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  1. #21
    Quote Originally Posted by Jonathan View Post
    I think your theory is worth exploring and (you guessed it) I'd like to confirm that your measurement method is sound since we don't want to be victims of correlation not implying causation.
    I agree - it's all a hypothesis at the moment, and I'm still doing some measurements to get a bit more data. Silyavski's comments are interesting although I'm not sure that they agree with my data, and in one of his posts he comments on a 10% shrinkage (although I'm not sure which resin he was using at the time).

    Quote Originally Posted by Jonathan View Post
    Have you measured the shrinkage of your resin directly, since clearly if the resin shrinkage is far from 10% the rest of your analysis is potentially invalid (though not necessarily invalid, since it could still exist as a small effect masked by something else)? If you've not measured or got the data elsewhere, then perhaps pour some into a fairly thin tube, mark the level and when it sets measure the change, or think of a better method as I expect there will be one.
    I'm trying to work out the best way to do that. It's the only way to remove other complicating factors.

    Quote Originally Posted by Jonathan View Post
    The west system resin datasheet says it 'does not shrink after curing', but does that mean it doesn't shrink during curing? I guess not else they'd surely specify. Adding thinners does increase the shrinkage, so one should be careful when selecting the resin to ensure it doesn't contain them as that option might seem attractive to reduce the viscosity.
    I'm not using West System, and I have not found any shrinkage data for my resin.

    Quote Originally Posted by Jonathan View Post
    Please could you elaborate on your measurement method to get the "height error from horizontal plane"? Did you add the feeler gauge under one end of the level until the level read zero, then note down the thickness of the gauge, or have I completely missed the point? If it's the former then the measurement is an angle from the horizontal plane (or strictly speaking the Earth's center of mass, but lets not worry about the earth curvature error), not height deviation without more careful interpretation of the readings.
    I set two carriages on the profiled rail, so that I was not relying on the rail's surface. The carriages were set so that their centres were at the measurement points (every 200mm along the rail) and my 200mm level placed on them. I then measured the height difference between the two ends from a combination of feelers under one end (to bring the bubble "on scale") and the level's scale. I have checked the scale and as far as I can judge, the readings are reasonably accurate. The level has a nominal sensitivity of 1 div = 0.02mm/metre. I have remeasured my rail, working from both ends to check for systematic errors, and the two curves match pretty well so I'm reasonably happy that the technique does not introduce excessive cumulative errors. I shall post my data later; I have also tried to measure twist in the rail and I shall give that as well. I have also shimmed the rail but have not yet measured the finished result (it's so cold in the garage at the moment!)

    Maybe I should try a simple test using my surface plate - create three channels, one directly on the surface plate with the surface plate horizontal (the 'control' experiment) and one with the surface plate tilted so the resin is significantly deeper at one end. Measure the linearity of both and see if there's a discernible difference between the samples.
    Someone needs to do this! I had thought about a small clear plastic pot - I'll try both if I get the chance (and it's warm enough for the epoxy to set).

    I can also try to get some data off my other X rail - I haven't examined that one yet.

  2. #22
    Very interesting thread. I'm with Sylyavski on this one and I don't think shrinking is that much. Considering the viscosity of the epoxy, pouring a few thin layers is not going to improve things much and I believe your'e better off with one thick layer. As an extreme example start with an uneven surface and try to paint layers it until its perfectly flat... But please carry on testing as its the one way we'll really know. Its difficult to test though as there are so many factors for example some epoxy likes to be cured cured at higher temps e.g. 60 or 80 deg Celsius so temperature might also have a big effect on the actual results.

  3. #23
    Has anyone

  4. #24
    Has anyone considered the temperature expansion/contraction characteristics and it's effect (or its lack of) change with differing bed thickness? I know some epoxies can move quite a bit with relatively small temperature changes

  5. #25
    Here are the "before and after" rail level measurements. That is, the results from the epoxy levelling (two curves, one measured going left to right and the other in the opposite direction, just to try to validate the measurement technique as obviously an error at one point will carry right through the remaining measurements). The third, flatter, curve is after shimming. Shims were calculated from the first pair of curves, and were to about the nearest "thou" as this was the thickness of the aluminium foil I used (with some 8 thou shimstock where it could be used). I could have used thinner foil but my wife wouldn't let me eat her Christmas chocolates which had thinner wrappers, even when I said that it was in the name of science and I wouldn't enjoy them...
    Click image for larger version. 

