Screw with inclined milling ribs, use of such screw and a method for countersinking the screw

Abstract

In order to prevent wood fringes at the edge of a rounded hole from a countersunk screw (1), the screw-head (2) is provided with a tapering underside and inclined inward-pressing milling-ribs and cavities between the milling-ribs.

Description

[0001]Screw with inclined milling ribs, use of such screw and a method for countersinking the screw FIELD OF THE INVENTION The present invention relates to a screw, especially a wood-screw, wherein the under- side of the screw-head comprises a tapered surface with a plurality of milling-ribs that are inclined for pressing milled material inwards towards the shank during countersink- ing of the screw. BACKGROUND OF THE INVENTION For a proper and smooth countersinking of a screw-head into a material, it is common practice to provide the underside of the screw-head with milling-ribs. In some cases, the milling-ribs are often oriented parallel to the central axis of the screw. In other cases, they are inclined to such plane. In even further cases, serrations are curved, for example along a spiral path or S-curved. Some screws have inclined milling-ribs that are outwards-pressing in the sense that the material that is milled by the milling-ribs is pressed in an outwards direction as seen from the screw shank. Other screws have inclined milling-ribs that are inwards-pressing in the sense that the material that is milled by the milling-ribs is pressed in an inwards direction as seen from the screw shank. Some alternative screws have both inward pressing milling ribs as well as and outward pressing milling ribs, for example as dis- closed in EP3067575A1. Inclined, inward pressing ribs on a tapering part of the crew under uppermost portion of a screw head are disclosed in WO2019 / 086091 and WO2020 / 125890 by Dissing. Both have the objective of preventing wood fringes at the edge of a hole from a coun- tersunk screw and disclose milling ribs with inclined planar front-side. However, whereas the screw head in WO2019 / 086091 results in a hemispherical. countersunk hole, WO2020 / 125890 results in a conical countersunk hole. In WO2019 / 086091, the ribs curve smoothly convex from the planar front-side to a frustoconical transition region at the shank in order to form a hemispherical countersunk hole. In WO2020 / 125890, the milling ribs have a width between the planar front-side and a pla- nar back side and are straight for milling a conical countersunk hole. From the planar back side of the rib, a concavely bending cavity extends to the next rib for better accu- mulation of the inwardly milled material. Whereas WO2019 / 086091 discloses curving ribs and convex cavities between the ribs, WO2020 / 125890 discloses straight ribs and concave cavities between the ribs. Each technical solution provides certain advantages. However, it would be desirable to provide further improvements. For a proper counter- sinking, even small variations in the shape of the screw-head and the milling-ribs can have substantial effect. For example, the effect of the screw-head shape and the shape of the milling-ribs may result in the countersunk hole to appear clean or may result in broken wood fibers as fringes around the hole, which is not desired. Additional effect are experienced with respect to stopping of the screwing motion, which should be quickly after countersinking in order for the screw head not to sink too deep into the material. There is a constant need for improvement. DESCRIPTION / SUMMARY OF THE INVENTION It is the objective of the invention to provide an improvement in the art. In particular, it is an objective to provide a screw with improved capabilities for countersinking in a material, for example in wood. This is achieved with a screw as described in more detail in the following. As an alternative to wood, the screw is also useful for countersinking other materials, including certain composite materials, for example composites based on wood fibers, and polymers, for example fiber reinforced polymers. Useful application of the screw is in Oriented Strand Boards (OSB) and also gypsum materials. The screw comprises a screw-head at a first end of the screw and a shank extending from the screw-head towards an opposite, second end of the screw. A thread is provided on the shank for screwing the screw into a material by a fastening-tool. The screw has a longitudinal central axis extending between the first end and the second end. The screw-head has an upper side and an underside, wherein the upper side comprises a tool- receiver for engagement with the fastening-tool. The underside of the screw-head has a tapered surface with a taper-direction towards the shank. The taper-direction is defined in a plane that contains the central axis, and, thus, is parallel with the central axis and extends through the center of the screw. A plurality of such milling ribs are provided, for example in the range of 4 to 8, and an equal number of cavities between the ribs. The ribs and the cavities form a tapering portion on the underside of the screw head. Each of the milling-ribs comprises a planar front-side that is oriented towards a milling direction and delimited by a milling-edge for milling material during countersinking of the screw-head. The term “front-side” of the milling-rib is used herein for the side of the milling-rib that is pushed against the material when the screw during rotation is countersunk into the material. The term “back side” is the opposite side of the milling-rib relatively to the front-side. The front-side is inclined relatively to the taper-direction. In particular, it is inclined by a first angle V1 relatively to a plane P parallel with the central axis. The inclination of the front-side and the milling-edge is oriented such that the milling-rib is inward-press- ing milled material towards the shank during countersinking of the screw-head in the material. The milling-ribs have a planar front-side and the milling-edge, accordingly, is in a plane of the planar front-side of the milling rib. Both aspects have turned out in practice to result in an improved efficient and aggressive cutting of the wood fibers during coun- tersinking. The planar front-side has turned out to be by far more efficient than rounded ones. In particular, the sharp milling-edge of the planar front-side is smoothly and