Thank you for the compliments, the video quality can be attributed to my Canon EOS 7D… ;-)
Well, when I was talking about tolerances, I was referring to the parts, mostly how accurately I could drill and tap. Not very much, but thanks to the tolerance tolerant design (pun intended), and the assembly instructions, the lead screws are not moving much, hardly at all, actually. There seems to be no tension, at least, no matter which end the lead nut is closer to. I think I could improve on the ball bearing mounts, they are a bit sloppy. I can see (and hear) them wobble a bit between those three 6-32 screws holding them from three sides.
As much as the end accuracy is concerned, the only way I so far "measured" it was that I drilled those holes on that copper clad board, and then tried the top solder printed transparency on (all originated from the same Eagle design). By the naked eye, the holes and the pattern on the transparency perfectly matched! :-) I know it doesn't mean much, since the board is small (about 2"x2"), neither my eyesight is as good as when I was 20, but I'm happy with it. This was my primary objective when building this machine. I, and my eyes, were tired of drilling those 0.024" vias, on the small boards you can design with the free version of Eagle. There were 127 of them on the control board…
I have an idea about how to measure the hypothetical top speed of the control board. I will hook a scope on the step pin of one axis (motor not connected), and start increasing the feed rate with G1 commands.
About the ACME lead screw. Yes, I was thinking about them, but boy aren't they expensive! :-( And of course, you would want matching lead nuts, equally expensive.
The only reason I am trying to increase the speed is that I want to fabricate aluminum parts, and with the minimum RPM my Dewalt 611 router can do (16000), even with a 1/8" Solid Carbide One Flute Upcut O Flute for Aluminum from Onsrud, I am close to the speed limit. I guess I will order one of those bits, and try it out. Maybe 600mm/min will eventually be enough.