Abstract:
A locking system for mechanical joining of floorboards, each of the floorboards comprising a body comprising plywood with several layers; a locking groove which is formed in an underside of and extends in parallel with a first joint edge at a distance from the joint plane, the locking groove having an opening, a bottom, and two side walls; a portion projecting from a lower part of the second joint edge and below the first joint edge and integrated with the body of the board; said projecting portion supporting, at a distance from the joint plane, a locking element for cooperating with the locking groove; said projecting portion being located entirely outside the joint plane as seen from the side of the second joint edge; and the walls of the locking groove comprise at least two layers of the body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation of Ser. No. 10/906,109, filed on Feb. 3, 2005, and which is a continuation-in-part of Ser. No. 10/361,815, which is a continuation of Ser. No. 10/100,032, which is a continuation of Ser. No. 09/679,300, which is a continuation of PCT/SE99/00934. The entire contents of Ser. No. 10/361,815, Ser. No. 10/100,032, Ser. No. 09/679,300, and PCT/SE99/00934 are incorporated herein by reference.  
         [0002]     The invention generally relates to a locking system for providing mechanical joining of floorboards. More specifically, the invention concerns an improvement of a locking system of the type described and shown in WO 94/26999. The invention also relates to a floorboard provided with such a locking system. According to one more aspect of the invention, a floorboard with different designs of the locking system on long side and short side is provided. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The invention is particularly suited for mechanical joining of thin floating floorboards, such as laminate and parquet flooring, and therefore the following description of prior art and the objects and features of the invention will be directed to this field of application, in particular rectangular floorboards that are joined on long sides as well as short sides. The features distinguishing the invention concern in the first place parts of the locking system which are related to horizontal locking transversely of the joint edges of the boards. In practice, floorboards will be manufactured according to the inventive principles of also having locking means for mutual vertical locking of the boards.  
       BACKGROUND ART  
       [0004]     WO 94/26999 discloses a locking system for mechanical joining of building boards, especially floorboards. A mechanical locking system permits locking together of the boards both perpendicular to and in parallel with the principal plane of the boards on long sides as well as short sides. Methods for making such floorboards are described in SE 9604484-7 and SE 9604483-9. The principles of designing and laying the floorboards as well as the methods for making the same that are described in the above three documents are applicable also to the present invention, and therefore the contents of these documents are incorporated by reference in present description.  
         [0005]     With a view to facilitating the understanding and description of the present invention as well as the understanding of the problems behind the invention, now follows with reference to  FIGS. 1-3  a brief description of floorboards according to WO 94/26999. This description of prior art should in applicable parts be considered to apply also to the following description of embodiments of the present invention.  
         [0006]     A floorboard  1  of known design is shown from below and from above in  FIGS. 3   a  and  3   b , respectively. The board is rectangular and has a top side  2 , an underside  3 , two opposite long sides  4   a ,  4   b  which form joint edges, and two opposite short sides  5   a ,  5   b  which form joint edges.  
         [0007]     Both the long sides  4   a ,  4   b  and the short sides  5   a ,  5   b  can be joined mechanically without any glue in the direction D 2  in  FIG. 1   c . To this end, the board  1  has a planar strip  6  which is mounted at the factory and which extends horizontally from one long side  4   a , the strip extending along the entire long side  4   a  and being made of a flexible, resilient aluminum sheet. The strip  6  can be mechanically fixed according to the illustrated embodiment, or fixed by means of glue or in some other fashion. Other strip materials can be used, such as sheet of some other metal, and aluminum or plastic sections. Alternatively, the strip  6  can be integrally formed with the board  1 , for instance by some suitable working of the body of the board  1 . The strip, however, is always integrated with the board  1 , i.e. it is not mounted on the board  1  in connection with laying. The width of the strip  6  can be about 30 mm and its thickness about 0.5 mm. A similar, although shorter strip  6 ′ is arranged also along one short side  5   a  of the board  1 . The edge side of the strip  4  facing away from the joint edge  4   a  is formed with a locking element  8  extending along the entire strip  6 . The locking element  8  has an active locking surface  10  facing the joint edge  4   a  and having a height of e.g. 0.5 mm. In connection with laying, the locking element  8  cooperates with a locking groove  14 , which is formed in the underside  3  of the opposite long side  4   b  of an adjacent board  1 ′. The short side strip  6 ′ is provided with a corresponding locking element  8 ′, and the opposite short side  5   b  has a corresponding locking groove  14 ′.  
