Epoxy granite metal lathe - videos

[tonyfoale]

Robert, glad that you chimed in, welcome.

...... based on that I believe 11% by weight is close enough to optimal that chasing lower epoxy percents would have diminishing returns. ckelloug took stiffness measurements on many samples of eg and although around 7-8% by weight was better, 10-11% was very similar stiffness.

Stiffness is important although that can be controlled by using a steel frame cast into the EG.

The look of the side of my casting looks like a total disaster lol, I absolutely see why you think it had terrible settling problems. I actually switched aggregate sources for one of my sizes of granite, the original source was black the new one white. So it looks like there was bad settling, but that was just me gradually switching to the white aggregate as I ran out of black (graded to the same size range of course).

It is good to know that settling was not a problem.

As for thermal issues, honestly Im very concerned about that as I do not have a conditioned shop to work in, so my temps range from around 10F to 100F through the year. I think I can minimize warp with strips of steel attached to the bottom of the casting so when there’s a temp change it at least has even pressure on the top and bottom and doesn’t tend to “banana” so badly

Yes, that is cause for concern. I have read a few of the CNCzone posts and saw that stiffness was of more concern than thermal stability. This is a big mistake because, as you have found, thermal distortion can easily exceed any potential distortion from load. When I saw the problems that you had I was shocked because I have often seen that EG had a similar thermal expansion coefficient to steel. It was simple calculation to see that based on the coefficients of the constituents an 11% resin content by volume would match the coefficient of steel. There would be no commercial machines made if they had the same problems that you have.

As I mentioned before, your lathe design/construction is very high up on my list of best DIY lathes, pity about the thermal issues. If you do add steel underneath I would suggest that you mirror what you have on top, that would give a more balanced structure. Perhaps a better solution, although not easy to do, would be to drill one or two holes through the length of the base and fit steel rods threaded on the ends so that you can pretension them like prestressed concrete. The tension could be adjusted, after experimentation to provide the minimum distort over the temp range.

[Frank S]

Here's a thought, call it random brain flatulence if you like. When considering making castings out of a composite of materials would it not be feasible and indeed possibly prudent to create a wireframe steel endoskeleton out of something like small rebar maybe even as small as 1/4inch or an open woven wire mesh with a minimal mesh weave size of about 1 inch square for larger thicker castings possibly 2 or more layers, My line of thinking is the added tensile strength of the steel going in so many different directions within the cast member would serve to stiffen and possibly help to stabilize movement of flex due to climatical or temperature variances . To help insure a good cohesive bond between the mixture and the steel i think I would sand blast the steel to remove any mill scale as well as to create a rougher surface for the bonding.

[tonyfoale]

Here's a thought, call it random brain flatulence if you like. When considering making castings out of a composite of materials would it not be feasible and indeed possibly prudent to create a wireframe steel endoskeleton out of something like small rebar maybe even as small as 1/4inch or an open woven wire mesh with a minimal mesh weave size of about 1 inch square for larger thicker castings possibly 2 or more layers, My line of thinking is the added tensile strength of the steel going in so many different directions within the cast member would serve to stiffen and possibly help to stabilize movement of flex due to climatical or temperature variances . To help insure a good cohesive bond between the mixture and the steel i think I would sand blast the steel to remove any mill scale as well as to create a rougher surface for the bonding.

Frank, that is not a brain fart. It is not uncommon with these constructions to have a cast in frame and in fact is what I plan for my upcoming surface grinder. It is not the tensile strength that is important but the rigidity of steel. There another poster on this forum who made a large router, I think that it was with concrete not epoxy granite, in which he had a steel frame work. Search this forum for Bongo and you'll probably find it. Interesting chap who enjoys his work.

i get the idea that you are not all that familiar with the use of epoxy granite for machine tools but it is a well proven technique used commercially as well as for DIY stuff. Commercially it is used for prototypes and one offs because it is cheaper than getting one offs in cast iron, the traditional material. For DIY it is a method that can be used at home. When done properly it can result in a superior machine than cast iron because it has a lot more damping. Unfortunately a lot of DIY efforts are spoilt because the builders use aluminium instead of steel. Aluminium has 3 times the compliance of steel and a higher coefficient of thermal expansion. However, aluminium is easier to machine if you have a router or low quality mill.

Even though epoxy granite is a technique that can be used for homebuilt, that does not mean that it is always done well. Youtube is full of poor quality builds and a small number of quality builds. I intend to use a steel structure for my precision surface grinder embedded in epoxy granite but the steel will be bolted together not welded in order to eliminate the possibility of internal stresses disturbing the initial alignment. I shall be using linear rails like Robert used on his lathe.

[Robert Steinwandel]

Tony- thanks for all the kind words

Not sure if you’ve already been testing mixes, or maybe you’ve worked with EG for a million years and I have nothing useful to say here, but let me trumpet opinions into the void and see what comes back hahaha

I found that 11% by weight was about the lowest epoxy percent I could do without starting to get more voids. Before HEAVY vibration with a vibrating table it is like damp sand and rocks (and extremely stiff to mix), it barely looks to have any epoxy on it at all. It only gets harder to mix the aggregate and epoxy together as the epoxy percentage drops, so 11% was about all my poor hands could take lol.

I was going to just epoxy the same cross section of steel onto the bottom of my lathe as it has on the top, but I think your tensioning rod idea is much better. I can’t make a 48in deep hole though, so I may cut slots for threaded rods in the bottom of the base, using a angle grinder diamond wheel.

[Robert Steinwandel]

Frank- that sounds like a great call, and something like some rebar running the length of my base would probably have solved most of the problems I have now with temperatures.

