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.

[Sven]

Very nice to be able to read about this kind of process, I may need it in the future.

I've not yet read in to the particular shrinkage of epoxy but it seems to me that 10% is a number that is way too high, and that the measured gap is the result of something else.

I'll see if I can find any info on shrinkage and report back.

[Boyan Silyavski]

If you have read carefully my build, and implemented the technique i suggested that would not have happened. Epoxy shrinks and that is life.

Read from post #77 on, or jump directly at the solution. i think i should write an epoxy guide and make it sticky. As i already paid the price to learn/~6kg of wasted expensive epoxy, 2 pours wasted for learning purposes , so no need for others to repeat the mistakes.


Your epoxy shrunk at the bridge, cause the bridge shrunk lengthwise and sucked the epoxy from the center of the rail. Thats all.
I confirmed that in my build by calculating the given shrinkage of the epoxy, calculating the volume, length and so and it gave me as a result the exact shrinkage that happened.


One thing more. The glass transition temp of WS is 50C. So take care for direct sunlight in summer not to fall onto the epoxy, especially with heavy gantry. It will f^^ck up. I made some experiments and it becomes like a chewing gum quite fast. Faster than i liked.

[njhussey]

If you have read carefully my build, and implemented the technique i suggested that would not have happened. Epoxy shrinks and that is life.

Read from post #77 on, or jump directly at the solution. i think i should write an epoxy guide and make it sticky. As i already paid the price to learn/~6kg of wasted expensive epoxy, 2 pours wasted for learning purposes , so no need for others to repeat the mistakes.


Your epoxy shrunk at the bridge, cause the bridge shrunk lengthwise and sucked the epoxy from the center of the rail. Thats all.
I confirmed that in my build by calculating the given shrinkage of the epoxy, calculating the volume, length and so and it gave me as a result the exact shrinkage that happened.


One thing more. The glass transition temp of WS is 50C. So take care for direct sunlight in summer not to fall onto the epoxy, especially with heavy gantry. It will f^^ck up. I made some experiments and it becomes like a chewing gum quite fast. Faster than i liked.

50 degree C.....In the UK hahaha........not likely

[Neale]

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).

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.

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.

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.

[mitchejc]

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.

[Neale]

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...

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.