continu- ously convex curving between opposite ends of the planar front-side, so that the width of the front-side is varying between an inner edge of the planar front-side and the milling edge. This contributes to a good cutting result and a rounded countersunk hole, which is advantageous and in contrast to the screw disclosed in the aforementioned WO2020 / 125890, which result in a conical countersunk hole. Between the milling-ribs, cavities are provided. These cavities extend in a longitudinal direction of the screw from a first position near the shank towards at second position near the rim of the head. In a lateral direction, the cavities are smoothly curving from the one milling rib to the planar front-side of the neighboring milling rib. In particular, the cavities comprise a smoothly concave curved cavity portion. This is in contrast to the aforementioned WO2019 / 086091 where all cavities are convex only. For example, the cavities extend from the milling edge and to the inner edge of the planar surface of a neighboring milling rib. Optionally, each of the cavities has a first cavity portion that is smoothly convex near or at the milling edge and form a back-side of the milling-rib and a concave second cavity portion near or at the inner edge of the planar surface of the neighboring milling edge. In this embodiment, the cavity in cross section resemble a weakly bending S- curve with transition from the convex cavity portion to the concave cavity portion. As an option, the width of the planar front-side is narrowest at a first end near the shank and wider at a second end near the rim of the head. In some embodiments, the planar front-side is widest at the second end near the rim of the head. In alternative embodi- ments, the planar front side is widest at a location between the opposite ends, for exam- ple within a portion which is remote from both ends by at least 20% of its total length. Optionally, the width of the planar front-side is smoothly and continuously decreasing towards the shank. The combination of the inclined inward-pressing orientation and the concave cavities has revealed surprising results when used in practice as explained in more detail in the following. The front-side with the cutting-edge, especially if the front-side is planar, is aggressively efficient when countersinking the screw-head into a material, for example wood. Due to the planar front-side being oriented such that the milled material is pressed inwards towards the shank, an efficient stopping of the screw in the countersunk hole is achieved despite the aggressive milling. Due to the S-shaped cavity that extends convex bending backwards from the milling edge during countersinking, the milled material slides smoothly away from the milling edge and into the cavities behind the milling edge, where the milled material is accumulated during countersinking and compressed therein due to the inwards pressing of the material. During this compression, a layer of the compact material is gradually formed on the tapering portion at the underside of the screw-head and accumulated in the concave cavities between the milling-ribs until the volume between the milling-ribs is so filled with compacted material that the milling- ribs are prevented from further milling. As compared to the convex cavities of the prior art, more material is compressed in the concave cavities, which leads to a more compact and solid connection between the screw-head and the material without causing the ma- terial, especially wood, to break. Thus, the additional material taken up in the concave cavities act as a soft but efficient damper during stopping of the screw and acts as a binder between the screw head and the material into which the screw-head has been countersunk. Especially for wood, the result during countersinking is proper stopping of the screw as soon as the screw-head is countersunk into the material without causing breakage of the wood. In addition, due to the compacted material in the concave cavities, the screw has a high tightening force against an underlying sub-structure when a piece of material, for ex- ample a wood element, such as timber, is screwed against such sub-structure, for exam- ple a second wood element, such as a beam. For this reason, the planar front-side and the inclined orientation thereof that is inward-milling has a surprising positive syner- gistic effect. The cavities need not extends all the way to the rim of the head. Optionally, a flat, narrow edge-region on the underside of the head at the rim is provided to cut the wood fringes. For example, the flat edge-region is connecting the rim with the tapered surface. At the rim of the flat edge-region, a cutting edge is provided to assists in cutting wood fibers at the edge of the countersunk hole. In some embodiment, the flat edge-region has a surface in a plane perpendicular to the central axis. In some cases, the width of the flat edge-region, as measured radially from the rim of the screw-head to the start of the ribs near the rim of the screw-head, is in the range of 1-25% of the diameter of the screw-head, for example 1-5% of the head diam- eter. Optionally, along the rim of the screw-head, alternatively or in addition to the flat edge- region, there is provided a collar extending a distance towards the second end, the dis- tance typically being in the range of 0.1-5 mm, depending on the size of the screw. Such collar makes the cutting of wood fibers at the edge of the countersunk hole more effi- cient. As a further alternative, a sharp cutting-edge is provided at the rim, which cutting- edge is directed towards the second end. When countersinking the screw, it has been experienced that wooden fringes around the countersunk screw are minimized and much less than when using similar prior art screws. Optionally, the screw comprises a cutting-notch in a longitudinal direction of the screw along the front part of the thread in order to cut or mill its way into the material. The front part is at the second end of the screw. Optionally, in order to reduce drag on the screw, the screw comprises a knurled shoulder between the thread and the head. Typically, the knurls are provided in direct extension of the thread. For example, the knurled shoulder is provided at a distance to the head, optionally at a distance corresponding to between 1 and 40% of the total length of the screw. Although, such