New CNC Router - Welded steel

Discussion in 'Hobby Engineering' started by pippin88, Oct 26, 2014.

  1. OP
    OP
    pippin88

    pippin88 Member

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    Work is progressing. I had a set back while milling Z axis parts: the brass motor gear in my mill shredded itself leaving only flakes and dust where the teeth had been. I'd started a belt drive conversion ages ago but abandoned it as my lathe blew a gear and had some other issues. So after much struggling and kludge fixes on the lathe I now have the mill belt drive done.

    In the meantime I've got the main router epoxy pour done. It went ok and looks good now. I had a couple of stuff ups:
    1. I tried to cover it with a suspended cloth to keep dust and chips out - the cloth dumped a load of fibres all over the epoxy... It didn't do that when I used it for the gantry epoxy cover.
    2. I managed to droop the cloth on to the epoxy. It'd been curing for a few hours and so was starting to gel and a few humps were created by the contact, but still seemed to start to level / smooth.
    3. Watch the heat - I'm using a pocket butane blow touch to get rid of bubbles. It also temporarily makes the epoxy flow easier (viscosity falls with rising temperature). However I went a bit overboard trying to fix the spots the cloth had touched and managed to cook a few small areas of epoxy. This meant I had to fish these cured discs / thin bits of epoxy out and the epoxy had to self level again.
    4. I switched to fast epoxy for the dam sealing step on this last pour. This works much better, but pay attention to the pot time. I was a bit slow and had a pot start smoking then suddenly go off. Had to drop the cup on the concrete and the disposable cup melted.
    Despite those problems it looks like it's gone well overall.

    Tips for self leveling epoxy
    You need the right epoxy - slow, low viscosity for self levelling. I used West System 105 + 209 hardener. Certainly not cheap, but others have used with good results.
    Get the object as level as possible as you'll need less epoxy
    Clean the object then let it dry
    There are two main stages:
    A) Building a dam and sealing it
    B) Pouring the main epoxy

    A) The dam
    You need something the epoxy doesn't stick too. It won't stick to the non sticky side of most tapes. It will stick to metal and wood.
    I tried gaffer tape, duct tape, and blue painters masking tape for various parts.
    Definitely use the blue tape (also called 14 day painters tape).
    Advantages:
    1. It's stiff and therefore forms a nice straight dam
    2. It's low tack so doesn't pull your paint off, and it's possible to reposition if you don't apply it perfectly
    Gaffer tape works but is too sticky and pulls off paint, and is also hard to apply as well as the blue tape.
    Duct tape stretches and curls, so doesn't give a nice flat edge.
    I found a single layer of blue tape carefully applied worked well, with particular attention to the corners.

    You must then seal the dam. This is done by applying a small amount of epoxy around the join between the dam and the object. You only want a small amount - this is done by applying with a spoon or knife, not pouring. If you stick to a small amount it will bridge small gaps and hold in place with surface tension - if you pour too much gravity will overcome this and it will flow through the gaps. Use fast curing epoxy for this. I used West System 105+205 hardener. The fast curing epoxy tends to be more viscous (an advantage at this stage) and works quickly to seal gaps. I used very slow epoxy for my first couple of dam sealing and it was a pain - it's runnier and it has time to all flow out before it cures.
    Be ready for leaks. Tape doesn't really stick where the epoxy is running, but if you whack enough on, the tape sticking to either side of the area will be enough to hold it and you can create an effective barrier. Blutack works also and is of course mouldable, but try to avoid it. It won't adhere where the epoxy already is, but does get incorporated in to the epoxy edge. However I did use it effectively to physically cover small leaks that would have been unfixable otherwise. You'll need big lumps of it.
    Now you have to wait till the sealing epoxy is solid enough. It doesn't have to be fully cured / full strength. If you used fast curing epoxy (seriously don't even consider using slow) you can proceed to the main pour a couple of hours later.

    The main pour
    As mentioned above, you need slow curing low viscosity epoxy.
    Mixing in a large shallow dish gives you more pot time.
    Mix thoroughly but avoid vigorous mixing as you want to avoid introducing bubbles.
    I mixed in batches based on my mixing container size.
    When each batch is mixed, just pour it on. Spread out where you pour fairly evenly so the epoxy doesn't have to travel as far.

    Be aware that surface tension is a significant factor and there is a minimum thickness to self level. This will vary with the epoxy, and I'm not sure what it is, but I guess and good 2mm at least. You won't get self levelling with 0.5mm thickness. I used to spoon to spread the epoxy around a bit also.
    Fingers crossed you have no leaks.
    Now you need to get rid of bubbles. This is done with heat, either a small blow torch / gas soldering iron, or a heat gun. Do not use a hair dryer as the air flow is too much and will blow dust in. I opted for a small blow torch as I can use it for other things. Wait a little for the bubbles to rise then gently waft the torch over the surface. Don't stay in one area too long - you'll cook the epoxy and I'm sure burning epoxy fumes aren't good you for either. You'll probably need a few passes, it's surprising how much bubbles you'll have. Bigger bubbles cause significant surface irregularities. Often you'll only see that bubbles are present if you use look at the epoxy with a bright light, such that the epoxy is a mirror and you are looking at the light bulb on the surface of the epoxy.

