-free lower head section (F) and of the retaining thread section (E) is between 2 and 5 mm in each case.

5. Screw (100, 200) according to one or more of the preceding claims, characterized in that the main thread (110) is designed symmetrically with a flank angle of 60° ±3°.

6. Screw (100, 200) according to one or more of the preceding claims, characterized in that the secondary thread (120) begins in the tip section (190) of the cone (A), the thread height increases continuously and decreases again after half the length of the cone (A) and ends in the transition region of the cone (A) and the milling section (B).

7. Screw (100, 200) according to one or more of the preceding claims, characterized in that the outside diameter DN of the main thread (110) in the shank section (C) is greater than the outside diameter DB of the main thread in the milling section (B).

8. Screw (100, 200) according to one or more of the preceding claims, characterized in that the flank height of the main thread (110) at the transition from the milling section (B) to the cone (A), starting from the outside diameter DB, decreases continuously over the tapering cone (A) and ends in the tip section (190) of the cone (A).

9. Screw (100, 200) according to one or more of the preceding claims, characterized in that the tip angle of a conical envelope end (300), formed by the tangents at the thread tips of the main thread (110) on the cone (A), which form a common intersection (310) with the longitudinal axis (180) of the screw, is 35° ±5°.

10. Screw (100, 200) according to one or more of the preceding claims, characterized in that the projection (330) of a respective thread crest (320) of a milling rib (160) onto the longitudinal axis (180) of the screw forms an angle of 30° ±10° therewith.

11. Screw (100, 200) according to one or more of the preceding claims, characterized in that the screw (100, 200) is designed as a bimetallic screw, wherein the cylindrical shank sections (D), and if present (E) and (F), and the head region (G) are made of stainless steel and the cone (A) and the milling section (B) of the threaded tip are made of carbon steel and the transition point between stainless steel and carbon steel are located in the main threaded shank section (C).