Method of and apparatus for screwing together parts with little strength
This application is a continuation, of application Ser. No. 07/768,735, filed Oct. 10, 1991and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to an apparatus for screwing together parts with little strength, in particular an apparatus for joining shells, molds, foundry cores, preferably bottom cores and water-jacketed cores, as well as a corresponding method. 
     The joining of parts with little strength is always a problem, since a complicated handling or a costly transportation of such parts to a screwing station can lead already to a breakage of these parts. To avoid a breakage of the parts to be joined, it is also necessary to provide for both a fine feed and a slight tightening torque of the screw. 
     Cores as are employed in the foundry practice, are such parts with little strength, so that the problems arising in the practice will be described below with reference to such cores. 
     In the foundry practice, cores are formed by hand or with the aid of core molding machines in special, multipart core boxes or core dies. In general, the cores are manufactured by blowing, plugging and shooting. The individual cores are assembled or joined in separate operations by bonding or nailing. As a result, the method of manufacture is very complicated, labor-intensive and time-consuming (German Patent DE 35 26 265 and German Utility Model GM 75 15 919). 
     Further it has been found in practice when joining parts with little strength or when assembling individual cores by nailing or screwing that the material of the parts to be joined breaks easily as a result of shearing forces which occur in this process. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the present invention to first provide for an apparatus for joining parts with little strength, in particular for joining shells, molds and foundry cores, preferably bottom cores and water-jacketed cores, which allows to bring the parts to be joined, without any intermediate handling, directly from the manufacturing station to the screwing station. In so doing, it is intended to reduce considerably the risk of damage to the parts to be joined or even of breakage of the material. Furthermore, it is the object to provide for a corresponding method. 
     According to the invention, the foregoing problem is solved by providing a gripping device for gripping the parts to be joined and at least one screw driver comprising a screw spindle and a drive means for the screw spindle. In a particularly advantageous manner, the screw spindle is spring mounted at least in the direction of the screw feed. 
     It has been recognized, first, that a proper screwing together of the parts concerned requires a gripping device for gripping the parts to be screwed together and at least one screw driver comprising a screw spindle and a drive means therefor. It has further been recognized that the damage to the material or the breakages of the material which frequently occur when parts with little strength are screwed together, is primarily due to a too &#34;coarse&#34; screw feed. According to the invention, this &#34;coarse&#34; screw feed is reduced by a resilient support of the screw spindle at least in the direction of the screw feed, when the screw meets with a resistance on the side of the material. 
     In a particularly advantageous manner, a spring housing is arranged between the drive means of the screw spindle and the screw spindle to provide for a resilient support of the latter. This spring housing accommodates at least one spring element which is operative between the drive means and the screw spindle. 
     For purposes of balancing the feed differences between the advance of the screw spindle and the screw insertion speed which is defined by both the speed and the thread pitch of the screw to be driven in, without adversely affecting the contact pressure of the screw spindle on the screw and thus on the material of the parts to be screwed together, a driving pin is provided in a further advantageous manner, which is arranged on the spring element, and, thus, represents a connecting member or an operative connection between the spring element and the screw. 
     Furthermore, it has been recognized that besides a &#34;coarse&#34; screw feed, a too high tightening torque can lead likewise to a breakage of material in parts with little strength. It was therefore necessary to reduce the tightening torque, so as to avoid a breakage of the material, in such a manner, that tolerance ranges are no longer possible. It would be possible, though, to hold the screws by clamping jaws when they are injected automatically by means of compressed air, in which event the clamping jaws could apply the holding pressure, for example, via leaf springs. However, in so doing, the tightening torque would be considerably influenced when the screw head is pushed through the clamping jaws and against the holding pressure of the leaf springs, so that it would become impossible to maintain any longer the uniform tightening torque which is necessary to apply to the screw. 
     With respect to a limitation of the tightening torque, it is therefore of special advantage that the torque of the drive means or respectively the screw driver is defined by a preferably adjustable, automatic cut-off clutch which is provided in the drive means. 
     When the screws are injected automatically, clamping jaws are provided for holding the screws. The holding pressure is generated by means of pneumatic or hydraulic cylinder-piston arrangements. The clamping jaws open for the passage of the screws. 
     However, it happens specifically when parts with little strength, in particular brittle parts are joined, that particles separate when the screw is driven in, which subsequently penetrate into the region of the clamping jaws and increase there the frictional values, or even result in damage to the jaws. Therefore, in a further embodiment of the teaching of the present invention, the particles of the parts to be screwed together, which enter during the screwing into the region of the apparatus, are blown out or sucked off by means of a blower. 
     For purposes of being able to insert in a proper manner, with the apparatus of the present invention, also screws which are not in alignment with the original axis of the screw spindle, the screw spindle is resiliently supported in a further advantageous manner also in transverse direction to the direction of the screw feed. Thus, a small clearance of the screw is created orthogonally to the direction of the screw feed, which permits the screw spindle to tilt slightly. 
     As regards the preparation of the screws to be driven in, and having particularly in mind an automatic joining of the parts concerned, an injector is provided which feeds the screws to be entered by the screw driver. For purposes of ensuring a troublefree advance of the individual screws to the screw spindle or respectively to the parts to be screwed together, the injector is provided with a reflected light barrier for monitoring and controlling these operations. 
