Abstract:
A band saw, a band saw assembly, and a method of spatially positioning a band saw blade are suggested. The band saw comprises a band saw blade against which a sawing material is adapted to be guided in a feed direction. A guide for the band saw blade has at least one magnet exerting a force on the band saw blade determining the spatial position thereof. The at least one magnet is adjustable in its force effect and the force is directed transversely to the feed direction. The guide, as viewed in the feed direction of the sawing material, has a front magnet and a rear magnet, the magnets facing a front area and a rear area, respectively, of the band saw blade. A method of spatially positioning a band saw blade is executed while a sawing material is guided against the band saw blade in a feed direction. A magnet force is exerted on the band saw blade. The magnet force, and thereby the position of the band saw blade is adjusted transversely to the feed direction. The magnet force, as viewed in the feed direction of the sawing material, is exerted on a front area and on a rear area of the band saw blade.

Description:
FIELD OF THE INVENTION 
     The invention is related to the field of band saws and methods for positioning a band saw blade. 
     More specifically, the invention is related to a band saw comprising a band saw blade against which a sawing material is adapted to be guided in a feed direction, and a guide for the band saw blade, the guide having at least one magnet exerting a force on the band saw blade determining the spatial position thereof, the at least one magnet being, further adjustable in its force effect and the force being directed transversely to the feed direction. 
     Correspondingly, the invention is related to a method of spatially positioning a band saw blade while a sawing material is guided against the band saw blade in a feed direction, in which a magnet force is exerted on the band saw blade, and the magnet force, and thereby the position of the band saw blade is adjusted transversely to the feed direction. 
     BACKGROUND OF THE INVENTION 
     Band saws mostly consist of two wheels arranged one above the other, sometimes also one besides the other, with a horizontal axis of rotation, over which a band saw blade is guided. One of the wheels is driven and, hence, moves the band saw blade in a longitudinal direction. Band saw blades are, for example, 10 m long and run with a velocity of about 30 to 45 m/s. 
     For making sure that the band saw blade runs stably, even if a sawing material, for example wood, is guided with a certain force with its front against a narrow, toothed side of the band saw blade, the band saw blade is mechanically held under tension with high forces. This is done by increasing the distance between the wheel axes after having applied the band saw blade upon the wheels. 
     Moreover, it is well known to push the band saw blade, which otherwise would run along a common tangent line interconnecting the wheels, in an outward direction by means of two mechanical guide elements arranged at a distance along the respective strand, such that the band saw blade extends parallel to the tangent line over a certain section. This results in that the free length of the band saw blade is reduced to the distance between the two guide elements, and that the band saw blade reacts with a higher resistance to a force acting laterally on it. 
     Band saw blades are exposed to different mechanical loads not only by such a bias but also by the sawing itself. These loads cause the band saw blade to evade. Depending on how the forces acting chaotically and irregularly on the band saw blade during sawing engage same, various evasion movements occur. 
     One first such evasion movement is directed opposite the feed movement. This evasion movement is conventionally countered by guiding the band saw blade over wheels being configured crowned at their periphery. The evasion movement is quite critical in view of the dimensional accuracy of the sawing operation and the quality of the surface generated during the sawing. 
     A second such evasion movement is directed laterally. This evasion movement is significantly more critical because it influences both the dimensional accuracy and the surface quality. In conventional band saws, this evasion movement, as already mentioned, is countered essentially only by a high tension of the band saw blade and by shortening the free length thereof. 
     Finally, it may happen that the band saw blade is twisted around its longitudinal axis. 
     All these evasive movements are disadvantageous in operation. On the one hand, they result in a stretching of the band saw blade and, on the other hand result in an increased wear. Furthermore, also the quality, i.e. the dimensional accuracy and the quality of the generated surface, i.e. the saw cut, are negatively affected when the band saw blade evades laterally during sawing or twists. 
     In order to keep such movements and deformations as small as possible, one has suggested various mechanical guides for the band saw blade. These guides are mostly configured as slide guides or as roller guides. These guides, however, have the disadvantage that they likewise cause wear due to friction. 
     For this reason one has already suggested a magnetic guide for a band saw blade. 
     Printed citation DE 201 05 845 U1 describes such a magnetic band saw positioning apparatus. This prior art 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 via springs against a machine-mounted bearing in the feed direction of the sawing material. 
