I've been asked to do this again, so I hope it doesn't get buried, or at least that someone bookmarks it.
Anyway, what I do is not really "welding", it's more "brazing" or high temperature soldering using a silver alloy that's a little more than half pure silver and the rest other metals, designed to melt/fuse at around 1200F. Resistance type welding IS done for many armatures, and is probably the most widely used method, but doing it well/consistently/reliably requires some expensive equipment. Using a silver alloy for high temperature soldering or "brazing" produces a very strong and electrically sound connection and can be done on the cheap. I like that because, I'm cheap.
For a power source, you need a lot of current. Low voltage is best because it reduces the chance of striking an arc and vaporizing the com tab, but the car-starter type thing I use is cheap...just like me.
For a way to control when and where you deliver all that current, I use a boat ignition switch I got from some website. It's contacts are rated to handle all that current, which is important. It's a momentary switch, meaning it only "trips" while you press it, and is normally open, meaning that when you press it, it closes the circuit it's attached to. I mounted it in a plastic project box and just ran the two contacts to a pair of brass bolts that act as terminals on the top of the box.
The positive wire from the car charger goes to one of the box's terminals. When you trip the box's switch, the current runs through the other terminal to an alligator clip I use to hold a piece of gouging rod held against the com tab...and that will heat it up PRONTO. The negative from the car charger goes to a smaller alligator clip that's attached to the com. When the circuit is tripped, the current flows through the gouging rod to the tab and back to the charger. The gouging rod gets hot fast because of it's resistance, causing the silver to melt and make the permanent connection, a connection that should be good to 1200F. Copper melts at around 1800-1900F, so the silver should melt before the copper com tab. ***The reality is that the difference in melting points gets small when you trip the switch because it takes only a few short blips to melt the silver. If you feed only a bit too much current, the copper will melt and you can strike an arc that will instantly ruin the com. The best way to have at this is to quickly pulse the thing on and off with your foot until you see the silver fuse. Just holding the circuit closed will melt both the silver and the tab.
Here's the gouging rod in the package:
Here it is out of the package:
I cut it down to small 2 1/2" or 3" pieces that I sharpen, but not too sharp, or they produce too much heat in too small of an area too quickly to control. I also put a small "groove" on the tip to help prevent it from slipping off the end of the com tab like so (the tip shape gets tuned-up after every couple of coms):
The silver alloy I use comes from a nifty jewelry-making website called Rio Grande, and you can buy it by the foot. A little goes a long way, so it's cheap...did I mention I like cheap?
I also use the same alloy in a syringe that comes as a powder mixed with brazing flux:
I put a very small amount of the flux/silver goop on the tab and then put a teensy weensy bit of the silver wire on that. The goop holds the silver in place and helps the silver flow, as well as adding a bit more of silver. When ready to hit the current, the tab looks like this:
With the negative from the car starter attached and the gouging tip touching:
NOTE: I have the back jaw of the clip insulated with some high temp tape just for a little more insurance that the current flows through the tab I want it to. If you're not careful, the current might not do that...no bueno.
When done, you get something like this. I usually get a smoother joint, but the arm meters perfectly (*it's a #27 for the "A Walnut Is Born" thread) at .138 ohms per pole, so I know it's good:
You just have to lather/rinse/repeat for the other two poles and you get this:
The arm is ready (after testing on a meter after welding) for being tied:
Then it gets epoxied. After all that, it gets a fresh com cut, and then balanced and ground. This one is a hot little #27 wind and will spin very fast, so it needs all the good stuff!