Hi Rob
Gotcha now, i was looking into this the other weekend on one of the US CNC forums for a cheap ballscrew alternative. so all you need is 3 pieces of the bar threaded with 2mm pitch, diameter will have to be bigger than the smallest bearings you can get unless you were just going to put oilite bushes in the end plates, still relying on accuracy of the studrod but as it's rolled it should be near enough for making bits of plastic.
Mike