Abstract:
A band saw comprises a band saw blade that is moved along a running direction against which a sawing material is guided in a feed direction. The band saw further includes a guide mechanism for the band saw blade comprising a plurality of magnet guide elements that are adjustable in their force for exerting a force on the band saw blade to influence its position in space. The magnet guide elements are positioned upstream and downstream of the sawing material and have at least two magnets one beside the other, as viewed along the running direction.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     The present application is a continuation of pending international patent application PCT/EP2009/008926, filed Dec. 14, 2009 designating the United States, which international patent application has been published in German language and claims priority from German patent application 10 2008 063 696.7, filed Dec. 19, 2008. The entire contents of these prior applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention is related to the field of band saws. 
     More specifically, the invention is related to band saws having guide means for magnetically controlling the band saw blade position in space. 
     Still more specifically, the invention is related to a band saw comprising a band saw blade moved along a running direction, a sawing material adapted to be guided against the band saw blade along a feed direction, and guide means for the band saw blade, wherein the guide means has a plurality of magnet guide elements being adjustable in their force and exerting a force on the band saw blade for influencing a position of the band saw blade in space. 
     BACKGROUND OF THE INVENTION 
     It is well known that band saw blades during the operation of a band saw make evasive movements when the sawing material, for example an entire log or a beam or a board is guided from the front against the toothed side of the band saw blade. These evasive movements may be laterally inclined with respect to the feed direction, or the band saw blade may twist about an axis parallel to its running direction. The evasive movement may also appear periodically, i.e. as oscillations with one or more frequencies. 
     For avoiding such evasive movements numerous band saw blade guides have been proposed. These guides, in most cases, are configured as mechanical guides. 
     U.S. Patent Application Publication US 2009/0126549 A1 discloses a band saw as well as a method for positioning a band saw blade in space. This prior art band saw utilizes a magnetic guide in which a magnetic force is exerted on the band saw blade transversely to the feed direction and to the running direction by means of magnetic guide elements. Within one magnetic guide element two separate magnets may also be provided which are positioned one beside another along the feed direction and exert a first force on a front area as well as a second force on a rear area of the band saw blade. When these forces are unequal, then they exert a torque on the band saw blade about an axis parallel to its running direction. Therefore, one may counteract torsions within the band saw blade. 
     Within the course of the present invention one has now found that with the use of the above-described approach one may make substantial contributions to the stability and to the precision of the predetermined sawing direction of the band saw blade, however, it has turned out that there exist, moreover, higher-order components of movement, the contribution of which to the instability of the band saw blade increasing with the running velocity of the band saw blade. These components of movement, for example consist of undulated movements in the running direction of the band saw blade. 
     German Utility Model publication DE 201 05 845 U1 describes a magnetic band saw positioning apparatus. This apparatus essentially consists of a U-shaped guide, the legs of which extending on both sides of the band saw blade to be positioned. The guide as a whole is supported against a stationary base via springs in the feed direction of the sawing material. A row of opposing magnets, obviously permanent magnets, is housed in the two legs of the guide, the rows extending parallel to the longitudinal direction of the band saw blade. In a first embodiment of the apparatus, the row is positioned beside the tooth base of teeth of the band saw blade. In a second embodiment of the apparatus the row is positioned behind the rear edges of elongate saw blade holes arranged along the longitudinal direction. Nothing is said within the publication as to the polarization of the magnets and their interaction with the band saw blade. The legs of the guide are dimensioned so long and the guide is positioned relatively to the band saw blade such that the rear side of the band saw blade keeps a distance from the flange interconnecting the legs. Thereby, the band saw blade may be somewhat shifted in the feed direction in spite of the magnets action when high feed forces occur, wherein, finally, also the elastic support acts as a limit. The guide exclusively affects a support of the band saw blade against the feed direction and, hence, counteracts only the little critical evasive movements. Lateral evasive movements or a torsion of the band saw blade are not avoided by this prior art guide which, hence, does not contribute to the quality of the sawing cuts with respect to dimensional precision or surface quality. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object underlying the invention, to improve a band saw of the kind specified at the outset such that the afore-mentioned problems are removed or at least are minimized as far as possible. In particular, a band saw shall be provided, in which also higher-order components of movement may be effectively compensated for. 
     In a band saw of the type specified at the outset, this object is achieved in that the magnet guide elements have each at least two magnets one beside another upstream or downstream of the sawing material, as seen along the running direction. 
     The object underlying the invention is, thus, entirely solved. The provision of two individually controllable magnets one close beside another along the running direction either upstream or downstream the sawing material allows to compensate for higher order effects, in particular of oscillations propagating along the saw blade in the running direction. 
     In a preferred embodiment of the invention, the guide comprises magnet guide elements on both sides of the band saw blade as known per se. 
     Moreover, it is preferred when the guide comprises pairs of magnet guide elements positioned one beside another in the feed direction. 
     These measures have the advantage that the advantages of prior art magnetic guides may be combined with the above advantages. 
     Finally, the invention encompasses the use of magnet guide elements both upstream and downstream the sawing material. 
     Further advantages will become apparent from the description and the enclosed drawing. 