     Finally, the screw drivers can be changed in their position by means of spindles, whereby the screw drivers are adjusted to different screw-inserting positions. 
     By the method of joining parts with little strength, in particular of joining shells, molds, foundry cores, preferably bottom cores and water-jacketed cores, as is provided by the present invention, the parts to be joined are first picked up by means of a gripping device and jointly stayed. Then, the parts are positioned in the working range of a screw driver. In the case of screwing together foundry cores, the parts or respectively cores are positioned in an orderly fashion above one or several screw drivers. Subsequently, one or several screws are injected into the screw driver. The parts are screwed together, with a resilient support of the screw spindle balancing the screw feed, i.e., the screw insertion speed, in a particularly advantageous manner for the purpose of avoiding damage to the parts or avoiding a brittle fracture, and without adversely affecting the contact pressure. 
     For the purpose of avoiding damage to the parts to be joined, the operation of driving in the screw is further made advantageous in such a manner that the torque of the screw driver is defined or respectively controlled by a preferably adjustable, automatic cutoff clutch. 
     There are various other possibilities of configuring and further developing the teaching of the present invention in an advantageous manner. To this end, it should be referred, first, to the subsequent claims and, second, to the following description of an embodiment of the invention with reference to the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some objects and advantages of the present invention having been stated, others will appear when considered in conjunction with the accompanying drawings, in which 
     FIG. 1 a schematic side view of an embodiment of the apparatus according to the present invention; 
     FIG. 2 a schematic front view of the apparatus of FIG. 1; 
     FIG. 3 a schematic top view of the apparatus of FIG. 1; 
     FIG. 4 an enlarged exploded view of the parts to be screwed together, the screw driver and the screw; 
     FIG. 5 a longitudinal sectional view, partially enlarged, of the apparatus according to the present invention; 
     FIG. 5A is a fragmentary sectional view illustrating the screw clamping jaws and the air blowing means of the invention; 
     FIG. 6 an axial schematic view of a screw driver of the apparatus of FIG. 5. and; 
     FIG. 6A is a view similar to FIG. 6 but sectioned and rotated 90°. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1-3, 5 and 6 illustrate an embodiment of an apparatus of the present invention for screwing together parts 1, 2, with little strength. The apparatus is a screwing device for screwing together foundry cores, namely bottom cores and water-jacketed cores 1, 2. 
     The apparatus of the present invention includes a gripping device 3 for picking up the foundry cores 1, 2 which are to be screwed together, and a screw driver 6 comprising a screw spindle 10 and a drive means 11 for the screw spindle 10. 
     As shown in the illustrations of FIGS. 6, and 6A the screw spindle 10 is spring mounted at least in the direction of the screw feed. As can be noted, this is accomplished in that a spring housing 12 with a spring element 13 which is operative between the drive means 11 and the screw spindle 10, is arranged between the drive means 11 of the screw spindle 10 and the screw spindle 10 for a resilient support of the latter. Arranged on the spring element 13 is a driving pin 14. 
     Furthermore, the torque of the drive means 11 or respectively the screw driver 6 is defined by an adjustable, automatic cutoff clutch provided in the drive means 11. 
     For holding the screws 5 when they are automatically injected, clamping jaws 15 are provided. The necessary holding pressure is generated by means of pneumatic or hydraulic cylinder-piston arrangements 16. The clamping jaws 15 open only for the passage of the screws 5. 
     Particles of the parts 1, 2, which may enter into the region of the apparatus during the screwing operation, are removed by means of a blower 17 or via a suction device as illustrated in FIGS. 5 and 5A. 
     As shown schematically at 22 in FIG. 4, the screw spindle 10 can also be spring mounted transversely to the direction of the screw feed, so that it will be easy to also drive in screws 5 which are not in alignment with the original axis of the screw spindle 10. 
     Indicated at 4 in FIGS. 1 and 5 is an injector for feeding the screws 5 to be driven in by the screw driver 6. The injector 4 can also be provided with a device for singling the screws 5, which is not shown in the figures. Also, the apparatus preferably includes a reflected light sensor 20 for monitoring and controlling the feed of the screws 5. 
     The operation of the apparatus according to the present invention is as follows: The parts 1, 2 or respectively the bottom core 1 and the water-jacketed core 2 are assembled or joined, picked up by the gripping device 3, and jointly stayed. Subsequently, the stayed parts 1, 2 are moved above the screw driver 6. A screw 5 or screws 5 are pneumatically fed by means of the injector 4 through feed hoses 8 into the clamping jaws 15. The parts 1, 2 are then screwed together. The respective screwing position of the spindle 10 can be preset via an adjusting spindle 7. The screw spindles 10 with drive motor 11, spring housing 12, spring 13, and driving pin 14 serve to gently screw together parts 1, 2, with the screw feed or respectively the screw inserting speed being balanced, without adversely affecting the contact pressure which the screw spindle 10 exerts on the screw 5. The screws are held by the clamping jaws 15 which are actuated via a pneumatic cylinder 16. Only for the purpose of a complete illustration, ball bearings and a bearing sleeve are indicated at numerals 18, 19. 
     Finally, it should here be emphasized one more time that the teaching of the present invention is concerned with the joining of parts having little strength. The screwing together of foundry cores is here brought up only as an example.