Use of epoxy for levelling

Thanks to various contributors, I have been able to get a lot of useful information about using epoxy for rail levelling. However, it is usually buried deep within build logs, so I wanted to start a thread just for this topic, if only to make it easier to find later!

After reading lots of views on the subject, I ended up with Reactive Resins Syntac EPAFD low-viscosity resin and matching Synamin 201-c slow hardener. I calculated the required volume based on width/length of area to be filled and a guess at a sensible depth (5mm), and based on 1ml weighing 1g (which I now realise is a big assumption and not based on any known density...) I mixed up 500g of resin and 250g hardener, which should have filled the required area with a bit left over. To my surprise, this is exactly what happened.

This is an overall view of my machine (very much work-in-progress).

Attachment 14029

Top rails are about 1800mm, spaced at about 1000mm. Construction is all-welded, a mix of 50x50x3 and 100x50x3. As far as I can judge, the X rails dip in the middle by about 1.3mm. I don't know if this is welding distortion, or if the original box section was slightly bent.

I used a single central bridge, an odd length of MDF that was about the right size. It is screwed to some odd bits of batten, clamped to the rails so it is level with the rail surface.

Attachment 14031

The dam uses Screwfix thick draught excluder, which stuck well. To avoid leaks in the corners, I used small dabs of hot-melt glue. I tested this first, in case it melted the draught excluder, but it seemed OK and worked well. The only leak, in fact, was over a couple of the plug welds that hold a 25x5 strip in the top of the rails (to take the tapped holes later). There was clearly a very slight weep through these. This shows the hot-melt glue, and the sink hole close to one end over a plug weld.

Attachment 14030

I don't expect this to be a big problem later.

There were a couple of things that were worrying me before I started. One was the horror stories of shrinkage, especially at rail/bridge joins. I didn't see any evidence of this.

Attachment 14032

One thing that I did do was to run up and down the epoxy with a small stick once I had poured it, to encourage it to find its own level. I think that that might explain the visible stripes in the epoxy at the join, although you can't feel them on the surface. I could also remove almost all the bubbles by poking them with the stick.

The other main worry was whether the epoxy would set or not, given the drop in UK temperatures over the last week or so. I decided to go ahead anyway, and for a small part of the time built a tent over the machine and blew in hot air from a small fan heater. However, I wasn't happy running this when I wasn't in the garage, so it didn't get a lot of this. In practice, this only took it up to 15C or so. However, as far as I could tell, the temperature in the garage varied between about 8C and 12C; the minimum temperature for the resin is given as 8C so it wasn't too bad. After 24hr, I could leave a fingerprint on the resin which largely disappeared over a few hours. After 72hr, it would mark with a fingernail. After 6d, I peeled back some of the dam on the bridge and tried scraping the meniscus as an experiment and it seemed pretty hard.

Attachment 14033

I'm now feeling fairly comfortable about being able to start drilling/tapping and mounting the rails.

Rather than copy it all out I thought it would be acceptable to put a link to my epoxy experience. This is where it starts;
http://www.mycncuk.com/threads/6565-...4024#post54024

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Originally Posted by EddyCurrent

Rather than copy it all out I thought it would be acceptable to put a link to my epoxy experience.

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Thanks, Eddy - even the search facility on the forum just takes you to the start of a 34-page thread! It's the problem with build logs - the most useful ones are the ones where it's most difficult to find what you know is there...

NASA calls it "conformal shimming"
dave

I had no idea just how level epoxy levelling left the surface so I checked one of my rails this evening. About 1750mm long, joined to the opposite rail about 1000mm away via a single central bridge. Epoxy around 4-5mm deep (there was a dip in the middle of the 100x50 box section rail support of about 1.5mm). Once set, I removed the meniscus from the epoxy with a coarse file and also ran the file over the top of a few bubbles that had formed.

For checking, I used a precision level plus feeler gauges, resting on a couple of ball carriages on the profile rail bolted directly on top of the epoxy. Using a 200mm level, I measured the deviation from level at 200mm intervals along the rail, then calculated the variation from a straight line between the two ends of the rail (which were about 4 thou different in height. This was because I had moved the frame slightly from when the epoxy was first poured.) I'm sure that there was plenty of scope for errors to creep in, but at least it gives a feel for the overall magnitude of the errors. I'm sorry for the mixture of imperial and metric units in this post but I still can't get a grasp of the magnitude of small measurements once you are down to tens of microns and I have to go back to "thou".

