Hi Jonathan,

Bearings:
I've not opened the bearing packets yet, but I'm expecting to have to make up a shim like I did last time. I've been able to make them work on the current machine so am not too concerned.

Ballnut mounts:
The spindle is as true as I can get it - note I'm at the mercy of the flatness of the supported rail on X, and on the flatness of the extrusion for the Y rail. Based on things I've made from the MK3 machine I think it will be OK. If not there are ways and means . . .

Stiffness:
Double Y ballscrew design went in at the outset because of the reasons you mention - it cannot rack so you can push the bearings closer together. But you are right, and it bothered me for ages, that for loads in the Y direction, where it rotates around X, there is a moment between the cutting tool (reaction) and the ballscrew (input) which is resolved in the bearings. As they are close together this moment is higher, meaning more deflection.
Ways around this are to lower the ballscrew and reduce the moment - I tried between the box sections but this makes the gantry even wider fore/aft. I tried underneath, which is very good for forces, but it would get swarf on it and if it hit a clamp it would be nasty. So it went on top. The mori-seki is like that so that sealed it.


p.s. What did you mean here?
". . .Just noticed that's you've put the ballscrews on the ends of the X and Y carriages, not at the center (of stiffness), so the stiffness will be a bit worse. . . "