Technical Field

The present disclosure relates to a joint system and to a method for forming such a joint system.

The joint system described herein finds particular application for the joining of wood or wood based materials, including laminated wood and wood based materials, such as glulam-type materials.

Background

In many applications it is desirable to join butt ends of wood pieces, timber, boards, poles, beams and/or planks in order to provide longer units.

One concept for such joining is known as "finger joints" and is disclosed in e.g. US33888020A and US3262723A.

In practice, wood pieces which are to be joined by a finger joint present a principal direction which is approximately parallel with a grain direction of the wood and a substantially rectangular cross section. Each wood piece thus presents a first pair of parallel and opposing side surfaces and a second pair of parallel and opposing side surfaces. The side surfaces are substantially parallel with the principal direction. The first and second pairs of surfaces are mutually orthogonal relative to each other. Each wood piece also presents an end surface, which is parallel with the cross section and orthogonal to both pairs of side surfaces.

A traditional finger joint is recognized as a comb- or saw tooth-like pattern of alternating protrusions and notches on one of the side surfaces, termed the first side surface. This pattern extends throughout the wood piece orthogonally from the side surface. When seen from the adjacent side surface, i.e. one of the second side surfaces, the finger joint will generally be perceived as a straight joint which extends orthogonally through the wood piece. Conventional finger joints have their protrusions and notches extending parallel with each other and transversely at right angles to two parallel side walls in the timber or plank.

There is a general desire to further increase the strength of finger joints. One concept for increasing the strength is disclosed in US3692340A. Yet another concept is disclosed in US2003026955A.

Summary

An object of the present disclosure is to provide an improved joint for wood and wood based materials.

The invention is defined by the appended independent claims.

Embodiments are set forth in the dependent claims, in the following description and in the attached drawings.

According to a first aspect, there is provided a joint system comprising first and second members formed of wood or wood based material, the members being joined in an end-to-end relationship along a longitudinal direction by means of a finger joint. An end portion of the first member presents a plurality of parallel straight finger-like projections, which enter and engage a plurality of complementary parallel straight notches provided between projections on the second member. A plane comprising substantially the entire length of each projection tips, or a plane comprising substantially the entire length of each notch bottom, extends at an angle other than a right angle to the longitudinal direction.

A longitudinal direction may be a direction of major extension of the members. For members having a rectangular or square cross section, the longitudinal direction may be parallel with the side surfaces of the members.

A "wood based material" should be understood as a material comprising wood fibers. Such materials may have anisotropic strength properties, as would be the case where the material exhibits a principal fiber direction, which is substantially parallel with the longitudinal direction.

By decreasing the angle of the plane(s) to the longitudinal direction, the effective contact surface between the grooves and notches may be

increased, which provides a stronger finger joint. The invention is applicable both in end-to-end joining of wood and glulam materials in order to provide beams, planks or poles which are longer than the individual material pieces forming the beam or pole.

The invention is also applicable when joining together ends of one material piece, from which a section has been cut away, as would be the case when removing for example a portion having a defect, such as a knot or crack or other weak point.

Hence, a wood piece, timber piece boards, pole, beam or plank may be provided presenting a joint system as described herein.

The invention finds particular application in the joining of wood and wood based materials into elongate structures, such as beams, poles, planks, strips or moldings, which have a major direction that extends along the longitudinal direction.

In most applications, this means that the plane(s) will extend at an angle other than a right angle to at least one side face of the members, or to a pair of opposing side faces.

The plane(s) may extend at right angle to one side face, or to a pair of opposing side faces.

The plane may extend at an angle of 80° or less, 70° or less, 60° or less, 50°, 45° or less, 40° or less, or 30° or less, to the longitudinal direction.

The plane(s) may extend at an angle of 10° or more, 20° or more, 30° or more or 40° or more, to the longitudinal direction.

The members may have a major fiber direction which substantially coincides with the longitudinal direction.

The projections may be tapered in cross section as seen from the member and outwardly along the longitudinal direction.

A glue or adhesive may be applied to the surfaces of at least parts of the projections and/or notches.

At least one of the members may be formed of solid wood.

At least one of the members may be formed of at least two laminated material pieces with a lamination plane extending parallel to the longitudinal direction. The members may present four major side faces, all of which being substantially parallel with the longitudinal direction.

