Axis stiffening with concrete?


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    Default Axis stiffening with concrete?

    If I was to build an x axis out of aluminum extrusion, say a 40x40mm profile like this one, would filling that hole in the middle with concrete be an effective way of adding weight and stiffness to it? I know a lot proposals for concrete cnc machines have been put out, with the main issue being shrinkage, but I think that since the actual mounting would be done on aluminum extrusion shrinkage wouldn't be an issue here.


    Also beyond this, what about making a truss out of PVC/gas pipe and filling it with concrete?

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    Default Re: Axis stiffening with concrete?

    Concrete will make it heavier, but not much stiffer. If you want it stiffer, use a bigger extrusion.

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    Default Re: Axis stiffening with concrete?

    Shrinkage would still be an issue, since the concrete would shrink inside the tube and just rattle around in there rather than reinforcing it. People that go that route usually use a granite/epoxy mixture, which doesn't have that problem. But you wouldn't want to use it on any parts that need to move, since it increases weight and momentum, which would work against you.

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    Default Re: Axis stiffening with concrete?

    using a rebar in the middle and non-shrinkable grout might help if weight is not an issue...compromise between rigidity and weight is needed.I would use larger rectangular extrusion.



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    Default Re: Axis stiffening with concrete?

    Quote Originally Posted by Scum View Post
    If I was to build an x axis out of aluminum extrusion, say a 40x40mm profile like this one, would filling that hole in the middle with concrete be an effective way of adding weight and stiffness to it? I know a lot proposals for concrete cnc machines have been put out, with the main issue being shrinkage, but I think that since the actual mounting would be done on aluminum extrusion shrinkage wouldn't be an issue here.


    Also beyond this, what about making a truss out of PVC/gas pipe and filling it with concrete?
    If you are talking the portland cement group of concretes, you don't want to mix that material with aluminum as concrete will corrode aluminum. Obviously the right environmental conditions pushes the corrosion process along, but in a nut shell the materials aren't compatible.

    Now epoxy concretes are another matter but they will not add a lot of stiffness to your structure either. The usual reason to fill any tubular structure is to impact vibration more than changing the stiffness of the structure. Sometimes the added mass is an advantage too.


    ""Also beyond this, what about making a truss out of PVC/gas pipe and filling it with concrete?""
    I'm not an engineer so it is very likely that somebody well versed in truss design might have a different opinion. In this sort of machine design, that is of routers that are generally of a smaller scale, I don't see a trust adding a lot of advantages over simply buying a steel beam of the right dimensions. This especially since steel beams come in so many sizes and are basically off the shelf in most locations. Now if you are doing a thirty foot gantry then there is likely a different answer here. AT that point though a good mechanical engineer would be able to nail down the advantages and disadvantages of each approach. I just don't see any advantage at all in a truss based gantry for a home shop or even a small commercial shop CNC router.



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    Default Re: Axis stiffening with concrete?

    What is the length of the X axis you want to build?

    What is the weight of your Z axis?

    What kind of linear rail and motion control (ballscrew, rack and pinion, other)?

    What kind of machine is this? Laser, router, 3d printer, plasma?

    People use 8020 aluminum extrusions to build CNC machines because they are generally flat and easy to work with. Not because they are the best choice as far as stiffness is concerned.

    40mm x 40mm is eensy weensy. That's super small for a gantry of any length.

    I believe that the concrete idea for a moving part is a bad idea.

    Steel is much stiffer than aluminum.

    If you need the T slots to bolt things onto, such as your linear rails or R&P, you can bolt some square steel tube on the two faces you're not using. That's a better idea. Get some square tubing and drill some holes in it, then bolt it onto the T slot aluminum. I did this for my first build. I believe I used 80mm x 80mm 8020 T slot, and I bolted 3 inch by 1.5 inch by 1/8 square steel tube on the bottom and one side. That left one face for me to bolt the V groove rail, and one face to bolt the gear rack for the rack and pinion. It worked out fairly well. Straight as an arrow and negligible deflection based on my Z axis weight. In retrospect, this is an ideal setup for a CNC plasma.

    As far as using plastic pipe filled with concrete, I think its safe to say that you are better off to build a structure out of wood. If this is an extreme budget build for a light duty hobby machine (nothing wrong with that) then instead of using plastic pipes filled with concrete, just make it out of wood.

    My 2 cents.



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    Default Re: Axis stiffening with concrete?

    If you're looking to reduce vibrations, I saw another post where somebody commented on what they found in driveshafts on large trucks.
    Try taking a slightly larger diameter carboard packing tube (the really thick wall kind) and cut a slit in it. Compress it till it's small enough to fit into your tube then trum the excess so the ends almost meet up, but when left out they "spring" back into the original size. This creates tension to hold the cardboard in place, which will also act as sort of a shock absorber.
    I haven't tried this myself yet but it makes nearly perfect sense in my head and I would love to see someone try it out on a machine. If you're going to take this and test it, might I request you build the machine without it firat, run it and observe vibrations and other issues, then insert your carboard and retest the same toolpaths just to see what difference it made?

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Axis stiffening with concrete?

Axis stiffening with concrete?