The invention relates to a method for stiffening a sheet of solid wood consisting of an integral piece of wood or a plurality of wood strips which are joined together and have substantially parallel grains, said sheet comprising two substantially parallel sides.

The method allows a sheet of wood to be stiffened in order to prevent the substantially straight sides of the sheet from bending.

The invention further relates to a sheet of solid wood consisting of an integral piece of wood or a plurality of wood strips which are joined together and have substantially parallel grains, said sheet comprising two substantially parallel sides.

One of the biggest problem with the use of sheets of solid wood is that they tend to bend as a result of variations in the moisture conditions or other changes affecting the internal stress. It is previously known to manu¬ facture, for example, panel doors comprising one or more panels made of a sheet of solid wood. Each panel is surrounded by frame portions; the frame portions that are perpendicular to the grain of the sheet of wood are intended to stiffen the sheet and thereby prevent the sheet from bending. It is also known to stiffen for instance table-tops of solid wood by attaching a stiffening bar of wood or metal to the lower surface of the table-top.

The above-mentioned known solutions are, however, difficult to manufacture and comprise awkward frames or bars on the surface of the sheet of wood, which complicates the use of the sheet. In addition, such projections make it difficult to maintain, e.g. to clean, the constructions. It is also difficult to attach a frame construction tightly to a sheet of wood, and thus cracks are formed in course of time in the joints between the sheet of wood and the frame portions. Crack formation allows moisture to penetrate into the sheet of wood, which makes the sheet bend. Furthermore, the manufacture of the known stiffening constructions is difficult to automate, which makes the products expensive.

The present invention obviates the problems associated with the known solutions and provides a method for stiffening sheets of solid wood which can be easily automated.

A solution to the above-mentioned problems has been achieved by the present invention, which has the characteristics disclosed in the appended claims.

The method of the invention is characterized by providing the sheet of wood with at least one hole which is substantially perpendicular to the grain of the sheet of wood and extends through the sheet of wood either all the way or part of the way in parallel with its side, inserting a stiffening member in the hole, and closing the hole with a closing means.

The sheet of wood of the invention is characterized in that the sheet of wood is provided with at least one hole which is substantially perpendicular to the grain of the sheet of wood and extends through the sheet of wood either all the way or part of the way in parallel with its side, a stiffening member is inserted in the hole, and the hole is closed with a closing means.

The present invention is based on the idea that the stiffening of a sheet of wood should not involve any projections or cause any disadvant¬ ageous crack formation in the sheet of wood, which could damage the protective surface treatment of the sheet. The invention also counteracts for example changes in the stress conditions of a sheet during profile milling of the sheet surface. The sheet of wood in question is usually of glulam, in which the wood strips joined together with an adhesive can be of any kind of wood.

The method and device of the present invention have many significant advantages over the prior art. The manufacture of the sheet of wood of the invention can be fully automated in a simple manner. The invention also provides a sheet of wood which retains its form in different environments much better than the prior art solutions, since the construction has a better moisture resistance on account of the considerably reduced crack formation.

The sheets of wood of the invention can be used for the manufacture of doors and cupboard doors, in particular. It is important that these products remain substantially straight, since a bent door does not close a doorway or cupboard properly.

Doors of solid wood are conventionally often panel doors with an upper and a lower vertical frame portion surrounding a door panel. These frame portions serve as stiffening members. The idea of the present invention is to provide a stiffening member which is perpendicular to the grain of the

door panel and which can replace the separate frame portions. According to the invention, a door corresponding to a conventional panel door with frames could be produced by milling distinctly profiled outer edges to a solid piece of wood. Since the manufacture of doors can be automated to a great extent, the products are considerably less expensive to manufacture with the method of the invention than with the conventional methods. In addition, the products manufactured according to the present invention are easier to handle. Since the product consists of a uniform sheet of wood in which the surface details are formed by milling, the surface of the product comprises no laths or frame portions which can loosen as a result of careless handling. The doors of the invention are also much easier to clean, since there are no cavities or gaps which gather dirt or moisture, and thus the product of the invention has lower servicing costs than conventional products. In the following, the invention will be described in greater detail with reference to the accompanying drawings, in which

Figures 1A to 1D are schematic illustrations of the method of the invention,

Figure 2 shows a longitudinal section of a product of the invention, Figure 3 shows a cross-section of the product of Figure 2 when a stiffening member is partly inserted therein, and

Figure 4 is a sectional side view of another product in which stiffening members are inserted.

Figures 1A to 1D are schematic views of a preferred embodiment for stiffening a sheet 1 of wood. In the figures, the sheet of wood is a cupboard door, but the method can be applied to any sheet of wood.

As shown in Figure 1A, the method of the invention comprises making at least one hole 3 by a drill 2 in a sheet 1 of wood, i.e. the so-called raw sheet, comprising two substantially parallel sides 4, two lateral edges 5, an upper edge 6 and a lower edge 7. The sheet of wood may be an integral piece of solid wood, but most often it consists of wood strips joined together with an adhesive, i.e. so-called gluelam. The lateral edges 5 are substantially parallel to the grain of the piece of wood or the wood strips in gluelam. The holes 3 extend from one lateral edge towards the other lateral edge substantially perpendicularly to the grain of the sheet 1 of wood; the holes can extend through the sheet of wood either all the way or only part of the way.

In the case of doors or cupboard doors, the sheet 1 of wood is preferably provided with holes 3 both near the upper edge 6 and near the lower edge 7.

A member 8 preferably shaped as a tube or rod is inserted in the hole 3 as shown in Figure 1 B, whereafter the hole is closed with a closing means 9, such as a pin. In order to be easily insertable, the member should have slightly smaller dimensions than the space formed by the hole.

The member 8 which provides the stiffening in conjunction with the hole 3 is preferably a metal tube or a metal rod, but it is also possible to use wood strips or various plastic or composite materials.

When doors or cupboard doors, for example, are manufactured, a sheet 1 of wood is milled with a milling device 10 known per se, as shown in Figures 1C and 1 D, and subsequently varnished to form a final product.

Figures 2 and 3 are sectional views of an embodiment of a product manufactured according to the method of the invention. The product comprises a sheet 1 of wood with sides 4, lateral edges 5, an upper edge 6 and a lower edge 7. The sheet of wood is provided with at least one hole 3 in which a stiffing member 8 is inserted.

Figure 4 illustrates an alternative embodiment of the product. In this embodiment, the holes 3 do not extend all the way through the product but are arranged for instance in such a way that stiffing members 8 inserted in the holes from the opposite lateral edges 5 of the product partly overlap each other in the middle of the product. The holes 3 are closed by closing means 9, such as pins, which are mechanically attached to the sheet 1 of wood, preferably with an adhesive.

When a very rigid product is made, the stiffening members 8 are mechanically attached to the holes 3, for example by an adhesive joint.

The invention is not limited to the embodiment described above but can be modified in many ways within the scope of the inventive concept disclosed in the appended claims.