The present invention relates to a screw, especially a wood-screw, according to the introductory part of the claims, wherein the underside 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 countersinking 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 serrations, for example milling-ribs. The milling-ribs are often oriented such that they are lying in a plane that also contains 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. Examples of these various shapes are disclosed in German publication DE3344048A1, German utility model DE9106746U1, European patent applications EP0705987, EP1182367, EP2522865, French patent application FR2784722, British patent applications GB2227540, GB972653, GB2336415, U.S. Pat. Nos. 586,232, 1,151,861, 5,199,839, 7,293,947, U.S. D708509, and Chinese patent application CN101498334A.

In all of the above prior art, the inclined milling-ribs 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.

For better understanding, the term “outwards-pressing milling-ribs” is used herein for skew milling-ribs by which material is pressed outwards during countersinking of the screw into the material. Similarly, the term “inwards-pressing milling-ribs” is used for skew milling-ribs by which material is pressed inwards towards the shank during countersinking of the screw into the material. Inward-pressing milling-ribs have a front-side that is inclined by an angle in the direction of the screwing-in direction of the screw relatively to a cross-sectional plane that contains the central axis of the screw and which crosses the front-side nearest to the shank. Outward-pressing milling-ribs have a front-side that is inclined by an angle opposite to the screwing-in direction of the screw, when the angle is measured relatively to a cross-sectional plane that contains the central axis of the screw and which crosses the front-side nearest to the shank.

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 two different principles are illustrated inFIG. 7a, in which the screw has outwards-pressing milling-ribs262by which wood is pressed outwards during clockwise-rotational countersinking of the screw, andFIG. 7b, in which the screw has inwards-pressing milling-ribs262by which wood is pressed inwards towards the shank during clockwise-rotational countersinking. Both drawings inFIG. 7aandFIG. 7bare copies from EP3067575A1 by Kwantex.

This European patent application EP3067575A1 differs from the other above-mentioned prior art documents remarkably in that it discloses not only outwards-pressing but also inwards-pressing milling-ribs that are arranged in an incline orientation that causes the wood to be forced towards the shank during countersinking.

The drawingsFIG. 7aandFIG. 7billustrate a spirally curved milling-edge on the front-side of the milling-ribs. For more detailed illustration of the spiral milling-ribs of the prior art EP3067575A1,FIG. 7candFIG. 7dshow that the inwards-pressing milling-ribs262have a front-side264of the milling-rib262that does not vary in thickness along the milling-rib262but has a constant width.

GB15102 discloses a screw with straight milling edges integrated in the underside of the head. The milling edges are formed by cavities inside the surface of the conical underside and are angled such that the milled wood is pressed inwards towards the stem. The countersunk hole formed by this screw is conical. It is emphasized in this disclosure for the milling edges that “in no case do they come out at the face of the head”. This integration in the head as cavities is done in order to retain strength and have the appearance as an ordinary screw.

In contrast to cavities that form ribs inside the head, EP3022468 discloses ribs that protrude from the conical surface. The ribs are curved helically and offset and also pressing the wood inwards towards the stem during countersinking. The curving of the ribs is such that the edges are tangential and parallel to the rotational direction when the ribs start engaging with the wood surface during countersinking, so that the initial action is rather cutting than milling during the first phase of the countersinking. Only during further countersinking, when other parts of the ribs engage with the wood, the angle gradually deviates from a tangential direction and the initial cutting of the wood surface is followed by a milling action. During continued countersinking, deeper layers of the wood are first cut and then milled by the curve ribs.

For a proper countersinking, 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.

DESCRIPTION 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, in particular in wood. This is achieved with a screw as described in more detail in the following.

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, for example frusto-conical, 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. The tapered surface is provided with a plurality of milling-ribs. Each of the milling-ribs comprises a 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 front-side is inclined relatively to the taper-direction. The inclination of the front-side and the milling-edge is oriented such that the milling-rib is inward-pressing milled material towards the shank during countersinking of the screw-head in the material. The term inward-pressing was explained in more detail in the introduction above.