One thing to keep in mind though, is you will need more epoxy the more surface area there is in the mold. The large aggregate doesn’t pack well against walls and other features, so you end up needing slightly more epoxy to not have voids. This is why they generally say that your largest aggregate should be at most about 1/4 the size of your smallest feature (although not bigger than 1in or so usually since bigger rocks tend to crack more easily)

So if you add like a 1/4in wire mesh inside the mold, you add a lot of surface area and small features, which would dramatically increase the epoxy you’d need in my experience. Personally I might put like rebar or threaded rod in the mold as the steel framing, but Id probably stay away from mesh screens (the surface roughness of the rods/rebar shouldn’t effect the epoxy too much more than a smooth rod and getting a mechanical grip from the epoxy is always nice)

I could absolutely be 180 degrees totally wrong here, but just my thoughts based on my experience with EG so far

[tonyfoale]

I found that 11% by weight was about the lowest epoxy percent I could do without starting to get more voids. Before HEAVY vibration with a vibrating table it is like damp sand and rocks (and extremely stiff to mix), it barely looks to have any epoxy on it at all. It only gets harder to mix the aggregate and epoxy together as the epoxy percentage drops, so 11% was about all my poor hands could take lol.

In an earlier post I did say " mixing 90% with 10% resin would not be physically easy."

I was going to just epoxy the same cross section of steel onto the bottom of my lathe as it has on the top, but I think your tensioning rod idea is much better. I can’t make a 48in deep hole though, so I may cut slots for threaded rods in the bottom of the base, using a angle grinder diamond wheel.

I did not think of it but cutting slots would be a very practical way of doing this. Good luck with it.

[Frank S]

Just as a thought I personally think if I were to consider a build such as one of these that I would first screen my aggerate to insure a more consistent size any larger rocks could be cracked or broken into smaller pieces this would reduce the possibility of having rocks in the mix with natural cracks hidden in them.
I know this is totally off base from what we are discussing but I have seen concrete structures crack in cold temperatures. The causes were determined the cracking was form moisture trapped in minute fissures of the larger rocks of the aggerate. Enough fissures with enough trapped moisture once fully cold soaked and boom the whole side of a concrete wall would look like mortar rounds went off in it

[tonyfoale]

...... you will need more epoxy the more surface area there is in the mold. The large aggregate doesn’t pack well against walls and other features, so you end up needing slightly more epoxy to not have voids.

Voids on the surface are a cosmetic issue only, you should not compromise the bulk to the needs of appearance. It is easy to get a nice finish after mould release by filling the surface with a resin and glass micro-balloons mix.

So if you add like a 1/4in wire mesh inside the mold, you add a lot of surface area and small features, which would dramatically increase the epoxy you’d need in my experience. Personally I might put like rebar or threaded rod in the mold as the steel framing, but Id probably stay away from mesh screens (the surface roughness of the rods/rebar shouldn’t effect the epoxy too much more than a smooth rod and getting a mechanical grip from the epoxy is always nice)

I would stay away from any mesh, that makes it way too hard to avoid voids. Rebar set in along the length of the casting is a good method but I would sand blast it first and maybe paint it with neat resin just before casting. I see that several people advocate using a fast cure hardener, I go the other way and use the slowest hardener. Apart from the time benefits of working with a slow set, the exothermic reaction will significantly heat a large casting during a fast cure. If the cure takes 24 hours then the heat has so much time to dissipate that the temperature rise is very low, which results in a more stable casting.

[tonyfoale]

Just as a thought I personally think if I were to consider a build such as one of these that I would first screen my aggerate to insure a more consistent size any larger rocks could be cracked or broken into smaller pieces this would reduce the possibility of having rocks in the mix with natural cracks hidden in them.

Frank, In order to a high enough granite percentage it is necessary to have a mixture of aggregate sizes. The voids between the large pieces get filled with the medium size pieces and the voids in the medium size pieces get filled with the small pieces. Other voids are hopefully filled with epoxy.

I know this is totally off base from what we are discussing but I have seen concrete structures crack in cold temperatures. The causes were determined the cracking was form moisture trapped in minute fissures of the larger rocks of the aggerate. Enough fissures with enough trapped moisture once fully cold soaked and boom the whole side of a concrete wall would look like mortar rounds went off in it

Very interesting. it wasn't until I came to live in NH for 3 years that I began to appreciate the effects that water/ice and very cold weather could have on everyday things. It was an education but I am glad to be back in warmer climes, with the cold being a distant memory.

[Robert Steinwandel]

Sorry tony, I must have missed that in your earlier post, but I certainly agree lol

When I was talking about voids I was thinking of interior voids. Reading back though I phrased it very poorly. What I noticed -when there were lots of small features, the features sort of “used up” more of the epoxy content of the EG, and there were more voids throughout because there was less epoxy for the remainder of the casting. That was my impression at least, I wish I had been taking videos for YouTube back then as it was a really interesting effect.

Frank- I bought my stone from a landscaping store where it was stored outside, so everything even my sand was quite wet. I rinsed the fine dust off everything then dried it in trays in the oven which seemed very effective

I don’t know how much the fine dust effects things, but I had read that theres a minimum coating thickness of epoxy. Like no matter how small the particle, to coat it completely you need at least a layer of epoxy around it that’s a certain thickness. So very tiny particles end up not being very effective for lowering epoxy percentage, and just make the EG harder to mix. Has anyone heard anything similar to this? I think it was a paper by LeGarde or something like that (old paper like at least 50 years old I think)