A crazy thought and potential project.

[therbig]

Well, now that's very impressive! I wonder how stiff the setup is of the two I-beams compared to a cast-iron bed.

 

[John Hasler]

Well, now that's very impressive! I wonder how stiff the setup is of the two I-beams compared to a cast-iron bed.

That's the homemade lathe I wish I'd bought. The I-beams would be stiffer filled with concrete, though.

 

[cjtoombs]

There seems to be a lot of talk, and some misconception about concrete use in home built machine tools. The modulus of elasticity of steel is around 30 million PSI, the modulus of elasticity of 10,000 psi concrete is about 6 million (depending on the density, that number is probably near the high end of what it could be). So in general, while the concrete will make it stiffer, you would have to use either quite a lot of it or us it in such a way as to have very deep structures, which would compensate for the concretes low modulus to make a noticeable difference. Of course, a deep structure would have to have steel reinforcement in the tensile direction, or it would just crack. What concrete does bring to the table is vibration damping. I think adding concrete to a steel structure without specifically designing the structure to use the concrete effectively is a waste of time. Adding it for vibration dampening may be worthwhile, but building a steel structure that is very stiff may head off vibration problems anyway, by moving the natural frequency of the structure above the vibration frequencies that would be generated anyway. In short, if you didn't design the tool to begin with to use concrete to best advantage, it may not do any noticeable good. For a machine like this, if I were going to use concrete, I would probably make the bases for it out of steel reinforced concrete and bolt the machine structure to it and it to the floor to maximize it’s effect.