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IndustryArena Forum > Mechanical Engineering > Epoxy Granite > epoxy granite sliding on epoxy granite.
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  1. #1
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    epoxy granite sliding on epoxy granite.

    Grind the dovetail or box ways with diamond wheels...

    Then brush some really nasty chemicals on it that will selectively eat away either one of the additives, the epoxy, or both.
    (or use a diamond wheel to make all the oil pockets.)

    Next grind oil groves.


    Thoughts?

  2. #2
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    Re: epoxy granite sliding on epoxy granite.

    Disclaimer: I have zero knowledge of EG in practice.
    But, I rather doubt you need to remove the epoxy, and I strongly suspect that trying to do so would let some of the 'granite' fall out, either immediately or some time later. I would simply go diamond wheel for the lot.

    Just how well EG on EG would work - I haven't a clue. Sounds bad ... but you can get the surface really shiny smooth. Maybe pressurised lubrication?

    Cheers
    Roger

  3. #3
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    Re: epoxy granite sliding on epoxy granite.

    Quote Originally Posted by RCaffin View Post
    Disclaimer: I have zero knowledge of EG in practice.


    Just how well EG on EG would work - I haven't a clue. Sounds bad ... but you can get the surface really shiny smooth. Maybe pressurised lubrication?

    Cheers
    Roger
    My opinion is this would work so long as there's a film of lubricant between the two. Without it, I can see the re-plasticizing of the epoxy and subsequent weld together due to friction. I guess alot of it depends on the weight and pressure of the two surfaces sliding together.
    I too, don't work with EG, although I see it as a viable alternative to cast iron. This is interesting and if someone knows more, please enlighten us all.

  4. #4
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    Re: epoxy granite sliding on epoxy granite.

    The only machine I have seen that used granite was Excellon, and they used air, under pressure, as the "lubricant".

  5. #5
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    Re: epoxy granite sliding on epoxy granite.

    Of course, with a pressurisd lube system, you would need really close tolerances to get any accuracy.

    Cheers
    Roger

  6. #6
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    Re: epoxy granite sliding on epoxy granite.

    I've made some lean samples (4% at 2 mm grain) with only one grain size.
    I can use those to take the sharp edges of of other samples.

    So I'd say, leave epoxy in the matrix!

    Maybe even better, mix in graphite with the mix?
    Sven
    http://www.puresven.com/?q=building-cnc-router

  7. #7
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    Re: epoxy granite sliding on epoxy granite.

    > mix in graphite with the mix?
    Tempting. That's what cast iron does after all.
    But the graphite from cast iron is SO MESSY, compared to (say) linear bearings.
    Would it work? Dunno. SIAS.

    Cheers
    Roger

  8. #8
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    Re: epoxy granite sliding on epoxy granite.

    What i meant by brush some nasty chemicals on the surface, was to remove the epoxy from the surface to rough it up, leaving room for oil to get between the larger grains which will be the last to wear down.

    The smaller, sand sized particles I might expect to be removed, or be broken loose when the epoxy is etched away from the surface.

    You would be left with the same 20-40 points per inch (according to the largest aggregate you used) that the cast iron machines of the finest quality are scraped to. except instead of cast iron, you have quartz, silica, etc.

    Such a project would be just as labor intensive as cast iron, except you don't have to pay for the castings, you don't have to wait 20 years for the iron to relax/stress relief (allegedly*)

    *i do not believe cast iron creeps.. not in your lifetime under the stresses that are found in machine tools, but yes it must be properly heat treated and that can be expensive.

  9. #9
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    Re: epoxy granite sliding on epoxy granite.

    Personal opinion only: I don't like the 'nasty chemicals' such as epoxy solvents.

    But more to the point, they do penetrate, and just because you have scoured the loose epoxy off the surface does not mean that you haven't weakened the epoxy further in. A slow decay is almost inevitable imho.

