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Thread: Epoxy-Granite machine bases (was Polymer concrete frame?)

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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Has anyone tried Rust-Oleum TurboKrete with other aggregate mixes





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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Wow, very interesting thread for sure! Has there been a determination on the best epoxy? I saw several variants used, but not a lot of comparisons. It looks like West 105 and epon 813 from Heliox are being used a few times.

    Out of curiosity, what would you estimate the required volume of epoxy to be for a 10-12% by weight per cubic foot given a fine rock powder or silica carbide through 8-10mm aggregate? Just trying to do some cost analysis on a fairly large machine. Of anyone had some quantity per volume that would be great!

    Sent from my XT1080 using Tapatalk



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi all,

    So I recently spent 2 days reading through the whole thread to get a background and understanding.

    I'm looking to build a lathe soon, so I'm brushing up on lots of things as well as this materials science. Can I assume that Cameron's recipe on page 404 is the current "recipe" ?

    Also, when making a base/machine, has anyone used a concrete vibrator during their cast? It seems like I can rent one fairly cheaply/easily... but will this be sufficient for the packing?

    thanks!

    --
    Scott



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Scott,

    Can you actually source the ingedients of Camerons ideal packing density recipe? If you can, it would be cool if you could do a test peice and measure the Youngs modulus. Based on academic studies which , this sort of mix should produce a cured material stiffer than 80gPa.

    I wouldn't get too hung up on a certain recipe, unless you are certain you can get exactly the same ingredients. There are plenty of examples (at least on the German CNCEcke Forum) of E/G builds.

    So far I have only done test peices, but can already see that a vibrator with variable frequency is the essential tool in this process. Does the concrete compactor have variable frequency? 8% epoxy E/G is way stiffer than concrete, and needs to be hit with the right vibration to get it to sag. Nucky and Thomas, who started a company in Germany making E/G gantries have reported needing strong vibration at between 68 and 72 hz, with the exact frequency set within a few tenths of a Hz based on the form, mix and the temperature. They describe their set up here...
    Peters CNCECKE

    The other thing which my tests made clear is that you want a spread of aggregates down to really fine powder, not just stopping at sand. For my recipe, the sand and gravel is extremely cheap, but the only fine powder I have been able to source in sensible quanitities is powdered alumina from a pottery supply shop.

    Thomas and Nucky moved on from making their own recipes to Silamix. In their words it was simply easier and quicker to work with.

    I bought the aggreagates I could easily source first, than plugged them into Thomas's spreadsheet and got a recipe (see page 405) :

    From the local Hardware store
    5-8mm pebbles @ €4/25 kg
    2-4mm crushed granite@ €2.50/25kg (unfortunately I can't fins rounded gravel in this size, so this is sharp broken Stone which is not ideal)
    0-2mm quarze sand @€4/25kg
    0.1-0.3mm quarze sand @€3/25kg

    I end up at 0.12c/ Kg for the bulk of the aggregate.

    The expensive bits for me are:
    Powdered Alumina at €16/5kg from the pottery supplier
    Casting Epoxy resin €40/2.5kg from Ebay.

    Alumina 34c per kg E/G
    Epoxy €1,28 per kg E/G

    Total cost of E/G =€1.68/kg

    Epoxy gets cheaper in larger qualities. If I buy 7.5kg over ebay, the total E/G Price per kg Drops to €1.15 If I could find a source of quarz flour willing to sell in quantities of less than a ton, It should be pretty easy to get the price below €1/kg (about $0.40/lb)

    Regards,
    Mark


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hey Mark,

    Unfortunately I don't have any way to do any sort of load testing myself. In terms of the concrete vibrator, it'd likely be an oztec electric unit.

    Cameron,

    I'm not sure if you're still testing away or not, but have you thought about trying a piece using a 1-3mm gelcoat with re-enforcing fibers (of some sort) and then using the normal filler mix? I would have thought that that would make a reasonable improvement to tensile strength.

    thanks!



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hey Mark,

    So... I've actually found an ebay seller who deals in silicon carbide grit at a reasonable price and who also ships worldwide.

    I contacted him to see if they were interested in providing a pre-mixed version of the "recipe". He seems rather keen... http://stores.ebay.ca/mineralabrasives/

    "First of all, since I make the mixtures to order, it would be easy for me to create custom mixtures from the grits I have available (60, 90, 120, 220, 500). If you think there is a demand, I would be happy to offer such mixtures as products on E-bay, or on my website."

    So if we can get a good mix together for him, we should be able to have a source for the mix available by the lb.

    @Cameron

    The person seems to have some questions about spherical vs jagged media. We want the jagged variety, correct?

    Thanks,

    Scott



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Why would you want to use silicon carbide for epoxy concrete?

    I have used it for filling gaps after setting the gantry straight because it was the finest dense powder I had on hand.

    Once set you can not do much more then heat it to change the result.
    I removed some edges using a chisel and it was blunt in one stroke.

    Sven http://www.cnczone.com/forums/diy-cnc-router-table-machines/320812-aluminium-1250x1250x250-router.html


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    For best packing density you want rounded aggregate, not sharp/jagged.

    Ordering aggregate mixes in useable quantities from a long distance vender is normally an expensive proposition due to freight costs. When I contacted Euroquarz about buying Silimix 282, which is an Aggregate graded especially for this purpose, they offered to gift me the 60kg of aggregate, for my hobby purpose, but the freight from Germany to Vienna was a show stopper.

    Remember the whole E/G thing works because you can reach adequate stiffness despite the low Youngs modulus, as you can cast thick sections without the shrinkage issues which make thick metal castings difficult. Thick sections = weight. Not sure what you want to make, but getting hundreds of KG's of aggregate sent from Oregan to anywhere is going to cost a lot.

    Unless you can get silicon carbide for the price of sand, I wouldn't bother. It's Youngs modulus is 8x that of granite, but I doubt it would make a measurable difference once mixed and cast as E/G.

    Regards,
    Mark


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    The shipping is pretty good. it's about $15 from Oregon to British Columbia and that includes handling of customs/duty.



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    So,

    I was thinking about this again and while rounded aggregate is best in terms of total volume, there is a large downside. With rounded media, you're now more at the whims of fracture propagation. With sharp/jagged media, a fracture shouldn't propagate as far since the fracture should run into a perpendicular surface, unlike with round media.

    The promise behind EG is that you take the innate strength of a compressed aggregate and ensure that you're locking it into place using epoxy. At this point, the youngs modulus for a structure is now mainly based on the strength of the compacted aggregate. So, at this point... you have the mix of aggregate sizes and shapes to take into account. I believe that regardless of the media used at larger sizes, the extraneous volume can be filled with smaller rounded media to take up gaps. But the larger media is what should be assumed as the strength point.

    I also think that a gel coat with reenforcing fibers would make quite a difference in terms of fracture propagation from the surface (perhaps to the point where embedded features could be inlaid closer to edges.

    Mark,

    Did you have a look at the cost/pricing on the media available from the seller at all? I'm only able to get pricing on shipping to myself, but would be interested in hearing what it's like further afield.

    Thanks,


    --
    Scott



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Don' t fuss over strength.
    It is extremely hard if not impossible to build a cnc that is rigid enough for milling and then to have it break because it is not strong enough.

    Sven http://www.cnczone.com/forums/diy-cnc-router-table-machines/320812-aluminium-1250x1250x250-router.html


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Sven, who said anything about CNC? =0)



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