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vtsteam:
Really enjoying this project Norman, nice casting work. too!

I have a concern about concrete filling -- which may or may not be valid. Up to those with experience combining these two materials to say.

Portland cement is highly caustic and aluminum is particularly subject to attack by alkalis. Will this cause problems, not only during the filling stage, but also down the road? I think even cured concrete remains basic for quite a long time. With moisture from condensation (aluminum a great conductor, concrete much less so, and porous) I just wonder if problems like corrosion would cause either separation, or the opposite, expansion, or both in different places?

Could be I'm wrong -- hope so -- concrete would be a nice inexpensive vibration damping mass if not.

ps, I remember reading in an old ME about a large lathe built after the War out of concrete as the main material. And here in Vermont, at the Precision Museum in Windsor, there is a late constructed of marble (quarried here).

RotarySMP:
Your castings are much better than mine. Nice work.

NormanV:
Thanks SMP. I have been reading, on another machining website, of problems that people have had with bubbles in their castings. There has been a discussion about the various degassing chemicals that can be used. I do not do any degassing, I just scrape off the slag and pour. I haven't noticed any bubbles in my castings. I am using mild steel for the crucibles, apparently that can cause problems with the castings. I haven't seen any problem with that either.
Steve, I share your concern about using concrete as a filler. I rather like Andrew's idea of using lead shot, First I will finish the machine and try it out to see if it suffers from vibration and then decide what to do.

mattinker:
Apart from the chemical problems Al concrete, concrete shrinks, if you put it inside a tube, it will shrink away from the walls, having no damping effect! To use concrete effectively, it needs to be outside. If you make the bed out of concrete, with the reinforcing bars and sleeves to bolt the ways to you won't have any problems. Lead shot sounds much better than concrete.

Regards, Matthew

vtsteam:
Lead shot does seem like the ideal.

I was thinking about the problem of concrete and it seems like there could be inexpensive solutions if cost was the determining factor.

Coating the inside of the aluminum with epoxy before doing the concrete pour would protect the aluminum, while using a relatively small amount of epoxy.

To deal with the shrinkage problem, there are a few possibilities -- one would be to use "hydraulic cement" which doesn't shrink.

Another would be to actually cast the concrete separately and just fasten it in place -- after all the rest of the machine is held together with fasteners. This would also allow you to remove it, if ever needed, which might be kind of handy. You might cast in pipe, or studs or nuts to facilitate fastening -- lots of different possibilities.

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