[genixia]

Check out what this guy says on the topic. It might help:

https://www.cnccookbook.com/epoxy-gr...-machine-fill/


If you just want to make it heavier then filling it with anything heavy would work. The 50lb sand and granite aggregate mixes from Home Depot are very cheap. I think I paid around $5 per bag.

I was toying with the crazy idea of filling steel or aluminum tubes with oil (for steel) or water (for aluminum) to increase weight. I then wanted to fit a valve to connect a vacuum pump so it could be emptied when it was being moved. I abandoned that idea for various reasons but the main one was that just being heavier was not necessarily helpful.

After doing (far too much) research on this, it seems like there is some confusion on this topic. I.e. The desired end goal is maximum stiffness and strength with minimum vibration. Just being heavier doesn't necessarily help on it's own. Some people seem to assume that extra weight is the end goal. It just happens that thicker walls in iron castings increase weight but stiffness is the aim.

Cast iron, the typical material of choice for mills, is not as strong as steel so thicker castings are necessary to achieve comparable strength which makes good cast iron machines heavier. I don't know how much benefit you get from increasing weight by adding a material that is not strong or stiff (like loose sand or a liquid). It's worth a little research though.

I don't know nearly as much about CNC machines as some of the experts here but one thing I do know is that many of these questions (like what should I fill steel tubes with) become a lot easier to research when you are clear on what the end goal is.

Based on what I have been told here and confirmed with my own research, I think you were on the right track when you were looking for ways to make your frame stiffer.

True, the filling isn't going to add strength. It is going to damp vibrations leading to better rigidity. Resonant frequencies exist for every beam based upon its length and mass. If you are taking a cut where the cutter tooth hits the workpiece at a resonant frequency then that beam will start resonating. A small amount of energy at the resonant frequency can lead to a large deflection. Think of a child on a swing set - a small push each swing results in a large seeing movement. That large beam vibration will cause the finish to suffer, could cause the machine to walk across whatever surface it sits on, and in extreme cases with enough time, cause the beam or weld to work-harden and crack. The filler absorbs vibrational energy across a large frequency range and attenuates that resonance. Finishes are improved, and more extreme outcomes prevented.

Even doing this or using EG frames won't entirely eliminate resonance in the machine. Linear rails have resonance, balllscrews have resonance, spindle shafts have resonance. Running into any of them can affect finish and shorten the lifespan of that component. Spindle resonance is especially tough on expensive spindle bearings, and cutting tool life.
At the high end of CNC machining it is becoming increasingly common to map the machine's resonant frequencies and to tweak speeds and feeds to avoid them. In a production environment where a machine is making one part for years, tool life and spindle rebuilds go straight to the bottom line.

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