    Cheers
    Roger

  10. #10
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    Re: epoxy granite sliding on epoxy granite.

    yes i am concerned about that as well.

    a hexagonal mesh made of polyethylene, which is then pulled loose of the casting, or grinding the divots in with a diamond wheel are probably better options.

  11. #11
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    Re: epoxy granite sliding on epoxy granite.

    Oil grooves work OK on cast iron. Very likely they would work here too?
    Cheers
    Roger

  12. #12
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    Re: epoxy granite sliding on epoxy granite.

    Hi Eldon and others - Old thread but:
    1) Epoxy is not usually used for bearings as its not very hard. High performance plastic bearings are usually made from phenolic resins as they are V hard and can survive very high temperatures due to the friction in plain bearings
    2) I have added graphite, silicon oil, mineral oil to epoxy investigating this sort of thing. Small amounts of all of these help to improve its slipperiness. You can add <5% by weight usually but over that the epoxy starts to not cure. Your own test with your own mixes will prove or disprove this ratio. Silicon was best result in making a self lub bearing with epoxy
    3) Not sure why you would make your own ways these days with rails being so easy and cost effective
    4) If you want to machine EG then make parts using aluminium powder and epoxy. Same elastic properties as EG as aluminium and sand and granite have same stiffness but the mix can be machined using std metal tooling, even threads
    5) The main fault with most of the EG info is that people are using laminating resin which is too thick. To make the best EG use infusion grade resin or clear casting epoxy resin with very low viscosity. This will flow, wet out, degas very easily. The other thing is that the mixing of particle sizes is old theory and is not used anymore. You can make denser and easier mixes by just using one small size like 1mm sand.

    cheers Peter

  13. #13
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    Re: epoxy granite sliding on epoxy granite.

    Peter,

    I'm interested in your last comment about single particle sizes.

    Can you elaborate? Any references for testing stiffness and damping?
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  14. #14
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    Re: epoxy granite sliding on epoxy granite.

    Hi Pippin -

    https://www.cnczone.com/forums/mecha...ml#post2341842 follow some of this. Ivan spends some time in doing dry packs then filling with water to determine actual packing factor of various mixes.

    In summary:
    1) historically and logically there have been many theories about packing multi size particles to achieve high packing factors. Relatively recently for instance in the concrete industry they have decided that these theories did not work in practice as they are based on spheres and in reality stones etc are not spheres. Viscosity prevents distribution and segregation occurs so the very high packing ratios are not achieved in practice
    2) Ivan shows that by using one grade of sand you get 65% pack and that's about it and thats what the concrete guys get as well
    3) My own history in making high packing factor composites (glass and carbon fibre structures) shows its tough to get past 60% fiber volumes (even in autoclave at 6bar pressure) and they come highly organised to begin with
    4) In the past with EG people have used laminating resins which have thixotropes and are non newtonian so they thicken when worked. This is by design so resins dont run downhill and stick to surfaces etc so this is counter productive for the EG process
    5) These days you should use a very thin epoxy designed for infusion or encapsulation. These have no thix's and being very thin wick and flow really well. They are designed to flow.
    6) Using a single sand or particle is easier and gets you to the same packing factor as using a mutli size mix and is in fact more uniform.

    I'm about to test the stiffness of some of my mixes but if Ivans numbers are correct and we look at industry published numbers (look up Zanite by basetek) then we know Silicon dioxide (sand 100Gpa) at 65% pack will get you 35GPa stiffness and thats a good figure. If you use Al2O3 grit (300GPa) then you get 3x that figure. I have costed damping tests are a local uni and its $800AUD per test so I'll save some pennies for that one. But most articles say composites are 10X damper then metals so thats good info. EG is damper then cast iron for instance and many very precise exquisite machines are made using EG so its a controllable material vs metal which has various negatives for that sort of thing. I'm designing a benchtop mill and it will use an epoxy metal fibre composite so I can cast and post machine it. Difficult to machine ceramic EG. Cheers Peter

  15. #15
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    Re: epoxy granite sliding on epoxy granite.