I was surprised by how good the results were. Remembering that these figures are variations from a straight line between the two rail ends, the maximum variation from "flat" was about 7 thou corresponding to a dip in the middle of the rail and smoothly rising towards each end. Given that this resulted from an epoxy self-levelling along a long, narrow 1750x30x5 "channel", and even though fairly runny by epoxy standards it's still not quite like water, that seems pretty good. Knowing the numbers gives me more confidence in moving forwards, and I'm planning now to use epoxy levelling on the gantry as well.

Would it be possible to use a shallower bed of epoxy? 5mm seem like a lot?

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Originally Posted by gavztheouch

Would it be possible to use a shallower bed of epoxy? 5mm seem like a lot?

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Yes just mix Less epoxy.!!.:hysterical:

As long as there's enough to level then that's all that really matters as long as there's enough steel to tap into. The epoxy adds to the depth of the tapping...

Cool sounds good, this is going to be for a laser cutter. I need quite a large surface area to mount the rails, laser tube and optics so the thinner the better. I think I might drill through holes rather than tap the holes so I have a bit of jiggle room, nylock bolts would be nice to use too.

I wasn't sure how deep to make the epoxy but 5mm, maybe a touch less, at the deepest seemed to be about right while leaving an adequate depth at the shallowest points. I wonder if the roughly 7 thou variation is due to the change in depth along the length of the rail - there is a slight dip in the epoxy surface where the epoxy is deepest. Might be coincidence - don't know. I was concerned that the epoxy should be deep enough that the self-levelling would work; I had a feeling that the epoxy might not flow quite as freely if it were too shallow. Again, this is conjecture and not tested. I was not concerned about depth as far as tapping was concerned; the epoxy drills easily (rather more easily than the steel, anyway) and I would not expect unfilled epoxy to hold a decent thread. Although I used 3mm box section, there is a 25x5mm strip inside the box section to give depth for threading. I drilled the box section and plug-welded the strip in place using an improvised wedge arrangement to hold the strip against the inside of the box for welding. In retrospect, I should have used Eddycurrent's "glue it in place" technique. Once tapped and bolted, it ain't going anywhere...

Another variable to play with is epoxy strip width. I used draught excluder tape to make the dam and with the aid of hot-melt glue to plug any joins, it worked well. However, it leads to a narrower strip. Njhussey's technique with MDF walls glued to the sides of the box section clearly gives a wider strip. Personally, I believe that the narrower strip (as long as it is wide enough to support the rail) works fine and is perfectly adequate but it does make it essential to remove the meniscus which has to be done without damaging the surface that will take the rail. A wider strip means that the meniscus will not interfere with the rail mounting, and in any case there is more room to hack at the meniscus without damaging the surface. My way uses less epoxy, and I didn't have any problems taking a coarse file to the edges but that doesn't mean that it's the best way.

I'm sure that the epoxy technique would work well for a differently-shaped area. My experience suggests that the epoxy does not level itself absolutely but it might be better over a square area rather than a long thin area. Additionally, I wonder if large variations in depth affect the top surface so doing the filling in two passes - the first gives a "nearly there" surface and the second corrects any minor residual error - might help. Or be ready to shim out the odd thou or two at the end if you need a really flat surface, but the epoxy will still get you closer than any technique other than machining or grinding. Even a slab of Ecocast will need to be mounted carefully to avoid deflection when bolted down.

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Originally Posted by beone

NASA calls it "conformal shimming"

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...and I shall be doing that with epoxy putty when it comes to mounting the gantry feet to the plates bolted to the tops of the X carriages!

Neale, your readings of the resin accuracy are interesting. Seves thou is 7*0.0254=0.18mm which is a bit more than I'd expect (though by no means a showstopper). Did you see the readings for the expoxy 'straightness' I took in my build log?

http://www.mycncuk.com/attachment.ph...id=14208&stc=1
Towards the end of that graph there is a large negative deviation. It turns out this was not actually the case - the bearing block was rubbing on the aluminium epoxy barrier which pushed it out of line for the last 100mm or so. Either way, the deviation is very small...

