Use of epoxy for levelling

[Neale]

This post is probably going to open me up to derision from the practical community ("that's close enough, why are you bothering about 7 thou?") and the more academic ("your sums are wrong and your measurements are suspect!") but I'm feeling brave so here's my theory.

I believe that the dip in the surface of my epoxy is due to shrinkage, and there is a lesson to be drawn from that in how you should use epoxy. This evening, I measured, as closely as my measuring kit would allow, the depth of the epoxy bed at the end and the centre of my X rail. If I assume that the difference in level is due to shrinkage, the difference in levels of the hardened epoxy corresponds to a shrinkage amount of approximately 10%. That is, after setting, the epoxy bed will end up about 90% of the depth of the liquid resin. That's using the resin as per the first post of this thread. I suspect that a nominal 10% linear shrinkage will not lead to an exact 10% reduction in depth as there will be other smaller effects due to shrinkage across as well as down through the epoxy but I'm ignoring three-dimensional effects for the moment. I'm happy to go through how I did my calculations if asked, but for the moment I'll skip directly to my conclusions.

An application of this epoxy will lead to a reduction in "depth" errors by an amount dependent on the shrinkage factor of the epoxy used. In my case, for example, I started with a dip in the rail surface of about 1.7mm; after epoxy treatment I ended up with a dip of about 0.18mm, a reduction of about 90%. If you think about it, this will happen whatever the depth of epoxy used (unless there is some other effect associated with shrinkage). However, if I now used a second layer of epoxy on top of the first, then I would reduce this dip by about 90% again, and I would expect to see an dip of about 0.02mm after that. In other words, for a given total depth of epoxy, you would do better to use it in two thin layers than one thick layer.

Does this also explain the effect that some people have seen where there is a noticeable shrinkage effect at rail/bridge joins - if you are not careful to get the bridge at the same height as the rail, then there will be a different amount of absolute shrinkage either side of the join and that might affect the resulting levels?

Jonathan's measured error curves are not quite in agreement with this principle, but his measurement technique is dependent both on rail height variation and rail twist, and once we are getting down to small numbers the measurement technique becomes important. However, it is interesting (but possibly coincidental) that his initial error was reduced from about +-0.3mm to about +-0.03mm. That's the kind of reduction that my thinking would have suggested although it doesn't prove anything due to, as I say, the different measurement technique.

If I had thought about it beforehand, then all this would have been blindingly obvious but then, I would not have expected quite as much shrinkage. Looks like epoxy is great, but it reduces rather than removes build alignment errors and the more accurately you can build the initial structure, the better the result after using epoxy. Shimming, on the other hand, should be able to take out arbitrary errors, although it's going to be much more difficult to measure and remove rail twist.

[EddyCurrent]

eHowever, if I now used a second layer of epoxy on top of the first, then I would reduce this dip by about 90% again, and I would expect to see an dip of about 0.02mm after that. In other words, for a given total depth of epoxy, you would do better to use it in two thin layers than one thick layer.

That's what the bloke in the video said, do a small pour first ; https://www.youtube.com/watch?v=NlifRFChReY

[Jonathan]

I'm happy to go through how I did my calculations if asked, but for the moment I'll skip directly to my conclusions.

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.

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.

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.

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.

Jonathan's measured error curves are not quite in agreement with this principle, but his measurement technique is dependent both on rail height variation and rail twist, and once we are getting down to small numbers the measurement technique becomes important.

I could have indicated from the surface plate to the resin, or from the rail to surface plate at two distances to separate the readings of linear and angular error ... but time was not on my side.

However, it is interesting (but possibly coincidental) that his initial error was reduced from about +-0.3mm to about +-0.03mm. That's the kind of reduction that my thinking would have suggested although it doesn't prove anything due to, as I say, the different measurement technique.

I'm not sure where you got the +-0.3mm from, but from the graph I posted in the thread it was more like +-0.5mm, then 0.3mm after the failed compensation method using a machined strip. However the graph I posted is a combination of the height and twist errors, so we can't actually conclude from this what the height error was.

We could gain confidence in your theory about the resin shrinkage by measuring if the height error follows the original profile, just with a smaller magnitude. My readings do not seem to follow the original profile, but as mentioned already the readings are a combination of the rotational and linear error, so we can't draw any conclusion from this.

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.

[mike os]

Has anyone

[mike os]

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

[Neale]

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...

[Boyan Silyavski]

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.

[mekanik]

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

[mitchejc]

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.

[Sven]

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.