The screw according to the invention provides that the portion which penetrates into the base panel has a segment, or sub-portion, without threading, and that the threading is present only for a short segment of the tip, corresponding to a very limited number of threads, in the terminal part of the stem, to ensure that the threaded part of the screw never extends into the second panel to be attached.

BACKGROUND OF THE INVENTION There are known self-threading screws to join and attach thin elements to a panel, or two panels or elements in general.

There are various types of screws according to the thickness of the panels to which they are to be screwed and the material of which these panels consist. Screws have been codified in reference tables available to any technician in the field.

Conventional screws consist of a cylindrical or conical stem on which a helical thread is made, which forms the threading.

This thread usually extends for the whole length of the stem, or at least the greater part of the length in the case of long screws. The thread engages between the ridges of the corresponding furrow generated by the rotation of the screw itself.

It is known that, in order to use wood or other material for carpentry or other applications, the behavior of the

material used when screws are inserted therein is very important.

When a screw is driven into a hole, made in the base panel by the insertion of the screw itself, it causes stresses to the material which lead to the breaking up of the walls around the hole. In limit cases, this can even lead to the breakage of the zone adjacent to the hole, especially when the screw turns on itself without penetrating any further, causing the so-called"stripping"of the hole. This occurs particularly in the case of serial jobs or jobs made by inexpert staff.

At present, moreover, in carpentry and in the field of furniture in particular, it is more and more common to use base panels made of pressed or composite chipboard, such as MDF or suchlike; for such materials using screws of a traditional type entails that the attachment does not hold very well. In fact, the upper part of the threading of the screw creates a considerable swelling and raising of material around the hole in the surface area of the base panel, which is usually harder than the inner part.

Therefore, to prevent risks of the holes being stripped, and consequently the limited hold of the attachment and the low resistance to traction of the screw, either every individual screw has to be screwed in manually, taking great care not to make it turn more than necessary, or panels made of a more consistent material have to be used. In both cases, this entails an increase in the production costs.

Document EP-A-0.108. 334 describes a screw with an expanding plastic rawlplug, to attach together a metal element with a cement construction element. This screw has a core impact section of the threaded part which is at least partly bigger than at the front end, opposite the head.

Because it has a very long threaded part with respect to

the non-threaded part, and a core having a big diameter towards the front part of the screw, almost equal to the diameter of the threading, this screw does not solve the problems pointed out above. In fact, the small threading near the tip and the high number of threads create, when the screw is inserted into a wooden panel, on the one hand a very limited hold at the tip, and on the other hand the disintegration of the furrows in the segment behind, particularly if the panel is made of chipboard material or in any case of limited consistency. Therefore, the screw described in EP'334 is not suitable to attach the panel to be fixed onto the material directly, but only if it is associated with an expanding rawlplug.

Document FR-A-1.099. 620 describes a screw which is characterized in that it functions only if its threaded terminal part passes beyond the total thickness of the elements to be attached. Moreover, this screw is not of the self-threading type since its flattened tip can cooperate only with pre-holed materials, particularly metal materials.

Furthermore, the threaded part covers a considerable part of its extension, about half, and a part of the thread is suitable to deform, that is, to bend without breaking, after the screw has been screwed so that, during the unscrewing, the thread can no longer be inserted into the threaded part of the hole of the elements to be joined.

For all these features, the screw described in FR'620 does not solve the problems described above either.

Document GB-A-2.356. 028 describes a screw which has a threading only on an intermediate part of its length, while the tip has no threading.

Document US-A-5,649, 798 describes a screw for metals which, at an intermediate position of the stem, has a ring- type washer which abuts, during screwing, on the plane of

one of the elements to be attached. The screw has a drilling tip without threading and the threading extends on a substantial part, equal to about half, of the entire longitudinal extension of the screw.

This type of screw does not solve the shortcomings and problems described above either.

The present Applicant has devised and embodied this invention to overcome such shortcomings and to obtain further advantages as will be shown hereafter.

SUMMARY OF THE INVENTION The present invention is set forth and characterized essentially in the main claim, while the dependent claims describe other innovative characteristics of the invention.

One purpose of the present invention is to make an attachment screw to join together a base panel with a second panel, in such a manner as to improve its ability to hold the join, compared with traditional type screws and given the same material used. Another purpose is to obtain a screw which will allow to increase resistance to traction, particularly when materials made of lower quality wood are used, and/or materials with limited compactness and resistance.

