Use of epoxy for levelling

[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.