Calculating forces on linear bearings ...

[m_c]

For 4 rails, the forces are still the same, just the force is applied over more rails/bearings.

For simplicity, I'd assume each pair of rails/bearings will see an equal load, as the flex in the gantry/bearing mounts on each side is going to be pretty negligible. Of course, that depends on how you plan on mounting each rail in relation to the ballscrew.

[Jonathan]

Of course, that depends on how you plan on mounting each rail in relation to the ballscrew.

I don't follow that last sentence?

I'm using supported round rail, with the supported surface mounted flat on the bed.

Just realised I should also include the additional load on the bearings from the gantry accelerating. Need to know the where center of mass of the gantry is for that...can get it from the CAD model I suppose. Might be taking it too far!

[m_c]

Actually rephrase that, the pivot point for any cutting forces parallel with the x-axis is going to be the lead screw.