     It goes without saying that the features mentioned before and those that will be explained hereinafter may not only be used in the particularly given combination but also in other combinations or alone without leaving the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are depicted in the drawings and will be explained in further detail in the subsequent description. 
         FIG. 1  shows a schematic front elevational view of an embodiment of a band saw according to the invention; 
         FIG. 2  on a highly enlarged scale shows a side elevational view of a band saw blade of the band saw of  FIG. 1  as well as a magnet guide element according to the present invention; and 
         FIG. 3  shows a top plan view along line of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , reference numeral  10  as a whole designates a band saw of conventional design. Band saw  10  has an upper wheel  12  with an upper axis  14 , and a lower wheel  16  with a lower axis  18 . A band saw blade  20  is guided around wheels  12  and  16 . 
     As shown in  FIG. 2 , band saw blade  20  has a toothed front side  22  as well as a smooth rear side  24 . However, it goes without saying that the invention may likewise be used with band saw blades having teeth on both sides. An arrow  26  indicates the feed direction between the band saw blade  20  and a sawing material, and an arrow  28  indicates the running direction of band saw blade  20 . Feed direction  26  in the depicted embodiment extends horizontally and perpendicularly with regard to the plane of  FIG. 1 . Running direction  28  extends vertically. Also insofar it is understood that the invention is not limited to these directions. 
     A sawing material  32 , for example a board, lies on a sawing table  30 . Band saw blade  20  in the conventional manner runs through a corresponding opening within sawing table  30  and sawing material  32  is guided in the feed direction  26  with a certain force against toothed side  22  of band saw blade  20 . 
     When band saw blade  20  comes into engagement with sawing material  32 , evasive movements of band saw blade  20  may occur. Such evasive movements may be continuous or discontinuous. If, for example, sawing material  32  is inadvertently guided against band saw blade  20  at a slant angle, band saw blade  20  reacts with a continuous evasive movement and the sawing becomes untrue. If band saw blade  20  meets a discontinuity within sawing material  32 , for example a knot, then band saw blade  20  will evade discontinuously. 
     Further, band saw blade  20  may begin to oscillate, in particular begin torsional oscillations about an axis parallel to running direction  28  or oscillations along such an axis. Such oscillations occur, in particular, at high running speeds of band saw blade  20 . Natural resonances of band saw  10  and the periodical running by of discontinuities within band saw blade  20 , for example the band joint running over stationary guides, may likewise induce such oscillations. 
     In order to counteract this complex motional action, the depicted preferred embodiment, to start with, is provided with four magnet guide elements, as known per se, namely in the depiction of  FIG. 1  and relative to sawing material  32  with a first, upper left magnet guide element  40   11 , a second, upper right magnet guide element  40   12 , a third, lower left magnet guide element  40   21 , and a fourth, lower right magnet guide element  40   22 . Magnet guide elements  40   11 ,  40   12 ,  40   21 ,  40   22  are positioned on both sides of band saw blade  20 , in particular at the same height, and magnet guide elements  40   11 / 40   21  and  40   12 / 40   22 , resp. are located above and below, resp. of sawing material  32 . 
     Insofar, it is understood that, as an alternative, only one pair of magnet guide elements right/left and/or one pair of magnet guide elements upper/lower may be used, wherein the then missing magnet guide elements may be replaced by mechanical guides, in particular stationary guide blocks. Band saw blade  20  may also, as shown, be guided along a tangent line to wheels  12  and  16  or, as an alternative, be guided at a lateral distance to such tangent line, wherein band saw blade  20  in its rest position would be under a mechanical bias tension. Further details with regard to such alternatives are described in already cited DE 10 2007 005 581 A1 to which reference is made. 
     According to the present invention four magnets each are provided within magnet guide elements, preferably in all magnet guide elements  40   11 ,  40   12 ,  40   21 , and  40   22 , as shown in  FIG. 2  with respect to magnet guide element  40   11 . In the illustration of  FIG. 2  we have a first, upper left magnet  42   11 , a second, upper right magnet  42   12 , a third, lower left magnet  42   21 , and a fourth, lower right magnet  42   22 . Magnets  42   11 ,  42   12 ,  42   21 , and  42   22 , therefore, are positioned at the corners of an imaginary rectangle. They are adapted to be controlled individually. Accordingly, magnets  42   11  and  42   21 , as well as magnets  42   12  and  42   22  are positioned one beside another in the running direction  28  and magnets  42   11  and  42   12  as well as magnets  42   21  and  42   22  are positioned one beside another in the feed direction  26 . In the embodiment shown, band saw blade  20 , therefore, when running by each of the magnet guide elements  40   11 - 40   22 , every time runs by four magnets. Preferably, the magnets of magnet pairs  42   11 / 42   12  and  42   21 / 42   22  are positioned in the front and in the rear area of band saw blade  20 , resp. Magnet guide elements  40   11 ,  40   12  and  40   21 ,  40   22 , resp., are, preferably, positioned just above and below, resp., sawing material  32 . 
