Originally Posted by
RCaffin
Distance between rails
I do not agree that you should reduce the distance between rails. The bigger the distance, the less wobble there can be.
Mind you, for a machine that size you will need BIG rails, to carry the weight if nothing else. Given that, and given good quality rails, adding a few extra cm to the distance between might not be too significant.
Position of axis motor
I definitely don't like where you have put the X axis motor - over to one side. It might in fact work, but I would very strongly urge that you put all motors midway between their rails. You will probably need something like a 25 mm double-nut ball screw anyhow, at least for the X axis. You will probably need a 20 or 25 mm ball screw for the vertical axis as well: it is carrying a bit of a load.
On the other hand, I do not think you will need dual axis motors for this machine, not on any axis.
Base and column
You specified these as granite I think? Do you have any idea what it is going to cost to get the channels or slots milled out of solid granite? Rather a lot I think! I machine granite and basalt, and I have some idea of what it takes. You might consider getting polished slabs and fabricating both the base and the column, with bolts, rawlplugs (or whatever brand) and epoxy. The end result would be about the same but much less expensive. Any monumental stone mason could set these simple slabs up for you: they would just be custom tombstones. (I had a long and interesting chat to one stone mason years ago.) YOU would have to do the alignment of the slabs of course, but you would have to do the alignment of the vertical relative to the base anyhow.
Axis Motors
Whether you are over-speccing the motors or not - not sure. I have a smaller CNC mill (NOT a router), with 500 W motors on the axes. I also have current meters on the motors, and I very rarely see any deflection on any axis meter. By the time the torque goes through a 3:1 belt reduction and then down a 5 mm pitch lead screw, it does not need much to destroy anything in the way. (Errr - yes.)
Cheers
Roger