In short, I'm looking for ways to reasonably precisely machine a "+"-axle hole, Lego compatible sockets, Lego compatible knobs, Lego compatible round peg/axle holes, and other cutouts for Lego-compatible parts on a small/hobby scale. Preferably by removing material from existing slightly over-size knobs / slightly under-size holes.
I've been really enjoying making lego-compatible parts, which I mostly make with a 3D printer then modify by hand to achieve the desired fit. This is cumbersome and it's very hard to get good results.
My FDM 3D printer isn't proving precise enough to produce a good fit for parts straight off the print bed. While I'm working on improving that, I'm looking for ways I can more easily "finish" or refine prints to produce a better fit for real Lego.
"+"-axle hole: Most importantly I'm looking for a way to cut a Lego-compatible "+"-shaped axle hole, or trim/expand an existing hole to improve fit. I just can't figure out a sensible way to cut this in a sufficiently uniform, reliable manner. Filing it out is just not an option. Tips for making helpful jigs or custom tools would be more than welcome.
Lego knob: Knobs to go into Lego sockets are close to 4.9mm in diameter. I can not for the life of me figure out a good way to cut a nice accurate and smooth sided/topped 4.9mm knob from a slightly over-size knob printed in plastic.
For ideal results the knob should actually be slightly non-round (by a something like 100μm) so it deforms slightly into place, but I can't see that being practical to do at home. A nice round smooth fit would be a good start.
socket to accept a Lego knob: I haven't found any sensible way to do this. The knob diameter is 4.9mm. Most places that accept knobs aren't a round socket, they're usually 4 pegs, but round sockets a bit over 4.9mm are also used. A 5mm metric drill bit is too large and produces a loose, sloppy fit.
smooth bore round axle hole: A 3/16 inch imperial drill bit (4.76 mm) seems to be about perfect for a Lego round axle/peg shaft. So that much is working well, I can just print slightly undersized with thick walls, then drill it out and get a great result.
Note that there's a disturbingly thorough workup on Lego dimensions available here.