1. Field of the Invention
The invention relates to a CNC controlled buttonhole sewing machine, comprising a needle drivable via an arm shaft; an x-y table, which is displaceable placeable in an x direction by an x drive and in a y direction by a y drive, and which supports a first and a second work piece clamp mounted for displacement relative to each other, and which comprises a pneumatically actuated displacement drive for displacement of the work piece clamps relative to each other from an initial position of spread by a length of spread into a final position of spread; and a control unit for control of the drives.
2. Background Art
The JUKI MEB-3200 Instruction Manual leaflet no. 02 29343316 describes a CNC controlled buttonhole sewing machine of the generic type. It comprises an x-y table which is displaceable on the sewing plane by two stepper motors as x and y drives. Two work piece clamps are disposed on the x-y table, holding a work piece thereon. By means of a pneumatic cylinder, they are displaceable in the x direction on the sewing plane, which is formed by the x-y plane, in mirror symmetry to a center plane. Both pneumatic cylinders are operable by means of a control command that is stored in the control unit. Provided on each side of the x-y table are setscrews, enabling the length of spread of each work piece clamp to be set from a non-modifiable inner initial position of spread to a final position of spread.
Shifting the work piece clamps enables the tightly clamped work piece to be stretched and spread into a tautened plain position. It is possible in this way to produce high quality buttonholes. Spreading the work piece also creates sufficient room for the needle, in case the buttonhole is first cut and then sewn i.e., with the sewing machine working in a so-called preconditioning mode. The displacement of each work piece clamp is in the range of 1.0 mm so that the clamps can be moved apart by a length of spread of 2.0 mm. The length of spread depends on various sewing parameters such as the structure of the work piece i.e., material, thickness and the like, the kind of threads used for sewing, thread tightening, needle size, possible use of a gimp thread and further parameters.
Drawbacks of the known sewing machine reside in that the initial position of spread and the length of spread i.e., the final position of spread, are attainable only by complicated adjustment jobs with the aid of implements such as a screw driver and measuring instrument. These jobs need a learned mechanic. The numerous components needed for adjusting are susceptible to wear. It is difficult to retrieve the adjustments that have been determined empirically and make them reproducible. Each buttonhole seam of varying width needs renewed adjustment; otherwise it is not possible to clamp the work piece in vicinity to the buttonhole seam that is to be sewn.
German patent 457 750 teaches a spreading device for the work piece clamp of buttonhole sewing machines; it has work piece clamps which are relocatable in symmetry to a center plane by means of a mechanism. Provision is made for a lever mechanism with a manually adjustable stop, by adjustment of which the length of spread is modifiable. This known device also includes the mentioned drawbacks.
U.S. Pat. No. 4,312,283 describes an automatic sewing machine, in which a CNC control unit produces coordinate data for motion of a sewing machine relative to an adjustable-size work piece holder. So as to avoid any collision of the needle with the work piece holder, information on the current setting of the work piece holder is fed to the control unit for it to automatically correct a sewing program.
It is an object of the invention to develop a buttonhole sewing machine of the generic type such that adjustment of the length of spread by an operator is easily feasible by simple means.
According to the invention, this object is attained by the features in which only the first work piece clamp is displaceable relative to the x-y table, and the second work piece clamp is non-displaceable relative to the x-y table; and means are provided for defining the displacement of the first work piece clamp relative to the x-y table by a given length of spread between a first stop position and a second stop position of the first work piece clamp; and the control unit stores data for triggering the x drive for reversed displacement of the x-y table by half the given length of spread. The solution according to the invention ensures that the length of spread can be preset very conveniently. Only one work piece clamp is movable in relation to the x-y table, compensating motions being produced by the possibility of displacement of the x-y table. This simple solution is possible although only the movable work piece clamp is designed for displacement by means of a pneumatically actuated displacement drive.