Method for fastening wpc panels to a wooden support and a combination of a drill-bit and a screw

Abstract

Method for fastening WPC panels to a wooden support and screw for it. For fastening a panel (1) made of a wood-plastic composite, WPC, on an underlying wooden support (2), a screw (4) is extending through an oversized bore hole (5) in the panel (1) and flexes (14B) when the panel (1) moves (14A) relatively to the support (2) due to thermal contraction or expansion of the panel (1). This prevents damage to the panel (1).

Description

[0001]Method for fastening WPC panels to a wooden support and a combination of a drill-bit and a screw FIELD OF THE INVENTION The present invention relates to a screw / drill-bit combination and a method for fastening plastic or wood plastic composite workpieces, especially panels, to a support, in partic- ular wooden support structure. BACKGROUND OF THE INVENTION Fascia Boards are panels for covering the exposed sections of terrace decks, stairs or where walls and rooftop meet around a house. Whereas, those fascia boards traditionally were made of wood, this tradition has recently changed into use of plastic panels or wood-plastic composite WPC boards. As discussed in US8714887, such composite fascia materials change size when heated. Due to such thermal expansion and contraction, caused by varying temperatures, screws may break and / or the fascia material becoming deformed, also called buckling, resulting in unwanted appearance. For counteracting such deformation of the fascia panels, US8714887 suggests drilling a hole in the fascia board with a diameter 1.5-2.5 times larger than the diameter of the non-threaded portion of the screw that holds the fascia panel to the underlying wooden support structure. US8714887 also discloses a special tool which drills not only the oversized hole for the screw through the panel but which also drills a recess a short distance into the panel along the rim of the bore hole for accommodating the screw head, so that the screw appears countersunk into a corre- spondingly sized recess in the panel around the oversized hole. When the screw head is accommodated in the correspondingly sized countersunk recess and the thread is fas- tened to the underlying wooden support structure, lateral movements of the panel rela- tively to the support structure results in movement of the head of the screw together with the panel relatively to the wooden support structure, but since the thread is fixed in the support structure, the screw is bending with its non-threaded portion in the over- sized hole without breaking. Although, the principle as disclosed in US8714887 with the bending screw is appealing at first sight, it unfortunately requires use of a special tool. Each additional tool required for mounting panels implies additional weight to carry along with the other necessary tools and adds complexity for the worker that is doing the construction. In particular, such tool is designed particularly for fascia panels and is, thus, not a standard tool that a worker has normally in the tool box. Accordingly, it would be desirable to provide an alternative mounting method not requiring such tool. DESCRIPTION / SUMMARY OF THE INVENTION It is therefore an objective of the invention to provide an improvement in the art. It is also an objective to provide an alternative mounting method for plastic panels or wood- plastic composite, WPC, panels, in particular fascia panels, to a wooden support struc- ture. These objectives are achieved by a method and a screw / drill-bit combination de- scribed in the following. For fastening a panel made of a plastic or wood-plastic composite, WPC, on an under- lying wooden support according to the invention, dimensioning is important. In partic- ular, the screw that is used for the fastening is extending through an oversized bore hole in the panel and flexes when the panel moves relatively to the support due to thermal contraction or expansion of the panel. This prevents damage to the panel. Notice that the term “underlying” support is used herein also for vertical wooden supports on the fastened side of a vertically oriented panel, as this terminology is common in the corre- sponding technical field. The panel has a thickness TP, typically in the range of 12-17 mm, and a first side from which is receives the screw and a second, opposite side for abutting the wooden support. A cylindrical bore hole is drilled through the panel with a drill-bit. However, in contrast to the aforementioned disclosure of US8714887, the bore hole is not drilled with a re- cess for the screw head, but has, instead, a constant width W through the entire panel from the first side to the second side. This makes the preparation for the final mounting easier as compared to US8714887 in that no special drill-bit is needed. Standard drill- bits of cylindrical shape are sufficient. This simplifies the work for many workers in that standard drill-bits are also standard tools that wood-workers always bring along. As the bore hole is provided by such standard drill-bit, the bore hole also has a rim at the first side with a width equal to the width W of the bore hole. Advantageously, the width W is in the range of 4 mm to 15 mm, and typically in the range of 6 mm to 10 mm, depending on the screw diameter, which is smaller, for example up to 4.5 mm but, typically, 2-4 mm and typically 2.5-6 mm smaller than W. The screw that is provided is shaped and dimensioned according to the bore hole, as explained in more detail in the following. The screw has a screw head at one of its ends and a thread for screwing into the wood support at the opposite end, which is pointed. A non-threaded shank portion is provided between the head and the thread. The screw head has a connector to a tool, optionally