    I left each pour a full week to achieve good strength before disturbing.

    Heating the epoxy components reduces viscosity. This is done by putting the bottles in a bucket of warm water. However, this reduces pot life / speeds up curing. Apparently heating the object to be poured on still gives the benefit of better flow but doesn't speed up curing, which would be ideal.

    Once your epoxy is cured you'll need to remove the fairly large meniscus. Be careful, the edges will be sharp. My hands are covered in cuts from my clean up.
    Use a file to break the edges then take down the meniscus. Epoxy files easily and doesn't clog a file like it will sand paper.
     
    Last edited: Dec 4, 2014
  2. RobRoySyd

    RobRoySyd Member

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    I got the bubbles out of polyester casting resin by putting the pot into a chamber and applying vacuum. I made the chamber out of a length of 150mm PVC sewer pipe and caps but I hear an old pressure cooker also works very well. I already had an old rotary air pump that I converted to a vacuum pump. Pretty sure you can pickup vacuum pumps pretty cheap on eBay.
     
  3. 2xCPU

    2xCPU Member

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    You need to be careful with multiple pours or you'll end up with very poor bond strength at the interface.
    I realize it's a trade off against shrinkage and distortion if the volume is large.

    2.
     
  4. OP
    OP
    pippin88

    pippin88 Member

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    Each pour is one area, not multiple pours over each other.

    So I did a pour on the gantry, and then one for the table area, and one for the Y axis beams.
     
  5. 2xCPU

    2xCPU Member

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    Ah! - All good then.

    2.
     
  6. OP
    OP
    pippin88

    pippin88 Member

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    Progress slowed a bit as I was away for a couple of weeks.

    I've done the epoxy for the 4th (rotary) axis on the front of the machine.

    Major work has been the Z axis, which is largely done.
    Fairly happy with how things have gone. The large backplate is bigger than the working area of my mill, so required a fair few setups to do the edges. Fortunately the holes on the face are all just within the area so could be done in one go.

    It's loosely assembled below.

    Click to view full size!



    Click to view full size!
     
  7. elvis

    elvis Old school old fool

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    Great build. Subscribed. :thumbup:
     
  8. OP
    OP
    pippin88

    pippin88 Member

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    Work has progressed fairly significantly in a number of areas, but I have found a major problem. I finally got around to checking the epoxy for level and it's not good.

    I poured 3 separate areas.
    1) Two beams 200mm wide, 800mm long (the beams the Y rails mount to, this is the most critical one). The final epoxy thickness is 3-4mm and these seem to have worked fairly well.
    2) One beam 150mm wide, 1300mm long (the front rotary axis area). The epoxy thickness varies from ~1.5mm to ~5mm in a fairly linear fashion from one end of the beam to the other. There is a significant depression in the middle (lengthwise) of the beam of about 0.75mm.
    3) A grid with members 50mm wide, 1300mm long with cross beams at 1/3 and 2/3 along length (the table area). Again minimum epoxy thickness of ~2mm. Significant depressions / not level across the grid. The grid before pouring would have had areas 1-2mm low / high.
    (I also did the gantry but do have a good way to check that)

    I mixed carefully and thoroughly based on weight as per the directions.
    I suspect that the problems relate to surface tension / a minimum required epoxy thickness. In 2) the area with the thinnest epoxy (~1.5mm) is proud of the depression, where the epoxy is ~3mm thick and I wonder if the epoxy clung to the area rather than flowing.
    With each pour I poured the epoxy over a broad area. I used a pocket blow torch to remove bubbles. The epoxy appeared uniform.

    Unfortunately I've now got a few litres of not very level cured epoxy.
    I've sent an email to West System to see if they can give any advice / shed some more light.
     
  9. srl

    srl Member

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    West system is a two part epoxy formulated so that it will be adsorbed into wood. Its used by boat builders. I use it for building and strengthening wood parts in RC aircraft.
    I use it for propeller molds but without fibers it chips and over time shrinks and over a big area that will mean cracks.
    In winter its thick and needs warming to use but in a Sydney summer its thin and goes off like 5 minute epoxy using the long cure.
    Its very good as a finishing and laminating resin.
    I've been following your build but could not understand this part of the design.
    At this size you need all metal. Can you get it milled somewhere ?.

    I had my last CNC rebuild 18 months ago, heavy metal is the way to go!

    Click to view full size!

    SRL
     
    Last edited: Jan 23, 2015
  10. OP
    OP
    pippin88

    pippin88 Member

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    I used 209 extra slow hardener, it takes 24 hours to cure in a thin film.

