20 years ago I bought a Century MIG welder. It came with a spool of wire marked "ER70-S6." It was easy to grind, and soft enough to mill, drill, and tap, like when welding up and putting a new threaded hole back in the proper place.
When it ran out I bought a spool of "Forney" brand ER70-S6 at the local hardware store. It's hard to grind, and hard enough a file won't bite without considerable pressure. A high-speed steel drill bit just dulls. Forney wire is about all I can buy locally.
I've been through a couple of other brands of ER70-S6, plus a roll of something called "EZ-Grind", which wasn't, particularly. The specs I've found for ER70-S6 *say* it's supposed to be soft and machineable like the first roll, but that's not how it's working for me.
All I want is some .028" or .035" "chicken-wire grade" MIG wire. It doesn't have to have umpty-thousand PSI of tensile strength or be harder than a file; it just has to stick stuff together and be about as machineable as plain old hot-rolled angle-iron-grade steel.
What kind of wire should I be looking for? The "local" welding supply store is 30 miles from here; I have to make a run next week to get the gas bottle refilled. They were unhelpful when I asked them about machineable wire, but if I know what to ask for, they might have what I need on the shelf.