a hexalobular insertion tool. For example, the screw has a flat upper side for being flush with the surface of the panel, especially if countersunk. However, this is not strictly necessary, as the head optionally may have a convex upper side. The latter can be advantageous for providing a thicker head for a deeper tool receiver inside the head. The thread has an outer diameter that is equal to or smaller than the width W of the bore hole for easy insertion through the bore hole in the panel. Advantageously, the screw thread is provided with a cutting-flute for ease of drilling into the wood. Typically, the cutting-flute extends less than the length of the thread, for example corresponding to 25-75% of the length of the thread when measures along the longitudinal axis A of the screw. The cutting-flute has a sharp cutting edge, for example parallel with the central longitudinal axis A of the screw, cutting the fibers of the wood during drilling. Advan- tageously, in order for the screw to have a tip that is screwed easy into the wood of the support, the thread starts at the pointed tip but the flute start only at a distance of 1-2 thread turns from the pointed tip. The non-threaded shank portion has a length L1 that largely corresponds to the thickness TP of the panel. Typically, it is slightly shorter due to a fixation disc, provided between the head non-threaded shank portion, and the also taking up a portion of the length TP of the bore hole. Typically, the screw is dimensioned such that the length L1 of the non- threaded shank portion plus the thickness T2 of the fixation disc corresponds to the thickness TP of the panel. In principle, the non-threaded shank portion can be made slightly longer than the bore hole, depending on the thickness of the wooden support, but it should leave enough length for the thread to securely fastening the screw in the wooden support. The non-threaded shank portion has a diameter D3, typically constant diameter D3, for example in the range of 2 mm to 4.5 mm, optionally in the range of 2.5 to 4 mm, and typically such that D3 is 2.5 to 6 mm smaller than the width W of the bore hole. How- ever, the width W of the bore hole can be larger, for example up to 15 mm, for boards that expand much. The shorter the borehole, the thinner the diameter D3 of the non- threaded shank portion in order for achieving sufficient flexing. This results in sufficient space in the bore hole for the lateral movement of the panel, primarily caused by thermal expansion and contraction but also potentially cause by variations in humidity, which may cause dimensional variations not only of the panel but also of the wooden support structure. As discussed above in relation to US8714887, but now achieved in a simpler way due to the simpler bore hole, damage to the panel at the bore hole is prevented by providing the non-threaded shank portion in flexing metal, in particular steel, such as carbon steel or stainless steel, and by dimensioning the non-threaded shank portion for flexing when the panel moves relatively to the support. The fixation disc is provided underneath at the screw head between the non-threaded shank portion and the screw head. It is provided in extension of the screw head. Advan- tageously, the fixation disc has a thickness T2 in the range of 2 mm to 5 mm, for exam- ple in the range of 2 mm to 4 mm, when measured parallel with the central longitudinal axis A of the screw. The fixation disc has a diameter D2 equal to the width W of the borehole, as the fixation disc is used to center and fix the screw head at the entrance of the bore hole on the first side of the panel, which is the side facing away from the wooden support. Optionally, the fixation disc is tapered, typically conically tapered, narrowing in the direction towards the non-threaded shank portion, in order to ease insertion of the fixation disc into the bore hole and proper centering and fixation when fully inserted. For example, the outer rim of the fixation disc is tapering an angle V as measured rela- tively to a line parallel with the central axis, where the angle V is in the range of 1 to 10 degrees, but typically in the order of 1-7 degrees. The head has a width D1, measured laterally to the central longitudinal axis A, which is typically in the range of 5 to 18 mm and usually in the range of 2 to 10 mm larger than the width W of the bore hole. The head has a rim and a ring-shaped underside that extends from the rim to the fixation disc in a direction lateral to the axis A. When the panel, for mounting, is abutting the support, the screw is inserted through the bore hole and screwed into the support until the fixation disc is inside the bore hole and the ring- formed underside, typically flat underside, of the head is abutting the first side of the panel and resting against the rim of the bore hole or until the ring-formed underside of the head is countersunk a distance into the panel by compressing and displacing the material of the panel at the rim. Typically, the head has a thickness T1 in the range of 0.4 to 4 mm, when measured along the axis A. However, in order for the head not to extend much from the panel surface in the case where the head is not countersunk, the head advantageously has a thickness T1 in the range of 0.4 to 2 mm. This thickness T1 is also useful when coun- tersinking the head by screwing the screw until the head is driven into the panel by deformation of the panel at the rim of the borehole. However, the screw is typically not countersunk more than a distance equal to T1, at which point the screw head it is flush with the panel surface. Panels that comprise a WPC bulk material, are typically covered with a protective and often structured plastic film that forms a visually appealing outer side of the panel and protect