     Two parallel rows each of opposing magnets, apparently permanent magnets, are integrated into the two legs of the guide, wherein the rows extend parallel to the longitudinal direction of the band saw blade. The one row is positioned besides the tooth base of the band saw blade teeth and the other row is positioned besides the rear edges of saw blade holes extending in a longitudinal direction. Nothing is said in the printed citation neither about the polarisation of the magnets nor their interaction with the band saw blade. 
     The legs of the guide are dimensioned so long and the guide is positioned relative to the band saw blade such that the rear side of the band saw blade keeps a distance to the flange interconnecting the legs. Thereby, with large feed forces, the band saw blade can be somewhat displaced in the feed direction against the action of the magnets, wherein also the resilient support finally has a limiting function. 
     The guide effects exclusively a support of the band saw blade opposite the feed direction, and, hence, only counteracts the not so critical evasive movements of the band saw blade in the feed direction. Lateral evasive movements and a twisting of the band saw blade are not prevented by the prior art guide which, therefore, does not contribute to the improvement of the saw cut quality with regard to dimensional accuracy and surface quality. 
     Printed citation SE 436 849 B describes a circular or band saw in which a force is exerted on the saw blade by means of two electromagnets positioned on opposite sides of the saw blade. By means of a sensor the lateral position of the saw blade is detected, is compared with a desired position, and the saw blade, as the case may be, is redirected into the desired position by corresponding excitation of the magnets. By doing so, a fluttering of the saw blade is prevented. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object underlying the invention, to improve a band saw as well as a method for positioning a saw blade of the type mentioned at the outset such that the aforementioned disadvantages are avoided. In particular, the invention shall make it possible to guide band saw blades in a contactless manner and precisely in their position, wherein, in particular, a lateral evasion and a twisting of the band saw blade are avoided or reduced to a no more disturbing extent. 
     In a band saw of the type mentioned at the outset, this object is achieved in that the guide, as viewed in the feed direction of the sawing material, has a front magnet and a rear magnet, the magnets facing a front area and a rear area, respectively, of the band saw blade. 
     In a method of the type mentioned at the outset, this object is achieved in that differently set magnet forces, as viewed in the feed direction of the sawing material, are exerted on a front area and on a rear area, respectively, of the band saw blade. 
     The object underlying the invention is, thus, entirely solved. 
     The specific type of the inventive control, namely, allows for the first time to laterally guide a band saw blade in a contactless manner. Thereby not only the wear on the band saw blade is minimized but also the quality of the executed saw cuts is optimized. Moreover, band velocities up to more than 100 m/s are achieved. 
     The measure, to excite the magnets with different magnet forces has the advantage that an oblique position and a twisting, respectively, of the band saw blade can be compensated for by an individual action on the front and on the rear area thereof. One, therefore, exerts a torque on the band saw blade which compensates the twisting. On the other hand, the option is open to intentionally twist the saw blade and, thus, to orient it obliquely relative to the feed direction of the sawing material, in order to make straight, but oblique or arc-shaped saw cuts. This is of particular advantage for a sawing material having a conical or an arc-shaped form as is the case with naturally grown logs. 
     In a preferred embodiment of the invention, the at least one magnet is an electromagnet. 
     This measure has the advantage that components may be used which are available as commercial products in the required dimensions and precision and at low cost. 
     Moreover, an embodiment of the inventive band saw is preferred in which the guide has sensors for detecting the position of the band saw blade in a direction transverse to the feed direction, the sensors being operatively connected with the magnets via a controller, and, preferably, a desired value for the position being adapted to be fed to the controller. Correspondingly, according to the method the position of the band saw blade is detected and is controlled on a desired value by the adjusting of an amount of the magnet force. 
     These measures have the advantage that a closed control loop is provided allowing a precise positioning of the band saw blade, thereby eliminating all occurring disturbance variables, among which are also thermal and other influences. 
     In a particularly preferred improvement of this embodiment which may likewise be used alone, i.e. without the other mentioned features, means for detecting a natural frequency of the band saw blade circulating in engagement and/or out of engagement with the sawing material are associated to the sensors, the means feeding control signals for compensating periodical movements of the band saw blade directed transversely to the feed direction to the at least one magnet in synchronism with the natural frequency. According to the method a natural frequency of the circulating band saw blade when in engagement and/or out of engagement with the sawing material is detected, the magnet force being adjusted for compensating periodical movements of the band saw blade directed transversely to the feed direction in synchronism with the natural frequency. 
