Abstract:
A buttonhole sewing machine for the production of buttonholes on a work-piece comprises a thread feeding mechanism on the path of a needle thread which is fed in a direction of delivery. The thread feeding mechanism comprises a pivotal thread lever which has an opening for the needle thread to be led through. Further, the thread level is drivable by a triggerable positioning motor to pivot between a zero position and several positions of thread extraction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a buttonhole sewing machine for the production of buttonholes on a workpeice of fabric, comprising a needle mounted in an arm, which needle is reciprocatingly drivable in a Z direction by a driving motor, forming a series of stitches, which needle is drivable by a jogging drive for the production of a zigzag seam by a motion of the needle relative to the workpiece, and which needle is drivable to pivot about an axis by means of a pivot drive; a hook bearing, which is disposed in a base plate, and which is drivable by a pivot drive to pivot synchronously and equiangularly relative to the needle about a pivot axis which extends in the Z direction; and a thread feeding mechanism in the path of a needle thread which is fed in a direction of delivery. 
     2. Background Art 
     U.S. Pat. No. 1,372,473 teaches a buttonhole sewing machine of the generic type for the production of eye-type buttonholes, in which stitch forming takes place in customary and known manner in a zigzagging sequence of stitches with a single thread chain stitch and a double thread chain stitch alternating. Single thread chain stitching only requires a needle thread, whereas double thread chain stitching needs both the needle thread and a hook thread or under-thread. This known buttonhole sewing machine is provided with a device for the control of the needle thread, in which, on a shaft that rotates at half the speed of the arm shaft, a pair of disks, which co-rotate therewith, and a cam are disposed for the control of a thread clamp. 
     U.S. Pat. No. 4,590,879 teaches a thread feeding mechanism of a sewing machine, in which a cam disk is provided, which rotates at half the speed of the arm shaft and which, by two portions on its periphery that are remote from the axis of rotation and by two portions that are close to the axis of rotation, acts on the thread supplied to the needle in such a way that the thread is tensioned i.e., it is pulled or loosened i.e., released. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to embody a buttonhole sewing machine of the generic type such that by simple means varying thread feedings are attained for the alternating production of a single thread chain stitch and a double thread chain stitch. 
     According to the invention, this object is attained by the features wherein the thread feeding mechanism comprises a pivotal thread lever with an opening for the needle thread to be led through; and wherein the thread lever is drivable by a triggerable positioning motor to pivot between a zero position and several positions of thread delivery. The measures according to the invention help ensure that, by means of a thread lever to be triggered by a positioning motor, handling the needle thread i.e., feeding the thread or advancing the thread and tightening or withdrawing the thread, can be suited to given conditions of sewing, which helps obtain an optimum appearance of the array of stitches. Sewing conditions comprise for example the motion of the needle relative to the workpiece i.e., the factual consumption of thread. Further factors of influence are the type of workpiece, the thickness of the workpiece, the thickness of the needle thread, the conditions of friction between the workpiece and the needle thread, the work tolerances of the elements of stitch formation i.e., needle and hook. The solution according to the invention also enables the thread to be advanced for, and in combination with, thread cutting. 
     Details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an illustration, partially broken away, of an elevation of a buttonhole sewing machine; 
     FIG. 2 is a view, on an enlarged scale, of a section, on the line II—II of FIG. 1, through a thread feeder disposed in the arm of the sewing machine; 
     FIG. 3 is a plan view of the thread feeder along the arrow III of FIG. 1 on an enlarged scale; and 
     FIG. 4 is an illustration of operational diagram. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The single/double thread chain stitch sewing machine seen in FIG. 1 comprises a housing  1 , which substantially consists of a so-called base plate  2 , a standard  3  and an upper arm  4 . An arm shaft  5  is rotatably run in the arm  4  and can be driven in rotation by means of a driving motor  6  via a belt drive  7 . 
     Mounted in the arm  4  in bearings  9 ,  10  is a substantially vertical and hollow needle bar  8 , which can be driven to reciprocate by the arm shaft  5  via a crank drive  11 . At its lower end, the needle bar  8  is provided with a needle  5 . 
     Underneath the needle bar  8 , a hook bearing  13 , which comprises two commercial chain stitch hooks (only roughly outlined in FIG. 4) known for example from U.S. Pat. No. 1,372,473, is mounted in bearings  15 ,  16  for rotation by approximately 400° about a vertical pivot axis  17  which extends in the Z direction. Actuation of the hooks takes place via a driving connection  14  derived from the driving motor  6 . Rotary actuation of the hook bearing  13  takes place via two belt drives  19 ,  20  by means of a stepper motor which serves as a pivot drive  18 . The needle bar  8  is mounted in the bearings  9 ,  10  not only for displacement in the longitudinal direction, but also for rotation about a pivot axis  17 . It is driven synchronously and equiangularly relative to the hook bearing  13  by the pivot drive  18  via a setting shaft  21 , which is drivable by the belt drive  19  and extends in the Z direction, and by a further belt drive  22  so that the needle  12  and the hook bearing  13  are synchronously and equiangularly pivoted abut the pivot axis  17 . 