The invention may be embodied in an elongate member of wood or wood based material comprising at least one joint system as described above. Such a wood or wood based member may be a beam, pole, planks, strip or molding. The elongate member may be of solid wood or laminated wood, such as glulam, where a lamination plane may be substantially parallel with the longitudinal direction.

According to a second aspect, there is provided a method of making a joint system. The method comprises providing a first member formed of wood or wood based material and forming a plurality of parallel, straight notches, which run across an end portion of the first member, whereby a plurality of parallel straight finger-like projections are formed. The method further comprises forming the notches such that a plane comprising substantially the entire length of each protrusion tip, or a plane comprising substantially the entire length of each notch bottom, extends at an angle other than a right angle to the longitudinal direction.

The method may further comprise providing a second member formed of wood or wood based material, forming a plurality of parallel, straight notches, which run across an end portion of the second member, whereby a plurality of parallel straight finger-like projections are formed, and joining the first and second members by causing the projections of the first member to enter and engage the notches of the second member and vice versa. In this method, the notches of the second member may be formed such that a plane comprising substantially the entire length of each protrusion tip, or a plane comprising substantially the entire length of each notch bottom, extends at an angle other than a right angle to the longitudinal direction.

The method may further comprise forming an end plane at the end portion of the first and/or second member, at an angle other than a right angle to the longitudinal direction and subsequently forming the notches in the end plane. Such forming may include a cutting operation performed during a movement of the cutter in a direction parallel with the end plane.

The end plane may be formed prior to the forming of the notches. The method may further comprise providing the first member by arranging first and second laminated material pieces such that the second material piece extends further than the first material piece by a length substantially corresponding to T ^* tan(a), where T is the thickness of the second material piece, and subsequently cutting the first member along a cutting plane which substantially tangents an end part of the first member at a line directly adjacent the second member.

Hence, the loss of material can be reduced when joining ends of so- called glulam materials.

Brief Description of the Drawings

Fig. 1 is a schematic perspective view of a pair of joined pieces 1 , 2 of wood based material.

Fig. 2 is a schematic side view of the pair of joined pieces 1 , 2 of wood based material in Fig. 1.

Fig. 3 is a schematic top view of an end portion of a piece 1 of wood based material.

Fig. 4 is a schematic top view of a pair of end portions 1 , 2 in a nearly joined state.

Fig. 5 is a schematic perspective view of a pair of joined pieces 1', 2' of laminated material.

Fig. 6 is a schematic perspective view of a pair of joined pieces 1", 2" of laminated material.

Figs 7a-7d schematically illustrate a method of producing a finger joint. Fig. 8 is a schematic side view of a laminated piece of material.

Fig. 9 is a graph showing test results of embodiments according to the present disclosure.

Description of Embodiments

Fig. 1 is a schematic three dimensional view of a pair of joined-together pieces of material 1 , 2. Fig. 1 also illustrates the coordinate system which will be used in the description below. As can be seen from Fig. 1 , the pieces of material 1 , 2 have been joined by a so-called finger joint 3. The pieces of material 1 , 2 present a respective major side surface 11a, 11b; 21a, 21b and a respective minor side surface 12a, 12b; 22a, 22b.

The pieces of material, 1 , 2 are joined to form a common member which extends along a longitudinal direction L.

The pieces of material may have a substantially rectangular or square cross section, optionally with rounded corners. Other cross sections are possible, such as circular, elliptic, prismatic, polygonal, etc.. the cross section may be substantially constant along the longitudinal direction L.

Referring to Fig. 2, it is noted that the joint 3 presents a joint angle a, which is less than 90° relative to the major side surface 11a, 21a.

Referring to Fig. 3, an end portion of the first member 1 presents a plurality of alternating arranged finger-like projections 31 and notches 32. The projections 31 present a respective side surface 311 , 313 and a tip surface 312. At the bottom of each notch 32, there is a notch bottom surface 321.

The projections 31 form parallel ridges between which parallel notches are formed.

The projections 31 may be formed such that all projections extend substantially equally far in the longitudinal direction L, such that the tips or tip surfaces 312 define a common tip plane Pt.

The notches 32 may be substantially equally deep in the longitudinal direction L, such that the notch bottom surfaces 321 also define a common notch plane Pn.

In the embodiment shown in Fig. 3, the tip plane Pt and the notch plane Pn are substantially parallel and extend at a joint angle a, relative to the major surface 1 a, 21a (and the longitudinal direction L), which is less than 90°. Moreover the planes Pt, Pn are substantially orthogonal to the minor side surface 12a, 22a.