In accordance with the purposes set out above, an attachment screw according to the present invention, to attach a first base element, or panel, and a second element, comprises a portion able to penetrate into the base panel, and provided with a terminal sub-portion equipped with a threading, made around a stem, and a second upper sub- portion without a threading.

In other words, the screw according to the invention has only a terminal part which is threaded, whereas all the remaining part of the screw stem, both the part which penetrates into the base panel and also the part which

penetrates only into the second panel, has no threading.

Moreover, according to a characteristic of the present invention, starting from the tip, this threading occupies a very limited extension of the stem, less than 25% of the whole length of the screw, advantageously between 10 and 20%. The number of threads on the screw is therefore very low, preferably between 1 and 5, advantageously between 2 and 4, according to the pitch, the length and the diameter of the screw. Advantageously, the number of threads is calculated with the criterion of having 1 thread always present, plus 1/25 of a thread for every millimeter of length of the screw.

Thanks to this configuration, it is guaranteed that the threaded part penetrates completely into the base panel so that, even if the screw is tightened more than necessary, it does not in any case cause damage to the material where the screw is screwed. Moreover, thanks to the limited number of threads, the threaded part of the screw does not emerge from the base panel into the second panel which is to be attached.

According to a first embodiment, the sub-portion not provided with the threading has a diameter equal to, or at most slightly less than, the diameter of the core of the threaded part.

According to a variant, the sub-portion not provided with the threading has a first segment, adjacent to the threading, with a diameter equal to that of the core of the threaded part, and a second segment, which can even reach the head of the screw, with a diameter substantially equal to the diameter of the threading. This segment with a greater diameter on the one hand has a centering function during the screwing, and on the other hand it functions as a stopper on the material where the screw is inserted, thus

improving the functionality of the screw itself.

The screw according to the invention is able to ensure a better hold than that of traditional screws, which are threaded uniformly over the entire length of the portion which penetrates into the first base element, or which in any case have part of the thread which also occupies part of the second element to be joined.

The hold of the screw thus achieved is ensured by the short terminal sub-portion equipped with threading, which, penetrating by rotation, is able to compress and partly break up the material of the walls of the hole. The impediment to the screw coming out is increased because the short threaded terminal sub-portion of the screw finds, as an obstacle to its coming out, the material of the panel which surrounds the upper sub-portion, which is not threaded and, at least in part, has a smaller diameter than the diameter of the threading.

When the screw according to the invention is driven in, the short threaded terminal sub-portion penetrates into the material by rotation, creating around itself a helical furrow in the hole of the panel; this furrow is maintained also in correspondence with the second sub-portion of the screw not provided with the threading, which penetrates afterwards.

When the head of the screw is tightened against a second element which is joined to the first base element, a force of compression is exerted in an axial direction against the base panel. In this way, the material around the non- threaded portion of the screw of the first base element is thus compressed against the second element and performs a "stopper"function or further obstacle to the removal of the screw.

Moreover, with the screw thus configured, in the case of

empty turns, no damage is done to the first base element to be joined since only the thread of the short terminal sub- portion possibly strips the first base element, and only in depth, while the upper portion, without any threading, rotates on itself without damaging the furrow and without displacing material towards the outside.

The screws according to the invention are made according to the reference norms with regard to the unified data according to the tables of a conventional screw available to a technician in the field. For this reason the present invention is not restricted either by the shape of the section of the screw, for example conical, cylindrical or otherwise, or by the shape of the head of the screw, for example countersunk, cylindrical with a cap or otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the present invention will be apparent from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached drawings wherein: - Fig. la is a three-dimensional view of a conventional screw which joins a base panel, preferably made of wood, to a thin second panel; - Fig. lb shows another conventional screw wherein the second panel to be joined is of a greater thickness; - Fig. 2a is a three-dimensional view of a screw in accordance with a preferential form of embodiment according to the invention; - Fig. 2b is a three-dimensional view of a screw in accordance with a variant of the invention; - Figs. 3a and 3b show the screw in Fig. 2a in two different sizes ; - Figs. 4a and 4b show the screw in Fig. 2b in two different sizes.

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT OF THE INVENTION With reference to the drawings, two forms of embodiment of a screw 12 according to the invention, shown respectively in Figs. 2a, 3a and 3b, and 2b, 4a and 4b, are compared with corresponding conventional screws respectively in Figs. la and lb.