     These measures have the advantage that an effective compensation of a substantial disturbance variable becomes possible, namely the natural resonance of the band saw. As all moving systems a band saw exhibits one or more such natural resonances with a fundamental frequency and harmonics. This natural resonance results in an oscillation of the band saw blade in a lateral direction, also as a torsional oscillation, at high frequencies. Within the scope of this embodiment, the frequencies of the fundamental and the harmonic waves are determined beforehand. The oscillation of the band saw blade is then extinguished by interference, in that an oscillating force of like frequency but opposite direction is exerted on the band saw blade. 
     Analogously one can proceed with still another phenomenon of band saws, namely the interference due to the circulating butt joint of the band saw blade. This butt joint interconnecting both ends of the band saw blade and being made by soldering or welding configures a discontinuity shaped as a bump which during each circulation generates an evasive movement when it runs over the guide. In the above-mentioned example of a band saw blade of 10 m length and a velocity of 40 m/s this event has a frequency of 4 Hz or a clock period of 250 ms. When the band saw velocity is 100 m/s, the frequency would be 10 Hz and the clock period 100 ms. 
     Within the scope of still another embodiment which may likewise be used alone, i.e. without the other mentioned features, means for detecting a periodical evasive movement of the circulating band saw blade caused by a butt joint running by the guide and being directed transversely to the feed direction are associated to the sensors according to the invention, the means feeding control signals for compensating such movements of the band saw blade to the at least one magnet in synchronism with the butt joint running by the guide. According to the method a periodical evasive movement of the circulating band saw blade caused by a butt joint running by the guide, and being directed transversely to the feed direction is detected, and the magnet force is adjusted for compensating such movements of the band saw blade in synchronism with the butt joint running by the guide. 
     This measure has the advantage that also these periodically occurring evasive movements can be effectively compensated for. 
     Within the scope of the present invention a good effect is achieved in that the guides, as viewed in the sawing direction of the band saw blade have a guide module in front of the sawing material and a guide module behind the sawing material. According to the method the magnet force, as viewed in a sawing direction of the band saw blade, is exerted on the band saw blade in front of and behind the sawing material. 
     This measure has the advantage that the band saw blade is stabilized within the sawing area. 
     In still other embodiments of the invention the magnets, as viewed transversely to the feed direction, are positioned on both sides of the band saw blade. According to the method the magnet force, as viewed transversely to the feed direction, is exerted on both sides of the band saw blade. 
     This measure has the advantage that the band saw blade may extend freely between the wheels along a tangent line common for both wheels. Accordingly, in the rest position of the band saw blade no basic force must be exerted from the magnets of the magnet guide on the band saw blade extending symmetrically between them. Further, the control speed in both directions is very high because it only depends on the rise rate of the magnet force, i.e. an electronically controllable value. By means of the magnets arranged on both sides of the band saw blade, the band saw blade may be specifically twisted about its longitudinal axis. 
     As an alternative, the magnets, as viewed transversely to the feed direction, may also be positioned only on one side of the band saw blade. According to the method, the magnet force, as viewed transversely to the feed direction, is exerted on one side of the band saw blade only. 
     If, when doing so, the band saw blade is guided over two wheels, several alternatives are possible. 
     In a first alternative, the guides extend beyond a common tangent line interconnecting the wheels. 
     This measure has the advantage that known and well-proven concepts for biasing a band saw blade by lateral deflection may be used. 
     In a second alternative, the guides extend inwardly beyond a common tangent line interconnecting the wheels. 
     This measure has the advantage that band saw assemblies of the type already mentioned may be put into practice in which two individual band saws may be positioned one adjacent the other. 
     According to another embodiment of the invention which may also be used alone without the other mentioned features, guides, as viewed transversely to the feed direction are positioned on one side of the band saw blade. 
     In that case it is preferred when mechanical guide blocks are provided on the opposite side of the band saw blade. 
