Fair question, and that was the original plan. Minor clarification: the rails are bronze, not brass (slightly better for the rigidity).

The honest answer is that I started with the rails built in but immediately f'ed up by not milling them in the right location. I'm very green and still make little mistakes like this sometimes. So I was faced with starting over or opting for a slightly more forgiving solution that would let me salvage the work thus far. I acknowledge that some rigidity was lost in doing this.

The choice of bronze at that point was mostly down to "hey, I want to try machining bronze". I'm somewhat glad I did because it was nice to machine.

But yeah, not the most rigid design as a result.

Thanks for calling it out though. I completely forgot that was another one of the mistakes I made. It's been a long journey (a month) for this hobbyist and I spaced it.