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How-to Guide on Building Cast Epoxy Granite Machine Bases
Hello Everyone!
I have spent COUNTLESS hours experimenting and building cast epoxy granite bases. Here is an example of a finished product:
Attachment 356366
I wrote a blog post how to design epoxy granite machine bases:
https://www.adambender.info/single-p...e-Frame-How-To
I cover the theory, my experimentation, how to build a mold, and finally a few pictures of the finished product.
I'd be happy to answer any questions or comments people have. My next goal is to build a cast epoxy CNC frame!
Cheers,
Adam
Re: How-to Guide on Building Cast Epoxy Granite Machine Bases
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Three popular metals are highlighted; aluminum (Al), steels and cast iron. All have very poor loss coefficients, but are very stiff.
Cast iron has at least ten times the dampening coefficient of steel, perhaps 100 fold, and steel has a higher dampening coefficient than aluminum by perhaps a factor of 10, except for certain cast aluminum alloys which have high dampening coefficients. Epoxy granite may or may not have a higher dampening coefficient than cast iron, when you account for its density and stiffness. it depends. its modulus of elasticity is also a huge variable. epoxy for instance iirc is 100 - 200 times more flexible than steel. so if there's 10% more epoxy than needed your epoxy granite may be half as stiff as it could be, as the modulus of elasticity of granite is 40 times that of epoxy and one 8th that of steel. so until you measure both the stiffness and the dampening coefficient of the EG and make it a repeatable process its not really appropriate to make statements as general as you have made, and of the documents i've read, i've found a variation on the order of 10 one person reports 40GPA, the other 6 for the modulus of elasticity of epoxy granite.
But it is the modulus of elasticity divided by density, multiplied by the dampening coefficient that matters.
Cast iron actually wins for the most part. There are a couple other materials that can do better but they are expensive. like a mixture of indium and carbide particles iirc which has a dampening coefficient of something like 0.5 but its horribly expensive.
where epoxy granite is of most use is when its not possible to get a cast iron casting made (mostly limitations regarding what the foundry is capable of doing). so for years now many machine tools are made by filling the inside of the cast iron castings with epoxy granite. -but you only apply as much as you need. filling the machine solid lowers its resonant frequencies and that is often not what you want.
Re: How-to Guide on Building Cast Epoxy Granite Machine Bases
Thanks for the reply Eldon_Jon!
I would certainly agree that cast iron certainly has a much higher internal damping coefficient when compared to steel and aluminum. It too makes a fantastic machine base material, although the cost of creating a custom cast iron base is very high. If you can find a few thousand people to sell a machine base too, then the cast iron base certainly makes sense!
To make up for the reduced Young's modulus of the epoxy granite, I've suggested to have an internal frame structure made of a stiffer material, and pour the epoxy granite around this. This gives you the benefit of the vibration damping properties of the epoxy granite, but also the stiffness of the aluminum internal structure. The epoxy granite can be cast into a thick member, which drastically increases the moment of inertia (Inertia goes up with thickness ^ 3), which in itself will lower the deflection amount significantly under identical loading conditions. This helps make up for the reduced Young's modulus. It can get expensive to produce the same machine base shapes with solid steel/aluminum, since the material price is very high for this!
For example, there was about $100 of epoxy and $50 of aluminum in the machine base of the picture I linked in my original post. The mold was about $150 worth of aluminum, although this is reusable every time. If I had machined that same geometry out of 3 thick pieces of 3-4" thick aluminum aluminum, and bolted them together, the cost would have been between $1000-$2000, never mind I didn't have the tools to CNC mill such large pieces)
The density of epoxy granite will vary depending on the exact composition. The density of my formulation was around 1650 kg/m^3, or around ~40% less than aluminum.
Cheers!
Adam
Re: How-to Guide on Building Cast Epoxy Granite Machine Bases
Nice write up Adam. A very nice machine design. The first EG sample that I ever made was with just sand and epoxy and turned out pretty good. Who knew that it would be about the best mix possible.
I think that the smoother roundish sand particles may increase the vibration loss even more than with rocks with sharp edges.
Re: How-to Guide on Building Cast Epoxy Granite Machine Bases
Hello Adam,
It is a very nice looking machine you have made.
