home made epoxy granite cnc – the mixture

The motivation of this is that I found myself answering the same questions on youtube and facebook over and over again. Lots of people are quite interested in the mixture of the epoxy resin, what resin to use and how I handled it.

Lets start in the beginning. Before I was getting serious on this build I started playing around in CAD and I liked the idea of casting the structure of a machine. There are two ways of easily casting parts at home. Concrete (HPC) and epoxy granite or mineral cast. Somehow I decided to start playing around with epoxy granite. Just out of curiosity.

I think it was in December 2018 when I ordered some epoxy at amazon.
I just picked one that’s supposed for casting with a slow hardener. Slow hardener because I don’t want to work under time pressure and I guess a slower hardening process should also result in less heat and less deformation. But that’s just my guess. I don’t have any professional knowledge of resins.

At a local hardware store I purchased some fine sand (for mixing concrete) and a bag of gravel. The sand came really humid, so I had to dry it first. Did this in a cooking pan in the kitchen (at this point I hope so much that my girlfriend isn’t reading this …).

I used such a Tupperware like plastic container as a mold. They are pretty handy because epoxy doesn’t stick to them and they got and draft angle. Plus they are a little flexible, so the test piece can be easily released.

First I started mixing sand and gravel to see what mixing ratio looks good. I used a kitchen scale and made notes in order to be able to reproduce the results.
First test I made was with 17% of resin. Then I lowered the amount of resin to 15 and 10 percent.

The 17 percent piece came out almost perfect. But there was quite a layer of resin on top (in this pic it’s on the button). That’s a clear indicator that there was more resin then the sand/gravel mix can absorb.

But on the other hand while casting it behaved extremely pleasant. Very thin, almost liquid. That’s the reason why I didn’t find any air bubbles in this cast.

The 15 percent piece looked a bit more rough. Some small air bubbles, the surface not as smooth as the first test.

The 10 percent piece looks pretty rough with a few air pockets and a not very nice surface finish.

My conclusion was that I should use as little resin as possible, but still enough to ensure a good handling and as little air pockets as possible.
Using as little as possible because of less heat while hardening, and even if epoxy is only shrinking a very little, it’s better to be on the safe side here.
Little heat and little shrinking should result in a part with little tension and little distortion.

Little distortion is a time saver later on. As straighter the rail mounting surfaces come out, as less effort is necessary for lapping them flat afterwards.

At this point I’d point out that those results are depending on lots of parameters, the grain of the sand, the epoxy and so on.
Doing some tests is the only way to figure out a working recipe.

home made epoxy granite cnc – the mixture
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