         [0008]     For mechanical joining of both long sides and short sides also in the vertical direction (direction D 1  in  FIG. 1   c ), the board  1  is further along its one long side  4   a  and its one short side  5   a  formed with a laterally open recess  16 . The recess  16  is defined downwards by the associated strip  6 ,  6 ′. At the opposite edges  4   b  and  5   b  there is an upper recess  18  defining a locking tongue  20  (see  FIG. 2   a ) cooperating with the recess  16  to form a tongue-and-groove joint.  
         [0009]      FIGS. 1   a - 1   c  show how two such boards  1 ,  1 ′ can be joined by downwards angling.  FIGS. 2   a - 2   c  show how the boards  1 ,  1 ′ can instead be joined by snap action. The long sides  4   a ,  4   b  can be joined by both methods whereas the short sides  5   a ,  5   b —after laying of the first row—are normally joined after joining of the long sides and merely by snap action. When a new board  1 ′ and a previously laid board  1  are to be joined along their long sides according to  FIGS. 1   a - 1   c , the long side  4   b  of the new board  1 ′ is pressed against the long side  4   a  of the previously laid board  1  according to  FIG. 1   a , so that the locking tongue  20  is inserted into the recess  16 . The board  1 ′ is then angled downwards to the subfloor  12  according to  FIG. 1   b . Now the locking tongue  20  completely enters the recess  16  while at the same time the locking element  8  of the strip  6  enters the locking groove  14 . During this downwards angling, the upper part of the locking element  8  can be active and accomplish a guiding of the new board  1 ′ towards the previously laid board  1 . In the joined state according to  FIG. 1   c , the boards  1 ,  1 ′ are locked in both D 1  direction and D 2  direction, but may be displaced relative to each other in the longitudinal direction of the joint.  
         [0010]      FIGS. 2   a - 2   c  illustrate how also the short sides  5   a  and  5   b  of the boards  1 ,  1 ′ can be mechanically joined in both D 1  and D 2  direction by the new board  1 ′ being moved essentially horizontally towards the previously laid board  1 . This can be carried out after the long side  4   b  of the new board  1 ′ has been joined as described above. In the first step in  FIG. 2   a , bevelled surfaces adjacent to the recess  16  and the locking tongue  20  cooperate so that the strip  6 ′ is forced downwards as a direct consequence of the joining of the short sides  5   a ,  5   b . During the final joining, the strip  6 ′ snaps upwards as the locking element  8 ′ enters the locking groove  14 ′. By repeating the operations shown in  FIGS. 1 and 2 , the entire floor can be laid without glue and along all joint edges. Thus, prior-art floorboards of the above-mentioned type are joined mechanically by, as a rule, first being angled downwards on the long side, and when the long side is locked, the short sides are snapped together by horizontal displacement along the long side. The boards  1 ,  1 ′ can be taken up again in reverse order, without the joint being damaged, and be laid once more.  
         [0011]     For optimal function, it should be possible for the boards, after being joined, along their long sides to take a position where there is a possibility of a small play between the locking surface  10  and the locking groove  14 . For a more detailed description of this play, reference is made to WO 94/26999.  
         [0012]     In addition to the disclosure of the above-mentioned patent specifications, Norske Skog Flooring AS (licensee of Valinge Aluminum AB) introduced a laminate flooring with a mechanical joining system according to WO 94/29699 in January 1996 in connection with the Domotex fair in Hannover, Germany. This laminate flooring marketed under the trademark Alloc®, is 7.6 mm thick, has a 0.6 mm aluminum strip  6  which is mechanically fixed to the tongue side and the active locking surface  10  of the locking element  8  has an inclination of about 70°-80° to the plane of the board. The joint edges are impregnated with wax and the underside is provided with underlay board which is mounted at the factory. The vertical joint is designed as a modified tongue-and-groove joint. The strips  6 ,  6 ′ on long side and short side are largely identical, but slightly bent upwards to different degrees on long side and short side. The inclination of the active locking surface varies between long side and short side. The distance of the locking groove  14  from the joint edge, however, is somewhat smaller on the short side than on the long side. The boards are made with a nominal play on the long side which is about 0.05-0.10 mm. This enables displacement of the long sides and bridges width tolerances of the boards. Boards of this brand have been manufactured and sold with zero play on the short sides, which is possible since the short sides need not be displaced in connection with the locking which is effected by snap action. Boards of this brand have also been made with more beveled portions on the short side to facilitate snapping in according to  FIGS. 2   a - c  above. It is thus known that the mechanical locking system can be designed in various ways and that long side and short side can be of different design.  