Here is the corresponding error plot for my rail (as I have only managed to fix down one rail so far. Once visitors have left...)

Attachment 14209

I have converted to mm for ease of comparison. We used different resins, although I'm inclined to dismiss that one as a significant factor - they are both at the runny end of the epoxy viscosity scale. However, I was working at an ambient temperature of around 10C at the time of pouring and although I had kept the resin indoors for a few days beforehand, it was a thin film sitting on steel and will have been down to garage ambient almost immediately. That won't have done wonders for viscosity and maybe it didn't flow as well. I think my rail section error was also rather greater to start with, and as my errors are roughly aligned with the initial error (the bends in the rail) I wonder if these are linked. Better flow where the epoxy is deeper?
There is also the measurement technique to consider. I don't have a surface plate big enough to be useful - mine's a sheet of thick plate glass about 300mm square - so I used a level at a series of points, combining level scale readings with feeler gauges to get a "difference in height" value. I dropped all these into a spreadsheet to get the graph shown here. It wouldn't be difficult to calculate shim values to bring the rail into line. Is it worth it? Assuming, obviously, that the bed is finished to the rails, then a hump in the middle wouldn't be a problem as the workpiece would tend to droop over it, being clamped at the edges. However, it is more likely to bridge a hollow in the bed. And I'm quite certain that my sophisticated woodworking techniques would dramatically fail with an error of 0.18mm in depth over a 1.8m length...

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Originally Posted by Neale

Here is the corresponding error plot for my rail (as I have only managed to fix down one rail so far. Once visitors have left...)

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Slip of the keyboard I expect, but Jonathan's was 'Height Deviation' i.e. vertical, while your graph says 'Error from Horizontal'.

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Originally Posted by EddyCurrent

Slip of the keyboard I expect, but Jonathan's was 'Height Deviation' i.e. vertical, while your graph says 'Error from Horizontal'.

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What was going through my mind was "height error from horizontal plane". What I should have said is beyond me at this hour, but I knew what I meant:beer:

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.

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Originally Posted by Neale

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.

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

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Originally Posted by Neale

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

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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.

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Originally Posted by Neale

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.

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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.

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Originally Posted by Neale

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.

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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.

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.

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 :beaten:, 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.

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Originally Posted by 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 :beaten:, 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.

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50 degree C.....In the UK hahaha........not likely [emoji6]

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Originally Posted by Jonathan

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.

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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:

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Originally Posted by Jonathan

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.

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I'm trying to work out the best way to do that. It's the only way to remove other complicating factors.

Quote:

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Originally Posted by Jonathan

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.

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I'm not using West System, and I have not found any shrinkage data for my resin.

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Originally Posted by Jonathan

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.

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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!)

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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.

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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.

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.

Has anyone

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

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...
Attachment 14258
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.

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Originally Posted by 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

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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...

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.

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

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.

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.

Still haven't finished my second round of epoxy. All the results I gave earlier were from just one rail, and were looking good. Then I looked closely at the second rail...

Main problem was leakage through a couple of places where I had plug-welded through to attach an internal extra steel strip to increase the depth for tapping. There was clearly a slight weep which seriously distorted the surface, leaving a virtual step at one point which was just too difficult to shim. So, I tried to recover by adding more epoxy. One problem with the first rail was that the dam was made with draught excluder strip. This left the epoxy strip on the narrow side, the meniscus needed to be removed to get a flat surface, and it was difficult to do that without impinging into the area carrying the profiled rail. So, I decided to pour a second coat of epoxy on both sides. That meant plugging all the bolt holes (short lengths of dowel tapered using a pencil sharpener and tapped into the holes), and I used gaffer tape to build a dam around the outside of the steel box. I put it on so that it stuck up above the box section by about 8mm. I did this on both rails, even the one that I had shimmed earlier, just so that both sides were the same. Biggest problem was that you get a meniscus around the site of each plug which was difficult to remove without damaging the surface. Unfortunately, I didn't leave the epoxy long enough to really harden so when I bolted down the first profiled rail, it dented the surface which left grooves which were difficult to shim. In the end, I stripped all the epoxy off, and have started again. This epoxy has only been on for a couple of days so not hard enough yet to want to start drilling and bolting.

