There is no real good reason to have fixed ends on both sides of the ballscrews. You're assuming that the screw and cast iron will be the same temperature if you're trying to match CTEs, and this simply won't be the case. The ballnut will have a higher temp than everything else, followed by the ballscrew. And even then it'll be more localized in the area you're performing the work in. The cast iron itself should have very little heat if it's lubricated properly.