Field of the Invention

The present invention relates to a screw with a head having a peripheral cutting edge for cutting into a workpiece. Background of the Invention

Screws are used more and more in favor of nails for fastening of wooden and other building elements, generally referred to as workpieces. Most screws have conical heads that are supposed to sink into the workpiece at the final stage of installation of the screw. Such screws, however, tend to penetrate too far into the workpiece unless the installation torque is very precisely controlled. This requires a high level of attention from the installer, and may be rather time-consuming. Moreover, radial displacement of material in the workpiece by the conical head of traditional screws may result in an unwanted appearance of the finished result and/or possibly a weaker than desired fastening of the workpiece.

Mainly to solve the problem of an unwanted finished appearance when the workpiece is plastic composite lumber, US 6 616 391 discloses a screw having a head with an undercut head and radially projecting helical flutes on the shank. According to US 6 616 391 , the undercut head may be provided with a series of unidirectional cutting teeth around the underside of the head periphery. Between the notches defining the teeth, the peripheral lip of the head is flat and perpendicular to the body of the screw.

Although the screw according to US 6 616 391 seems to represent a step forward for installation in plastic composite lumber, there appears to be room for improvement, in particular for workpieces made of fibrous materials, such as wood. Summary

In view of the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved screw, and in particular to provide for improved interaction between the head of the screw and the workpiece.

According to the present invention, it is therefore provided a screw for fastening a workpiece to a structural member by rotating the screw in a rotational direction around a rotational axis of the screw, the screw

comprising: a tip; a head having a top side for accommodating a tool to rotate the screw, and a bottom side having a peripheral cutting edge for cutting into the workpiece; and a thread extending from the tip towards the head, the thread having a thread lead, wherein the peripheral cutting edge exhibits a serrated profile with a plurality of cutting teeth, each of the cutting teeth being defined by a protrusion preceded by a leading recess and followed by a trailing recess when the screw is rotated in the rotational direction, and wherein, for each of the cutting teeth, a ratio between a lateral distance along a circumference of the head at the peripheral cutting edge from a bottom of the leading recess to a bottom of the trailing recess, and the circumference is less than or equal to a ratio between an axial distance from a tip of the protrusion between the leading recess and the trailing recess to the bottom of the trailing recess and the thread lead.

The thread lead defines the axial (downward) movement of the screw per turn (revolution) of the screw when the bottom side of the head touches the workpiece. The thread lead, in combination with the configuration of the serrated profile of the peripheral cutting edge, will determine the angle(s) between the cutting edges of the cutting teeth and the workpiece.

It should be understood that the thread need not extend uninterrupted from the tip towards the head of the screw, but that the thread may be wholly or partly absent at one or several locations from the tip to an end of the thread.

Moreover, the screw may comprise an unthreaded portion adjacent to the head of the screw. The present invention is based upon the realization that cutting action, in particular for cutting fibers such as wood fibers, can be improved by configuring the peripheral cutting edge at the bottom side of the screw head to depend on the thread lead of the screw. In particular, the present inventors have realized that efficient and substantially non-compressing cutting of fibers in the workpiece can be achieved by configuring the peripheral cutting edge, in relation to the thread lead, in such a way that a purely axially directed force is not used for cutting fibers in the workpiece anywhere along the peripheral cutting edge.

To achieve this, the present inventors have found that, for each cutting tooth in the peripheral cutting edge, the ratio between the lateral distance from the bottom of the leading recess of the cutting tooth to the bottom of the trailing recess of the cutting tooth along the circumference of the head at the peripheral cutting edge, and the circumference should be less than or equal to the ratio between the axial distance from the tip of the protrusion of the cutting tooth to the bottom of the trailing recess of the cutting tooth and the thread lead. In this way, it is ensured that no part of the peripheral cutting edge will exert a purely axially directed force on the workpiece, but that the cutting edge of the next protrusion starts to cut fibers with an attack angle defined by the thread lead and the configuration of the protrusion before any portion between adjacent cutting teeth or notches is used for cutting fibers in the workpiece.

In addition to efficient cutting of fibers in the workpiece, the screw according to embodiments of the present invention compresses the portion of the workpiece circumscribed by the peripheral cutting edge substantially in an axial direction towards the tip of the screw. When the workpiece material that has been cut by the peripheral cutting edge is compacted, the resistance to further insertion of the screw increases much more distinctly than is the case for a conical screw head. As a result, there will be a relatively large torque range resulting in the screw being inserted to the proper depth in the workpiece. This, in turn, greatly facilitates installation of the screw. In addition, the force exerted by the head of the screw for pressing the workpiece to the structural member is substantially axially directed, which provides for more efficient and strong fastening of the workpiece. In addition, the risk of splitting a wooden workpiece, in particular close to an end of a plank or similar, is considerably reduced.

