Quote Originally Posted by devmonkey View Post
another option is just to mill a few mm each side of the slot so the rail can sit flat, this would also preserve the 3 planar points (both corners and centre) in case I mess up.
That's likely not such a bad idea as it could give some lateral stabilit - Hiwin suggest sitting the rails in such a groove in the manual. However you'll need to do it accurately as it removes the option for tweaking alignment to some extent.
.Then again the centre of this extrusion will be stiffer than either slot as it i supported equally whereas the slots are supported on one side by the unsupported corner. No idea really.
Routercnc's point about using a tapped strip rather than individual floating nuts might help this.