screw can be used for various materials, the screw is especially useful for screwing into wood. A typical overall length L of the screw is in the range 10-1000 mm, although, it can also be longer. The thread corresponds typically to a length of at least 10% of L. If knurls are provided, these are typically over a range of 1-40% of L. Typically, the milling edges are shaped such that the final countersunk hole is smoothly concave curving, for example hemispherical hole. SHORT DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail with reference to the drawing, where FIG. 1 is a principle-sketch of an example of a screw; FIG. 2 illustrates a screw head in side view, FIG. 3 illustrates the screw head in cross sectional side view; FIG. 4 illustrates the screw head as seen from the underside; FIG. 5 illustrates the screw head with a tool socket seen from the upper side; FIG. 6 is a shaded perspective view of the screw head. DETAILED DESCRIPTION / PREFERRED EMBODIMENT FIG. 1 illustrates a screw that is explained in more detail in the following. The screw 1 comprises a screw-head 2 at a first end 1a of the screw 1 and a shank 3 extending from the screw-head 2 towards an opposite, second end 1b of the screw 1. A thread 5 is pro- vided on the shank 3 for screwing the screw 1 into a material by a fastening-tool. For the fastening tool, the screw-head 2 comprises a tool-receiver 4, for example a cross socket or a Hexalobular (Torx®) socket as illustrated in FIG. 5, for engagement with a corresponding fastening-tool for screwing of the screw 1. Optionally, the screw 1 comprises a cutting-notch 9 in the thread 5 in order for the screw 1 to cut its way easier into the material. The cutting-notch 9 extends along the shaft 3 and is exemplified as extending over seven windings of the thread and with sharp edges. The cutting-notch 9 could extend over fewer or more windings. Optionally, the screw 1 comprises a knurled shoulder 10 to reduce drag when the screw 1 is driven into the material, for example wood. The knurled shoulder 10 has an outer diameter larger than the diameter of the shank 3 near the screw-head 2 in order to create a hole wider than the shank 3. Typically, the knurled shoulder 10 is provided in imme- diate extension of the thread 5. FIG. 2 illustrates the screw head 2 in side view and FIG. 3 in cross sectional side view along the line A-A of FIG. 2. FIG. 4 illustrates the screw head 2 as seen from the un- derside 2B of the screw head 2 in a direction parallel with the screw axis 6, FIG. 5 illustrates the screw head 2 with a tool socket 4 seen onto the upper side 2A of the screw head 2, and FIG. 6 is a shaded perspective view of the screw head. As illustrated in FIG. 2, a plurality of milling-ribs 8 are provided on the tapered surface 7 on the underside 2b of the screw head 2. The milling-ribs 8 are inclined by an angle V1 relatively to a plane P parallel with the screw axis 6 for inward-pressing of the ma- terial towards the shank 3 during countersinking of the screw-head 2. The angle V1 is exemplified as 37.5°, but could vary, for example in the interval of 30°-45°. Also indi- cated is the direction of movement 19 of the inclined milling-rib 8. The angle between the direction 19 of by rotation and the inclination of the milling rib is 90°-V1. As also clearly seen in the side view of FIG. 2 is the straight planar front-side 15. At the rim 12, the head has a thickness X1, typically in the range of 0.1 mm to 5 mm. The height X2 of the tapering part in typically in the range of 2-12 mm. The diameter X3 of the head 2 is typically in the range of 6 mm to 19 mm. A typical number of milling-ribs 8 are 3-8, for example the exemplified 6 milling-ribs 8. For example, the milling-ribs 8 are arranged with identical mutual angular distance V3. In the present case with 6 milling-ribs, the angular distance V3, as indicated in FIG. 4, is 60°. As best seen in FIG. 3, the milling edge 17 of the milling ribs 8 in this side projection is shaped such that final countersunk hole is concavely rounded, for example as part of a hemisphere. As also seen in detail in FIG. 4, the milling ribs 8 end within a circle 20 that has a smaller diameter than the diameter X3, shown in FIG. 5, of the screw head 2 at its rim 12. This leaves a flat edge-region 11 between the rim 12 on the underside 2b of the screw-head 2 and the circle 20. During countersinking, the rim 12 cuts-off remaining material, for example wood fibers. For example, the diameter of the circle 20 is in the range of 0.4 mm to 2 mm smaller than the diameter X3 of the head 2. This leaves a minimum width X5 of the flat edge region in the range of 0.2 mm to 1 mm. The distance X4 from the most inner side of the cavities 14 to the rim 12, when meas- ured along the underside 2B of the head 2, is typically in the range of 0.7 to 2.5 mm. Typically, this distance X4 is at least 0.5 mm larger than X5. Whereas the front-side 15 of the milling ribs 8 is straight planar, the rear side 16 is smoothly convex curving. This rear side 16 is also a convex first cavity portion 14A of a smoothly curving cavity 14. The cavity 14 extends with its first cavity portion 14A smoothly convex curving from the cutting edge 17 at the planar front-side 15 of one milling rib 8, then shifts into a smoothly concave curving second portion 14B, which continues to the inner edge 18 of the planar front-side 15 of the neighboring milling rib 8, the inner edge 18 being a border of the planar front-side 15 opposite to the milling edge 17. Notice that this weakly S-curved cavity 14 with both convex and concave portions 14A, 14B between the milling ribs 8 is different from both of the aforementioned disclosures WO2019 / 086091 and WO2020 / 125890. The planar front-side 15 extends and narrows, for example continuously narrows, as exemplified in FIG. 4, from a middle part 15C towards the front part 15B near the rim 12 and from the middle part 15C towards a rear part 15A near or at the shank 3. The position of the widest portion of the flat planar surface 15 is indicated by arrow 21 for this example, where the widest portion is in the middle part 15C. During the countersinking of this screw, the inward-pressing orientation of the milling- ribs resulted in a compaction of the wood fibers in between the milling-ribs. As an example of a useful embodiment, X1=0.7 mm, X2=4 mm, X3=9 mm, V1=37.5 mm, and a max width of 1 mm of the planar front-side

