Plant for finger-jointing wood-boards
BACKGROUND OF THE INVENTION 
     The invention pertains to a plant for finger-jointing wood boards in an end-to-end relationship. Plants for finger-jointing wood-boards are already known and comprise means for transporting the wood-boards through the plant, a finger-cutting device cutting wedge-like fingers into the ends of the boards, a glueing device for applying glue on the cutted fingers and an assembling and pressing device wherein the boards are assembled in an end-to-end relationship with the glue-coated fingers engaged in one another and afterwards firmly pressed together. After leaving the press, the assembled long board is cut to desired lengths and the glue in the joints between the fingers has to be cured, for instance during storage of the jointed boards in a warm room 15° C or more. This known finger-jointing plant has several disadvantages. It is not capable of working with cold or even frozen boards because glueing cannot be performed below a certain temperature i.e. 15° C for most glues. Further, in order to reduce storage time, the temperature of the storage-room has to be chosen sufficiently high i.e. 20° C or more. 
     It is already known to use dielectric heating of the glued joints in order to eliminate storage. In such a known plant the glued boards at the output of the press are conveyed through a dielectric heating device and heated to a temperature sufficient to obtain very rapid curing, such that afterwards the long assembled board may be fed into a planing machine without danger of damaging the joints. In this known plant the whole boards, and not only the ends of the boards have to pass through a very long dielectric heating device so that an important amount of heating energy is needed. Further, as the glued joints are conveyed through the dielectric heating device, the curing of the glue is influenced by the dielectric properties of the glue. Moreover, excess glue falling on the electrodes may disturb the operation of the dielectric heating device. 
     SUMMARY OF THE INVENTION 
     In order to overcome the disadvantages of the known finger-jointing plants in accordance with the invention, the curing of the glue is not performed by dielectric heating of the glue in the joints, but dielectric heating is applied to the crude ends of the boards even before the fingers are cut in them. The ends of the boards only are fed in and out of the dielectric heating means and conveyed to finger-cutting means where wedgelike fingers are cut in the heated ends. Glue is applied to the hot cut fingers and assembling and pressing together of the hot glue-coated fingers is performed rapidly. The curing of the glue is provided by the heat accumulated in the ends of the boards and no further heating or storage in warm rooms is necessary, even if the ambient temperature is below the freezing-point. The heating energy is relatively low as only the ends are heated, whilst the middle parts between both ends of the boards may remain unheated and even frozen. The length of the end to be heated by dielectric heating and the temperature at the output of the dielectric heating means are chosen sufficient in order to accumulate in these ends the amount of calories necessary for the curing of the glue and to compensate for the heat-losses before the curing process is completed. 
     As the curing of the joints is very rapid, a machining device may operate on the jointed board at the output of the pressing means. The short conveying time of the board between the pressing means and the machining device is sufficient for curing the glue to a point at which the joints are able to resist the forces exerted by the machining device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference may be had to the preferred embodiment exemplary of the invention shown in the accompanying drawings, in which: 
     FIG. 1 is a perspective view, partially cut, of the part of the finger-jointing plant comprising the dielectric heating device. 
