AVOR – a steel-framed medium-size router

[Neale]

Epoxy levelling

I want to be a bit controversial here.

First – using epoxy to level isn’t as easy as it looks. It’s not that difficult either, but there are a few gotchas.

Secondly - you don’t need a bridge between the two X rails. At least, not with the adjustment I built into my gantry mountings. Leaving out the bridge really does simplify the X rail epoxy levelling.

First point first. I didn’t allow for the effect of ambient temperature. It was a big mistake to try to do the epoxy application during a spell of cold weather. The stuff cures OK (although it takes a bit longer), but it doesn’t particularly want to flow and you really need low viscosity for the process to work. Levelling is all based on the idea that the fluid epoxy will “find its own level” and it doesn’t want to do that readily if it is too cold. I used the thinnest resin from Reactive Resins [note – it appears that the supplier has now gone into liquidation and this resin is no longer available]. No problems with the resin, it was my technique that was at fault. I tried building a tent over the machine frame with a fan heater underneath but I couldn’t really get enough heat into it to make the resin thin enough. The other thing to watch is that the resin seems to like at least a couple of millimetres or so of depth to flow; it doesn’t particularly want to run into very shallow areas. Allow for this when you make up your batch of resin – have enough to give adequate depth over the whole area. That was another of my mistakes. Yet another was not checking for leaks before filling the area with resin. I used a simple dam around the X rails made by sticking gaffer tape to the vertical faces of the rails. Works better than you might think but there will be leaks! And once a leak has started, you can’t do anything about it – gaffer tape does not stick to steel covered with resin. Easiest way, I reckon, is to fill the holes before you do the main epoxy pour. Make up a small amount of resin, then run it around the rail/gaffer tape join. A small amount will run into any gaps but without much pressure behind it, it will stay there. Once cured, no more gaps and you can go ahead with the main pour. Note that without a bridge between rails, there are fewer joints to leak…

Why don’t you need a bridge? Because with my design, the two X rails do not actually need to be at exactly the same height, and it’s pretty easy to get them to less than a millimetre with simple tools. A builder’s spirit level will probably claim to be within 1mm per metre, but you can calibrate them to quite a bit better than this – place on “flat” surface, note bubble position, turn level around and repeat. “True” level is with the bubble halfway between the two bubble positions, whatever the little lines on the bubble tube say. Use the calibrated level to get the X rails as level as you can, using the adjustable feet you have already fitted. You have fitted adjustable feet, haven’t you? This levelling is going to be approximate anyway. In my case, one X rail dipped in the middle by about 1.5mm, the other by about 2mm. Don’t know if that was welding distortion or if the tube was like that to begin with. If you start with the rails as level as you reasonably can and add equal amounts of resin to both sides, they are going to end up pretty close in height. What is more important is that the level of the finished epoxy is flat in both planes (along and across) and self-levelling using gravity is the easiest way to achieve this. Both X rails need to be parallel along their length but this is easy to achieve if the rails are sitting on accurately flat, horizontal, surfaces which is what the epoxy bed gives you. Hiwin rails are great, but they are so accurate that they are not very tolerant of misalignment. If a rail twists along its length, or the X rails are not parallel in all axes, they will bind. In my case, my gantry to X carriage mount fixing allowed for small differences in height, so I was not worried about these and could scrap the whole bridge thing.

Having said that, I have to admit to having two attempts at the epoxy levelling. The first, with a bridge (which was why I wasn’t convinced that it contributed much to levelling between rails anyway), leaked badly, to the point that I lost so much resin there wasn’t enough left to self-level. Second attempt was better, without the bridge and after chipping off the remains of the first.

I levelled the top of the gantry using epoxy, with the gantry mounted on the machine. I had intended to then remove the gantry, turn it over, and rest the epoxy surface on one of the (epoxy-levelled) X rails, propping it into position. The idea was then to epoxy level the lower rail mounting surface so that both surfaces would be parallel. In the end, I left the gantry in place and fitted the lower rail using shims between mounting bolts. I think that plan A would have been better but it’s all fitted and working now although it took quite a time to do.

I am very glad that the Z axis uses Ecocast plates which are more than accurate enough to take rails and carriages without all that faffing about with epoxy…

Here’s another thought. If you are using steel tube that is too thin to take tapped holes for rail hold-down bolts, you are going to need an extra strip of steel to thicken the surface. The usual way to do this is to add the steel strip inside the tube. I did this for the X rails (the long ones) by drilling holes through the box section, avoiding the points where I was going to put the rail fixing bolt holes, and making up a “wedge-on-a-stick” arrangement which held the strip against relevant face inside the tube. I then welded through the holes to keep the strip in place. This was a bit awkward to do, although it worked OK. However, I forgot to fit the strips in the Y (gantry) box sections before welding the gantry together, so I just welded the strips to the outside of the tubes. If you are going to use epoxy anyway, this is a perfectly practical way to do things and much easier than fitting the strips inside the tubes. This is a good time to repeat that when you are tapping for the rail fixing bolts, you really want to use spiral-point machine taps, not hand taps. I found that the spiral-point taps worked fine through steel using a cordless drill but I did use a tapping guide to help keep them square. Don’t use spiral-point taps for blind holes as they are designed to push the swarf ahead of them; use spiral-flute instead.