A conventional screw 110 (Fig. la) comprises a head 117 and a portion 118 provided with a stem 122 and a corresponding threading 114, able to penetrate into a base panel or element 15. The threading 114 extends along the whole portion 118.

The screw 110 is used in particular when a base panel 15 has to be joined to a second element or panel 16, significantly less thick, for example a sheet of wood or metal. To be more exact, the head of the screw 117, with a countersunk section, penetrates into the second panel 16, while the threaded portion 118 penetrates completely into the base panel 15 and the screw 110 is driven due to the adherence of the threading 114 in the corresponding furrow created by the screw 110.

In a first form of embodiment of the present invention, a screw 12 (Fig. 2a) has a stem 22 with a terminal or lower sub-portion 20, comprising the tip 18, provided with a threading 14 and a second, or upper sub-portion 19, advantageously cylindrical and with a smooth surface.

When the screw 12 is driven, the threaded lower sub- portion 20 penetrates into the material by rotation, creating around itself a helical furrow in the base panel 15; this furrow is maintained also in correspondence with the upper sub-portion 19, without the threading 14. When the head 17 of the screw 12 is tightened against the panel 16, a further compression is exerted in an axial direction on the

furrowed and partly broken up material of the panel 15 which surrounds the upper sub-portion 19, without the threading 14.

This furrowed and compressed material of the base panel 15 creates an impediment or"stopper"which increases the hold of the screw 12.

In a preferential embodiment, the lower sub-portion 20 has between 1 and 5 threads, according to the pitch of the threads, the diameter and overall length"1"of the screw 12; for smaller screws, for example 30-40 mm long, the number of threads is advantageously near 2, while for longer screws, for example 90-100 mm, the number of threads can be equal to a decimal number between 4 and 5.

Irrespective of the number of threads, the length"t"of the threaded lower sub-portion 20 is in any case less than 25% of the overall length"1"of the screw 12. This ensures that, substantially in all circumstances, the sub-portion 20 never extends into the panel to be joined 16, irrespective of its thickness. Thanks to this configuration, it is guaranteed that the lower sub-portion 20 penetrates into the base panel 15 in such a manner that the upper sub-portion 19 also enters for at least a few millimeters into the base panel 15.

In the embodiment shown in Figs. 2a, 3a and 3b, the stem 22 has a constant diameter which, in the upper, unthreaded portion 19, coincides with the diameter of the core of the threaded part 20.

A variant of the invention provides that, when the screw 12 is used to join two panels or elements 15,16 of comparable thickness (Fig. 2b), the stem 22 has a first segment adjacent to the threading 14 with a diameter equal to the diameter of the core followed by a segment 24 having an enlarged diameter which extends as far as the head 17 of

the screw 12.

If we compare the screw 12 with a corresponding conventional screw 111 (Fig. lb), we shall see that the latter also has an unthreaded portion 124 adjacent to the head 117, which is positioned, after the screw has been inserted, into the seating defined by a through hole 23 of the second panel 16, but which never goes inside the base panel 15.

The length of the unthreaded portion 124 depends on the thickness b of the second panel 16. The screw 12 according to the present invention, on the contrary, has the segment 24 of an enlarged diameter which is inserted in depth into the base panel 15 too (Fig. 2b).

As with the screw 12 as shown in Figs. 2a, 3a and 3b, in this variant too the hold of the screw 12 is ensured by the impediment or"stopper"created by the furrowed and compressed material which surrounds the sub-portion 19, when the head 17 of the screw 12 is tightened against the two panels 15 and 16.

The segment 24 with an enlarged diameter not only has a centering function during the screwing, but also accentuates the stopper function on the material into which the threaded part is inserted.

In experiments it has been observed that, in the unwanted case of the furrow being stripped by a conventional screw 110, during the stripping the screw 110 lifts material and makes it emerge from the panel 15, since the furrow of the threading 114 is completely occupied by the threading 114, causing an unwanted detachment of the two panels 15 and 16.

The screw 12 according to the present invention, on the contrary, strips the furrow only in correspondence with the short terminal sub-portion 20 and displaces any possible material into the upper part of the furrow, in

correspondence with the sub-portion 19, without making it emerge from the panel 15, except for possibly an irrelevant amount.

It is obvious however that modifications and/or additions of parts can be made to the screw as described heretofore without departing from the field and scope of the present invention.

It is also obvious that, although the present invention has been described with reference to specific examples, a skilled person in the art shall certainly be able to achieve many other equivalent forms of screw, all of which shall come within the field of the present invention.