Warren,
A fixed shaft would be better and the smaller diameter would be ok as long as its load capacity met your needs.
I forgot something which Scott ( thanks Scott!) picked up first time round - vibration. You're way better off with fixed rails or shafts rather than end supported shafts when you consider the vibration caused by actually moving and cutting.
When working out the deflections under the weight of the axis its easy to forget the real loads are the forces of moving the cutter through the piece. A 1/2" bit in a 3hp router can produce *way* more than 50lbs of load and it will do so in all directions. If a Shaft is just end supported then it can and will be deflected under cutting side to side as well as just up and down.
Have you looked at hardened shafting at McMaster or MSC? their prices for the real stuff are not much more than drill rod and I think cheaper than the site you linked to. You could get the 20mm open cars from vxd and shafting from McMaster? For a 48" travel you need some pretty hefty rails..
Misumi has some good and fairly economical stuff too.
Andrew  |