Use of epoxy for levelling

[Neale]

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.

[gavztheouch]

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

[JAZZCNC]

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

Yes just mix Less epoxy.!!.

[Jonathan]

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?


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...

[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...)



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...

[EddyCurrent]

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...)

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

[Neale]

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

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