Re: Filling steel tubes with epoxy granite- machine before or after?
As Peter has pointed out, the first thing is to make the gantry / machine as stiff as possible.
Go big on the gantry tube. Increasing the section size is much more effective than increasing wall thickness. Look up a table for steel RHS moment of inertia.
However, I don't think EG fill is a waste of time for a metal machine at least. Chatter and cutting induced vibration are major limiters of performance, both in part quality and tool life. The purpose of EG fill in this case is vibration damping not structural. Steel is vastly stronger and stiffer than EG.
There is research data to show that filling steel with epoxy granite results in improved vibration damping.
I am planning a steel weldment with epoxy granite fill for a metal mill. However, the epoxy granite may end up a bit different. There are some interesting papers on using crumb rubber (rubber granules) in the mix to improve damping. If the EG is not structural (e.g. in a weldment) then the composition can be tailored optimally for vibration damping.
The greatest gain will be from the EG in contact with the steel tube.
An easy way to achieve this is to use a foam core, or a light tube in the middle. For example, a 200x200mm steel SHS tube, with a 100mm PVC pipe in the centre. You can then fill with EG between the steel and PVC.
Generally, material close to the neutral axis (i.e. the middle) is of very little benefit in engineering.
8x8" would be a better choice than 8x6". Gravity is not the only or most important force. Cutting is and there is usually more cutting force in X and Y and in torsion.
7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)