     The needle bar  8  and the needle  12  are drivable to job laterally, i.e. to swing, by means of a needle jogging drive  23 . The lateral jogging motion is accompanied with a deflection of the needle bar  8  relative to the pivot axis  17 . Due to the rotatability of the needle bar  8 , the jogging plane of the needle bar  8  with the needle  12  is displaceable synchronously and equiangularly relative to the position of rotation of the hook bearing  13 . A stepper motor  25  is provided for the lateral jogging of the needle bar  8 , this stepper motor  25  acting on the needle bar  8  by way of a jogging shaft  28 . To this end, provision is made for a transmission  29  (not shown in detail), which is known from U.S. Pat. No. 1,991,627 and U.S. Pat. No. 6,095,066. 
     An X-Y table  30  (only roughly outlined) is disposed on the base plate  2 . Design and actuation of the table  30  are also known from U.S. Pat. No. 6,095,066. A clamp  31  is mounted on the table  30 , fixing a workpiece  32 . 
     On the upper side of the arm  4 , provision is made for a needle thread feeding mechanism denoted as a thread feeder  33  for a needle thread  34 , which is fed to the needle  12  through the hollow needle bar  8  from the upper end thereof. The thread feeder  33  comprises two thread guide webs  35 ,  36 , which are disposed at a distance from each other in the X direction and each of which has a hole  37 ,  38  in the vicinity of its upper side for the thread  34  to be threaded through. The holes  37 ,  38  are in alignment in the X direction. The two thread guide webs  35 ,  36  are mounted on a joint support plate  39  which is fixed to the upper side of the arm  4  by means of screws  40 , closing a recess  24  in the arm  4 . A thread level  41  is disposed between the two thread guide webs  35 ,  36 . This thread lever  41  passes through an opening  42  in the support plate  39  of the thread feeder  33 . On its end located above the support plate  39 , the thread lever  41  has an eye  43 , through which the needle thread  34  is threaded between the holes  37 ,  38 . The thread level  41  is fastened on the shaft  44  of a stepper motor  45  which is mounted on the underside of the support plate  39 . 
     Needle thread tighteners  46 ,  47  are provided on the path traveled by the needle thread  34  on both sides of the thread feeder  33  i.e., one after the other in the X direction, namely a first needle thread tightener  46  disposed upstream of the thread feeder  33  in the director of delivery  48 , and a second needle thread tightening  47  disposed downstream of the thread feeder  33  in the direction of delivery  48 . The two thread tighteners  46 ,  47  are structured identically with the exception of the conical coil springs  52 ,  52   a  inserted therein; therefore, only the thread tightener  46  is described. It comprises a bearing plate  49 , which is mounted on the support plate  39  and serves as a first clamping jaw and from which a threaded bolt  50  projects upwards i.e., in the Z direction. Guided for displacement on the threaded bolt  50  is a clamping disk  51 , which bears against the plate  49  and serves as a second clamping jaw and against which a prestressed conical coil spring  52  bears from above, it being possible to prestress the conical coil spring  52  by means of a clamping nut  53 . Subject to friction that corresponds to the prestress of the second clamping jaw, the needle thread  34  travels between the clamping disk  51  and the plate  49 . The conical coil spring  52  is such that the needle thread  34  tautened by a tension force of 1.6 N. By way of contrast, the conical coil spring  52   a  inserted in the tightener  47  is designed for the needle thread  34  to be tautened by a force of approximately 0.3 N. On both sides of the tightener  46 ,  47 , needle thread guide webs  54  are provided, each having a hole  55 , so that guidance of the needle thread  34  in the respective tightener  46  or  47  is ensured. On the side turned toward the thread feeder  33 , this job can of course also be performed by the guide webs  35  or  36 . 
     Provided in the arm  4  is a moment of momentum transmitter  56  as a position transmitter, comprising a slotted disk  57 , which is mounted non-rotatably on the arm shaft  5 , and a forked light barrier  58 , which is mounted on the underside of the support plate  39 , with the slotted disk  57  engaging with the formed light barrier  58 . Moment of momentum transmitters  56  of this type are general practice and conventionally have a given number of slots, for instance 400, arranged at regular angular distances on the slotted disk so that, upon a rotation of the arm shaft  5 , the moment of momentum transmitter  56  emits a number of signals equal to the number of slots in the slotted disk  57 . 