Claims

CLAIMS1. A screw (1) comprising a screw-head (2) at a first end (1a) of the screw (1) and a shank (3) extending from the screw-head (2) towards an opposite, second end (1b) of the screw (1) where a thread (5) is provided on the shank (3) for screwing the screw (1) into a material by a fastening-tool; wherein the screw (1) has a longitudinal central axis (6) extending between the first end (1a) and the second end (1b); wherein the screw- head (2) has an upper side (2a) and an underside (2b), wherein the upper side (2a) com- prises a tool-receiver (4) for engagement with the fastening-tool; wherein the screw comprises a tapered surface (7) extending from the underside (2b) of the screw-head (2) towards the shank (3); wherein the tapered surface (7) comprises a plurality of milling-ribs (8), each milling-rib (8) comprising a straight planar front-side (15) that is oriented towards a milling direction (19) and which it outwards delimited by a milling-edge (17) for milling material during countersinking of the screw-head (2), wherein the planar front-side (15) is inclined by a first angle (V1) relatively to a plane (P) parallel with the central axis (6) for inward-pressing of milled material towards the shank (3) during countersinking of the screw-head (2) in the material; wherein the milling-edge (17) is smoothly and con- tinuously convex curving between opposite ends (15A, 15B) of the planar front-side (15) for varying width of the planar front-side (15) between an inner edge (18) of the planar front-side (15) and the milling edge (17) for providing a rounded concave shape of the hole during countersinking; wherein a smoothly curved cavities (14) is provided between each pair of neighboring milling-ribs (8), the cavities (14) comprising a smoothly concave curved cavity portion (14B).

2. A screw according to claim 1, wherein the cavities (14) extend from the milling edge (17) and to the inner edge (18) of the planar surface (15) of a neighboring milling rib (8), wherein each of the cavities (14) has a first cavity portion (14A) that is smoothly convex near the milling edge (7) and form a back-side (16) of the milling-rib (8) and a concave second cavity portion (14B) near the inner edge (18) of the planar surface (15) of the neighboring milling edge (8).

3. A screw according to any preceding claim, wherein the planar front-side (15) is nar- rowest at a first end near the shank (3).

4. A screw according to claim 3, wherein the planar front-side (15) is widest between opposite ends of the planar front-side (15).

5. A screw according to claim 4, wherein the width of the planar front-side (15) is smoothly and continuously decreasing towards the shank (3).

6. Use of a screw according to any of the preceding claims for screwing into wood 7. A method for countersinking of a screw with minimal wooden fringes around the countersunk screw, the method comprising providing a screw (1) according to any one of the claims 1-3 and driving the screw (1) into a piece of wood until the screw-head (2) is countersunk into the wood, wherein the method comprises forming a rounded countersunk hole by the screw.

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