Better late than never reply, I suppose, but this is my old machine, and I did that first retrofit! You're at least the fourth owner, as I had to sell it to another guy when I moved into a workshop without a solid enough floor. It's a shame, because I'd have loved to had the time to fully mod it as you've done. We retrofitted a bridge router with linear motors for that new place, so it's not all regrets . There's a bunch of functionality in KMotionCNC that got added in because of that machine and working together with Tom on features we needed.
Anyway, here's some answers to the mysteries:
1- I was running a job shop with that as my only machine when we retrofitted it, so you are correct that the second I had it cutting parts I was back to work. I was also younger and less detail oriented in my work and there was basically no information online in 2010. The programmability of the KFLOP (and Tom) was really important to being able to hack together a solution, at the time the only other solutions I could find were building something in Mach3 (haha), LinuxCNC, custom code with Galil boards, or something like a Centroid retro where prices started at 20K US. The KFLOP was a total game changer, and in a lot of ways it still has no peers.
2- The drives had a circuit which blended the tachometer output with the control input and did some sort of differential foolery to make them work. If you left the tachs in, then you could run them in velocity mode with a +/- 5V input. But as soon as you took the tachs out of the circuit it was a mess. We were originally working with Tom on maybe implementing some sort of tachometer emulation using the encoders and the Kanalog, but decided to trace the boards and see if we could get around it entirely. Messing with an undocumented way of driving an undocumented servo drive as the first person doing such a retrofit was...tense...but it was really awesome once we got it going! It was originally resolvers and tachometers. Every tech I talked to insisted there was no way to reuse the motors and drives, which of course sounded like a personal challenge.
3- The performance compared to stock is 100% worth the retrofit. Stock acceleration was about 30-50ips^2, we got it up to something around 10X that. That meant on a 6" wide part with an oscillating finish pass at 125IPM you were basically halving your cycle time. At 200IPM the difference is 4X. The machine stock also had feed rates capped at 375IPM and the following error was not great over 200-250. Rapids were at max 600IPM default 400IPM I think, but the machine would need to run over a lot of distance to ever reach that velocity. I machined at 400-600IPM routinely post retro. The difference in finish with the tracking we could pull using the encoders was also night and day. It was twice the machine post retrofit, and capable of much higher quality. 1994 was a year where they were transitioning from inch ballscrews (which were small) to metric ballscrews (which were big) and so the metric machines were much better mechanically. You won't be able to get the same performance out of one of the inch machines. I think they changed from the DC motors to AC motors in 1998.
4- Those inserts in the table are the same pattern they use at Taylor guitars to mount their tooling. I originally got the machine rebuilt by the guy who used to run their machining department, and he did the same mods they do to their machines as I was planning to build guitars with it. That included the table inserts drilled, the door interlock removal, the air blast in place of flood coolant, and the dust extraction ports in the base of the machine. The air was mainly so I could use air turbine spindles as I did a lot of micro work and needed the 65-90K RPM.
5- There's a vid of its former life, at the resolution of the day.
6- That double-crescent dent in the middle of the table is from my first day with the machine (which was my first day ever touching a mill). I got it shipped from San Diego to Halifax, NS and basically had to learn everything alone. Accidentally dropped a pristine 1" carbide ball out of the spindle. My machining education didn't come cheap!
Those Fadals are really great machines to work on, and they lost a lot of resale value when the company shut down and changed hands a few years back. If you've got the space, a retrofitted 4020 is about the best value you can get per dollar in a mill.