Electric heating – Metal heating – For bonding with pressure
Patent
1999-05-05
2000-12-05
Shaw, Clifford C.
Electric heating
Metal heating
For bonding with pressure
2191171, B23K 1125
Patent
active
061569925
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for controlling a welding device and to a welding means.
In electrical spot and resistance welding it is often difficult to maintain the required quality scale for the welding spots.
It has been shown that the quality, especially the load capacity of a finished welding spot, depends on a plurality of factors, among others, on the electrode force, i.e. on the force with which the electrodes lie pressed against the material to be welded during the welding process, on the size of the welding current, and also on the state of the electrodes or pole caps. The latter, during welding, are subject to heavy wear and change of their surfaces, for example, in the form of a flattening due to wear, but also metallurgically, by the material of the electrodes or pole caps changing on the surface, as is the case in welding of galvanized sheets with copper electrodes. This leads to changes in the thermal and electrical properties of the electrodes or pole caps, conventionally used to execute a plurality of welding spots in time succession.
The object of the invention is to devise a process and welding means ensuring flawless welding spots, in spite of inevitable changes which occur mechanically, metallurgically, etc. on the electrodes or their surfaces by use and hereinafter are generally called "ageing".
SUMMARY OF THE INVENTION
In the invention, the welding current is controlled such that there is at least one testing phase between two successive working phases executing a given number of welding spots. Proceeding from the current intensity used in the working phase preceding the testing phase for the welding current, the welding current is increased in steps and, in doing so, at least one welding spot is executed until the spatter limit is reached. Then, in the new working phase following the testing phase, execution of the welding spots occurs with a welding current reduced by a value, compared to the maximum welding current of the preceding testing phase. This current drop is either determined empirically or by the fact that an adhesive limit must be reached. In each working phase the set value of the welding current is preferably kept constant.
Control sets an outermost maximum value for the welding current which cannot be exceeded under any conditions. In the testing phase, if the spatter limit is not reached for this absolute maximum value for the welding current, replacement of the electrodes is necessary. In the process, this is preferably displayed to the operator on a display.
In a typical embodiment of the process, the gradual increase of the welding current in each welding phase takes place in steps of 0.2 kA. At the end of each testing phase, the size of the welding current drops by roughly 0.4 kA. It is assumed that the distance between the spatter limit and the cement limit is roughly 1 kA. The number of welding spots executed in each working phase is, for example, 50.
In the process, specific feeling the way to the spatter limit is done and then, when the spatter limit is detected using a sensor is reached, the welding current is reduced by a value which is a fraction of the distance between the spatter limit and the cement limit. The pole caps or electrodes are selected and the basic value for the electrode force is set for new electrodes, i.e., electrodes which have not yet been flattened. In this way, welding spots with the required diameter are achieved. Ageing yields flattening of the electrodes and thus an enlargement of the possible welding spot. Optimum welding spots are ensured with consideration of this finding and the previously described control of the welding current for the entire service life of the electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is detailed below using the figures.
FIG. 1 shows, in a simplified schematic, a welding system which has spot welding tongs invention;
FIG. 2 shows, in a graphic representation, the change in the welding current in the welding current control process;
FIGS. 3-5, in a
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