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  1. #91
    Quote Originally Posted by jimbo_cnc View Post
    Never done anything like that, but I'd have thought you'd want to:

    1. As you say, cut the bridges. You'd do this as soon as any levelling flow has stopped.

    2. How about cutting the 3000mm length into sections as well?
    Daming the bridges after a time might be a good idea but you can't section the 3m sections otherwise it won't be level along the length.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  2. #92
    Eddy's right has to be done in one pour, how about forget the bridge mix equal quantities of epoxy and pour both channels, when set find the highest rail and shim the lowest to match ?
    Just a thought
    Mike

  3. #93
    Don't go to hard an your self Boyan. I find your dam idea to be a great one. The only drawback is that you lose the connection between the two sides hence they become two separate world's. I will definitely use this dam technic and will try to use something as thin as a raiser blade. Maybe cut open some beer cans and make some stripes of it. I think a thicker dam material could introduce a small unwanted wave. And as you seem to be nuts with precision you won't want that. :)
    Sorry for your second fail but thanks for sharing as it is informative. Now I know it requires 21 degrees Celsius and in my case I see a problem there. In fact it even gives me some confidence as the result you have achieved is better than what I aim for in my build do to the nature of application.
    Good luck with your next pour tho, if you go for one.

  4. #94
    As jazz has said before they don't necessarily have to be the same height but they do have to be level and in the same plane. Provided this criteria is met, when you skim the bed, this puts things right, and obviously the spindle must also be at right angles to the bed.
    Last edited by EddyCurrent; 15-10-2014 at 09:49 AM.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  5. #95
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,000. Received thanks 170 times, giving thanks to others 5 times.
    Is there anything to be learnt from casting techniques? Arrange a large sacrificial reservoir close to the rails by widening the bridge locally but which can be cut away afterwards? However, I suspect that the shrinkage happens as the epoxy starts to gel, by which time it is not going to flow anyway. And what's happened to the "negligible shrinkage" property that we're told epoxy has?

    I have a vested interest in this as I shall be putting epoxy on my own rails shortly...

  6. #96
    Remember these videos cncJim posted ? they talk about priming the job first with a mini pour.
    http://www.mycncuk.com/threads/7190-...ht=epoxy+video
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  7. #97
    Quote Originally Posted by mekanik View Post
    Eddy's right has to be done in one pour, how about forget the bridge mix equal quantities of epoxy and pour both channels, when set find the highest rail and shim the lowest to match ?
    Just a thought
    Mike
    It will work, but at a later point the gantry have to be squared. It will make things extremely difficult then.





    I am starting to think of something like that:

    or



    One problem of cutting the bridge is that if i shut them off, that has to be complete, to avoid pulling. And at the same time any spill will kill the purpose.


    There must be a way to do it properly, unluckily i dont know well the material/epoxy/. Sb who works all day with epoxy will know how. cause the key may be simply to make the bridge same, wider or even more thinner to avoid the pull. But which of them. When casting aluminum i know that the hole has to be as wide as possible, cause when all shrinks, the casting holes is like a reservoir , so it pull aluminum from its core and balances the thing.


    Or may be even like this, so the rails will be thinner than the bridge and situated so that they will pull from it and play its role:






    Now the question is if i decide to shim the 0.15mm, what to use, so that when the 150kg gantry goes over that it does not change. Mix epoxy when mounting and paint the bottom, then wait to dry a bit and tighten a bit when its elastic, so it stays put in that shape.
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  8. #98
    Trying to balance the flow with the shrinkage while it hardens sounds like a recipe for disaster.

    Pour, flow, cut off, harden!

    Cutting the rail into equal size/shape sections should give consistent shrinkage. If you do repeated bi-sections to cut the rail (eg into 4 or 8) then (with skill/slow cut) even the slight increase in pool height should be even in each pool.

    edit:
    More likely to work and easier: One big ring of equal cross section is the best answer. Ideally you would not have a 90deg corner near what will be the final end of the epoxy rail, a radius would be nice.

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    Last edited by jimbo_cnc; 15-10-2014 at 11:49 AM.

  9. #99
    Would this type of arrangement work, placed at the ends of the bridges where they meet the rail channels ?
    http://ocw.mit.edu/ans7870/2/2.25/as...5-3/index.html
    I'm thinking that when the epoxy is thin it will find the same height each side but as it hardens it will be more difficult for it to flow back into the bridge.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  10. #100
    cncJim's Avatar
    Lives in Reading, United Kingdom. Last Activity: 3 Weeks Ago Has been a member for 4-5 years. Has a total post count of 170. Received thanks 15 times, giving thanks to others 32 times.
    Hi silyavski,

    Sorry to hear about your problems.

    What about adding some nails/pins to the bridge before the pore? Could this maybe help the situation?

    Click image for larger version. 

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    Jim

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