     FIG. 2 is a schematic diagram of an entire finger-jointing plant of which FIG. 1 is a part. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     The plant showed in FIGS. 1 and 2 is fed through an input guide 1 with wood boards 2, generally of unequal length. If wood-boards with uneven end-faces have to be utilized, the plant may be equipped with a sawing device, not shown, located before the input guide 1, cutting away the uneven ends and delivering boards with end faces perpendicular to their long edges. The incoming boards 1 are pushed either by hand or by a motored device (not shown) against a stop bar 3 of a dielectric heating means. The dielectric heating means consists of an elongated, open, earthed casing 4 and a live electrode 5 surrounded by the casing 4. The live electrode 5 is connected to the not earthed bushing of a suitable high-frequency generator 6. The stop bar 3 is fixed to the wall of the casing 4 opposite to its longitudinal opening. Guiding and transporting means for the wood-boards include supporting rails 7, parallel to the casing 4, a pushing bar 8, a roller-conveyor 9, a ram 10, a roller-conveyor 11 and a ram 12. The pushing bar 8 and the rams 10 and 12 are energized by suitable i.e. pneumatic or hydraulic devices. The pushing bar 8 conveys the boards parallel to themselves on the supporting rails 7. During this movement the ends of the boards 2 remain in the casing between the bottom of the casing and the live electrode where a high-frequency field is established and heat is generated in the interior of the ends of the wood-boards located between the live electrode 5 and the bottom of the casing 4. The amount of heat generated may be controlled by the rate at which the boards are conveyed through the high-frequency field, by the length of the high-frequency field which is equal to the length of the live electrode and by the intensity of the high-frequency field. This intensity may be controlled by a controlling device, (not shown) of the voltage of the generator 6 provided in the interior of the generator 6 or by a positionning device 13 extending into the casing 4 and positioning the live electrode 5 relative to the bottom of the casing 4. A pressing bar 14 energized by a device 15 outside of the casing 4, extending parallel and adjacent to the casing, maintains the ends of the boards away from the live electrodes by pressing them against the bottom of the casing 4. A blowing means i.e. a feeding tube 16 of compressed air blows air into the space between the bottom of the casing and the live electrode. The end of the tube 16 points in the direction of the longitudinal extension of the live electrode 5 so that the air-stream leaving it continually cleans the space between the live electrode and the boards 2 in order to prevent breakdowns of the high-frequency field due to saw-dust particles aligning in the electric field. 
     The plant operates as follows; wood-boards 2 are fed through the input guide 1 either by hand or with the aid of an automatic device (not shown). The boards are pushed into the casing 4 against the stop bar 3. The pushing bar 8 operates by successive strokes at a rate that determines the speed with wich the ends of the boards travel through the high-frequency field between the live electrode 5 and the bottom of the casing 4. When leaving the casing 4 at the end opposite the input end, the boards are pushed on the roller-conveyor 9. There, they may be pushed by the ram 10 against a stop bar 23 of an other dielectric heating device consisting of a casing 24, a live electrode 25, a high frequency generator 26 and supporting rails 27 similar to the corresponding elements 4, 5, 6, 7 of the dielectric heating device described above. The boards continue to be conveyed in a direction perpendicular to their long edges either by the action of the pushing bar 8 or of an other convenient device, not shown. When leaving the casing 24, the boards are pushed on a second roller-conveyor 11 where they are pushed by a ram 12 into an installation 17 which may be of a conventional design and include finger-cutting means for cutting wedge-like fingers into the ends of the boards, means for applying glue on the fingered ends and assembling and pressing means for assembling the glue-coated, fingered ends of the boards in an end-to-end relationship and pressing them one against the other, so that at the output of installation 17 a jointed board of great length is produced, from which desired lengths may be cut at will. 
     According to the invention the ends of the boards are heated in the dielectric heating devices throughout their mass to a more or less uniform temperature of, say 90° C, whilst the whole length outside the ends is not heated and may even remain frozen. The length of the heated end is determined by the dimensions of the casing 4 and the live electrode. As dielectric heating generates heat uniformly in the heart of the board exposed to the high-frequency field, a relatively high temperature and uniform temperature-distribution is obtained within a very short time so that a considerable amount of heat is accumulated in the heated ends of the boards 2. 
     In certain special cases, only one end of each board has to be heated. The heat accumulated in this one end must be sufficient for curing the glue of a joint between a cold end of one board and a hot end of another board and to compensate for the heat losses from the joint before the curing process is completed. In this case the heating device 4, 5, 6, 7 the conveying rollers 9 and the ram 10 in FIGS. 1 and 2 are not necessary. 
     Generally, however, it is better to heat both ends of the boards that have to be jointed and this is done, preferably, in two distinct dielectric heating devices as shown in the drawing. Thanks to such sufficient heating and to rapid conveying and operation in the installation 17, the curing of the glue in the joints takes place when these joints are at a relatively high temperature, say about 60° C and is therefore very rapid. Then, a machining device, such as a planing machine 18 may operate at the output of the pressing means of the installation 17. In such a case, the short conveying time of the board between the output of the pressing means and the input of the planing machine adds sufficient time for curing the glue to a point at which the joints are able to resist the forces exerted by the planing machine. 
     At the output of the planing machine a sawing device for cutting desired lengths of the outcoming board, (not shown) may be provided, as is the case of conventional finger-jointing plants.