In various embodiments of the screw according to the present invention, each of the cutting teeth has a cutting edge between the bottom of the leading recess and the tip of the protrusion; the cutting edge moves in a cutting direction defined by the thread lead when the screw is rotated in the rotational direction, the cutting direction forming a first angle in relation to a plane perpendicular to the rotational axis of the screw; and an average angle along the cutting edge, between the cutting edge and the cutting direction may be at least one and a half times the first angle.

Through this configuration of the cutting teeth, even more efficient cutting of the workpiece is provided for, in particular when the workpiece is fibrous.

To ensure efficient cutting through the fibers of the workpiece, the width of the peripheral cutting edge may be less than 0.2 mm, at least at the tip of each of the protrusions, and advantageously also along the above- mentioned cutting edge of each cutting tooth.

In addition to efficiently cutting through fibers in the workpiece, the peripheral cutting edge may advantageously be configured in such a way that the user of the screw does not cut his or her finger or experiences discomfort from handling the screw. To that end, the tip of each of the protrusions may be rounded.

In embodiments, the tip of each of the protrusions may advantageously exhibit a radius of curvature that is greater than 0.5 mm.

According to various embodiments, the lateral distance along the circumference of the head at the peripheral cutting edge from the bottom of the leading recess to the bottom of the trailing recess may be substantially the same for each of the cutting teeth. In other words, the serrated profile may be periodic. Advantageously, the peripheral cutting edge may exhibit at least eight cutting teeth.

In embodiments, the serrated profile may be undulating, such as sine- wave like. The smooth profile effectively reduces the risk of injury or discomfort when handling the screw.

To further reduce the risk of injury or discomfort, the above-mentioned axial distance from the tip of each protrusion to the bottom of an adjacent recess may be less than 1 mm.

To provide for easy and fast installation of the screw in the workpiece, it is, as was described further above, beneficial with a head that ensures a distinct stop for a suitable insertion depth of the screw in the workpiece.

Another beneficial feature contributing to easy and fast installation of the screw is a low axial starting force.

To provide for the desired low axial starting force, the configuration of the thread in the close vicinity of the tip of the screw has been found to be important. In particular, the present inventors have realized that a rapid increase at the tip of the axially directed (towards the head of the screw) surface of the thread will result in a very efficient initial conversion of torque to axial force driving the screw into the workpiece.

By configuring the thread to quickly approach its maximum thread depth, the axial driving force of the screw will increase rapidly from the start of the rotation of the screw, resulting in a quick entry of the screw with reduced effort.

In particular, the inventors have found that a screw having a thread portion at the tip (the tip thread portion) that extends along one half turn or less around the rotational axis of the screw and exhibits a thread depth increase within the tip thread portion corresponding to at least one half of the maximum thread depth, will require a substantially lower axial force for initiating penetration into a workpiece than a traditional wood screw with similar overall dimensions. This in turn provides for improved ergonomics and increased productivity. Even more advantageously, the thread depth increase within the tip thread portion may correspond to at least seventy percent of the maximum thread depth.

The thread depth increase within the tip thread portion may preferably be a monotonic increase to provide for the desired rapid increase of the axially directed surface of the thread.

To provide for the desired rapid starting action, the tip thread portion may advantageously be located at or very close to the tip of the screw. For instance, the tip thread portion may be located within three millimeters from the tip of the screw.

In summary, the present invention thus relates to a screw comprising a tip; a head with a bottom side having a peripheral cutting edge for cutting into the workpiece; and a thread extending from the tip towards the head, the thread having a thread lead. The peripheral cutting edge exhibits a serrated profile with a plurality of cutting teeth, each defined by a protrusion preceded by a leading recess and followed by a trailing recess when the screw is rotated. For each of the cutting teeth, a ratio between a lateral distance along a circumference of the head at the peripheral cutting edge from a bottom of the leading recess to a bottom of the trailing recess, and the circumference is less than or equal to a ratio between an axial distance from a tip of the protrusion between the leading recess and the trailing recess to the bottom of the trailing recess and the thread lead.

Brief Description of the Drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

Fig 1 a schematically illustrates a screw according to an embodiment of the present invention;

Fig 1 b is a close-up of the head of the screw in fig 1 a, when the peripheral cutting edge of the head just makes contact with the surface of the workpiece; Figs 2a-d are cross-section views of the head of the screw in fig 1 a in different stages of penetration into the workpiece;

Fig 3 is a diagram illustrating the insertion torque for the screw in fig 1 a as a function of insertion depth, in comparison with a traditional screw having a conical head;

Figs 4a-b schematically show the tip of the screw in fig 1 a; and

Fig 5 is a diagram illustrating the thread depth of the screw in figs 4a-b as a function of the rotational angle from the thread starting point. Detailed Description of Example Embodiments

In the present detailed description, various embodiments of the screw according to the present invention are mainly discussed with reference to a wood screw having an unthreaded shank portion directly adjacent to the screw head.