In contrast to the above-mentioned European patent application EP3067575A1 by Kwantex, the milling-ribs do not have a spiral front-side and cutting-edge, but each milling-rib has a planar front-side, wherein the front-side comprises a sharp milling-edge for cutting the material during countersinking of the screw-head. It has been shown in practice that the flat and straight planar front-side of the milling-ribs results in an improved efficient and aggressive cutting of the wood fibers during countersinking. The planar front-side has turned out to be by far more efficient than rounded ones, for example as the scallop-formed serrations disclosed in U.S. Pat. No. 5,199,839.

Advantageously, the sharp milling-edge of the planar front-side is smoothly and continuously convex curving between opposite ends of the front-side for varying width of the front-side, the width of the front-side being narrowest at the ends. This contributes to a good cutting result. The curving milling-edge is advantageous relatively to the straight edges of EP0705987. The result is a curved countersunk hole.

The combination of a planar front-side and the inclined orientation thereof that is inward-milling has revealed surprising results when used in practice, especially if the milling-edge of the planar front-side is smoothly and continuously convex curving between opposite ends of the front-side. On the one hand, the planar front-side with the cutting-edge is aggressively efficient when countersinking the screw-head into a material, for example wood. On the other hand, 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. This is understood by the following. The milled material is accumulated in the countersunk hole 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 underside of the screw-head and accumulated between the milling-ribs during countersinking until the space between the milling-ribs is so filled with compacted material that the milling-ribs are prevented from further milling. The result is a proper stopping of the screw as soon as the screw-head is countersunk into the material. In addition, the screw has a high tightening force against an underlying sub-structure when a piece of material, for example a wood element, such as timber, is screwed against such sub-structure, for example 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 synergistic effect.

In some embodiments, the planar front-side is widest at a center of the front-side and smoothly and continuously decreasing from the center towards the opposite ends. This way, the front-side of the milling-rib is thickest in the middle and gets narrow towards the first end and towards the shank.

The milling-edge can be smoothly curving in different ways. For example, the milling-edge, when projected onto a plane that extends along the central axis and through the center of the flat surface, follows a section of a circle, which results in a partly-spherical hole for the screw-head during countersinking. For example, such circular section ends with a tangent parallel with the central axis, in which case, the milling results in a hemi-spherical hole for the screw-head during countersinking. Alternatively, the milling-edge in such projection deviates from a section of a circle, for example has a shape of a parabola. The countersunk hole is also rounded in this case.

In some embodiments, the front-side has opposite ends and a center between the opposite ends, wherein the center forms a right angle with the tapering surface in a plane perpendicular to the central axis and through the center of the flat surface. As will be shown below, in some embodiments, the milling-rib can be oriented such that the center is aligned with the central axis when viewed along the planar front-side in a direction perpendicular to the central axis.

In some embodiments, each milling-rib also has a back side that has a larger angle with the tapering surface than the front-side.

In some embodiments, a flat edge is formed on an underside of the screw-head along a rim of the screw-head. For example, the flat edge is connecting the rim with the tapered surface. Such edge assists in cutting wood fibers at the edge of the countersunk hole.

In some embodiment, the flat edge has a surface in a plane perpendicular to the central axis. In some cases, the width of the flat edge, as measured from 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 diameter. Optionally, along the rim of the screw-head, alternatively or in addition to the flat edge, there is provided a collar extending a distance towards the second end, the distance typically being in the range of 1-5 mm. Such collar makes the cutting of wood fibers at the edge of the countersunk hole more efficient. As a further alternative, the flat edge is tapering from the rim inwards in order to provide a sharp cutting-edge 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.

In order to cut or drill its way into the material, advantageously, the screw comprises a cutting-edge in a longitudinal direction of the screw along the front part of the thread. The front part is at the second end of the screw.

In order to reduce drag on the screw, optionally, 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.

Typical values for the tapering angle are 20°-60° with the longitudinal axis. Thus, the tapered area, for example frusto-conical area, spans 40°-120° in total

Typically, the tapered surface at the underside of the head is frusto-conical. However, a more rounded surface would also be possible, for example convex tapered surface, including an ellipsoidal or semispherical surface or concavely tapered surfaces. Typically, the tapered surface is a solid of revolution, although, this is not strictly necessary. For example, the tapered surface has a non-circular cross-section in a plane perpendicular to the central axis.

