I started thinking about making a big milling machine a long time ago, and one of the materials I thought about using was reinforced concrete. This is the material I have ended up wanting to use because it's cheap, and I don't want to spend a lot of time figuring out how to work with other materials. I also thought about making it from cast iron or welded steel. One of those ideas was to get some old train rails, or something similar and just weld a base and machine it afterwards. Getting the machines to do that would not be an easy task, so I dropped it.
You can sand concrete to achieve flat surfaces after it has cured and "stopped" creeping. Concrete will always move around a little, but after a couple months it's reduced to so little that other factors in the machine play a bigger part in it. So I will cast inserts into the concrete. These will be welded to the rebar inside the frame for extra support. To give the concrete some more tensile strength, I will use threaded rods to post-tension it.
There are two designs that I have been thinking about. I'm not yet done with the 2nd one, so I will only post a picture of the first.
Travels are:
X: 800mm
Y: 350mm
Z: 450mm
The base of this design weights 500kg, and the table weights 100kg. With rails, motors and everything else, it should be around 750-800kg.
I have kind of dropped this design because of something I got from a friend of mine today (PS. if you are reading this, then thanks a billion! I owe you a beer!).
These rails are 1200mm long, and the trucks are 200mm long. They are heavily preloaded, and they are from a high end VMC (Mazak). AS you can see in the last picture, they are pretty big. Because theres only one truck on each rail, I will be making something more like a moving gantry machine. Because the trucks are so heavily preloaded, and they are as long as they are, I bet that they will be enough for this machine. I hope to have a picture up of the new design in a couple weeks.