against weathering and water ingress. When the ring-shaped underside of the head presses against the film at the rim of the bore hole, a fluid tight connection is formed. In case of countersinking the head, the ring-shaped underside of the head de- forms such plastic film, and if care is taken, without damaging the plastic film. This is useful in that this technique provides, in a simple way, a water tight sealing between the underside of the head and the film. This is in contrast to the disclosure in the aforemen- tioned US8714887, in which the drilling of the recess along the rim of the through- going bore hole for accommodating the screw head in the recess also drills through such protective plastic film cover and creates a small ring-shaped water-entrance passage around the screw head. Whereas, in this case, water may enter the WPC bulk material around the bore hole and create damage and become visually unappealing, the method according to the invention is different in that the underside of the screw head presses against an outer side of the protective plastic cover film, which is a substantial improve- ment over US8714887, despite its simplicity, as the underside of the screw head due to the pressure tightens against the outer side of the film along the rim of the bore hole. The head, the fixation disc, the non-threaded shank portion, and the thread are made as a single piece made of metal, in particular stainless steel. This is a traditional way of providing a steel screw. As mentioned above, the method is particularly useful for fastening fascia panels onto a vertical wooden support. However, the method can be generalized to mounting facade covers or decking panels on a terrace or balcony onto a wooden support structure. For example, the screw for the invention has the following parameters: Diameter D1 of the head: 5 – 18 mm Thickness T1 of head: 0.4 – 4.0 mm Shank diameter D3: 2 – 4.5 mm Length of screw: 30 – 60 mm Disc diameter D2: 4 – 15 mm Disc thickness T2: 2 – 5 mm Conus angle V of disc: 1° - 10° In some practical examples for a bore hole of W=6.5 mm equal to the disc diameter D2=6,5, a shank diameter of 3.5 mm, a conus angle of 5°, and a length in the range of 40-45 mm length, for example 40 mm for a 12 mm panel, 43 for a 15 mm panel and 45 mm for a 17 mm panel, and a disc thickness T2 in the range of 3-4.5 mm, for example 3 mm for a 12 mm panel, 3.8 for a 15 mm panel and 4.3 mm for a 17 mm panel. SHORT DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail with reference to the drawing, wherein FIG 1A illustrates a first mounting step, FIG. 1B illustrates a second mounting step, FIG. 2A illustrates a screw in a first perspective, FIG. 2B illustrates a projection view onto the head of the screw of FIG 2A, FIG. 2C illustrates the screw of FIG 2A in a side view. DETAILED DESCRIPTION / PREFERRED EMBODIMENT FIG. 1 illustrates a mounting method of a panel 1 onto an underlying wooden support structure 2. The panel 1 is a plastic panel or a wood-plastic composite panel. In the first mounting step, as illustrated in FIG. 1A, a drill-bit 3 is used to bore a hole with a con- stant width W into the panel 1 but not into the wooden support structure 2. Subse- quently, a screw 4 made of steel, such as carbon steel or stainless steel, is inserted through the drilled hole 5 and screwed into the wooden support structure 2. As illustrated in FIG. 1A, the drill-bit 3 is a standard drill-bit which has a front end comprising a drilling portion 3A for drilling the bore hole 5, which has a width W, and which has also an opposite cylindrical rear which is used for clamping and holding the drill-bit 3 in a drilling tool and which has a width of no more than W. The drill-bit 3 does not comprise a counter-bore head-sinking portion in addition to the primary drill- ing portion 3A, which is in contrast to the tool disclosed in US8714887. Instead, the drill-bit 3 necessary for the invention is merely a standard type drill-bit, readily availa- ble in tool stores and is part of standard equipment for wood workers, which makes the invention different from the disclosure of US8714887, which was discussed in the in- troduction. The screw is illustrated in more detail in FIG. 2A, 2B and 2C. The screw has a head 6 that is formed as a flat disc. The screw head 6 has an upper side 6A, in which there is provided a tool receiver 13, for example for a hexalobular tool, also called Torx®. Alt- hough, a flat upper side 6A of the head 6 may be advantageous for being flush with the surface of the panel 1, especially if countersunk, this is not strictly necessary. For example, the head 6 could also have a convex upper side. The latter can be advantageous for providing a thicker head 6 for a deeper tool receiver 13 inside the head 6. The screw head 6 has a diameter D1 that is larger than the width W of the drilled hole, so that the underside 6B of the screw head 6 rests on the rim of the hole 5 of the panel 1 when the screw 4 is mounted to hold the panel 1 against the wooden support 2, as illustrated in FIG.1B. The rim 6A of the head 6 has the thickness T1 in the range of 0.4 to 4 mm, when measured in parallel to the central axis A of the screw 4, however, typ- ically in the order of 1-2.5 mm in order for the head 6 not to extend much outside the panel 1. The thickness T1 of the rim 6A of the head 6 is equal to the thickness T1 of the head 6 if the head 6 is shaped a flat disc. A thickness of such flat head in the order of 1- 2.5 mm is useful for countersinking even into hard WPC panels. If sufficient force is used, the head 6 can be pressed a small distance into the surface of the panel 1 and become at least partially countersunk. Noticing that many modern wood-plastic panels have a plastic skin on their surface, it is readily recognized that the mounting