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    Jonathan - just for you!
    To calculate shrinkage, based on my hypothesis that this caused the variation in depth, I had three unknowns. These were S1, S2, and Z. These are the loss in height at the shallow end, the loss in height at the centre (deepest epoxy), and the shrinkage ratio expressed as final height/original "as poured" height. I could measure the depth of the epoxy bed at both points, and the drop in level between them. It was then just a matter of throwing together a set of three simultaneous equations in these variables and solving for each in turn. Plugging the values back in to the original equations gave the right answers, so I am assuming that my maths is OK. Obviously, the underlying assumptions might not be, but that's what we are exploring here.

  6. #26
    Quote Originally Posted by mike os View Post
    Has anyone considered the temperature expansion/contraction characteristics and it's effect (or its lack of) change with differing bed thickness? I know some epoxies can move quite a bit with relatively small temperature changes
    Could be an issue, I suppose, but I can't check at present. It's consistently b****y cold in my garage at the moment!

    And as an aside, I do have to say that I am chasing tiny "errors" here. My "with epoxy" flatness of about +-4 thou is well within any reasonably acceptable range for this class of machine. That's probably less than the manufacturing tolerance on thickness of any of the materials that I am likely to use on it. What started me on the current exercise is idly wondering why it wasn't "perfect" after using gravity and epoxy to set the surface.

    What is concerning me a little at present is the amount of twist in the rail. From what I have seen with some preliminary measurements, that exceeds the HiWin figures for this kind of error. Although the HiWin catalogue does not give a "max twist" value directly, it can be inferred from the published figures for maximum height difference between pairs of rails, and I believe that the twist in my rail at present is maybe 3-4 times that. I'm still investigating...

  7. #27
    Typical "non shrinkage epoxy" shrinkage is around 0.1% . I read that somewhere, don't ask me where, i read a lot of manuals and pdfs . 3000mmx0.1/100=3mm !!!! ,
    even 1m has longitudinal shrinkage of at least 1mm, if the epoxy is very good like WS.

    NON solvent epoxy means that the epoxy is 9x% something percent solid, not that its 100 % solid. And that small percentage is what f%%ks us.


    -So avoid the bridge to suck from the rail, let the rail be longer so when it shrinks it sucks from the bridge and that the lowered area is not inside the machine frame. That simple.

    -3mm minimum recommended epoxy thickness for the compression properties to be same as sheet specs, that's why the 5mm thickness.


    -the variation of vertical shrinkage IMO is due to not perfect mix, bubles and not perfectly clean/acetone clean/ surface, small invisible gaps where the epoxy leaks but does not leak out. That simple.
    project 1 , 2, Dust Shoe ...

  8. #28
    Hi Guys
    Loitering as usual.
    Spent the last half hour trying to find this http://www.moglice.com/
    Might be worth a look if you have the time it was hiding in a build log i had saved
    http://www.oneoceankayaks.com/madvac/machine_frame.htm
    Regards
    Mike

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  10. #29
    The accuracy you achieved after the shimming is phenomenal and I would be extremely happy if I can get mine remotely like that.
    I can't imagine why there would be twist in the rail and I hope you figure it out as I'm likely going to run into the same issue.

  11. #30
    Sven's Avatar
    Lives in a, Netherlands. Last Activity: 3 Weeks Ago Has been a member for 3-4 years. Has a total post count of 45. Received thanks 4 times, giving thanks to others 0 times.
    On a side note:
    So far i've only seen leveling systems utilizing "guttering" to let the epoxy settle.

    Could it also be done using a plastic hose as a "hose level"?
    Might be easier to set up.

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