Re: 'Reference Face' for a steel CNC Router build
Depends how serious you are and what tools you have available Steve.
Without a mill you'd need to pay someone to machine the RHS flat on preferably all 4 sides, and you'd need to be sure they actually do a good job within tolerance, which is a different matter again.
A lot of people automatically go to the hardest option to achieve the goal, I would say in this case if you're limited by tooling/costs, then a decent dial indicator can be your friend, and simply shim the RHS to achieve a consistent result across the length.
If you're using mounted supported rails (which you should) as opposed to unsupported rails (crap), then go up in thickness a size, this will greatly help alleviate the adjustments required, as the mounted rail will have great rigidity.
This is a few ideas for a start anyway. I'd strongly suggest try not to over think it. I'm all for over engineering, but I have a lathe and mill at my disposal so that's achievable for me, if it's not achievable for you, make some design changes to cater for the compromises.
A few dollars extra spent can mean the difference between failure, success, and a working but really crappy end result you may not be happy with.
cheers, Ian
It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!