         [0013]     WO 97/47834 (Unilin) discloses a mechanical joining system which is essentially based on the above known principles. In the corresponding product which this applicant began to market in the latter part of 1997, biasing between the boards is strived for. This leads to high friction and difficulties in angling together and displacing the boards. This document also shows that the mechanical locking on the short side can be designed in a manner different from the long side. In the described embodiments, the strip is integrated with the body of the board, i.e. made in one piece with and of the same material as the body of the board.  
       SUMMARY  
       [0014]     Although the flooring according to WO 94/26999 and the flooring marketed under the trademark Alloc® have great advantages compared with traditional, glued floorings, further improvements are desirable.  
         [0015]     Mechanical joints are very suitable for joining not only laminate floorings, but also wood floorings and composite floorings. Such floorboards may consist of a large number of different materials in the surface, the core and the rear side, and as described above these materials can also be included in the strip of the joining system, the locking element on the strip, fixing surfaces, vertical joints etc. This solution involving an integrated strip, however, leads to costs in the form of waste when the mechanical joint is being made. Alternatively, special materials, such as the aluminum strip  6  above, can be glued or mechanically fixed to the floorboard to be included as components in the joining system. Different joint designs affect the costs to a considerable extent.  
         [0016]     A strip made of the same material as the body of the board and formed by working of the body of the board can in some applications be less expensive than an aluminum strip, especially for floorboards in lower price ranges. Aluminum, however, is more advantageous in respect of flexibility, resilience and displaceability as well as accuracy in the positioning of the locking element. Aluminum also affords the possibility of making a stronger locking element. If the same strength is to be achieved with a locking element of wood fiber, it must be wide with a large shearing surface, which results in a large amount of waste material in manufacture, or it must be reinforced with a binder. Depending on the size of the boards, working of, for instance, 10 mm of a joint edge may result in six times higher cost of waste per m 2  of floor surface along the long sides compared with the short sides.  
         [0017]     In addition to the above problems relating to undesirable waste of material, the present invention is based on the insight that the long sides and short sides can be optimized with regard to the specific locking functions that should be present in these joint edges.  
         [0018]     As described above, locking of the long side is, as a rule, carried out by downwards angling. Also a small degree of bending down of the strip during locking can take place, as will be described in more detail below. Thanks to this downwards bending together with an inclination of the locking element, the boards can be angled down and up again with very tight joint edges. The locking element along the long sides should also have a high guiding capability so that the long side of a new board in connection with downwards angling is pushed towards the joint edge of the previously laid board. The locking element should have a large guiding part. For optimal function, the boards should along their long sides, after being joined, be able to take a mutual position transversely of the joint edges where there is a small play between locking element and locking groove.  
         [0019]     On the other hand, locking of the short side is carried out by the long side being displaced so that the strip of the short side can be bent down and snap into the locking groove. Thus the short side must have means which accomplish downwards bending of the strip in connection with lateral displacement. The strength requirement is also higher on the short side. Guiding and displaceability are less important.  
         [0020]     Summing up, there is a great need for providing a mechanical joint of the above type at a low cost and with optimal locking functions at each joint edge. It is not possible to achieve a low cost with prior-art solutions without also lowering the requirements as to strength and/or laying function. An object of the invention is to provide solutions which aim at lowering the cost with maintained strength and function.  
         [0021]     According to a first aspect of the invention, a locking system for mechanical joining of floorboards is thus provided, where immediately juxtaposed upper parts of two adjacent joint edges of two joined floorboards together define a joint plane perpendicular to the principal plane of the floor boards. To obtain a joining of the two joint edges perpendicular to the joint plane, the locking system comprises in a manner known per se a locking groove which is formed in the underside of and extends in parallel with the first joint edge at a distance from the joint plane, and a portion projecting from the lower part of the second joint edge and below the first joint edge and integrated with a body of the board, said projecting portion supporting at a distance from the joint plane a locking element cooperating with the locking groove and thus positioned entirely outside the joint plane seen from the side of the second joint edge, said projecting portion having a different composition of materials compared with the body of the board. The inventive locking system is characterized in that the projecting portion presents at least two horizontally juxtaposed parts, which differ from each other at least in respect of the parameters material composition and material properties.  
