Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
1998-11-09
2001-03-06
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S089000
Reexamination Certificate
active
06198065
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of resistance welding in which the welding electrode movement takes place at least by means of one working piston-cylinder unit.
Furthermore, the invention relates to a resistance welding plant with a working piston-cylinder unit for carrying out an electrode movement during the welding process.
Resistance welding is a generally known method in which, for example, two pieces of sheet metal are connected together, in that they are pressed together by two electrodes, of which at least one is movable, and fusing and subsequent solidification of the material takes place at the joint location by means of adequate electrical resistance heating through the welding current.
The quality of the welded connection is decisively influenced by how good the contact is between the electrode and the workpiece, because the contact resistances and thus the distribution of the heat produced are influenced by this contact.
The manner in which the electrode is placed onto the workpiece is thus, first of all, of significance, because if the placement movement is too rapid, the electrode rebounds so that the contact pressure force initially oscillates and the decay of this oscillation must be waited for prior to switching on the welding current, which, however, reduces the working speed in a disadvantageous manner. During spot welding rapid placement can lead to an uncontrolled high striking energy, which leads to cold deformation and rapid wear of the electrodes.
During projection welding the too rapid placement brings about an uncontrolled cold deformation of the projection prior to switching on of the welding current, and thus to an uncertain welding result.
One has hitherto attempted to regulate the speed of placement through the association of damping and restrictor valves. One has achieved a certain improvement of the placement behavior through the use of double stroke and feed stroke units.
The force responsible for the contact pressure between the electrode and the workpiece must, however, not only be made available at the start of welding but rather also during welding.
Of significance in this connection is the fact that the material to be welded becomes soft during the welding, i.e. pasty, and fuses at the joint position. When the electrode cannot be adequately quickly replenished, the force between the workpiece and the electrodes can, in unfavorable cases, reduce to such an extent that no adequate electrical contact is any longer present between the electrodes and the workpiece. This leads, having regard to the extremely high current density, to disadvantageous spray formation and premature electrode wear. If, during projection welding, the electrode is not adequately rapidly replenished, the contact pressure between the impressed projection of the one piece of sheet metal and the counter-sheet reduces, whereby the softened projection can spray away or pore formation can arise in the welding spot.
In the previously known methods, the electrode force is primarily made available during welding by single stroke cylinders, or by the working stroke of twin or feed stroke cylinders, which are, as a rule, pneumatically actuated.
SUMMARY OF THE INVENTION
It is an object of the present invention is to improve the welding of workpieces, and in particular the quality of the welding and/or the welding speed is to be increased. In this respect a rapid, precise follow-up movement of the welding electrode should be ensured during the welding procedure.
This object is satisfied in a method of the initially named kind in that the welding electrode is located prior to the start of the welding process in a position ready for use in which it gently contacts the workpiece or in which a distance in the range of tenths of millimeters exists between the welding electrode and the workpiece, and in that, during the subsequent welding procedure, a pneumatic loading of the working piston-cylinder unit takes place, with the maximum possible stroke of the working piston-cylinder unit starting from the position ready for use, corresponding to
1 to 40 times the follow-up movement path of the collapsing workpiece projections during projection welding, or
1 to 120 times the penetration path of the welding electrode into an originally hard workpiece, which becomes pasty during the welding process when spot welding, or
1 to 60 times the penetration path of the welding electrode into an originally soft workpiece which becomes pasty during the welding process when spot welding.
Furthermore, this object is satisfied in an apparatus of the initially named kind in that the maximum possible stroke of the working piston-cylinder unit corresponds to
1 to 40 times the follow-up movement path of the collapsing projections during projection welding, or
1 to 120 times the penetration path of the welding electrode into an originally hard workpiece, which becomes pasty during the welding process when spot welding, or
1 to 60 times the penetration path of the welding electrode into an originally soft workpiece which becomes pasty during the welding process when spot welding.
Both in the method of the invention and also in the plant of the invention it is of advantage
when the maximum possible stroke of the working piston-cylinder unit corresponds, starting from the position ready for use, to 1 to 20 times, in particular 1 to 10 times, and preferably to 1 to 5 times the follow-up movement of collapsing projections during projection welding, or
when the maximum possible stroke of the working piston-cylinder unit corresponds, starting from the position ready for use, to 1 to 60 times, in particular to 1 to 30 times, and preferably to 1 to 20 times the penetration path of the welding electrode into an originally hard material, which becomes pasty during the welding process when spot welding, or
when the maximum possible stroke of the working piston-cylinder unit corresponds, when starting from the position ready for use, to 1 to 30 times, and in particular to 1 to 15 times, and preferably to 1 to 10 times the penetration path of the welding electrode into an originally soft material, which becomes pasty during the welding process when spot welding.
In the context of the invention it has been recognized that in the previously known arrangements the maximum possible stroke lengths of the single, twin or feed stroke cylinders are of large dimensions, so that, during the welding process, a relatively large air volume flows into the cylinder space above the working piston and is compressed, and mainly so that a larger compressed air volume must escape from the cylinder space beneath the working piston. The long airflow times which arise through this influence the follow-up movement behavior of the electrode during the welding very unfavorably.
Since, in accordance with the invention, the maximum possible stroke of the pneumatically actuated piston-cylinder unit out of the position ready for use essentially only corresponds to the required working stroke of the welding electrode for the penetration path of the spot welding electrodes into the pasty material during spot welding and/or for the follow-up movement path of the collapsing projections during projection welding, only a very small working cylinder volume has to be loaded with compressed air in order to move the working piston and thus the working electrode in the direction of the workpiece. At the same time only a very small volume beneath the working piston need be displaced out of the working cylinder.
This requires less time than the previously known methods so that the pressure relationships required for the necessary welding electrode pressure force set in very rapidly in the working piston-cylinder unit. Moreover, through the low extent of the pressure loaded volume, only a smaller compression effect has to be brought about than with larger volumina, which leads to improved force action of the electrode.
Starting from the fact that the welding electrode gently contacts the workpiece in the position ready for use, o
Shaw Clifford C.
Townsend and Townsend / and Crew LLP
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