aluminium machining

[Jonathan]

Nothing difficult about that - there are various ways to get the bearing bores concentric and machine the bottom face parallel to their axis. It looks like you've chosen 7208 bearings, which are bigger than you need for a 25mm ballscrew - I'd use 7207 instead to reduce the moment of inertia and cost. Have you drawn the rest of it?

Edit: On the rotating ballnut mounts I have made I've tended to make the bearings a light interference fit, but I expect either would be fine. They need to be a sliding fit on the shaft to allow pre-loading.