         [0022]     In a first embodiment of the first aspect of the invention, said at least two parts of the projecting portion are located at different distances from the joint plane. In particular, they may comprise an inner part closest to the joint plane and an outer part at a distance from the joint plane. The inner part and the outer part are preferably, but not necessarily, of equal length in the joint direction. In this first aspect of the invention, a material other than that included in the body is thus included in the joining system, and in particular the outer part can be at least partially formed of a separate strip which is made of a material other than that of the body of the board and which is integrally connected with the board by being factory-mounted. The inner part can be formed at least partially of a worked part of the body of the board and partially of part of said separate strip. The separate strip can be attached to such a worked part of the board body. The strip can be located entirely outside said joint plane, but can also intersect the joint plane and extend under the joint edge to be attached to the body also inside the joint plane.  
         [0023]     This embodiment of the invention thus provides a kind of combination strip in terms of material, for example a projecting portion comprising an inner part with the material combination wood fiber/rear laminate/aluminum, and an outer part of aluminum sheet.  
         [0024]     It is also possible to make the projecting part from three parts which are different in terms of material: an inner part closest to the joint plane, a central part and an outer part furthest away from the joint plane. The inner part and the outer part can possibly be equal in terms of material.  
         [0025]     The portion projecting outside the joint plane need not necessarily be continuous or unbroken along the joint edge. A conceivable variant is that the projecting portion has a plurality of separate sections distributed along the joint edge. As an example, this can be accomplished by means of a separate strip with a continuous inner part and a toothed outer part, said strip being attachable to a part of the board body, said part being worked outside the joint plane.  
         [0026]     In an alternative embodiment of the first aspect of the invention, said at least two parts, which differ in respect of at least one of the parameters material composition and material properties, are instead juxtaposed seen in the direction parallel with the joint edges. For example, there may be a plurality of strip types on one and the same side, where each strip type is optimized for a special function, such as strength and guiding in connection with laying. As an example, the strips can be made of different aluminum alloys and/or of aluminum having different states (for instance, as a result of different types of heat treatment).  
         [0027]     According to a second aspect of the invention, a locking system for mechanical joining of floorboards is provided. In this second aspect of the invention, the projecting portion is instead formed in one piece with the body of the board and thus has the same material composition as the body of the board. This second aspect of the invention is characterized in that the projecting portion, as a direct consequence of machining of its upper side, presents at least two horizontally juxtaposed parts, which differ from each other in respect of at least one of the parameters material composition and material properties.  
         [0028]     The inventive principle of dividing the projecting portion into several parts which differ from each other in terms of material and/or material properties thus is applicable also to the prior-art “wood fiber strip”.  
         [0029]     In the same manner as described above for the first aspect of the invention, these two parts can be located at different distances from the joint plane, and especially there may be three or more parts with different material composition and/or material properties. Optionally, two such parts can be equal in respect of said parameters, but they may differ from a third.  
         [0030]     In one embodiment, said two parts may comprise an inner part closest to the joint plane and an outer part at a distance from the joint plane. There may be further parts outside the outer part. Specifically, an outer part can be formed of fewer materials than an inner part. For instance, the inner part may consist or wood fiber and rear laminate, whereas the outer part, by machining from above, consists of rear laminate only. In one embodiment, the projecting portion may comprise—seen from the joint plane outwards—an inner part, an outer part and, outside the outer part, a locking element supported by the outer part. The locking element may differ from both inner and outer part in respect of said material parameters.  
         [0031]     The projecting portion may consist of three laminated layers, and therefore it is possible, by working from above, to provide a locking system which, counted from the top, has a relatively soft upper guiding part which need not have any particular strength, a harder central part which forms a strong active locking surface and absorbs shear forces in the locking element, and a lower part which is connected with the rest of the projecting portion and which can be thin, strong and resilient.  
         [0032]     Laminated embodiments can be suitable in such floorboards where the body of the board consists of, for instance, plywood or particle board with several layers. Corresponding layers can be found in the walls of the locking groove. For plywood, the material properties can be varied by changing the direction of fibers in the layers. For particle board, the material properties can be varied by using different chip dimensions and/or a binder in the different layers. The board body can generally consist of layers of different plastic materials.  
         [0033]     In the definition of the invention, the term “projecting portion” relates to the part or parts of the board projecting outside the joint plane and having a function in the locking system in respect of supporting of locking element, strength, flexibility etc.  