It should be noted that this by no means limits the scope of the present invention, which equally well includes, for example, screws with other thread configurations and being suitable for other workpieces than wood-based products. For instance, the thread may run all the way from the tip to the head of the screw.

Moreover, the tip thread portion exhibiting a very rapid increase in thread depth has generally been illustrated as starting from the thread starting point. It should be noted that this need not necessarily be the case, but that the tip thread portion may start later along the thread than the thread starting point. Other configurations of the tip thread portion are also within the scope of the present invention.

Referring to fig 1 a, which is a side view of a screw 10 according to an example embodiment of the present invention, the screw 10 has a tip 1 1 and a head 12. As is indicated in fig 1 a, the screw 10 has a body 14 and a thread 15 extending from a thread starting point 16 at the tip 1 1 towards the head 12 of the screw 10.

The thread 15 spirals along the screw 10 to allow the screw 10 to be screwed into a workpiece through clockwise rotation, as seen from the head 12 towards the tip 1 1 , around a rotational axis 17. As is indicated in fig 1 a, the thread exhibits a thread lead L.

The top side 19 of the head is configured to accommodate a tool for rotating the screw 10. In this case, the top side 19 of the head 12 is

configured for insertion of a torx-tool, but obviously the top side 19 of the head 12 may be configured for any other suitable tool.

The bottom side 20 of the head 12 has a peripheral cutting edge 21 for cutting into the workpiece when the screw 10 is installed.

Between the peripheral cutting edge 21 and the body 14 of the screw, the bottom side 20 of the head 12 is undercut, such as concave, to collect and compress material from the workpiece when the head 12 of the screw 10 penetrates into the workpiece.

The peripheral cutting edge 21 , which exhibits a serrated profile with protrusions 22a-c and recesses 23a-b (only a few of the protrusions and recesses are indicated by reference numerals to avoid cluttering the drawing), will be described in greater detail below with reference to fig 1 b.

In fig 1 b, the head 12 of the screw 10 in fig 1 a is shown in an enlarged side view just when the protrusions 22a-c make contact with the surface of the workpiece 25.

As is schematically indicated in fig 1 b, each protrusion 22b and its neighboring recesses 23a-b define a cutting tooth having a cutting edge 27a and a trailing edge 27b.

When the screw 10 is rotated, the cutting edge 27a will move in relation to the surface of the workpiece 25 in a cutting direction r defined by the thread lead L. Through the configuration of the peripheral cutting edge 21 the cutting action by the cutting edge 27a of each cutting tooth on the fibers in the workpiece will be shearing, which provides for a cleaner cut with less depression at the peripheral cutting edge 21 than if purely compressing cutting action is used.

To ensure shearing cutting action along the entire peripheral cutting edge 21 , the peripheral cutting edge 21 is configured in such a way that the cutting edge of the next protrusion 22c starts to cut the workpiece before the bottom of the intermediate recess 23b contacts the workpiece 25.

In particular, the depth h of each recess 23a-b is sufficiently large to ensure that the screw is not moved an axial distance that is greater than this depth h when the screw is turned from the bottom of the leading recess 23a to the bottom of the trailing recess 23b. Mathematically, this is equivalent to requiring that the screw 10 fulfills the relation: h/L≥ I TTD, where h is the axial distance between the tip of a protrusion 22b and the bottom of its trailing recess 23b, L is the thread lead, / is the

circumferential distance from the bottom of the leading recess 23a to the bottom of the trailing recess 23b, and D is the diameter of the head 12 at the peripheral cutting edge 21 .

The effect of this configuration of the peripheral cutting edge 21 is a clean and well-defined cut through the workpiece 25. Due to the rounded shape of the protrusions 22a-c and the dimensions chosen (the distance h between tip and bottom is less than 1 mm), there is a reduced risk of injury or discomfort for the user handling the screw 10.

In fig 1 b, the cutting direction r is indicated as forming a first angle a in relation to a plane 29 perpendicular to the rotational axis 17 of the screw. Furthermore, the tangent to a point along the cutting edge 27a is indicated as forming a second angle β in relation to the cutting direction r. To provide for efficient cutting of fibers in the workpiece 25 at least partly through shearing cutting action, the average of the second angle Paverage is at least one and a half times the above-mentioned first angle a.

For a curved cutting edge, the above-mentioned average of the second angle may be defined by the integral of the second angle along the cutting edge divided by the length of the cutting edge: Paverage where S is the total length of the cutting edge 27a. P(s) is the angle between the tangent of the cutting edge 27a and the cutting direction r at position s along the cutting edge from the tip (s=0) of the protrusion 22b towards the bottom (s=S) of the leading recess 23a.