However, a few lessons learnt:

- Make the epoxy wide enough for your rails that you can grind or file back the meniscus without touching the area that will carry the rails. In fact, so that you are only removing the meniscus for cosmetic reasons.
- Gaffer tape makes a good dam/mould but watch out when you use a heat gun for agitating the surface/levelling and bubble removal. This heat gun technique is great but tends to soften the tape which sags a bit.
- I shall never drill before applying epoxy. Drill afterwards - epoxy drills really easily. If you find yourself in the position of having to epoxy an already-drilled surface (or ones with holes in for whatever reason) then I suggest that you don't try putting plugs in the holes as this just gives problems later. What does work is to cover the holes with small pieces of Sellotape. Leaves a much better surface, and the epoxy does not seem to attack the Sellotape.
- Epoxy doesn't stick that well to steel, even freshly wire-brushed and acetone-wiped steel. I was able to split it off by getting a chisel underneath it. Maybe that was just the epoxy I was using, though, but I won't be using it to make structural bonds on steel without more testing.
- Really, really, wait until the epoxy is hard before doing anything with it. Then wait a bit more, especially in this cold weather.
- (and this one is probably a bit more contentious) - consider very carefully if you really need a bridge. I'm not sure if it causes problems due to shrinkage or not - I don't think it does but others believe passionately that it does - but it is a pain to set up, block leaks, etc. What does a bridge buy you anyway? It cannot improve the "horizontalness" of an individual epoxy strip (but might make it worse). What it does, theoretically, do is make both rails the same height. Can you build a gantry that is so accurate that both "feet" are exactly flat, in the same plane, etc? Commercially, of course you can, but then you would probably be able to machine the tops of the rails anyway. I know that I am going to be shimming between the feet of my gantry and the X carriages, so I am happy to take up the odd mm or two height difference in the shimming. My latest epoxy layer hasn't used a bridge, and so far the epoxy surface is looking good when I look along it at reflections of the wall. I doubt if I'll be taking up more than the odd millimetre of height difference. Why make things difficult for no benefit?

I shall report back when this latest epoxy is fully cured and I have fitted the rails so that I can measure how well I've done. Or not...

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Originally Posted by Neale

- (and this one is probably a bit more contentious) - consider very carefully if you really need a bridge. I'm not sure if it causes problems due to shrinkage or not - I don't think it does but others believe passionately that it does - but it is a pain to set up, block leaks, etc. What does a bridge buy you anyway? It cannot improve the "horizontalness" of an individual epoxy strip (but might make it worse). What it does, theoretically, do is make both rails the same height. Can you build a gantry that is so accurate that both "feet" are exactly flat, in the same plane, etc? Commercially, of course you can, but then you would probably be able to machine the tops of the rails anyway. I know that I am going to be shimming between the feet of my gantry and the X carriages, so I am happy to take up the odd mm or two height difference in the shimming. My latest epoxy layer hasn't used a bridge, and so far the epoxy surface is looking good when I look along it at reflections of the wall. I doubt if I'll be taking up more than the odd millimetre of height difference. Why make things difficult for no benefit?

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In which case don't bother with epoxy at all and buy straight edge to straighten and level the top rails. If the surface isn't pefectly flat it takes nothing but a file to knock off high spots and bit of epoxy filler to raise low spots. this can be down in the time it takes to setup and build good dams.

The whole point of the bridge is that it allows both rails to level on the same plane and to be honest if anyone can't setup a decent bridge or dam that doesn't leak I'd be quetioning there abilty to build the machine in first place.!

To me epoxy is ok but to be honest I only use it in certain cases like recently when I'm correcting someone else frame work. Rest of time I find it more work than it's worth and can reach the same results or better quicker using straight edge and shimming. But for those without precision straight edges then realise it's a quick and easy method if done right. (thou you'll still need a straight edge at some point so don't bother and buy straight edge is my thinking. .Lol)

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Originally Posted by Neale

- (and this one is probably a bit more contentious) - consider very carefully if you really need a bridge. I'm not sure if it causes problems due to shrinkage or not - I don't think it does but ...

