Several friends asked for more info on my technique. Here's a photo collage showing work on a 16 gauge Fox. Same technique would apply to an Ithaca Flues, subject of this thread, except that the end mill would be sized same as the Flues hinge pin.
Just as an aside, I'll add that I started doing this work on my own guns because of sloppy workmanship, multiple times, by gunsmith XYZ who does rejointing work. I worked part time as a machinist while in high school and college, and nowadays have access to a large machine shop owned by two friends. Sorry, I won't badmouth this gunsmith on the world wide web. Please don't ask for his name. Maybe he does better work for others? Last comment, there's no reason why most any gunsmith who owns a vertical mill can't do similar high precision work.
After TIG welding
Milling machine adjusted for +.005" on the X (long axis). In this case it was .0035" for "off face" measurement + .0015" for final fitting
New radius milled in the hook using an end mill sized for a 16 gauge Fox hinge pin
New radius is cut and is ready for fitting. Note how little "heat" propagated to the barrel hook
Fitting about 1/2 way complete using oil lamp soot
I like to use Dykem (machinist layout blue) for final fitting. Dykem is much thinner than soot. Almost done here but I'll add that most factory done work will show even less contact than in this pic..