     This measure has the advantage that the band saw blade in the event of a wilful shutoff or an unwanted failure of the magnet comes into a defined rest position in which it may run down to a standstill under tension and is still held after standstill. The desired deflection of the band saw blade in an inward or an outward direction is effected in that the inner or outer, respectively, magnet in a standard position of the band saw blade exerts a certain basic force on the band saw blade, the basic force being modulated, i.e. increased or decreased depending on the particular evasive movement. 
     In a third alternative, the guides extend along a common tangent line interconnecting the wheels and concurrently configure mechanical guides. 
     Further advantages will become apparent from the drawing and the enclosed description. 
     It will be understood that the features of the invention mentioned above and those yet to be explained below can be used not only in the respective combination indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are explained in more detail in the following description and are represented in the drawings, in which: 
         FIG. 1 : is a highly schematical side elevational view of an embodiment of a band saw according to the invention with which the method of positioning a band saw blade according to the invention may be executed; 
         FIG. 2 : on an enlarged scale shows a guide of the band saw of  FIG. 1  in a view along line II-II; 
         FIG. 3 : shows a block diagram of an electronic control unit as may be used in the guide of  FIG. 2 ; 
         FIG. 4 : is a view, similar to that of  FIG. 1 , however for another embodiment of a band saw according to the invention, in an operational mode, in which the alternatingly increasing load of the band saw blade is low and the velocity of the position control is high; 
         FIG. 5 : is a view, similar to that of  FIG. 1 , but for still another embodiment of a band saw according to the invention, in which, preferably, two band saws are used one adjacent the other; and 
         FIG. 6 : is a view, similar to that of  FIG. 1 , however for still another embodiment of a band saw according to the invention, in which likewise the extent of alternatingly increasing loads is minimized. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , reference numeral  10  as a whole designates a band saw, as is typically used in saw mills for dissecting logs, for dissecting and edging boards and the like. Band saw  10  may be installed in the sawmill as an integrated stationary unit or as a mobile unit. 
     Band saw  10  comprises an upper wheel  12  and a lower wheel  14  rotating about a horizontal upper axis  16  and a lower axis  16 , respectively. A band saw blade  20  is stretched over wheels  12  and  14 . Band saw blade  20  is provided with teeth  21  on the front side of  FIG. 1  (see  FIG. 2 ). 
     The free ends of band saw blade  20  are interconnected with a butt joint  22  which may be generated by welding or soldering. With regard to band saw blade  20  butt joint  22  configures a discontinuity having the shape of a bump. 
     Arrows  23  and  24  indicate the sense of rotation of wheels  12  and  14 . The positioning of wheels  12  and  14  with regard to a vertical axis  26  intersecting axes  16  and  18  and with regard to a horizontal axis  28  extending centrally between axes  16  and  18  is symmetrical. 
     In  FIG. 1  the right hand strand of band saw blade  20  is designated with  32  and the left hand strand with  34 . From the senses of rotation  23  and  24  of wheels  12  and  14  follows a running direction of band saw blade  20  in its left strand  34  being directed downwardly as indicated by an arrow  37 . Whereas right strand  32  extends tangentially on the right hand side of wheels  12  and  14 , left strand  34  keeps a distance D from a tangent line  36  on the left side of wheels  12  and  14 . This is effected by an upper guide  40  as well as a lower guide  42 . Guides  40  and  42  are positioned such that a sawing table is located therebetween (not shown) on which, for example, a wood board is pushed through band saw  10 , namely in the illustration of  FIG. 1  perpendicularly to the drawing plane. 
     Insofar, band saw  10  corresponds essentially to the prior art. 
       FIG. 2 , in a view from above, shows details of upper guide  40 . Guide  40  on the right hand side of band saw blade  20  in  FIG. 2  comprises a machine-mounted guide block  44  and on the left side a machine-mounted magnet guide  46 . The arrangement right/left of guide block  44  and magnet guide  46  may, of course, also be the other way round. The term “machine-mounted” is to be understood to mean that during operation of band saw  10  elements  44  and  46  are rigidly connected with the machine base, however, may be adapted to be adjusted e.g. for calibration purposes. Machine-mounted guide block  44  may be provided with a low-friction coating  50 . 
     Magnet guide  46  comprises a housing  54 . Within housing  54  there are provided a front electromagnet  56   a  as well as a rear electromagnet  56   b  facing a front area  58   a  and a rear area  58   b , respectively, of saw blade  20 . The terms “front” and “rear” are related to a feed direction  60  of a sawing material indicated at  61 , for example a wood board as already mentioned. 
