Use of epoxy for levelling

[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.

[Neale]

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...

[JAZZCNC]

- (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?

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)

[mitchejc]

- (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 ...

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.

[JAZZCNC]

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.

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.?