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I totally agree that the benefits of a bridge between the X sides does not justify the pain. Like you said you can easily make up that small height difference when you mate your gantry and x base blocks with epoxy putty which one has to do anyway to ensure one doesn't put stress on the bearings when bolting the gantry to the x bases. I've done mine without a bridge and I did not have any issues or weird side effects afterwards with setting up my gantry, so go for it. Others might have had a different experience but my epoxy does not create a perfectly flat surface. The epoxy scrapes VERY nicely so I used a 260mm long planer/thicknesser blade to scrape it to spec.
I guess not needing epoxy casting is first prize and in hind sight, I'd be very surprised if the end result is more more accurate than using the filing and shimming process as described by Jazz.

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Originally Posted by mitchejc

I guess not needing epoxy casting is first prize and in hind sight, I'd be very surprised if the end result is more more accurate than using the filing and shimming process as described by Jazz.

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It's not more accurate it's actually much less accurate if scraping/shimming is done correctly but it is much easier and less time consuming for for those not equiped or experienced.

Regards not using a bridge can I ask how you know there is no difference in height between one side and the other.? Ie both rails on same plane.?

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Originally Posted by JAZZCNC

It's not more accurate it's actually much less accurate if scraping/shimming is done correctly but it is much easier and less time consuming for for those not equiped or experienced.

Regards not using a bridge can I ask how you know there is no difference in height between one side and the other.? Ie both rails on same plane.?

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epoxy method is nice because it makes sure your rails are on the same plane. How do you achieve this with a straight edge and shimming?

I liked the epoxy the first time on the 1000x400 machine. i hated it on the 3000x1300 , untill i figured my mistakes and made it right. The way as i see it if i have done correctly this one, i will stop using epoxy in the future when i am building smaller machines that fit inside that one.



I still don't see how sb will shim properly 3000x1300 machine. I mean to make try to make the machine not relatively straight but absolutely straight, which are 2 very different things. Though for 95% of the people relatively straight is more than straight enough.

By relatively straight i mean long rails not perfectly on 1 plane, one rail as a guide and the other straightened via the carriage, and so on. After the table is surfaced it will not show unless you have 500mm Z axis and are doing 500mm high details. If you get what i mean.

I spend a lot of time aligning and mounting my rails and even with all in one plane, even with a helper. Man, trying to make all absolutely straight is a big waste of time. It goes so slow. And you have to triple check everything. If sth not right do all again. Holding carefully 15kg straight edge does not speed the process up, i just can not imagine what will be if the rails were not in one plane. :dejection:

I know sb. could argue here but let me tell you something. Remember, i am speaking of trying to make an absolutely straight machine here. On a machine with 1350mm wide working area the rails are 1800mm separated. Using 2000mm square edge across is extremely difficult to square the second rail to the first. Why? Cause you move one side 2mm and the precision square still shows its square. You have to scratch the straight edge with precision square and judge by the friction if its square or not. There is no other way. So yeah, tell me now if at the same moment you have to check if the rail is not wavy, twisted and so on, and what happens if the rail itself is not straight from transport so you have to push and pull here and there while screwing. You need to grow some more hands. Now another thing. Even 1 tiny dust below the rail and it shows. So it slowed even further all.

So my suggestion is to speak of relative or absolute straightness when speaking of how it is done, cause its different. But as i said, my guess is that people most of the time are meaning in their heads absolute straightness and at most achieving in real life a relative one :hysterical:

Dean again i agree with you, in a way :semi-twins:. Absolute straightness /or a try at it/ in a DIY CNC is needed only if you make aluminum molds, surface aluminum machine beds and rails and generally intend serious production of aluminum pieces. Man i am not preaching OTT. But if sb spend considerate amount on BOB, Steel, motors, rails and so, he should at least give a try for an absolute straightness. Hence the bloody epoxy.

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Originally Posted by JAZZCNC

Regards not using a bridge can I ask how you know there is no difference in height between one side and the other.? Ie both rails on same plane.?

