Another update. I took the spindle apart this afternoon and was a bit surprised at what I found. The lower bearing inner bore was stuck fairly tightly to the shaft, and when I got it free there was brown staining on both the shaft and the bearing inner face. It cleaned off OK, but it's clear that the bearing was running much hotter internally than I thought - I think the staining was burnt oil.

Luckily I had a spare bearing, so I managed to bore a naffigravit off the inside of the bearing housing and reassemble the spindle. This has completely transformed it, as it now spins much more freely than it did before. It also runs more quietly and will reach a much higher off-load maximum rpm (it sounds like a jet when flat out!).

So far building this has taught me a few useful lessons:

1) Don't buy really cheap Chinese speed controllers unless you derate them a great deal.

2) Ball bearing fits are far more critical than I would have thought and make a significant difference to the way the spindle runs.

3) The 550 watt rated motor has more than enough power, even when derated by running it at 12V and 30 amps.

4) My machining skills are gradually getting good enough for precision work, even when using my extensively tweaked and modified Chinese mini-lathe.

Jeremy