In addition to achieving a clean and well-defined cut through a fibrous workpiece, such as a wooden workpiece 25, the screw 10 according to embodiments of the present invention also provides for rapid installation to a pre-defined depth and a reliable fastening.

These further effects will now be explained with reference to figs 2a-d, which are cross-section views illustrating penetration of the screw head 12 into the workpiece 25.

Fig 2a schematically illustrates the situation just after the peripheral cutting edge 21 has started to cut into the workpiece 25. As explained above, the cutting edges (27a in fig 1 b) have started to cut through fibers of the workpiece 25, but there is still air in the space 28 between the peripheral cutting edge 21 and the body 14 of the screw 10. The workpiece material under the bottom of the head 12 has not yet been compressed, and the Young's modulus of the workpiece material under the in the space 28 under the head 12 is still roughly the same as the Young's modulus of the workpiece material beside the head 12.

In fig 2b, the head 12 has penetrated further into the workpiece 25, so that the space 28 between the peripheral cutting edge 21 and the body 14 of the screw 10 is filled with somewhat compressed workpiece material that has been separated from the workpiece material beside the head 12 through the above-described cutting action of the peripheral cutting edge 21 . It should be noted that the compression of the workpiece material in and under the space 28 between the peripheral cutting edge 21 and the body 14 of the screw 10 is almost purely due to an axial force. This compression will gradually increase the Young's modulus of the workpiece material under the head 12, which means that the torque required for continued rotation of the screw 10 will increase.

This continued compression of the workpiece material in and underneath the space 28 between the peripheral cutting edge 21 and the body 14 of the screw 10 is schematically illustrated by dots with increasing density in figs 2b-d. In addition to providing for a distinct and sharp increase in the torque required for continued rotation of the screw 10, the compression of the workpiece material also provides sealing as well as improved strength due to the almost purely axial force between the screw head 12 and the workpiece 25.

In the diagram in fig 3, the installation torque M is shown as a function of installation depth y (penetration distance of the tip 16 of the screw into the workpiece 25) for the screw 10 in fig 1 a (the solid line 30). As a comparison, the same function is shown for a screw having the same screw body and thread configuration, but a conventional conical head with the same head diameter (the dashed line 32).

As can be seen in fig 3, the relation between installation torque M and installation depth y is the same for both screws until the installation depth y-i , where the peripheral cutting edge 21 first makes contact with the surface of the workpiece 25. The torque is still basically the same for both screws before workpiece material is compressed in the space 28 between the peripheral cutting edge 21 and the body 14 of the screw 10. Thereafter, the installation torque M increases much more rapidly with installation depth for the screw 10 in fig 1 a than for the screw with the conventional conical head. From the diagram in fig 3, it is easy to understand that the torque setting to achieve an installation depth that is substantially the same for each installation will be much easier to achieve for a screw according to embodiments of the present invention than for a screw with a conventional head.

As was mentioned in the Summary section, the installation of the screw 10 can be further facilitated and be made even more time-saving by providing for an easier installation start, in addition to the distinct end of installation explained above. The easier installation start is due to the configuration of the thread 15 at the tip 16 of the screw 10.

Referring to figs 4a-b, the thread 15 is configured to practically immediately present a substantial axially oriented driving surface to the workpiece. To that end, the screw 10 comprises a tip thread portion 18 exhibiting a very rapid increase in thread depth w. In the example

embodiment of figs 4a-b, the tip thread portion extends from the thread starting point 16.

The configuration of the thread 15 in the vicinity of the tip 1 1 will now be described further with reference to fig 4b, which is a plane view of the tip 1 1 as seen along the rotational axis 17, and fig 5, which is a diagram showing the thread depth w as a function of rotational angle Θ.

As can be seen in the diagram in fig 5, the thread depth w(0) of the screw 10 grows with increasing rotational angle Θ with a first average rate of increase Κ-ι(θ) during a first half turn (from 0° to 180°) around the rotational axis 17 from the thread starting point 16, and with a second average rate of increase Κ ₂ (θ) during the second half turn (from 180° to 360°).

For the screw 10 in figs 4a-b, the first average rate of increase

Κι(θ)«0.007 mm/°, and the second average rate of increase

Κ ₂ (θ)«0.001 mm/°.

Accordingly, the first average rate of increase is about seven times the second average rate of increase. This is a measure of the very rapid initial increase in the thread depth w. Due to this rapid initial increase in the thread depth, the torque applied to the screw 10 is translated to an axial driving force already from the very start of the rotation of the screw 10.

Furthermore, the tip thread portion 18 of the inventive screw 10 exhibits a thread depth increase Awi within the tip thread portion 18 corresponding to at least one half of the maximum thread depth w _max . This is schematically indicated in fig 5.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.