Thread: Use of epoxy for levelling
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...
Last edited by Neale; 29-01-2015 at 10:14 PM.
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)
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.
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.?
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.
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
Dean again i agree with you, in a way . 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.
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).
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.!
Last edited by JAZZCNC; 31-01-2015 at 01:53 AM.
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