You have published some information on optimising epoxy granite which is contrary to the industrial body of knowledge on this material though. Did you test the stiffness of the sample you made? I measured ~40GPa with a 10% epoxy, and ~50Gpa with my 8% epoxy mixes.
20% Epoxy is a very wet mix. For your application, with it's minimal loads, you could probably have filled your base with almost anything with the result of a well damped, stiff seeming machine. It would appear that you sacrificed mechnical properties for appearance and easy of moulding, which may be a decent trade off for such a lightly loaded machine.
Equating a mix of sand and 20% epoxy which flowed easily and looks nice, as being a successful E/G mix is an incorrect conclusion. Such a wet mix is going to have a pretty low Youngs Modulus , with strong separation, so you'll have a more dense sandy layer, and pooled epoxy resin on top. The porosity of your test pieces was an indication of insufficient different grades of aggregates, combined with insufficent vibration. The solution is not more epoxy, it should have been more grades of aggreate, down to finer grades, and vibration tuned to the moulded part.
An ideal E/G has a mix of aggregates graded so that each size fills the voids between the bigger grades, resulting in the space being nearly completely filled with granite. The finest aggregates have an essential role. The granite to granite contact forms the load paths and gives the stiffness. The minimal epoxy is not there to full voids, it is there to lock the aggregate together, although with a well graded aggregate, the epoxy will fill all remaining voids. The literature I could find indicates largest aggregates of 1/5 smallest cast feature, not 1/3. (1)
With tuned vibration of an adequate amplitude, and a optimised Fuller curve aggregate mix, you can compact 8-10% epoxy granite mixes. It is a pretty horrible material to work with. Hard to mix, it doesn't pour well, difficult to shovel out, differcult to deaerate, and only slumps (slowly) under strong vibration, ideally at the right frequency. On the german CNCecke there are a couple of guys who started a business making E/G frames, they are reporting frequencies around 68-72Hz as optimal. One of them, Tomas Zietz wrote a spreadsheet to calculate the best mix of available aggregates to approximate a Fuller curve.
(1) ISBN 3-478-93273-4 "Mineralguss für den Maschinenbau".
Re: How-to Guide on Building Cast Epoxy Granite Machine Bases
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Originally Posted by
Uguessedit
Im not sure I can agree with you the capacity to damp mechanical vibrations is one of the most important properties of granite epoxy and it is without a doubt superior to cast iron. For this reason, these materials have been adopted for manufacturing of tool machine foundations and precision instruments. Composite samples were prepared with different combinations of processing variables, like the weight fraction of epoxy in the mixture and size distributions of granite particles. The damping behavior of the material was investigated adopting the logarithmic decrement method. Samples, in the form of prismatic beams, were excited in the middle point and the output signal was measured in a point located at the extremity. The obtained results showed that composite samples, with weight fractions of about 80% of granite and 20% of epoxy, presented damping properties approximately three times greater than gray cast iron.
please post the source of this information. i don't doubt that 80% granite 20% epoxy would have a dampening coefficient 3 times better than grey cast iron. but a solid block of granite would have an even higher dampening coefficient, and it would be significantly stiffer than a 20% epoxy mix.
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To add I use EG in my machines and have filled an rf31 round column mill and base full and have less resonance and far more rigidity than any other mill in its class and far better than our PM833t and rf45 clone. Those machines are next in line to be filled. Absolutely worth the time and little money invested. I urge you to try Filling a machine and report back the results.
i know it works. but simply doubling the thickness of the cast iron in certain parts of those machines would have been less expense from the factory. as i've explained in other posts, significantly thick sidewall pipes and tubes, when completely filled with EG, are not significantly stiffer, because the stiffness follows the 4th power of the outside dimension, and you're filling the core with something that is about 1/8th as stiff as steel and its about 1/3rd as stiff as cast iron, and in the middle of of a beam (specifically, near the neutral axis) is exactly where you don't need the material.
btw, the 5 different samples of synthetic granite counter top material I have which are typically 93% aggregate and 7% polyester resin thermally cured after being compacted in a vacuum, have very little dampening coefficient. they would be suitable for making large, low frequency tuning forks.