     Electromagnets  56   a  and  56   b  are, preferably, of same design. The design with a U-shaped yoke indicated in  FIG. 2  is, of course, only to be understood as an example. As a matter of principle, any component may be used in the present context allowing to exert an adjustable force on band saw blade  20  in a contactless manner. 
     As two electromagnets  56   a  and  56   b  are used in any of the two magnet guides  46 , the entire assembly with two superimposed magnet guides ( FIG. 1 ) has four such electromagnets. 
     A front sensor  62   a  is associated to front electromagnet  56   a  and a rear sensor  62   b  is associated to rear magnet  56   b . Sensors  62   a  and  62   b  are adapted to detect a distance in a magnetic, capacitive, optical, acoustical or other manner. Within the magnetic guide  46  they measure a distance d between the right hand ( FIG. 2 ) surface  64  of magnet guide  46  and the left hand ( FIG. 2 ) surface  66  of band saw blade  22  in its front area  58   a  and its rear area  58   b , respectively. 
     When electromagnets  56   a  and  56   b  are excited with the same current intensity, i.e. when they exert the same magnet force on areas  58   a  and  58   b , then band saw blade  20 , as viewed in  FIG. 2 , will be displaced to the left or to the right, as indicated by a double arrow  70  while maintaining its orientation. If, however, the magnet forces of electromagnets  56   a  and  56   b  are different, then band saw blade  20  is twisted about its longitudinal axis as indicated by a pair of arrows  72 . By doing so it is possible to orient band saw blade  20  obliquely with regard to feed direction  60  of sawing material  61 . One can then make oblique or arc-shaped sawing cuts within sawing material  61 , in particular when sawing material  61  is conical or arc-shaped with regard to feed direction  60 , as is the case for naturally grown logs or parts thereof. 
     Accordingly, by selectively energizing electromagnets  56   a  and  56   b , one can as well compensate for lateral evasive movements as torsion of band saw blade  20 , being appropriate when sawing material  61  is guided with high power against teeth  21  of band saw blade  20  in feed direction  60  and saw blade  20  then buckles, or when band saw blade  20  enters into inhomogeneous areas of sawing material  61 , for example knots in a wood board. 
     The magnet force is preferably exerted as follows: 
     When band saw blade  20  is in its rest position, i.e. no magnet force is exerted, it rests on machine-mounted guide blocks  44 , for example by a conventional mechanical setting of a certain laterally oriented biasing force of about 100 to 1,000 N, e.g. 600 N. Directly before or after the starting of band saw  10 , band saw blade  20  is lifted off guide blocks  44  by a magnet force of e.g. 700 N being higher than the mechanical bias force of e.g. 600 N until it assumes a position between guide blocks  44  and magnet guides  46  as shown in  FIGS. 1 and 2  (distance d). This may be done irrespective of band saw blade  20 , as will be explained later, is twisted about its longitudinal axis or not. In this desired position band saw blade  20  is guided in a contactless manner. The position control is then effected around this desired magnet force of 700 N by modulation, e.g. by reducing or increasing the magnet force. 
     As an alternative it is, of course, also possible to manage without guide blocks  44  and to position magnet guides  46  on both sides of band saw blade  20  (not shown). In that case the control of the lateral position of the band saw blade would be effected through a selective excitation of magnet guides  46  on both sides of band saw blade  20 . Band saw blade  20  would then extend along a common tangent line of both wheels  12  and  14  as shown in the right half of  FIG. 1 . In this embodiment eight magnets altogether would be used at right/left, up/down and front/rear positions. 
       FIG. 3  shows a block diagram of an electronic control unit  74  which may be used for energizing electromagnets  56   a  and  56   b  and, further provides still other functions. 
     Control unit  74  comprises a controller  80 . Signals from sensors  62   a  and  62   b  as well as a desired value d s  indicating the desired distance between surfaces  64  and  66  are fed to inputs of controller  80 . From the actual values of distance d and from the given desired value d s  controller  80  in a manner known per se generates correcting variables for energizing electromagnets  56   a  and  56   b.    