         [0034]     An underlay of underlay board, foam, felt or the like can, for instance, be mounted even in the manufacture of the boards on the underside thereof. The underlay can cover the underside up to the locking element, so that the joint between the underlays will be offset relative to the joint plane F. Although such an underlay is positioned outside the joint plane, it should thus not be considered to be included in the definition of the projecting portion in the appended claims.  
         [0035]     In the aspect of the invention which relates to embodiments with a projecting portion of the same material as the body of the board, any thin material layers which remain after working from above should in the same manner not be considered to be included in the “projecting portion” in the cases where such layers do not contribute to the locking function in respect of strength, flexibility, etc. The same discussion applies to thin glue layers, binders, chemicals, etc. which are applied, for instance, to improve moisture proofing and strength.  
         [0036]     According to a third aspect of the invention, there is provided a floorboard presenting a locking system according to the first aspect or the second aspect of the invention as defined above. Several possibilities of combining prior-art separate strips, prior-art wood fiber strips and “combination strips” according to the invention are available. These possibilities can be used optionally on long side and short side.  
         [0037]     For the above aspects, the projecting portion of a given joint edge, for instance a long side, has at least two parts with different material composition and/or material properties. For optimization of a floorboard, such a difference in materials and/or material properties, however, may be considered to exist between the long sides and short sides of the board instead of within one and the same joint edge.  
         [0038]     According to a fourth aspect of the invention, a rectangular floorboard is thus provided, comprising a body and first and second locking means integrated with the body and adapted to provide a mechanical joining of adjacent joint edges of such floorboards along long sides and short sides, respectively, of the boards in a direction perpendicular to the respective joint edges and in parallel with the principal plane of the floorboards. According to this aspect of the invention, the floorboard is characterized in that said first and second locking means differ in respect of at least one of the parameters material composition and material properties. Preferably, said first and second locking means each comprise on the one hand a portion which projects from a joint edge and which at a distance from the joint edge supports a locking element and, on the other hand, a locking groove, which is formed in the underside of the body at an opposite joint edge for engaging such a locking element of an adjacent board. At least one of said locking means on the long side and the short side may comprise a separate element which is integrally fixed to the body of the board at the factory and is made of a material other than that included in the body of the board. The other locking means may comprise an element which is formed in one piece with the body of the board.  
         [0039]     Within the scope of the fourth aspect of the invention, there are several possibilities of combination. For example, it is possible to select an aluminum strip for the long side and a machined wood fiber strip for the short side or vice versa. Another example is that for the short side or the long side a “combination strip” according to the first and the second aspect of the invention is selected, and for the other side a “pure” aluminum strip or a “pure” worked wood fiber strip is selected.  
         [0040]     The above problem of undesirable costs of material is solved according to the invention by the projecting portion being made of different materials and/or material combinations and thus specially adaptable to the selected materials in the floorboard and the function and strength requirements that apply to the specific floorboard and that are specific for long side and short side. This advantage of the invention will be evident from the following description.  
         [0041]     Since different requirements are placed on the long side and the short side and also the cost of waste differs, improvements can also be achieved by the long side and the short side being made of different materials or combinations of materials. In some applications, the long side can have, for instance, an aluminum strip with high guiding capability and low friction whereas the short side can have a wood fiber strip. In other applications, the opposite is advantageous.  
         [0042]     In some applications, there may also be a need for different types of strip on the same side. The side may consist of, for instance, a plurality of different strips which are made of different aluminum alloys, have different thicknesses etc. and in which certain parts are intended to achieve high strength and others are intended to be used for guiding.  
         [0043]     Different aspects of the invention will now be described in more detail by way of examples with reference to the accompanying drawings. The parts of the inventive board which are equivalent to those of the prior-art board in.  FIGS. 1-3  are provided with the same reference numerals. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0044]      FIGS. 1   a - c  illustrate in three steps a downwards angling method for mechanical joining of long sides of floorboards according to WO 94/26999.  
         [0045]      FIGS. 2   a - c  illustrate in three steps a snap-in method for mechanical joining of short sides of floorboards according to WO 4/26999.  
         [0046]      FIGS. 3 and 3   b  show a floorboard according to WO 94/26999 seen from above and from below, respectively.  
         [0047]      FIG. 4  shows a floorboard with a locking system according to a first embodiment of the invention.  
         [0048]      FIG. 5  is a top plan view of a floorboard according to  FIG. 4 .  