The joint angle a may be 80° or less, 70° or less, 60° or less, 50°, 45° or less, 40° or less, or 30° or less to the major surface 11a, 21a. Moreover, the joint angle a may be 10° or more, 20° or more, 30° or more or 40° or more to the major surface 11a, 21a. In particular embodiments, the joint angle a may be on the order of 20°-40°, 40°-50° or 50°-70°.

The protrusions 31 are further designed, as is conventional, with respect to notch depth Dn, tip width Wt, notch bottom width Wn and side angle b.

Fig. 4 schematically illustrates the joining together of a first 1 and a second 2 member along the longitudinal direction L. In connection with the : joining, a glue may be provided on the side surfaces 3 1 , 313 of the ridges and notches and optionally also on the tip and bottom surfaces 312, 321.

The members may be forced towards each other while the glue is activated. Activation of the glue may be achieved by e.g. radiation, heating or vibrations.

Fig. 5 schematically illustrates the joining of a pair laminated members 1', 2'. In this embodiment, material pieces 13a, 13b, 13c, 13d to be laminated have their major surfaces being laminated to each other. The finger joint 3 is provided such that the joint angle a extends at an angle a other than a right angle to the major surfaces of the material pieces 13a, 13b, 13c, 13d and at a right angle to the minor surfaces of the material pieces 13a, 13b, 13c, 13d.

Fig. 6 schematically illustrates the joining of a pair of laminated members 1", 2". In this embodiment, material pieces 13a', 13b', 13c', 13d' to be laminated have their.major surfaces laminated to each other. The finger joint 3 is provided such that the joint angle a extends at an angle a other than a right angle to the minor surfaces of the material pieces 13a', 13b', 13c', 13d' and at a right angle to the major surfaces of the material pieces 13a', 13b', 13c', 13d'.

Referring to Figs 7a-7d, a joint 3 as disclosed herein may be produced according to the following.

As shown in Fig. 7a, a pair of members , 2 to be joined are provided. The members 1 , 2 may be in the form of relatively short pieces of material, e.g. on the order of 0.5-2 m in length or relatively long pieces of material, e.g. on the order of 2-5 m in length. As shown in Fig. 7b, a part of the end portion of each member 1 , 2 is then cut away, such that end planes 30 are provided, presenting an angle a corresponding to the joint angle of the finger joint that is to be produced.

As shown in Fig. 7c, notches 32 are then formed in the end surfaces 30. The notches may be formed by a cutting operation using a multi-tipped cutter tool (not shown), which is caused to move along the end plane, i.e. linearly along a path which makes the angle a relative to the longitudinal direction L.

After forming the notches, glue may be applied to all or parts of the side surfaces of the protrusions 31 and notches 32.

As shown in Fig. 7d, the members 1 , 2 are brought together along the longitudinal direction L and are optionally forced towards each other during at least part of the time it takes for the glue to cure.

While the above method is applicable to both solid material pieces (such as solid wood) and to laminated members (such as glulam), Fig. 8 schematically illustrates a method which may be used in the joining of laminated members.

As mentioned before, each laminated member may be formed of a plurality of material pieces 13a, 13b, 13c, 13d. The material pieces may be arranged such that a second material piece 13b extends further along the longitudinal direction than a first material piece 13a by a length E corresponding to about T ^* tan(a), where T is a thickness of the second material piece 13b. If the laminated member comprises further material 13c, 13d pieces, each subsequent material piece may thus be arranged to protrude beyond the previous one in a similar manner as the second material piece 13b.

The cutting operation discussed with respect to Fig. 7b above may then be performed along a plane which tangents intersections between the end side of the first member and the major side of the second member, such that substantially only triangular portions 14a, 14b, 14c, 14d are cut away.

The finger joint 3 may then be formed as described with respect to Figs

7c-7d. With the present invention the effective glue surface may be increased as compared to the prior art finger joints having an orthogonal joint angle. Hence, the amount of glue used will increase.

Referring to Fig. 9, the left bar represents a prior art 90° joint plane. The middle bar represents a 60° joint plane which provided a 13 % increase in bending strength at a 15 % increase in adhesive cost. The right bar represents a 45° joint plane which provided a 49 % increase in bending strength at a 41 % increase in adhesive cost.