Epoxy granite metal lathe - videos

[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.

[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.

[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]

...... 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.

[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)

[tonyfoale]

.........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,..........

This is true and the minimum useful size depends on surface tension and viscosity of the resin. I do not know what the optimum minimum grain size is for common epoxies.

[Robert Steinwandel]

I think I used .01in as my minimum but I can’t remember why I had that number honestly

[Robert Steinwandel]

A nice job, but he made the same mistake the mini lathe makers do. The spindle is way too short

I meant to ask about this much earlier, but would you mind explaining more about the spindle length issue? Im not sure I fully understand the different factors involved here

[Frank S]

I meant to ask about this much earlier, but would you mind explaining more about the spindle length issue? Im not sure I fully understand the different factors involved here

For this little lathe it may not be much of an issue, depending on how it is going to be used, how much power the Moter will have and the types of materials and cutters planned to be turned and used.
However the lathe makers of old had a general practice of placing the head stock bearings at a minimum distance of 1.5 times the diameter of the total swing, now some of this may have been due to the need for more space for the widths of the flat belt pulleys allowing for multiple speed changes, later being reserved for the transmission gears as in gearhead lathes. But a lot of it was to reduce the loading on the smaller bearing at the rear of the spindle and to help in controlling runout when turning or facing. This T reduce shaft flex due to the longer length many manufactures had spindles much thicker at one end not only because the bearing at the chuck end was larger since it sees more loading but since the forces were lower at the other end not as much material was required there,
Think of how the spindles on heavy duty truck and trailer axles are designed the outer bearing is smaller than the inner bearing because the loading is much less.
Very old heavy duty lathes had a split sleeve bearing at the big end of the spindle with a large flat thrust bearing sometimes made out of leaded bronze, and sometimes a caged ball bearing on the other end or just a smaller split sleeve bearing, Latter on the practice of utilizing tapered roller bearings on the spindles became the norm as they were easier to adjust by adding an amount of preloading the better lathes took this one step further and added a angular contact caged ball bearing between the larger tapered roller bearing and the spindle nose. Some lathe manufacturers did away with the tapered roller bearings altogether in lew of using multiple pairs of radial and angular contact bearings using an angular contact bearing facing the opposite direction at the rear of the spindle, this idea worked fine for drill presses but was very lacking in holding the spindle true as the bearings wore a little. Some car manufactures used ball bearings for their front wheel bearings that were not angular contact it proved to not be a good idea and abandoned the design. it wasn't until front wheel drive cars started becoming popular that ball bearings returned only now being angular contact.
So, since I am often accused of liking to type to summarize, when building a lathe or anything else for that matter look to the manufactures of old especially those who are still around after a 100 years and see how they have done it what they are still doing and in what way have they altered their designs to achieve their success .

[Robert Steinwandel]

Thanks for all the info here, that’s fantastic. My spindle I think is the weakest part of my design right now, but currently I have a two tapered roller bearings, 1 each side of my spindle. I can’t remember the precision class, I think relatively low which makes me think I’ll need to replace them with something better to be able to cut closer to true cylinders.
Im hoping to use a 2hp motor and vfd, and carbide insert tooling to cut steel. I have no real idea how realistic this actually is but I’m hopeful, the inserts seem really nice in that they’re so cheap to change out vs full carbide cutters and all.
I guess I’ll see before too long what kind of tolerances I can actually hit, and how well (if at all) this lathe works haha