    I considered getting it milled but there are a bunch of problems:
    1. Finding someone who can mill 1600x800mm
    2. Getting it to and from a machine shop
    3. I would need to get it stress relieved before milling to avoid warp as stress from welding is released - more cost and logistics.

    Epoxy has excellent compressive strength and has been used successfully both by DIY builders and in professional machines. West system epoxy has been used for this by others, and was the best / had the most previous use in this area, that I could find.

    The machine weighs ~300kg for a cutting area of 1300x675mm, it's not what I'd call light weight.
     
  11. RobRoySyd

    RobRoySyd Member

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    I'd think so. Even 3mm sounds way too thin to me to get something to self level over that area unless it's the viscosity of water and with something in it to break surface tension.

    I think you need to get the entire pour as level as possible and only rely on the self levelling as little as possible. Vibration would help.

    I would have rigid metal around the pour area that's as level as possible then use that as a guide to level the epoxy with a very rigid piece of metal then vibrate the entire table to encourage the bubbles to rise and the epoxy to flow.

    Another factor that could bring you unstuck is the epoxy curing at different rates over the area. As it cures it get hot so you have an issue of differential heating. I'd suggest you need fair slab of metal under the epoxy as it cures to avoid this problem. A quick search reveals that poured aggregates have been used for large machine beds such as granite epoxy mixtures but I bet they're quite thick.
     
  12. psychobunny

    psychobunny Member

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    I recently poured a self leveling cement in between the tiles in the patio and was able to make it quite viscous - similar to thickened cream

    http://www.aftek.com.au/flooring.html - maybe see if they have somethign suitable?
     
  13. OP
    OP
    pippin88

    pippin88 Member

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    I've been slowly chipping away at this.

    Everything takes a little longer than I expect...

    I've now got a working basic machine.
    Things I've knocked over:
    Linear rails: Drilled, tapped and aligned the linear rails. This took a fair while. For alignment I eventually used the tight wire method, with thin steel wire over some pulleys with weights hanging on the end. I searched for ages for piano wire, but couldn't find anywhere with thin enough stuff. In the end I found that fishing shops have thin stainless wire with rated breaking strain (fishing leader wire) and it worked great. I used 27lb stuff. It's pretty cheap too. I then used a simple circuit with an LED ring and a razor blade on a cart on one of the linear bearings. When the razor contacts the wire, the LED ring lights up. I was able to get it so along the whole length it just skimmed and only just lit up the ring. I think the rails are straight within 0.05mm per metre or so. This was a painstaking process. I had to turn the heads of the bolts down for the gantry, as I'd hand drilled using the rails as a guide, and the rails are not straight when not bolted down.

    Ballscrews mounted and aligned.

    Homing (and limit) switches.

    Wiring / electronics: Managed to kill a G540 driver (just one of the G250 drivers in my G540) due to making an incorrectly wired connector. I've since moved to a Mesanet 7i43 + 7i76 for step generation, and Leadshine DM856 digital drivers. I'm very happy with the combination. They are very quiet at low speeds, with the G540 the machine was loud at low speeds. I'm happily rapiding at 20m/min. I fit the PC and the electronics in an old aluminium mid tower PC case. I've got some buttons on the front: ESTOP, feed start, pause and stop, feed rate increase, decrease, spindle speed increase and decrease.

    Table top: Two layers of 18mm MDF, will have a spoilboard on top. Involved using a very long bit to skim the epoxy flat. Then, because the back table support beam is outside of my cutting area, I had to level it to the others. I used an angle grinder and a straight edge, and just kept grinding until it was flat compared to the other beams. Then drilled and tapped mounting holes. Then drilled the mounting holes in the MDF. Then I drilled a grid of holes, on 6cm centres, and milled pockets for tee nuts to sit in. I have 242 tee nuts in the table. They take a while to install - use a screw to pull them in to the MDF, don't use a hammer (I've seen some people recommend using a hammer).


    Click to view full size!

    Holes drilled and pockets routed for tee nuts


    Click to view full size!

    242 tee nuts installed - took a while!


    Click to view full size!

    Very bad photo of the table installed
     
    Last edited: Apr 11, 2015
  14. mad_mic3

    mad_mic3 Member

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    looking good:)
     
  15. Ch4iS

    Ch4iS Member

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    Very nice pippin :thumbup:
     
  16. OP
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    pippin88

    pippin88 Member

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    Did a bit of cutting today.

    6mm carbide single flute spiral, 6mm DOC, 24000rpm, 5metres/min (~200ipm) - cut beautifully.

    On my old machine I was cutting at 3mm DOC, 2metres/min with poor cut quality.

    Next step is definitely dust collection. I'm going to build a floating dust shoe.
     
  17. Jenova

    Jenova Member

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    Any Vids ?
     

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