The invention relates to a centering system for a screen printing apparatus, having abutment wheels arranged in a platform for plates or disks to be printed, which come into contact with the outside margin of the plates or disks and secure their position for further processing.
Centering systems of this kind are known. The positioning of the plates or disks to be printed is accomplished as a rule by urging the plates pneumatically against fixed abutment wheels. Tolerances in the plates or disk dimensions cannot in such a procedure be compensated.
It is also known, however, to couple together oppositely lying abutment wheels mechanically, e.g., via chain spindles or cogbelts, so that if the abutment wheels shift, the confronting abutment wheels will move the same distance, thus assuring centering on the center of the disk or plate. This procedure permits a compensation of dimensional tolerances of the disks or plates. What is disadvantageous is the very complicated mechanical structure, which must be configured so that twisting and shifting of the plates or disks is possible. Also, the abutment wheels are defined as to their direction of movement by the design. Plates or panels with certain outside dimensions, e.g., the side panels of automobiles, cannot be positioned by such systems.
The invention is based on the task of improving a centering system of the kind described above so that with comparatively little expense a centering of plates or disks of any shape on the center of the platform becomes possible.
To solve the problem the abutment wheels in a centering device of the kind referred to above are applied to longitudinally adjustable servo shafts which lie above the platform level and in turn are applied to supports which are arranged on a stationary frame and above the platform. In this configuration centering is possible in a relatively simple manner; the confronting abutment wheels can be driven synchronously, so that disk or plate tolerances can be equalized.
Of course, it is also possible to fix some of the abutment wheels in place if the application should require it. Then the positioning is performed, in a development of the invention, by detecting the torques exercised by the servo shafts, which are sensitively detected. When each abutment wheel comes in contact with the edge of the plate or panel the torque increases. Thus, by detecting the torque, the position of the plate can also be detected. The result is the possibility of equalizing the desired and the actual position, so that the centering becomes possible in a simple manner.
In further development of the invention, all records of movement and detections of position can be stored in a memory, so that they can be recalled in the event of a new print order. The centering force itself can be called from memory in the event of a new print order. The centering force itself can be adjusted by means of the torque of the servo shafts. It is also possible to transfer plate data to the positioning system via a CAD system. This eliminates the sensing of the plates by the servo shafts.
In further development of the invention, the servo shafts are mounted swivelingly on the beams so that their direction of action is also variable. The beams can have holes arranged at intervals for this purpose and the servo shafts can be provided with pins which can be anchored in them. In this way adjustments of the axes of action can be performed without great effort. This may be necessary in the case of certain panel or plate forms, as will be explained further on.
In further development of the invention a transport line can be placed before the work platform and a conveyor belt running parallel thereto can be provided, by which the plates or panels can be raised by a lifting means to the platform. This lifting means can be advantageously provided with a plurality of ball guides lying in one plane, which can be lifted together above the plane formed by the conveyor belt to the platform. The plates or panels then lie on the ball guides and can be adjusted easily to the desired centering position by operating the servo shafts.
The abutment wheels are best spaced away from the servo shafts. They can be arranged on the servo shafts at variable distances apart, swivelingly for example, so that in order to contact the external contours of the panels or plates they can be turned downwardly to the working position.
It is also possible, however, to mount the abutment wheels with their shafts fixedly on the servo shafts and to arrange the beams for the servo shafts for raising and lowering together on the frame. This configuration assures greater strength and thus more stable arrangement of servo shafts and abutment wheels. The lifting movement of the beam system itself offers no difficulty. It can be configured, for example, in a manner similar to the way provided for raising and lowering in screen printing machine superstructures.