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  1. #261
    Join Date
    Nov 2012
    Posts
    29

    Re: Large Epoxy Granite Vertical CNC Mill

    Any progress?

  2. #262
    Join Date
    Jun 2010
    Posts
    1414

    Re: Large Epoxy Granite Vertical CNC Mill

    see? and people thought I nagged :P
    going on 2 years now.

  3. #263
    Join Date
    Aug 2009
    Posts
    291

    Re: Large Epoxy Granite Vertical CNC Mill

    If you left click on the person's name, then select 'view profile', you can see their last activity date. June of 2017 in this case.

  4. #264
    Join Date
    Dec 2015
    Posts
    170

    Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by ger21 View Post
    Epoxy does not dissolve styrofoam. Polyester resin does.
    This is true. However it might make sense to use Polyester instead of Epoxy to make granite. It's hella' cheaper! Can live without styrofoam. Replace it with fibreboard structure or PUR foam (which can handle polyester, unlike styrofoam)... I am thinking about making milling machine using Polyester granite. Wonder why everybody uses expensive epoxy. Polyester might be little harder, which makes it kinda brittle. But i guess this can hardly affect thick pieces that are casted for milling machines. Such machines are commonly made using relatively fragile cast iron. They even use plain old concrete to cast such machines!

  5. #265
    Join Date
    Jun 2011
    Posts
    692

    Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by phucrobusl View Post
    This is true. However it might make sense to use Polyester instead of Epoxy to make granite.
    From what I've read all polyester resins shrink on curing, whereas some epoxy resins do not. If you're going for anything precise without post cure machining, shrinkage es no bueno.

  6. #266
    Join Date
    Jul 2009
    Posts
    419

    Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by skrubol View Post
    From what I've read all polyester resins shrink on curing, whereas some epoxy resins do not. If you're going for anything precise without post cure machining, shrinkage es no bueno.
    Indeed, polyester shrinks.
    So does epoxy, but by very tiny amounts.
    A friend of mine built a EG gantry with steel on one side to later connect the rails to.

    Even with setting up the cast on a surface plate, the steel was 0.1 to 0.2 mm convex over 600 mm after curing.
    That suggests a very small shrinkage indeed, but it required reworking the steel.
    Sven
    http://www.puresven.com/?q=building-cnc-router

  7. #267
    Join Date
    Jun 2011
    Posts
    692

    Re: Large Epoxy Granite Vertical CNC Mill

    He may have gotten the wrong type of epoxy. Some epoxies have more shrinkage than others. I think I remember reading somewhere on the huge epoxy granite thread on here that VOC's are the thing to look for. Any VOC's will evaporate and cause shrinkage.

  8. #268
    Join Date
    Jul 2009
    Posts
    419

    Re: Large Epoxy Granite Vertical CNC Mill

    This guy knew what he was doing and he had the right kind of epoxy.
    And lets be clear, to get this kind of curve the shrinkage is absolutely minute.

    All this means is that there is some shrinkage, not a lot, and most may not notice it.

    But if you need it to be zero for what you are going to do, test first and adjust your plans if needed.
    Sven
    http://www.puresven.com/?q=building-cnc-router

  9. #269

    Re: Large Epoxy Granite Vertical CNC Mill

    Is anyone of these guys making a DIY kit yet that you just need to supply your own epoxy/granite? It would be so much easier and cheaper to source the concrete locally.

  10. #270
    Join Date
    Apr 2006
    Posts
    1016

    Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by Hezz View Post
    Nice work Petruscad, it's coming together slowly but surely. Do you have a surface plate large enough to get readings on the other side once you turn it over?

    And will you be using the same scraping technique on the EG surface. Or do you have steel mounting rails on the other side?
    where did you source your precision flat straight edge?

  11. #271
    Join Date
    Oct 2010
    Posts
    16

    Re: Large Epoxy Granite Vertical CNC Mill

    Is it possible to get the moulds in Step or IGS files pls ?

  12. #272
    Join Date
    Jul 2018
    Posts
    6254

    Re: Large Epoxy Granite Vertical CNC Mill

    Hi All here - I'll discuss resin shrinkage. There are a few mechanisms to understand but in terms of mineral cast machine parts there are two main ones, then there's thermal expansion/contraction
    1) solvent desorption & 2) crystal re-organisation
    1) Resins such as polyester and vinylester use styrene as a diluent and a cross linker. The resin is supplied with about 8% more styrene then needed so that in transport and when open to air in the mould the evaporation of the styrene is not enough to leave it short for the required cross linking,. Excess styrene is trapped in the cured resin and over time desorbs from the solid creating large shrinkage 6-8% linear shrinkage is usual. 100% "solids" epoxy has no solvent or diluent in them so these do not suffer from this type of shrinkage
    2) Liquids have more space in them then solids plus when the resin molecules cross link they "entangle". Generally this entanglement is volumetrically smaller then when the molecules are floating around in liquid form. So there is a small volume change due to this mechanism. eg in some plastics and concrete the volume change can be an increase in volume. This effect is usually small say less then 0.5% linear shrinkage/growth
    3) during cure resins are exothermic ie they get rid of internal energy as the solid form requires less energy then the liquid form. This energy heats up the resin and this heat expands the resin and its formwork. If you have steel for instance it expands at about 13e6m/m/C vs the epoxy at 50m/m/C so the epoxy expands say 4x more then the steel. Normal epoxies can get to 60-100C when cured in mass. Granite and minerals typically used in EG expand around 10m/m/c so have a similar expansion rate to the metal. So the epoxy expands more during initial cure, then as the assembly cools they then shrink differentially and you end up with some distortion. Now this info has to be tempered a little as the epoxy is usually a small amount of the casting so you can volumetrically proportionally calculate all of these effects if you like. If you use resins that are designed for river casting like one I use it takes 2 days to cure so the heat release is over a very long time and the temperature buildup is nearly imperceptible, but it must exist.

    The mineral component went through these changes some billion years ago so has had a long time to achieve equilibrium!! Good luck with any EG projects out there, post your stuff always good to know what happening out there. Peter

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