In the last few days, I did a lot of testing and also a few changes to my setup.
First I did a test with a round 4mm brass electrode where you can see the wave pattern on the edge.
Then I did a test with a square-shaped 5*5mm brass electrode where you can see that the edge got cut with almost no wave pattern.
After that, I did a test with a smaller square-shaped 2x2mm copper electrode and also got a good edge cut.
While using the smaller electrode I noticed that the cutting speed increased, but also the wear of the electrode. The electrode that I used was 333mm long and for my plan to do workpieces with hundred of holes or for cutting thicker material it will likely be too short.
So I tried out to use 0.8mm welding wire, but because steel wire is not a good electrode material it did not work.
But after doing the test with the copper electrode I knew that copper is a good electrode material.
And so I got the idea to use just the copper wire from normal 1,5mm² power cables.
I also got a good edge cut with it.
With such a small 1.5mm² electrode the amount of removed material is much lower than with a 5*5mm electrode and so it's no longer a problem for the filter and that makes it possible again to do the cutting underwater.
With that I could get rid of the fumes and splashing and also the cutting quality got a bit better and less sooty.
At the moment I'm thinking about changing the design of the machine so that it will become closer to a normal 3D printer again where the 1.5mm² (∅1.38mm) replaces the filament.
So, maybe that could be the next step.