     FIG. 4 illustrates the overall control principle. A central control unit  59  is provided, having an input equipment  60 , for instance in the form of a keyboard. Via lines (seen in the drawing) this control unit  60  is connected to the stepper motor  45  of the thread feeder  33 , to the driving motor  6 , to the moment of momentum transmitter  56 , to the stepper motor  25  of the needle zigzag drive  61 , to the sensor of fabric thickness  62  disposed on the X-Y table  30 , to a Y driving motor  63  of the X-Y table  30  and to an X driving motor  64  of the X-Y table  30 . A thread cutter  71 , which is triggerably by the control unit  59 , is disposed in the hook bearing  13 . FIG. 4 also roughly outlines a first hook  65  for a double thread chain stitch and a second hook  66  for a single thread chain stitch. 
     The sewing machine specified serves to sew buttonholes into the workpiece  32 , for which two stitches are sewn crosswise of the lengthwise direction of the buttonhole seam at a distance from each other i.e., zigzagging; a first stitch of these two stitches is sewn as a single thread chain stitch, using the second hook  66 , whereas the second stitch is sewn as a double thread chain stitch, using the first hook  65 . The first stitch i.e., the single thread chain stitch, is made without the under-thread, whereas the second stitch, the double thread chain stitch, is made with a hook thread  67  being supplied. This is generally known practice. Since two different chain stitches are sewn alternately, also the needle thread  34  must alternately be supplied to the needle  12  or withdrawn. This takes place by means of the specified thread feeder  33 . 
     Referred to its zero position  68  i.e., referred to a vertical position, the thread lever  41  is pivotal about a pivoting angle a in one direction or about a pivoting angle b in the other direction. The zero position  68  is defined such that, in this position, the needle bar  8  together with the needle  12  takes its elevated position i.e., its position in the upper dead center. The thread lever  41  pivots by the smaller pivoting angle a into a first position of thread extraction  69  when the needle  12 , for sewing a single thread chain stitch, cooperates with the second hook  6  that has no thread. The thread lever  41  is pivoted by the greater pivoting angle b in the opposite direction into a second position of thread extraction  70  when the needle  12 , for sewing a double thread chain stitch cooperates with the first hook  65  that leads the thread. To this end, the stepper motor  45 , which triggers the thread lever  41 , is triggered by the control unit  59  in dependence on the signals, emitted by the moment of momentum transmitter  56 , of the angles of rotation of the arm shaft  5  and thus of the position of the needle  12 . The measure of the respective pivoting angles a or b and thus the measure of the extracted needle thread  34  is fixable to correspond to details given by the operator in accordance with a certain sewing program. Automatic adaptation may also take place in dependence on the thickness of the workpiece  32  detected by the sensor of fabric thickness  62 . Of course, the thread lever  41  will regularly pivot back into the zero position  68  when the needle thread  34  extracted during a pivoting motion into the position of thread extraction  69  or  70  is needed in the course of stitch formation. During the sewing job, a tension force of approximately 1.6 N+0.3 N=1.9 N is produced in the needle thread  34  by the action of the tighteners  46 ,  47 . As a rule, the tension force the tightener  46  exercises on the needle thread  34  will exceed by three to ten times the tension force the tightener  47  exercises on the needle thread  34 . 
     When the sewing job that serves for producing a buttonhole seam is finished, the needle  12  is stopped close to the upper dead center. While the needle  12  moves from the lower to the upper dead center, the thread level  41  is pivoted by a pivoting angle c into a position of thread extraction  72 . The pivoting angle c is greater than the pivoting angle b. Upon this process of thread extraction, the quantity of needle thread  34  taken from the thread supply (not shown) is greater than the quantity that is extracted upon a pivoting motion by the angle b into the position of extraction  70 . As the needle thread  34  is held in the vicinity of the stitch forming tools during this process of extraction, delivery from the thread supply  73  takes place although the tension force of the tightener  46 , which works as a counterforce in this regard, is higher than that of the tightener  47 . Once the quantity of thread has been extracted, the thread lever  41  is pivoted back into the zero position  68 . The extracted thread is located in the area between the tighteners  46  and  47 . 
     Then the thread cutter  71  is triggered by the control unit  59  and the needle thread  12  is cut. The tension force which, in this case, acts in the needle thread  34  is only approximately 0.3 N, because tension force is exercised on the needle thread  34  only by the tightener  47  neighboring the needle bar  8 . When the needle thread  34  is cut through by the thread cutter  71 , relief of the needle thread  34  takes place between the cut performed by the thread cutter  71  and the tightener  47  and that by the tension force of approximately 0.3 N exercised by the tightener  47 . Consequently, there is no unthreading the needle thread  34  from the needle  12 . 
     When a new buttonhole sewing job is started, sufficient needle thread  34  for the initial stitching jobs is available between the tighteners  46 ,  47 . The quantity of thread available after the thread cutting job corresponds to the one that has been pulled off in the position of thread extraction  70  by the thread lever  41 . Of course, also the size of the pivoting angle c can be set in the control unit  59 .