method explained herein has an advantage over the mounting method of US8714887, which was discussed initially. Due to the bore for the head in US8714887, water can enter the bore and the composite material at the rim of the bore around the screw head. In advantageous contrast thereto, as illustrated in FIG.1B, by the invention, the underside 6B of the head 6 is resting on top the plastic skin 11 along the rim 5A of the hole 5 and therefore provides a tight connection against water ingress. This is a major advantage of the method according to the invention over the prior art. As further illustrated in FIG. 2A and FIG. 1B, there is provided a fixation disc 7 under the head 6. The fixation disc 7 has a diameter D2 selected such that it fits snugly into the drilled hole 5. Accordingly, the fixation disc 7 has a diameter D2 that matches the width W of the hole 5. When the screw 4 is mounted in the hole 5, as illustrated in FIG. 1B, the fixation disc 7 and the head 6 are fixed to the panel 1. The thickness T2 of the fixation disc 7, as measured parallel to the central axis A of the screw 4, is selected such that it rests stable inside the bore hole 5, on the one hand, but does not prevent the lateral movement of the panel 1 relatively to the wooden support structure 2, on the other hand. Accordingly, the thickness T2 of the fixation disc is typically less than 30% of the length L1 of the non-threaded shank portion 8, for example less than 20% of L1. Typically, the sum L1+T2 is equal to or approximately equal to the thickness TP of the panel 1. The movement of the panel 1 relatively to the wooden support structure 2 is possible due to lateral flexing of the non-threaded shank portion 8 of the screw 4, similarly to the explanation given above in relation to US8714887. In order for the fixation disc 7 to easily enter the bore hole 5. it is advantageously slightly conical. Typically, the angle V of the conus relatively to the central axis A is in the range of 1° to 10°, for example in the order of 1° to 5°. For a conical fixation disc, the diameter of the fixation disc varies along the axis A, however, is chosen such that is has a diameter D2 that matches the width W of the bore hole 5, typically at the transition to the flat underside 6B of the head 6. Along the thread 9, the screw 4 further comprises a sharp-edged cutting-flute 10 that is used for easing drilling into the wood of the support structure 2. It may extend from the pointed end 12 or from a position near the pointed end 12 towards the head 6 but is typically shorter than the threaded portion 9. For example, the cutting flute 10 starts a small distance from the pointed end, the distance being in the order of one or two thread turns, in order for the screw to easily start the screwing action into the wooden support. In the example of FIG. 2, the flute 10 starts at a distance approximately corresponding to 1 winding from the pointed tip 12. In the illustrated embodiment, the flute 10 has a length about half the length of the thread 9. Typically, it has a length in the order of 25%-75% of the length of the thread 9. As illustrated in FIG. 2C, the screw 4 is exemplified as having a flute 10 with a cutting edge 10A defined by a thread cut 10B into the threaded portion of the screw 4, forming a cutting plane 10C extending from the cutting edge 10A towards the central axis A. Advantageously, as exemplified, this threads cut 10B does not extend into the lower half of the screw 4 in the projection of FIG.2C, which is head-on the cutting plane 10C. As illustrated in this head-on view onto the vertically oriented cutting edge 10A, this implies that the flute 10 including its thread cut 10A and cutting plane 10C is fully contained in only one quarter of the screw 4 when seen in a cross section that is perpen- dicular to the longitudinal axis A. Although, the mounting method is illustrated with the drill-bit 3 and screw 4 in a hori- zontal orientation, which is typical for mounting of fascia panels, the principle of the invention can also be used for mounting WPC panels that are used as a floor, in which situation, the drill-bit 3 and screw 4 would be vertically oriented. For example, the drilled hole 5 has a width W=6.5 mm, the non-threaded shank portion 8 has a diameter D3=3.5 mm so that there is a distance of 1.5 mm between the shank portion 8 and the walls of the hole 5. The distance of (W-D3) / 2, exemplified as (6.5 mm-3.5 mm) / 2=1.5 mm, is approximately equal to the distance that the panel 1 can move laterally relatively to the support structure 2. However, the exemplified dimen- sions are only for illustration and can be varied for such panels. As the length L1 of the non-threaded shank portion 8 is much longer than the thickness T2 of the fixation disc 7, for example at least 3 or 4 times as long, the non-threaded shank portion 8 can bend and flex, as indicated by arrow 14B in FIG. 1B, sufficiently to allow the fixation disc 7 and the panel 1 to move laterally, as indicated by arrow 14A in FIG. 1B, relatively to the wooden structure 2 in which the thread 9 is fixed. Typically, the non-threaded shank portion 8 has a thickness in the range of 2 to 4 mm in order to flex sufficiently without damaging the panel 1 during its lateral movement. Typical thicknesses TP of the panel are in the range of 10-50 mm. The width W of the hole should be at least 2 mm larger, for example in the range of 2.5-5 mm larger, than the diameter D1 of the shank portion 8. However, the clearance between the screw 4 and the panel is dependent on the thickness TP of the panel 1 and the desired flex of the screw 4. For example, for a thinner panel 1, the non-threaded shank portion 8 is shorter and should be thinner in order to flex sufficiently, for example only 2 to 4 mm thick for example 2.5 to 3.5 mm thick.

