I have had success drilling and tapping PLA, but only very carefully and by hand. if you go too fast the thread will melt and strip. (both whilst thread cutting and screwing in a bolt.) it will also obviously not hold as well, and will strip easier than prehaps expected.
At the moment i usually use a soldering iron on a brass insert, and melt the insert into a hole in the printed part.
this is very successful.
I have also put some on a lathe; using carbide metal cutting tools. Again only with PLA, and was pleased with the results. I did run coolant, and it was similar to machining some other plastics- too slow rubs and melts, needs to be fast enough to actually cut.
Hope that helps!