     In embodiments of the invention which may also be used alone, a frequency analyzer  82  is, further, associated to controller  80 . From e.g. the signals of sensors  62   a  and  62   b  frequency analyzer  82  continuously computes the natural frequency or, as the case may be, several natural frequencies f 0  of band saw  10  which, however, may also be given as fixed value or values, respectively, determined beforehand. Normally, one has different natural frequencies f 0  when the band saw blade  20  is out of engagement with the sawing material and in engagement therewith, respectively, when the tension of the band saw blade varies etc. 
     The natural frequency f 0  of band saw  10  becomes apparent as a periodical oscillation of band saw blade  20  which mostly is a superposition of lateral movements and torsional movements. These natural oscillations essentially depend on the free length of band saw blade  20  between wheels  12 , and  14 , on the tension force, on the modulus of elasticity of band saw blade  20 , as well as on the system saw/sawing material at the prevailing operation parameters. 
     On the basis of a command variable supplied by frequency analyzer  82  controller  80  now generates a periodical correcting signal of even frequency but opposed polarity for electromagnets  56   a  and  56   b , such that the natural oscillations of band saw blade  20  are extinguished through interference. One has found that this given control with frequency f 0  known beforehand is more effective than a control on the basis of measured instantaneous values. It goes without saying that while doing so, one may not only take into account the fundamental wave of the natural oscillation of the band saw blade but likewise harmonic waves. 
     In a similar manner, a clock  84 , also associated to controller  80  acts in embodiments of the invention which may also be used alone. Clock  84  governs controller  80  with a command variable characterizing the periodical running by of butt joint  22  configuring an uneven discontinuity at magnet guide  40 . If, for example, band saw blade  20  has a length of 10 m and is moved at a linear velocity of 40 m/s, then butt joint  22  runs by magnet guide  46  with a frequency of 4 Hz or a clock period of 250 ms. The running by effects an evasive movement which is compensated for by a correspondingly gated excitation of electromagnets  56   a  and  56   b  with a signal of sufficient amplitude and opposed polarity. 
     Here, too, the frequency and the clock period, respectively, may vary, for example when a high load acts on band saw blade  20  and, hence, its drive motor. For band saws, three-phase asynchronous motors are conventionally used as drives. Such motors, however, have a load-dependent slip, such that the rpm, and, hence, the velocity of band saw blade  20  may fluctuate by about 1 to 3%. Therefore, the clock period of the butt joint  22  running by is continuously detected such that a dynamic compensation is also possible here. 
     In  FIGS. 4 and 6  three more embodiments of band saws having a basic structure corresponding to that of band saw  10  of  FIG. 1  are shown, irrespective of whether band saw blade  20  is twisted about its longitudinal axis. In  FIGS. 4 to 6  like elements are designated bay like reference numerals. 
     In the embodiment of  FIG. 4  a band saw  110  having four magnetic machine-mounted guides  146   11 ,  146   12 ,  146   21 ,  146   22  are shown being positioned as two pairs above each other in the area of left strand  34 . The two guides of a pair are positioned on opposing sides of left strand  34 . In the embodiment shown, the positioning is made such that left strand  34  coincides with tangent line  36  of the two wheels  12  and  14 . The guides  146   11 ,  146   12 ,  146   21 ,  146   22  in their design preferably correspond to the illustration of  FIG. 2 . 
     The positioning of guides  146   11 ,  146   12 ,  146   21 ,  146   22  on both sides along band saw blade extending along a tangent line touching wheels  12  and  14  has the effect that contrary to the embodiment of  FIG. 1  no basic force must be exerted by magnets within guides  146   11 ,  146   12 ,  146   21 ,  146   22  on band saw blade  20  extending symmetrically between them in the rest position, i.e. when band saw blade  20  is not displaced. 
     Further, the lateral displacement of band saw blade  20  may be effected faster as is the case for the embodiment of  FIG. 1  for the displacement to the right. For the two-sided positioning of guides  146   11 ,  146   12 ,  146   21 ,  146   22  the speed of displacement namely depends primarily on the electronic control of the magnets, i.e. the rise rate of the magnet force which may be set very high. In the embodiment of  FIG. 1 , however, the displacement to the right, i.e. away from the magnet of magnet guide  46  is effected solely under the influence of the mechanical spring constant of the system, in particular of band saw blade  20 . 
