The development would appear to have been brought to an end with a special screw with a relatively thin screw shank with double-cut thread (3-5 mm), and with a screw head configured like the mouth of a trumpet ending with a quite thin outer edge with a diameter typically in the order of 8.5 mm. In that the back of the screw head will hereby extend out towards the edge while forming a relatively small angle with the front of the head, in connection with the small thickness of the outer edge this will mean that the slight degree of material compression which will arise under random areas of the screw head will be inwardly-directed in the material and only to a modest degree outwardly-directed from the screw ed- ge. This type of screw is widely accepted, but it is also ac- cepted, however, that a certain degree of ripping-up around the edge of the screw head will still normally arise, and even though this projecting ripping-up is only slight, it must still be subsequently removed by sanding.

With the invention it has been seen to be possible to provide a screw which does not give rise to any form of bulging-out or ripping-up whatsoever, which in practice will be extremely attractive. This screw is based on the quite different consideration that there is no reason to seek to achieve a short penetration depth of the outer edge area of the screw head, merely providing that this is configured in such a manner that it does not give rise whatsoever to any distinct outwardly-directed compression. A corresponding con- sideration is that a head of a screw, regardless of its pro- file, will be prevented from producing an outwardly-directed compression when it is configured with skirt part extending rearwards from the circumference of the screw head, which when the screw is screwed in will form a mechanical shiel- ding-off of that material area in which a compression of the material can possibly arise with the final screwing-in.

A screw according to the invention will thus be characterised by an appearance which is radically different

from the above-mentioned, hitherto-preferred screw, i. e. by being provided with a decidedly"thick"outer edge on the screw head. This will appear quite illogical until it is seen that the back of the screw head within the outer edge is "hollow", so that a compression of the material can arise without this in any way being able to spread to the area out- side the head of the screw.

In the following, the invention will be explained in more detail with reference to the drawing, in which fig. 1 is sectional view of a screw according to the invention, shown during the initial screwing-in of the screw head in a plaster board surface, while fig. 2 is a corresponding view of an almost comple- te screwing-in, and fig. 3 is a corresponding view of the finally com- pleted screwing-in.

The shown screw has a screw shank 2 with a thread 4, above which the neck 6 of the screw is configured with the same thickness as the core of the thread, i. e. preferably not with a greater thickness than this, and a screw head 8 which immediately appears as a"block head"with cylindrical outer side 10. The screw head has a slot 12 of any usable ty- pe.

The underside of the screw head has an annular re- cess 14 around the neck 6 of the screw, said recess being of curved cross-section with gradual transition to the screw neck and with a short, rounded-off transition 16 to the outer side 10 of the screw head.

In fig. 1 the screw 1 is shown screwed so far into a plaster board 20 that it has started to penetrate an under layer of wood W. Until a firm threaded engagement with the wood is achieved, there will typically occur a certain "over-turning"of the screw so that its thread has worked as an outwardly-directed milling tool which has brought about a pushing-out of crumbled plaster in the formation of a projecting collar 22 around the screw shank 2, with associa-

ted ripping-up of the covering paper or cardboard on the pla- ster board.

In fig. 2 the screw is shown screwed so far in that the outer under edge 16 of the screw head has just begun to press the paper-covered surface of the board inwards. The un- der edge 16 could be sharp, but a small chamfering is prefer- red for a milder handling of the cover paper, which will thus be quite slightly rounded-down just outside the cutting-in which will arise with the subsequent penetration of the screw head into the plaster material. It must be noted that the preferred diameter of the screw head is only 6-8 mm. It will be seen that the formation of the collar 22 will thereafter be accommodated completely within the annular recess 14. For use in softer materials, use can well be made of screw heads of substantially larger diameters.

During the further screwing-in, the head 8 will gradually penetrate deeper into the plaster, cf. fig. 3, whe- reby the plaster will be compressed locally. This compression will immediately manifest itself most strongly under the downwards-extending edge area of the screw head, but this most strongly compressed material will naturally seek inwards and upwards under the"dome"which is formed by the recess 14, as shown by the arrow in fig. 3, whereby the differenti- ated compression in under the screw head will be substantial- ly equalised to an acceptable level despite the large perip- heral thickness of the screw head.

A related and even more relevant result will be that no distinct material compression will arise at all imme- diately outside the screw head, and nor in the least near to the outer surface of the plaster board. Therefore, when the screw has been screwed in to its final position, where the outer side of the screw head lies flush with the surface pla- ne of the plaster board, or as shown by the stippled line in fig. 3 is slightly countersunk herein, there will not arise the slightest bulging-out or ripping-up of the board from this plane around the screw head, i. e. the board can subsequ- ently be treated with filling material and painted or wallpa- pered without any intermediate sanding.

It is of extra significance that the said sanding is avoided, not only in order to avoid the related work, but also because the filler can adhere poorly to the sanded parts which display crumbled material. It will be seen in fig. 3 that the broken surface of the plaster board is completely clean, in that it displays only the indented layer of cover material around the screw head. The resulting indentation can be treated with filler in a fully durable manner.

C L A I M S 1. Screw, in particular for the fastening of cladding plates of the plaster board type, with a screw shank and a screw head with a groove or the like in the top, c h a r a c t e r i s e d in that the screw head is configured with a substantially flat top and with a skirt part extending down- wards from the outer edge of said head top, and which within the underside of the screw head forms an annular cavity around the screw shank.

2. Screw according to claim 1, c h a r a c t e r i s e d in that the screw shank extends over into said annular ca- vity in a locally rounded-off manner.

3. Screw according to claim 1, c h a r a c t e r i s e d in that said cavity meets the under edge of the skirt part in a wholly or semi-sharp manner.