     Magnets  42   11 / 42   12  and  42   21 / 42   22 , resp., arranged one beside another along feed direction  26 , when energized differently, as known per se, exert a torque on band saw blade  20  about axis  44  shown in  FIG. 2  which extends parallel to running direction  28  as will be explained in further detail in connection with  FIG. 3 . 
     In contrast magnets  42   11 / 42   21  and  42   12 / 42   22 , resp., positioned one beside another along running direction  28  counteract non-homogenous movements of band saw blade  20  along the running direction, in particular oscillations propagating along running direction  28 . 
     It goes without saying that magnet pairs  42   11 / 42   12  and  42   21 / 42   22 , resp., positioned one beside another along feed direction  26  are advantageous and preferred, however, are not mandatory for the invention. Instead, the magnets may be provided only on one side of band saw blade  20 . 
       FIG. 3  shows a view along line of  FIG. 2 . 
     Magnet guide element  40   11  is provided with a housing  54 . Within housing  54  one can see first, front electromagnet  42   11  and second, rear electromagnet  42   12  arranged facing a front area  58   a  and a rear area  58   b , resp., of band saw blade  20 . The terms “front” and “rear” are related to feed direction  26  of sawing material  32 , for example a wood board. Below are provided third and fourth electromagnet  42   21  and  42   22 . 
     Electromagnets  42   11 - 42   22 , preferably, are of same design. The design with a U-shaped yoke indicated in  FIG. 3  is, of course, only to be understood as an example. As a matter of principle, any component may be used allowing to exert an adjustable force on band saw blade  20  in a contactless manner. 
     As four electromagnets  42   11 - 42   22  are used in any of the magnet guide elements  40   11 - 40   22 , the entire assembly with two magnet guide elements above each other ( FIG. 1 ), therefore, comprises as a whole eight such electromagnets or sixteen, resp., when two pairs each of magnet guide elements  40   11 / 40   21  and  40   12 / 40   22 , resp., arranged one above the other are used on both sides of band saw blade  20 . 
     A front sensor  62   a  is associated to front electromagnet  42   11  and a rear sensor  62   b  is associated to rear electromagnet  42   12 . Lower electromagnets  42   21  and  42   22  are switched correspondingly. Sensors  62   a ,  62   b  are adapted to measure a distance magnetically, capacitively, optically, acoustically or in any other manner. Within the magnet guide they measure a distance d between the right hand ( FIG. 3 ) surface  64  of magnet guide element  40   11  and the left ( FIG. 3 ) surface  66  of band saw blade  20  in its front area  58   a  and rear area  58   b , resp. 
     If electromagnets  42   11  and  42   12  are energized with the same current intensity, i.e. exert the same magnet force on areas  58   a  and  58   b , then band saw blade in the illustration of  FIG. 3  will be moved to the right or to the left while maintaining its orientation, as indicated by a double arrow  70 . If, however, the magnet forces of electromagnets  42   11  and  42   12  are set differently, then band saw blade  20  is twisted about its center axis, as shown by a pair of arrows  72 . In such a way it is possible to adjust band saw blade  20  inclined under an angle relative to feed direction  26  of sawing material  32 . One can then make inclined or arcuate saw cuts within sawing material  32 , for example when sawing material  32  is configured conical or arcuate along feed direction  26 , as is the case for naturally grown trunks or parts thereof. 
     Magnet pairs  42   11 / 42   21  and  42   12 / 42   22 , resp., arranged one above another, act correspondingly on band saw blade  20 . By differently energizing same, waves within band saw blade  20  propagating along running direction  28 , may be erased. 
     By selectively controlling electromagnets  42   11 - 42   22  one can, therefore, compensate for a lateral evasive movement and a torsion of band saw blade  20  as effectively as a propagation of waves along running direction  28 . This is of particular advantage when sawing material  32  is guided with high force along feed direction  26  against toothed side  22  of band saw blade  20  and band saw blade  20  then bends away, or when band saw blade  20  comes into non-homogenous areas of sawing material  32 , for example knots within a wood board. 
     In the arrangement of  FIG. 1  the control of the lateral position of band saw blade  20  is effected by selectively controlling magnet guides  40   11 / 40   12  and  40   21 / 40   22 , resp. on both sides of band saw blade  20 . Band saw blade  20  in that situation extends along a common tangent line between wheels  12  and  14 , as shown in  FIG. 1 . 
     As an alternative, the magnet guide elements, however, could also be positioned on one side of the band saw blade only, and the band saw blade on its other side could rest, as known per se, on stationary guide blocks when in its rest position, as illustrated, for example, in  FIG. 1  of DE 10 2007 005 581A. Then, in the rest position, i.e. in the absence of magnet force, the band saw blade rests e.g. on the stationary guide blocks due to the conventional mechanical setting of a certain, laterally oriented bias tension of between 100 and 1,000 N, e.g. 600 N. Immediately prior to or at the starting of the band saw, the band saw blade is lifted off the guide blocks by exerting a magnet force of e.g. 700 N being higher than the mechanical bias force of e.g. 600 N, until it assumes a desired position between the guide blocks and the magnet guide elements. In this desired position the band saw blade is guided in a contactless manner. The position control is then effected about the desired magnet force of 700 N by modulation, i.e. by decreasing or by increasing the magnet force.