I have many guys asking me why their welders burn holes every now and then and what the big deal is about the polarity of the probes.
Resistance = heat, meaning that the higher your resistance, the more heat you will have. Probe pressure is very important and the harder you push on the probes the better it will make contact and the lower your resistance will be, causing welds that are colder and with too much pressure too cold to melt the metals together. If the probe pressures are not firm enough, it will cause a higher resistance causing too much heat and burn holes. That is why many companies that sells these welders have Weld Heads that are adjustable and make welds when a settable pressure is achieved. That does not mean that we cannot make any good welds by holding the probes in our hands.
Polarities are also very important. when a weld is made it forms a nugget between the metals being welded. This nugget is the melted metal between the two metals that joins them together and you want this nugget to be in the middle of the metals being welded.
When you do a weld the nugget gets attracted to the positive electrode more than the negative electrode and also gets attracted to the metal with the higher resistance. When you weld copper and steel together the nugget will form in the steel with the higher resistance and little or nothing will form in the copper making the copper not stick to the steel or a very weak weld.A way to move the nugget to the middle of your weld is to always put your positive electrode on the material with the lowest resistance, like copper and your negative electrode on the steel with the higher resistance. If you weld thick and thin metals together you will have to put your positive electrode on the thicker material(thick metals take longer to melt) and your negative on the thinner material.
By looking at the photo below, we can clearly see the electrode positions.
The welds at the bottom of the "stock welds" and " My welds"(I got this pic from a guy that bought one of my boards) are deeper than the top ones and kind of went through the metal, making them weak welds. The positive electrodes were at the bottom of these welds and the negative electrodes were at the top. To overcome the deep welds at the bottom you have to put more pressure on the bottom probes (making colder welds) and put less pressure on the top probes (more resistance = more heat which will make deeper welds).