Invention concerns finger joint, particularly suitable for length superstructures of prismatic elements, to form infinitely long rods.

Background Art

To length joining of prismatic elements, particularly of wood, in order to form infinitely long rods by their linear adjustment, is generally known to use of the finger joint, also referred to as a "Wedge-finger jointed connection". Its common advantage is to allow the production of infinitely long rod products made only from the quality material, obtained by selecting lumber prepared from timber. The advantage of a finger joint is to allow effectively use lumber of non-standard sizes, which otherwise would form waste, on the production of wooden structural elements.

Normally used a finger joint is characterized by forming parallel teeth and grooves at the ends of bonding prismatic elements that fit into each other. The surfaces of the teeth and grooves form contact surfaces of the finger joint, which are coated by suitable adhesive. The finger joint is formed after pressing the adjacent prismatic elements opposite each other and glue dries. Tops of the teeth and/or bottoms of grooves lie in a perpendicular plane on the longitudinal axis of the prism bounding element. That design is basically satisfactory for the production of infinitely long glulam panels used in the manufacture of construction materials with a large surface area of cross section profile.

In the case of an infinitely long rod production, having the character of thin rods is a classic finger joint often not satisfactory in terms of strength and bending. Especially, it is true from the strength point of view, mainly for the product manufactured from such an infinitely long rod. Since the bonding strength increases with the size of a bonded area, one option to increase the bonding strength is to increase the finger profile. That option is limited by dimensions of the manufactured work piece and also the cutting tool design.

The solution of strength increase of finger via the increase of the size and complexity of the contact area is described in U.S. Pat. No. 5 985 415. The centre of solution lies in creation of large numbers of blind conical holes oriented along the longitudinal axis of the prism-seal element, to which conical protrusions formed at the opposite end of the adjacent prismatic element mesh together. Undoubtedly, the increase of size of the contact area will allow above mentioned solution. The problem, however, appears to be more complex and thus an expensive cutting tool, as well as high requirements for precision manufacturing such complex contact surfaces.

Disclosure of Invention

The finger joint eliminates largely mentioned disadvantages in accord to the invention, consisting of parallel teeth and grooves formed at the ends of the prismatic elements. The core lies in the fact that the tops of the teeth and/or bottoms of grooves lie in a plane intersecting with the plane perpendicular to the longitudinal axis of the prism element, thereby achieving the increase of a contact area of the finger joint.

Essence of the invention also lies in the fact that the plane, in which the tops of teeth lie and/or bottoms of grooves forming with the plane perpendicular to the longitudinal axis of the prism element is preferably an angle in the range from 10° to 80°, allowing the continuous production of infinitely long rods in automatic line production.

The advantage of the finger joint according to the invention is in comparison to the traditional finger joint that it allows a significant increase of strength of infinite long rods during bending. The joint strength can be adjusted, depending on the intended use of the product by selecting an appropriate inclination angle of the finger joint to plane perpendicular to longitudinal axis of an infinitely long rod.

Visually, in accord to the invention, the finger joint has an advantage that largely suppresses differences in the structure of the material between neighbouring prismatic elements, particularly in the case of an infinitely long rod produced from wood.

Brief Description of Drawings

The invention will be explained in detail by drawing, shown in figure 1 . It shows a perspective view of a prismatic element, the end of which formed teeth and a groove joint. Figure 2 shows a top view on an infinite long rod made up by use of the finger joint and figure 3 the view on such an infinite long rod from the side view.

Example of Carrying Out the Invention

Figure 1 shows a prismatic element 1, it has on both ends formed parallel teeth 2 and grooves 3 forming a finger joint V\_. Tops 21 of teeth 2 and bottoms 31 of grooves 3 lie in plane J3 parallel with a, perpendicular to longitudinal axis 4 of the prism element 1. The plane j3, in which the tops 21 of teeth 2 lie and bottoms 31 of grooves 3 as well, presses with the plane a perpendicular to longitudinal axis 4 of the prism element 1 is preferably the angle δ in the range from 10° to 80°. An infinitely long rod 5 made by use of the gear finger joint 11 connecting the adjacent prismatic elements 1 is shown in figure 2 and 3, where the angle δ has a size 45°.The contact area of finger joint H is approximately 40% larger than classic one formed in a plane perpendicular to the longitudinal axis 4 of the prism element 1 It follows that strength of such a finger joint 11 is higher about by 40%. Practical tests on strength bending, to which a wooden cylinder product made from an endless bar 5 made by use gear finger joint 11 was subjected, showed in accord to invention that the product never has not broken in the place of the gear finger joint 11, but always outside that place. Thus, the strength of the finger joint 11 in bending is higher than the strength of the original material. In addition, differences in the structure of the material between neighbouring prismatic elements after machining into a cylinder shape could not be visible.

Naturally, the invention is not limited only on above mentioned, shown in picture, however, in accord to technical documentation the solutions of finger joint in practical applications can differ in the scope of the invention and depends on the construction described in documentation. For instance, the shape of cross section of teeth and corresponding deep of grooves can completely differ and depend on the data mentioned in the documentation.

Industrial Applicability

The finger joint can be used everywhere, when is necessary to produce an endless rode formed by prism elements not only from wood, but also form similar materials.