         [0049]      FIG. 6   a  shows on a larger scale a broken-away corner portion C 1  of the board in  FIG. 5 , and  
         [0050]      FIGS. 6   b  and  6   c  are vertical sections of the joint edges along the long side  4   a  and the short side  5   a  of the board in  FIG. 5 , from which it is particularly evident that the long side and the short side different.  
         [0051]      FIGS. 7   a - c  show a downwards angling method for mechanical joining of long sides of the floorboard according to  FIGS. 4-6 .  
         [0052]      FIG. 8  shows two joined floorboards provided with a locking system according to a second embodiment of the invention.  
         [0053]      FIG. 9  shows two joined floorboards provided with a locking system according to a third embodiment of the invention.  
         [0054]      FIGS. 10-12  illustrate three different embodiments of floorboards according to the invention where the projecting portion is formed in one piece with the body of the board. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0055]     A first preferred embodiment of a floorboard  1  provided with a locking system according to the invention will now be described with reference to  FIGS. 4-7 . The shown example also illustrates the aspect of the invention which concerns differently designed locking systems for long side and short side.  
         [0056]      FIG. 4  is a cross-sectional view of a long side  4   a  of the board  1 . The body of the board  1  consists of a core  30  of, for instance, wood fiber which supports a surface laminate  32  on its front side and a balance layer  34  on its rear side. The board body  30 - 34  is rectangular with long sides  4   a ,  4   b  and short sides  5   a ,  5   b . A separate strip  6  with a formed locking element  8  is mounted at the factory on the body  30 - 34 , so that the strip  6  constitutes an integrated part of the completed floorboard  1 . In the shown example, the strip  6  is made of resilient aluminum sheet. As an illustrative, non-limiting example, the aluminum sheet can have a thickness in the order of 0.6 mm and the floorboard a thickness in the order of 7 mm. For further description of dimensions, possible materials, etc. for the strip  6 , reference is made to the above description of the prior-art board.  
         [0057]     The strip  6  is formed with a locking element  8 , whose active locking surface  10  cooperates with a locking groove  14  in an opposite joint edge  4   b  of an adjacent board  1 ′ for horizontal locking together of the boards  1 ,  1 ′ transversely of the joint edge (D 2 ). With a view to forming a vertical lock in the D 1  direction, the joint edge  4   a  has a laterally open groove  36  and the opposite joint edge  4   b  has a laterally projecting tongue  38  (corresponding to the locking tongue  20 ), which in the joined state is received in the groove  36  ( FIG. 7   c ). The free surface of the upper part  40  of the groove  36  has a vertical upper portion  41 , a bevelled portion  42  and an upper abutment surface  43  for the tongue  38 . The free surface of the lower part  44  of the groove  36  has a lower abutment surface  45  for the tongue  38 , a bevelled portion  46  and a lower vertical portion  47 . The opposite joint edge  4   b  (see  FIG. 7   a ) has an upper vertical portion  48 , and the tongue  38  has an upper abutment surface  49 , an upper bevelled portion  50 , a lower bevelled portion  51  and a lower abutment surface  52 .  
         [0058]     In the joined state ( FIG. 7   c ), the two juxtaposed vertical upper portions  41  and  48  define a vertical joint plane F. As is best seen from  FIG. 4 , the lower part  44  of the groove  36  is extended a distance outside the joint plane F. The joint edge  4   a  is in its underside formed with a continuous mounting groove  54  having a vertical lower gripping edge  56  and an inclined gripping edge  58 . The gripping edges formed of the surfaces  46 ,  47 ,  56 ,  58  together define a fixing shoulder  60  for mechanical fixing of the strip  6 . The fixing is carried out according to the same principle as in the prior-art board and can be carried out by means of the methods that are described in the above-mentioned documents. A continuous lip  62  of the strip  6  thus is bent round the gripping edges  56 ,  58  of the groove  54 , while a plurality of punched tongues  64  are bent round the surfaces  46 ,  47  of the projecting portion  44 . The tongues  64  and the associated punched holes  65  are shown in the broken-out view in  FIG. 6   a.    
