The above info has changed, I found a better way of attaching the tip. Of course the hollow tube used as a tong isn't the best but I don't have access to any machines to make a heavy duty tong. Other have posted how they have accomplished this.
Even though the hollow tube isn't the best, it does the job and I haven't had any spot welds fail.
Below is the link for additional info on the 115/230 VAC spot welders.
http://www.gunco.net/forums/showthread.php?t=2095
The new tip installation method better because when the tip becomes damaged (after doing about 10 spot welds) it's very easy to replace. I reccommend only one tip per receiver (the tips are cheap). Also the tighter and more even you get the tips (on the material) the better the weld.
Oh, I will be doing a tutorial on how I spot weld the lower rails when I get to that part of my build.