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  1. #1
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,005. Received thanks 170 times, giving thanks to others 5 times.
    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).

    Click image for larger version. 

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    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.

    Click image for larger version. 

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    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.

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    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.

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    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.

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    I'm now feeling fairly comfortable about being able to start drilling/tapping and mounting the rails.
    Last edited by Neale; 10-12-2014 at 01:46 PM.

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  3. #2
    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
    Last edited by EddyCurrent; 10-12-2014 at 02:53 PM.
    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  4. #3
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,005. Received thanks 170 times, giving thanks to others 5 times.
    Quote Originally Posted by EddyCurrent View Post
    Rather than copy it all out I thought it would be acceptable to put a link to my epoxy experience.
    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...

  5. #4
    NASA calls it "conformal shimming"
    dave

  6. #5
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,005. Received thanks 170 times, giving thanks to others 5 times.
    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.

  7. #6
    Would it be possible to use a shallower bed of epoxy? 5mm seem like a lot?
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  8. #7
    Quote Originally Posted by gavztheouch View Post
    Would it be possible to use a shallower bed of epoxy? 5mm seem like a lot?
    Yes just mix Less epoxy.!!.

  9. #8
    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...
    Neil...

    Build log...here

  10. #9
    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.
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  11. #10
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 4-5 years. Has a total post count of 1,005. Received thanks 170 times, giving thanks to others 5 times.
    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.

    Quote Originally Posted by beone
    NASA calls it "conformal shimming"
    ...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!

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