         [0059]     There is a significant difference between the inventive floorboard shown in  FIGS. 4-7  and the prior-art board according to  FIGS. 1-3 . The area P in  FIG. 4  designates the portion of the board  1  which is positioned outside the joint plane  1 . According to the invention, the portion P has two horizontally juxtaposed parts P 1  and P 2 , which differ in respect of at least one of the parameters material composition and material properties. More specifically, the inner part P 1  is, closest to the joint plane F, formed partially of the strip  6  and partially of the worked part  44  of the body. In this embodiment, the inner part P 1  thus comprises the material combination aluminum+wood fiber core+rear laminate whereas the outer part P 2  is a made of aluminum only. In the prior-art board  1  in  FIGS. 1   a - c , the corresponding portion outside the joint plane is made of aluminum only.  
         [0060]     As described above, this feature means that the cost of material can be reduced. Thanks to the fact that the fixing shoulder  60  is displaced towards the locking element  8  to such an extent that it is positioned at least partially outside the joint plane F, a considerable saving can be achieved in respect of the consumption of aluminum sheet. A saving in the order of 25% is possible. This embodiment is particularly advantageous in cheaper floorboards where waste of wood fiber as a result of machining of the body is preferred to a high consumption of aluminum sheet. The waste of material, however, is limited thanks to the fact that the projecting portion can also be used as abutment surface for the tongue, which can then be made correspondingly narrower perpendicular to the joint plane with the ensuing reduced waste of material on the tongue side.  
         [0061]     This constructional change to achieve saving in material does not have a detrimental effect on the possibility of resilient vertical motion that must exist in the projecting portion P. The strength of the locking element  8  is not affected either. The outer part P 2  of aluminum is still fully resilient in the vertical direction, and the short sides  5   a ,  5   b  can be snapped together according to the same principle as in  FIGS. 2   a - c . The locking element  8  is still made of aluminum and its strength is not reduced. However, it may be noted that the degree of resilience can be affected since it is essentially only the outer part P 2  that is resilient in the snap action. This can be an advantage in some cases if one wants to restrict the bending-down properties and increase the strength of the lock.  
         [0062]     The angling together of the long sides  4   a ,  4   b  can also be carried out according to the same principle as in  FIGS. 1   a - c . In general—not only in this embodiment—a small degree of downwards bending of the strip  6  may occur, as shown in the laying sequence in  FIGS. 7   a - c . This downwards bending of the strip  6  together with an inclination of the locking element  8  makes it possible for the boards  1 ,  1 ′ to be angled down and up again with very tight joint edges at the upper surfaces  41  and  48 . The locking element  8  should preferably have a high guiding capability so that the boards, in connection with downwards angling, are pushed towards the joint edge. The locking element  8  should have a large guiding part. For optimal function, the boards should, after being joined and along their long sides  4   a ,  4   b , be able to take a position where there is a small play between locking element and locking groove, which need not be greater than about 0.02-0.05 mm. This play permits displacement and bridges width tolerances. The friction in the joint should be low.  
         [0063]     In the joined state according to  FIG. 7   c , the boards  1 ,  1 ′ are locked relative to each other in The vertical direction D 1 . An upwards movement of the board  1 ′ is counteracted by engagement between the surfaces  43  and  49 , while a downwards movement of the board  1 ′ is counteracted on the one hand by engagement between the surfaces  45  and  52  and, on the other hand, by the board  1  resting on the upper side of the strip  6 .  
         [0064]      FIG. 8  shows a second embodiment of the invention. The board  1  in  FIG. 8  can be used for parquet flooring. The board  1  consists of an upper wear layer  32   a , a core  30  and a rear balance layer  34   a . In this embodiment, the projecting portion P outside the joint plane F is to a still greater extent made of different combinations of materials. The locking groove  14  is reinforced by the use of a separate component  70  of, for instance, wood fiber, which in a suitable manner is connected with the joint edge, for instance by gluing. This variant can be used, for instance, on the short side  5   b  of the board  1 . Moreover, a large part of the fixing shoulder  60  is positioned outside the joint F.  
         [0065]      FIG. 9  shows a third embodiment of the invention. The board  1  in  FIG. 9  is usable to provide a strong attachment of the aluminum strip  6 . In this embodiment, a separate part  72  is arranged on the joint edge supporting the locking element  8 . The part  72  can be made of, for instance, wood fiber. The entire fixing shoulder  60  and the entire strip  6  are located outside the joint plane F. Only a small part of the separate strip  6  is used for resilience. From the viewpoint of material, the portion P located outside the joint plane F has three different areas containing the combinations of materials “wood fiber only” (P 1 ), “wood fiber/balance layer/aluminum” (P 2 ) and “aluminum only” (P 3 ). This embodiment with the fixing shoulder  6  positioned entirely outside the joint plane F can also be accomplished merely by working the body of the board, i.e. without the separate part  72 . The embodiment in  FIG. 9  can be suitable for the long side. The locking element  8  has a large guiding part, and the projecting portion P outside the joint plane F has a reduced bending down capability.  