Claims

CLAIMS 1. Method of fastening a panel (1) made of a plastic or a wood-plastic composite on an underlying wooden support (2), the method comprising - providing a wooden support (2); - providing the panel (1) with a thickness TP and having a first side for receiving a screw (4) and a second side for abutting the wooden support (2); characterized in that the method comprises - drilling a cylindrical bore hole (5) through the panel (1) with a drill (3), wherein the bore hole (5) has a constant width W through the entire panel (1) from the first side to the second side, wherein the bore hole (5) also has a rim (5A) at the first side with a width equal to the width W of the bore hole (5); wherein the width W is in the range of 4 mm to 15 mm; - providing a screw (4) that has a screw head (6) at one of its ends and a wood-screw thread (9) at an opposite, pointed end (12), wherein the thread (9) has an outer diameter equal to or smaller than the width W of the bore hole (5), wherein the screw has a non- threaded shank portion (8) between the head (6) and the thread (9), wherein the screw (4) comprises a fixation disc (7) at the head (6) between the non-threaded shank portion (8) and the head (6); wherein the screw (4) has a central longitudinal axis A; wherein the fixation disc (7) has a thickness T2 in the range of 2 mm to 5 mm when measured parallel with the central longitudinal axis A and has a diameter D2 equal to the width W of the borehole (5); wherein the length L1 of the non-threaded portion is larger than the thickness T2 of the fixation disc (7); wherein the head (6) has a width D1 measured laterally to the central longitudinal axis A, wherein D1 is 2 to 10 mm larger than the width W of the bore hole (5) and wherein the head (6) has a rim and a ring-shaped underside (6B) that extends from the rim to the fixation disc (7) in a direction lateral to the central longitudinal axis A; - while the panel (1) is abutting the wooden support (2), fastening the panel (1) to the support by screwing the screw (4) through the bore hole (5) into the wooden support (2) until the fixation disc (7) is inside the bore hole (5) and the ring-formed underside (6B) of the head (6) is abutting the first side of the panel (1) and resting against the rim (5A) of the bore hole (5) or until the ring-formed underside (6B) of the head (6) is counter- sunk into the panel (1) by compressing and displacing the material of the panel (1) atthe rim (5A), wherein the head (6) has a thickness T1 in the range of 0.4 to 4 mm when measured along the axis A; - preventing damage to the panel (1) at the bore hole (5) by providing the screw (4) in flexing steel and providing the non-threaded shank portion (8) with a diameter D3 at least 2.5 mm smaller than the width W of the bore hole (5) and in the range of 2 mm to 4.5 mm for flexing (14B) of the non-threaded shank portion (8) when the panel (1) moves (14A) relatively to the wooden support (2) due to thermal contraction or expan- sion of the panel (1).