    Pete - I think you might have already posted it, but - what's your thoughts on a drastically different size ratio; like mixing in pea gravel (or similarly sized crushed granite or quartz or something) with the sand and epoxy? (just an off the top of my head notion, may be bad idea)

  16. #16
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    Re: epoxy granite sliding on epoxy granite.

    Peter,

    Thanks for the reply.

    I'm interested to see any references with experimental data.

    What I'm finding is lots of papers that report highest packing fraction when a multiple size particles are used with a large difference in size. That is >7x difference in size between particles. For example, 0.2mm and 1.6mm particles.
    Gap graded aggregates are superior to a continuous range of sizes. For example, better to have 0.1-0.2mm and 1.8-2.0mm rather than a continuous range of 0.1mm through to 2mm.

    One goal in epoxy granite is minimising the epoxy content as it is the expensive bit. More voids in the aggregate packing equals more epoxy. (Smaller particle size also requires more epoxy).

    You have vastly more knowledge and experience about all this than me, however I'm surprised to hear you think a single particle size is best. The surprise is because the many papers I've looked at about epoxy granite essentially all use multiple particle sizes.

    What particle size would be ideal?

    I do think people are over complicating DIY epoxy granite. We are not trying to build 5 axis machines holding single micrometer tolerance. Most of us are aiming to build a CNC machine that is better than a conversion of a cheap poorly made manual mill, and is more feasible than a 20ton worn out VMC.
    The perfect epoxy granite mix probably doesn't matter that much.
    It's quicker and easier to overbuild - large sections with good wall thickness will easily to compensate for modest differences in material properties.
    A bit of porosity probably doesn't matter.
    Tamping the mix down may be sufficient, rather than an elaborate vibration system for a 300kg casting.

    A single or perhaps two particle size mix could make things a lot easier than the 5 plus sizes talked about in a lot of threads.

    Cheers,
    Nick
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  17. #17
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    Re: epoxy granite sliding on epoxy granite.

    Hi Nick - Yes a two size mix seems to be as good as you can get. You automatically get this when you buy a graded single mix as there are fines in all mixes. These fines are the bits that get thru the mesh. The theory of multiple size mixes is flawed as they use spheres or ellipsoids for the math. In reality the particles are neither, have sharp edges have friction and either agglomerate or segregate into bands of mixtures vs a even dispersion of particles. The way to prove this is like Ivan has done. Pack a test dry mix into a known volume container, weigh or volume measure these and keep records. Then fill with water to determine the free space, then figure out your epoxy ratios to fill this space. Ivan found that using mixes he couldn't get past 65% volume ratio and by using many and graded approaches the pack separated vs homogenizing.
    As you say I think the EG camp is over thinking the issue and I think the one big issue is the use of laminating resins vs infusion or casting resins of low viscosity. The EG camp tends to make very large and heavy castings which make up for all these short comings for machine parts. In my case where I want to design a machine from first principles I need to know the density and stiffness of the material to do this. This requires testing and a bit more understanding of the material. I have found a student doing their final year engineering that is interested in doing some material testing so may have some good figures early next year.... Cheers Peter S

    Ideal particle size question - It would seem the smaller the better for making EG. Seems the pack is better the smaller the particle. Mixing tests would need to be done to determine the smallest size that a std in container mix would achieve. But a very thin epoxy with a very long gel time would mix quite well. Since I use vacuum casting the particle size is nearly irrelevant. I'm about to do some tests on aluminium powder and its 0.42mm 2% 0.23mm 96% and 0.15mm 2%. Its a commercial al powder.

    With sand I think 1-2mm is good.

  18. #18
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    Re: epoxy granite sliding on epoxy granite.

    Hi Britt - Search for koehler article in the forum. The concrete guys have thrown out the graded approach. A two size mix seems to result in the best packing factor. Peter

  19. #19
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    Re: epoxy granite sliding on epoxy granite.

    Thanks... reading it now.

  20. #20
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    Re: epoxy granite sliding on epoxy granite.

    Isn't this what linear rails are for.

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