         [0066]     When comparing the embodiments in  FIGS. 8 and 9 , it may be noted that in  FIG. 9  the tongues  64  are higher than the lip  62 . This results in a strong attachment of the strip  6  in the front edge of the fixing shoulder  60 , which is advantageous when bending down the strip  6 . This can be achieved without any extra cost of material since the tongues  64  are punched from the existing material. On the other hand, the lip  62  can be made lower, which is advantageous in respect of on the one hand consumption of material and, on the other hand, the weakening effect of the mounting groove  54  on the joint edge. It should further be noted that the locking element  8  in  FIG. 8  is lower, which facilitates the snapping in on the short sides.  
         [0067]      FIGS. 10-12  show three different embodiments of the invention, in which the projecting portion can be made in one piece with the board body or consists of separate materials which are glued to the edge of the board and are machined from above. Separate materials are particularly suitable on the short side where strength and resilience requirements are high. Such an embodiment means that the composition of materials on the long side and the short side can be different.  
         [0068]     The above technique of providing the edge of the body, on the long side and/or short sides with separate materials that are fixed to the body to achieve special functions, such as strength, moisture proofing, flexibility etc, can be used also without utilizing the principles of the invention. In other words, it is possible also in other joining systems, especially mechanical joining systems, to provide the body with separate materials in this way. In particular, this material can be applied as an edge portion, which in some suitable fashion is attached to the edge of the body and which can extend over the height of the entire board or parts thereof.  
         [0069]     In a preferred embodiment, the edge portion is applied to the body before the body is provided with all outer layers, such as top layer and rear balance layer. Especially, such layers can then be applied on top of the fixed, separate edge portion, whereupon the latter can be subjected to working in respect of form with a view to forming part of the joining system, such as the projecting portion with locking element and/or the tongue with locking groove.  
         [0070]     In  FIGS. 10 and 11 , the board body is composed of a top laminate  32 , a wood fiber core  30  and a rear laminate  34 . The locking element  8  is formed by the projecting portion P being worked from above in such manner that, seen from the joint plane F outwards, it has an inner part P 1  consisting of wood fiber  30  and laminate  34 , a central part P 2  consisting of laminate  34  only, and an outer part P 3  consisting of wood fiber and laminate  34 .  
         [0071]     The embodiments in  FIGS. 10 and 11  differ from each other owing to the fact that in  FIG. 10  the boundary between the wood fiber core  30  and the rear laminate  34  is on a vertical level with the lower edge of the active locking surface  10 . Thus, in  FIG. 10  no significant working of the rear laminate  34  has taken place in the central part P 2 . On the other hand, in  FIG. 11  also the rear laminate  34  has been worked in the central part P 2 , which gives the advantage that the active locking surface  10  of the locking element  8  is wholly or partly made of a harder material.  
         [0072]     The embodiment in  FIG. 12  differs from the embodiments in  FIGS. 10 and 11  by an additional intermediate layer  33  being arranged between the wood fiber core  30  and the rear laminate  34 . The intermediate layer  33  should be relatively hard and strong to reinforce the active locking surface  10  as shown in  FIG. 12 . For example, the immediate layer  33  can be made of a separate material which is glued to the inner core. Alternatively, the immediate layer  33  may constitute a part of, for instance, a particle board core, where chip material and binder have been specially adapted to the mechanical joining system. In this alternative, the core and the intermediate layer  33  can thus both be made of chip material, but with different properties. The layers can be optimized for the different functions of the locking system.  
         [0073]     Moreover, the aspects of the invention including a separate strip can preferably be implemented in combination with the use of an equalizing groove of the type described in WO 94/26999. Adjacent joint edges are equalized in the thickness direction by working of the underside, so that the upper sides of the floorboards are flush when the boards are joined. Reference letter E in  FIG. 1   a  indicates that the body of the boards after such working has the same thickness in adjacent joint edges. The strip  6  is received in the groove and will thus be partly flush-mounted in the underside of the floor. A corresponding arrangement can thus be accomplished also in combination with the invention as shown in the drawings.  
         [0074]     Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.