Wouldn't it be a lot easier and cheaper just to add a couple of bearings to a straight-shank ER collet-chuck?
Has anyone tried using a lathe live tooling head as a spindle for a small mill? They tend to accept ER collets and are fairly small. The issues seem to be mounting a pulley and mounting it as a spindle.
Say something like this:
Remove the gear and bore out a timing belt sprocket to fit. Maybe drill and tap some mounting holes. Would this work? What kind of speeds and loads are these rated for? Angular contact bearing or deep groove? (Not just these particular ones, but live tooling heads in general).
The Taig has a pretty small bearing set (smaller than Sherline) and an aluminum case. I'd buy the Sherline steel cartridge spindle before that. The ER16 holder looks interesting, but the bearing ID would be 20mm, same as Sherline. So it wouldn't be much more rigid when all is said and done. Also I'd have to make an accurate case for it, which isn't the easiest thing to do with a Sherline lathe (concentricity and cylindricity to a few tenths). That would also mean buying a 4 jaw chuck, .0001 indicator, bearings, etc... without certain success.
That ER25 live tool would have larger bearings and a cast iron/steel case. It's just adapting a pulley and mounting the thing.
I'm surprised no one has tried this out yet. There aren't exactly a lot of quality choices for small mills that don't cost a fortune. You have the Sherline & Taig spindle, then there is a jump to a $1,300 Finley. It goes up from there. Not much in between. The Tormach spindle is interesting, but's it's just too darn big for the design.