Now were getting on to some good discussions. I have some good experience in making and doing concrete work. I agree on the point of having too much aggregate in this application would be less helpful. The biggest factor in the strength and quality of concrete mostly lies in using too much water in the mix. Generally speaking though using less water makes the mix stiff and hard to work. Which leads to the necessity of using vibration to move the mix and get the air out and get the mix of sand and aggregate properly placed. The concrete lathe pictured would have come out world's better had the builder tapped the sides of the forms with a mallet for a while during and after for a bit.

There are a variety of other things that easily be added into the mix as well. Plasticizer would have helped for the flowability of the concrete while keeping water content minimal. Also some fiber would be very helpful here as well, metal reinforcement similar to chopped fiber is also available.

My thoughts on using concrete in a build mostly revolve around mass and vibration reduction of the finished lathe rather than a primary structural element. Another big sticking point for me is the adjustment of headstock, tailstock and ways to get everything square and straight. Then being able to keep it all in alignment over time. I think I'm starting to see some possible solutions to the challenges in building a useable machine.