     By this positioning, moreover, the above mentioned measure may be effected particularly well, namely to twist band saw blade about a vertical axis in the area between guides  146   11 ,  146   12 ,  146   21 ,  146   22  by a corresponding polarization of electromagnets  56   a  and  56   b  as indicated with arrow  72  in  FIG. 2 . The extent of the torsion is increased within the scope of elasticity of band saw blade  20  by positioning such electromagnets on opposite sides of band saw blade  20  according to  FIG. 4 . Should no sufficient torsion angle be achievable in view of, on the one hand, the width of a practically possible air gap between band saw blade  20  and guides  146   11 ,  146   12 ,  146   21 ,  146   22  and, on the other hand, the width of band saw blade  20  in the feed direction  60 , then, according to the invention, one may configure guides  146   11 ,  146   12 ,  146   21 ,  146   22  adapted to be rotated about a vertical axis, as will be explained below together with  FIG. 6 . 
     If a sawing material  61  shall be sawn by band saw  10  which is not straight in the feed direction  60 , one can effect by appropriate excitation of the electromagnets that band saw blade  20  makes a cut which does not extend parallel to feed direction  60 , but may extend, for example, obliquely or arc-shaped. 
     In the embodiment of  FIG. 5  a band saw assembly is provided which, besides a first band saw  210   1  of the type described with  FIG. 1  also comprises a second band saw  210   2  being essentially identical in design with fist band saw  210   1 , however arranged mirror-symmetrically. Second band saw  210   2  also comprises wheels  212  and  214  having axes  216  and  218  lying on a vertical axis  226  and a band saw blade  220  running thereover. Its right strand  232  runs in the vicinity of left strand  34  of first band saw  2101 . Such paired arrangements of band saws  210   1 ,  210   2  are used to apply two parallel saw cuts to a sawing material running therethrough in one run. When doing so still another pair of band saws may be provided perpendicularly to the drawing plane of  FIG. 5  so as to apply four such saw cuts in one run. Such tandem band saws are known to the person of ordinary skill, for example from U.S. Pat. No. 3,318,347 and, hence, need not to be explained in further detail here. 
     In order to achieve an arrangement being as compact as possible, wheels  12  and  14  or  212 ,  214  of the two band saws  210   1  and  210   2  in  FIG. 5  are arranged near to each other. In order to nevertheless provide band saw blade  20  or  220 , respectively, with a certain tension, strands  34  and  323  in the embodiment of  FIG. 5  are each drawn inwardly by means of magnetic, machine-mounted guides  246   11 ,  246   12 ,  246   21 ,  246   22  towards their respective vertical axis  26  and  226 , respectively, i.e. in the illustration of  FIG. 5  strand  34  is drawn by distance D from tangent line  36  to the right, and strand  232  by distance D from a corresponding tangent line  236  to the left. 
     If one would double the embodiment of  FIG. 1  into a tandem band saw of the type of  FIG. 5  by folding same about a vertical axis, then a close lateral approximation of the two band saws  10  would not be possible because the two laterally projecting magnet guides  46  would stand in the way. Further, in the transitional area between the two band saws  10  one could only saw relatively wide boards. With the arrangement of  FIG. 5 , however, an can make saw cuts which, as compared to the embodiment of  FIG. 1 , could be somewhat more approximated, and could be somewhat less approximated as compared to the embodiment of  FIG. 6  being still to be described. 
     It goes without saying that also for the embodiment of  FIG. 5  mechanical guide blocks may be used on the side of band saw blade  20  opposite magnetic guides  246   11 ,  246   12 ,  246   21 ,  246   22  as was described in connection with  FIG. 1  above. 
       FIG. 6 , finally, shows another embodiment of a band saw  310  in which only two machine-mounted magnet guides  346   1  and  346   2  are provided. These magnet guides  346   1  and  346   2  are positioned such that their left surface ( FIG. 6 ) is flush with tangent line  36 . When guides  346   1  and  346   2  are activated, they draw left strand  34  against these surfaces, such that left strand  34  coincides with tangent line  36 . 
     In order to be able to also make oblique or arc-shaped cuts, band saw blade  20  may be twisted in a manner already described several times (arrow  72 ). Considering, however, that guides  346   1  and  346   2  simultaneously guide mechanically, they must be rotated simultaneously as indicated in  FIG. 6  with an axis  348  and an arrow  349 .