After running through multiple repair cycles on the injector head, I finally gave up about two weeks ago and started building another injector. The one I have was designed to be bolted on to the chamber. I used that design because I didn't have a welder and I thought it would be nice to be able to remove the head and swap it out. Since I now have a welder and have been practicing with stainless steel, I've since come to the conclusion that welding is a MUCH simpler way to go (which is the advice that John Carmack gave a while back). If you do end up swapping injectors, its really not a big deal at all to cut the old one off and weld the new one on.
The new design also moves the spark plug off of the injector itself and onto the chamber. This was done to free up the space and make the injector much simpler to machine.
While I don't quite hate stainless as much as Paul Breed, I do find that it is much less forgiving than other materials. The main thing is to simply be patient - use the recommended cutting speeds and feed rates which are an order of magnitude less than aluminum. On the plus side, it is showing me some bad machining habits that I didn't know I had.
Below is a picture of the part I finished today, the cover for the fuel manifold on the injector. Also in my hand are two ruined 1/8" cobalt end mills. One I simply broke from over-feeding (feeding to fast/too much force). The other one I literally cooked from over-heating. It still cuts, but it more closely resembles a ball-end mill :)
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