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Thread: Large Format 5-Axis Gantry Router

  1. #49
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    Default Re: Large Format 5-Axis Gantry Router

    Hypermill 2018. Expensive, but good. A few years ago I considered building a machine similar to yours, to make moulds for small yachts, and at the time Delcam looked like the best bet.



  2. #50
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    Default Re: Large Format 5-Axis Gantry Router

    I think that I can officially declare that self leveling epoxy sucks.

    Maybe for small areas it is ok, but for longer beds it is just not possible to get it to flow out flat enough. Even when being very exact with putting the right amount in specific zones to ensure that the required flow is minimal and just local leveling is sought, I am still getting poor results. If I am not careful I can get high/low spots of +/-1.0 mm or more (which is I presume the form of a frozen flow wave) but even a good pour is high/low of +/- 0.1 mm or more over 300mm spans. Epoxy is a special low viscosity resin and gel time is two hours so that is all fine. Lots of hand work required (precision hand sanding, staight edge, precision level checking) to get it in any kind of shape. Across the bed is OK (level and flat) but longitudinally is rubbish.

    [Note: I am putting a gauged plate on top of the epoxy when flat. This plate follows the faults in the epoxy to a degree, for example +/- 0.05mm over 300mm]

    Question:

    Regarding flatness of beds for linear rail mounting (medium precision/preload) can anyone please tell me tolerances for flatness. Generally the linear rail manuals do not state tolerances on the bed flatness requirements, just stating "flat and straight mounting surfaces". I have found various info that would suggest max allowable values of 0.02mm out of flat over 500mm (for medium precision/preload rails up to 45mm) but this maybe seems a bit extreme to my thinking. Please if anyone has any knowledge here I would be grateful to hear it.

    At the moment it looks like I have a lot of hand scraping ahead of me.

    Last edited by jono5axe; 03-14-2019 at 04:36 PM. Reason: sp
    Jonathon Clarke
    www.solpont.com


  3. #51
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    Default Re: Large Format 5-Axis Gantry Router

    It sounds like your epoxy was too viscous. I'd suggest scraping it off and trying again; this time thin it with some acetone until it levels out immediately.

    [FONT=Verdana]Andrew Werby[/FONT]
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  4. #52
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    Default Re: Large Format 5-Axis Gantry Router

    What flatness values are you measuring with that method awerby?

    Jonathon Clarke
    www.solpont.com


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    Default Re: Large Format 5-Axis Gantry Router

    Heating both parts of the epoxy to about 115 degrees F, and then combining them, will reduce viscosity. On the downside, it will cause the mixture to set up faster.

    The preferred method, according to the folks at West Systems, is to heat the substrate and then add the epoxy at room temperature. The warmed metal will reduce the epoxy's viscosity and help it flow out. One problem, though. Your machine is a huge beast that probably needs several people to mix and pour at about the same time. It might be worth talking to the folks at West Systems for suggestions. Their international phone number is 1-989-684-7286. They are open 9:00 am to 5:00 pm US Eastern time. I believe that translates to 2:00 am to 10:00 am NZDT a day later.




  6. #54
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    Default Re: Large Format 5-Axis Gantry Router

    Hello Jono - sandwich structures are only stiffer in the thru thickness direction. Their global stiffness is actually less then if you only had the fibres on the outside. Sandwiches are great for boats that have to keep water (thru thickness pressure) out but not good for things like beams such as you are making. The only reason to make a sandwich in that case is to control local buckling. But your structure will not buckle in this application. There is also a comment to add acetone to make the epoxy thinner do not do this! The acetone is non functional and takes up volume within the epoxy. It then desorbs out of the epoxy over time and the epoxy shrinks. So your surface will change!! cheers Peter s



  7. #55
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    Default Re: Large Format 5-Axis Gantry Router

    Acetone can also inhibit the cure - so it remains permanently weak. I tried it some years ago to see if I could achieve very deep penetration into wood.