2. The method according to claim 1, wherein the method comprises countersinking the head a distance of no more than T1 into the panel (1) during screwing.

3. The method according to claim 2, wherein the panel (1) comprises a wood-plastic composite bulk material and a protective plastic film (11) which covers the bulk mate- rial at the first side of the panel (1), and wherein the method comprises deforming the plastic film (11) by the countersinking of the head (6) without damaging the plastic film (11) for providing a water tight sealing between the underside (6B) of the head (6) and the plastic film (11).

4. The method according to any preceding claim, wherein the method comprises provid- ing the fixation disc (7) conically tapered with a narrowing diameter in the direction towards the non-threaded shank portion (8) for easing insertion into the bore hole (5) and proper fixation when fully inserted, wherein the conus angle V of the disc is in the range of 1° to 10° measured from the central longitudinal axis A of the screw (4).

5. The method according to any one of the claims 1-4, wherein the head (6), the fixation disc (7), the non-threaded shank portion (8) and the thread (9) is a single piece made of steel.

6. The method according to any preceding claim, wherein the panel is a fascia panel, and the method comprises fastening the fascia panel onto a vertical wooden support.

7. The method according to any preceding claim, wherein the method comprises provid- ing the screw (4) with a cutting flute (10) with a cutting edge (10A) defined by a threadcut (10B) into the thread (9) of the screw (4), forming a cutting plane (10C) extending from the cutting edge (10A) towards the central axis A but not extending pass the central axis, wherein the flute (10) including its thread cut (10A) and cutting plane (10C) is fully contained in only one quarter of the screw (4) when seen in a cross section that is perpendicular to the longitudinal axis A.

8. The method according to any preceding claim, wherein the method comprises provid- ing the screw (4) relatively to the panel (1) such that the length L1 of the non-threaded shank portion (8) plus the thickness T2 of the fixation disc (7) corresponds to the thick- ness TP of the panel (1).

9. The method according to any preceding claim, wherein the method comprises provid- ing the screw (4) with the length L1 of the non-threaded portion (8) being at least two times the thickness T2 of the fixation disc (7) 10. A combination of a drill-bit (3) and a screw (4) for fastening a panel (1) made of a plastic or a wood-plastic composite on an underlying wooden support (2) according to a method according to any preceding claims, wherein the drill-bit (3) has a front end comprising a drilling portion (3A) for drilling a bore hole (5) that has a width W and an opposite cylindrical rear which has a width of no more than W for clamped holding by drilling tool, wherein the screw (4) has a screw head (6) at one of its ends and a wood- screw thread (9) at an opposite, pointed end (12), wherein the thread (9) has an outer diameter equal to or smaller than the width W of the drill-bit (3), wherein the screw (4) has a non-threaded shank portion (8) between the head (6) and the thread (9), characterized in that the screw (4) comprises a fixation disc (7) at the head (6) between the non-threaded shank portion (8) and the head (6); wherein the screw (4) has a central longitudinal axis A; wherein the fixation disc (7) has a thickness T2 in the range of 2 mm to 5 mm when measured parallel with the central longitudinal axis A and has a diameter D2 equal to the width W of the drill-bit (3); wherein the non-threaded shank portion (8) has a diameter D3 in the range of 2 mm to 4.5 mm, wherein D3 is at least 2.5 mm smaller than the width W of the drill-bit (3); wherein the head (6) has a width D1 measured laterally to the central longitudinal axis A, wherein the width D1 of the head (6) is in the range of 5 to 18 mm, and the width D1 is 2 to 10 mm larger than the width W of the drill-bit (3) and wherein the head (6) has a rim (6A) and a ring-shapedunderside (6B) that extends from the rim (6A) to the fixation disc (7) in a direction lateral to the central longitudinal axis A, wherein the head (6) has a thickness T1 in the range of 0.4 to 4 mm when measured along the axis A.

11. The combination according to claim 10, wherein the fixation disc (7) is conically tapered with a narrowing diameter in the direction towards the non-threaded shank por- tion (8) for easing insertion into the bore hole (5) and proper fixation when fully in- serted, wherein the conus angle V of the disc is in the range of 1° to 10° measured from the central longitudinal axis A of the screw (4).

12. The combination according to claim 10 or 11, wherein the ring-shaped underside (6B) of the head (6) is flat.

13. The combination according to any one of the claims 10-12, wherein the head (6), the fixation disc (7), the non-threaded shank portion (8), and the thread (9) is a single piece made of steel.

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