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Jazz, the way I see it the X rails don't have to be on the same horizontal plane as long as the two planes are parallel ie. if rail X1 is 2mm higher than X2 it must be 2mm higher everywhere along the length. There's probably other ways to do it but I epoxy casted a T shape reference surface on my base and used that to set up my Y and Z. The machine in the extreme example below will cut perfectly well as long as the y rails are parallel to the ref surface who's horizontal plane is parallel to the two x-planes. Mine, does not look like this as the x height difference was minimal:-) I used little skrew adjusters and then epoxy putty to shim my gantry bottom parallel to the ref surface and gantry front perpendicular to the ref surface and also used the ref surface to set up my Y rails and Z. Casting the ref surface does add a bit of extra work but once its there its very easy to set up Y and Z as opposed to some other methods I've seen.


http://www.mycncuk.com/attachment.ph...id=14566&stc=1

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Originally Posted by silyavski

not relatively straight but absolutely straight

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Silyavski, concerning rails and things there is no such a thing as absolutely straight. If it appears to be absolutely straight you are just not measuring it fine enough. Just getting back at you for saying my fans suck :cheerful:

Apologies to the OP for the digression.

Thanks for the picture, Mitchejc - that's exactly what I was meaning. X1 and X2 may be "level" in the sense of horizontal to the limits you can achieve with self-levelling epoxy, but they do not have to be in the same plane, just parallel. It's the same error and corrected the same way as not having the feet of the gantry exactly co-planar. It's not ideal, but once set up, your friends will never know the difference...

I have to agree that epoxy is a second-best. If we could use self-hardening water, I might believe that the surface would be ideal, but even low-viscosity epoxy doesn't run that well, and I suspect that you also get surface imperfections because of internal stresses caused by shrinkage. If I had a surface plate 1.8x1m, I would consider scraping (proper engineers' scrapers and everything, I've used them before) the top of the rails to it. In my case, I was looking to take out a dip in the 100x50 box sections of something around 1.5mm. I don't know why it was that bad, whether the box section was warped when I bought it or if it is welding distortion or what, but it would need quite a lot of filler and/or filing to get that flat to acceptable limits, and it just ain't that easy to do. As pointed out, even if you could use a straight edge to get the top level along its length, measuring twist is really difficult even though the profile rail manufacturer's specs are pretty tight and you need to get it right (or at least consistent along the length of rail).

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Originally Posted by silyavski

I still don't see how sb will shim properly 3000x1300 machine. I mean to make try to make the machine not relatively straight but absolutely straight, which are 2 very different things. Though for 95% of the people relatively straight is more than straight enough.

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Your fooling your self if you think you can build and more importantly measure to Absolute levels. Every DIY build is a compromise between accurecy and engineering abilty.

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Originally Posted by mitchejc

Jazz, the way I see it the X rails don't have to be on the same horizontal plane as long as the two planes are parallel ie. if rail X1 is 2mm higher than X2 it must be 2mm higher everywhere along the length. There's probably other ways to do it but I epoxy casted a T shape reference surface on my base and used that to set up my Y and Z. The machine in the extreme example below will cut perfectly well as long as the y rails are parallel to the ref surface who's horizontal plane is parallel to the two x-planes.

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Agree completely but again your only has accurate as your reference surface and abilty to measure. Not every one as large granite surface table for reference surface to work or cast from and without one then how sure are you your reference surface is accurate.?

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Originally Posted by Neale

If I had a surface plate 1.8x1m, I would consider scraping (proper engineers' scrapers and everything, I've used them before) the top of the rails to it. In my case, I was looking to take out a dip in the 100x50 box sections of something around 1.5mm. I don't know why it was that bad, whether the box section was warped when I bought it or if it is welding distortion or what, but it would need quite a lot of filler and/or filing to get that flat to acceptable limits, and it just ain't that easy to do. As pointed out, even if you could use a straight edge to get the top level along its length, measuring twist is really difficult even though the profile rail manufacturer's specs are pretty tight and you need to get it right (or at least consistent along the length of rail).

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It's all about having the right tools whether it's hard or not. With Precision straight edges and Engineers levels it aint that difficult. But here lies the problem becasue at DIY level 99% don't have or need these tools and the epoxy method with bridge makes it possible for less tooled up DIYers to achieve acceptable levels of accurecy for gantry router style machines.

The key word here being ROUTER. . Remember these are mostly Gantry based router machines being spoke about and the nature of the beast dictates lower accurecy is acceptable and high precision accurecy unobtainable.!! . . . .Again Horses for courses and use which ever method gets the job done for you in the easiest way possible.!