  8. #56
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    Default Re: Large Format 5-Axis Gantry Router

    Quote Originally Posted by peteeng View Post
    Hello Jono - sandwich structures are only stiffer in the thru thickness direction. Their global stiffness is actually less then if you only had the fibres on the outside. Sandwiches are great for boats that have to keep water (thru thickness pressure) out but not good for things like beams such as you are making. The only reason to make a sandwich in that case is to control local buckling. But your structure will not buckle in this application. There is also a comment to add acetone to make the epoxy thinner do not do this! The acetone is non functional and takes up volume within the epoxy. It then desorbs out of the epoxy over time and the epoxy shrinks. So your surface will change!! cheers Peter s
    Thanks for the comments Peter. I understand what you are saying, but i can say that:

    1) Most fibre is in fact on the outside.

    2) the method of constructing a large beam without a mold utilises the core to support for the fibre/laminate during construction.

    3) The skins need stiffness themselves, in order to mount precision equipment (linear bearings, gear racks).

    4) The critical consideration for the gantry beam is deflection in torsion (rather than bending) and I consider that my nice stiff sandwich rectangular-hollow-section is going to have less deflection than a floppy thin skin rectangular-hollow-section in torsion (bigger moment of gyration?). [circular section would be more pure solution, but not ideal for mounting rails on.]

    5) I wanted thickness/stiffness/substance to counteract resonance. Thin skin would be like a drum.

    6) I like my sandwich.


    Ps Acetone added to epoxy >> degraded cure and degraded mechanical properties of cured resin, and porosity.

    Jonathon Clarke
    www.solpont.com


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    Default Re: Large Format 5-Axis Gantry Router

    Hallo jono
    Very interesting and ambitious project here.The scale is impressive.
    On epoxy leveling ukcnc seems to be the forum where epoxy leveling is the go to method.Those builds go into considerable detail.
    If I recall accurately this build has some good detail as a starting point.

    BUILD LOG: A sufficiently strong machine

    I wish you good problem solving on this project.
    Robert.



  10. #58
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    Default Re: Large Format 5-Axis Gantry Router

    How thick is your epoxy pour?

    I found I needed quite a thick layer. A good 5mm. I remember reading a recommendation for 1/4" (6.35mm) thick layer - can't find the source however.

    There is a fair bit of surface tension.

    Sent from my SM-G970F using Tapatalk

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    Default Re: Large Format 5-Axis Gantry Router

    The linear rail spec sheets will have specifications for all the allowable tolerances.

    What matters is that the two rails are equal. That is why spec sheets don't concentrate on flatness - they talk about parallelism and height tolerance.

    Sent from my SM-G970F using Tapatalk

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  12. #60
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    Default Re: Large Format 5-Axis Gantry Router

    Hi Jono - I suggest you do a torsion proof test and measure its torsional deflection. I doubt if the bending stiffness will be the weak link as you say. Then back calculate the bulk shear modulus for the beam and see how you do compared to other structures eg riveted aluminium or steel (like aircraft construction) You'll only have one good opportunity to do this before you build it into the machine then you will know if you have made a good structure. The structure you describe has a small shear modulus and being square is not geometrical ideal. Then you'll have good info for when you build your next machine. I've been involved with composite structures for about 30 years and understand why you would build this way. Composite are great but they need to be executed correctly to get high mechanical properties. These days it's quite easy to get aerospace grade laminates in your garage using infusion methods.

    The discussion about mass as a damper is an interesting one. Adding mass is the easiest method to damp a structure, its harder to excite a heavy thing then a light thing. But if you want it to move fast this is a poor method as then you need more power to accelerate it. You won;t get a job in an F1 team or a Boeing design team if you say "let's add some mass here" So using the add mass approach we would make suspension elements on cars heavier or at least make the car heavier which is what the old american cars did. But if you want a performance structure then you have to lightweight the hell out of it and solve the resonance problem (if there is one) another way. There are many ways to dampen a thin structure, from adding creases, to adding soft stuff to it, to adding tuned dampers, to active dampers etc etc. If you want to up the game you have to up the technology. Cheers Peter S



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