Daniel,
Yes the 4th axis is made from scratch.
It is stepper driven through a 32 - 10 belt drive.
The spindle is a genuine cheap copy of an ER20 2 Morse Taper Collet Chuck (£5 from eBay)
The bearings are Deep groove Radial 6804-2RS bearings, I didn't need a lot of preload so deep groove bearings are perfectly adequate.
I don't use it as a spindle but as a rotary indexer and so have not tried it at speed. I guess it will be limited by the kernal frequency... 200 steps * 10 microstepping * 3.2 ratio = 6400 steps per rev. at 25khz that would be 234 rpm
I have attached a pic of the axis "transparent" so you can see the inside.
Yes I have used it extensively and have not found any problems with it. The stepper is pretty weak but that is to be expected. It is fine for cutting but does not have the power to hold the collet chuck in place while you try and ram the main spindle into it (Yes, it has tried it on a particularly odd hiccup) it just freewheels which is quite nice as it saved the chuck.
In the collet is just a small fixture I use for making RC Helicopter Rotor Cores, a picture of which can be seen here along side the plastic part it replaces....
http://www.byte-sized.com/ebay/DF4_Core_Both.jpgTo give an idea of scale, it is made from 1/2" bar.
The travel on the Boxford is 190mm in X and ~125mm in Y
Thanks
Simon