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Thread: Carbonfiber Gantry

  1. #13
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    I think what you're really looking for is stiffness. The problem with light weight stiffness is that it's susceptible to resonance. It's easier then to go with heavy stiffness as in cast iron, steel etc. If you can model the resonance issues accurately though there should be no problem with light weight stiff materials as in carbon fiber, titanium and of course unobtanium.

    Chris



  2. #14
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    All structures have resonant modes and resonance occurs under dynamic loads. Theoretically a mechanical structure has infinite number of modes due to essentially infinite degrees of freedom.

    A light and stiff structure has higher resonant frequencies than the same geometry of a low young's modulus and higher density material.

    Resonance is not an issue if dynamic loads are not at the resonant frequency. Sometimes you cant avoid resonance for example due to an impulse such as an instantaneous strike has all frequencies. But you can damp the vibrations out to make their magnitude of oscillation essentially negligible given the acceptable accuracy required.



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    I think though in the end aluminum and steel are so much cheaper, easier to work with, and proven for what most DIYers use CNC routers for.

    To give an example, Onsrud's Extreme Series fixed bridge router is capable of over 6000ipm rapids and up to 4000ipm cutting feedrate. The 40HP spindle alone has to weigh a couple hundred pounds, let alone the other parts of the carriage and other spindles (line boring head, etc...). There's no carbon fiber anywhere on the machine. If CF was such a benefit to a woodworking machine they'd save money and use smaller servos to move that carriage.

    I could see composite parts used in vinyl plotters, plasmas, lasers, waterjets, pick-and-place machinery, etc. where acceleration and speed are beneficial, and there is no heavy forces placed on the drive system.

    I guess you could wrap metal parts with CF to make a cool looking design, but I don't know if the benefit would outweigh the cost....



  4. #16
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    yeah and carbon fiber can be a PITA to create parts out of. Requires more steps and unconventional techniques to cut fibers and to impregnate with resin and cure etc. Also you are likely gonna need more assembly of parts when using carbon fiber vs machining it out of a single block which may cause engineering issues too due to need for more assembly.



  5. #17
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    Check out the links composite pro gives on the epoxy granite thread,post 4645 page 388.
    They show strength,weight and damping advantages of carbon fibre graphite materials.
    Great if your a big company with lots of dollars for r+d and good engineers.Cost,design
    and construction inexperence sadly put it beyond my understand the
    principle/empirical/experimental approach though.



  6. #18
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    Quote Originally Posted by ;1156855
    I think though in the end aluminum and steel are so much cheaper, easier to work with, and proven for what most DIYers use CNC routers for.

    If CF was such a benefit to a woodworking machine they'd save money and use smaller servos to move that carriage.

    I could see composite parts used in vinyl plotters, plasmas, lasers, waterjets, pick-and-place machinery, etc. where acceleration and speed are beneficial, and there is no heavy forces placed on the drive system.
    This and what others have written is conventional thinking to reason why new and unconventional ideas will not work.
    Not that conventional thinking is bad but if you want new ideas to work out you will need to use new ways to get to the desired result. Conventional thinking will be in your way.

    When the first boat was proposed to be made from steel the designer was laughed at because everyone knew that steel does not float.

    I have developed a system in composites for leisure boating that had been made in welded stainless steel and/or (cast) aluminium for decades. In the process I have reduced the number of custom made parts by about 50%, cutting weight and cost significantly.

    Based on what I know from that, and cnc, I think that resonance is the only problem you may face when building a cnc from CF.
    And that will only be known if someone will try it.

    And it may be unsure if that is a BETTER cnc (in any way) than heavy ones that are common.

    Sven http://www.cnczone.com/forums/diy-cnc-router-table-machines/320812-aluminium-1250x1250x250-router.html


  7. #19
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    Quote Originally Posted by shedbob View Post
    Check out the links composite pro gives on the epoxy granite thread,post 4645 page 388.
    They show strength,weight and damping advantages of carbon fibre graphite materials.
    Great if your a big company with lots of dollars for r+d and good engineers.Cost,design
    and construction inexperence sadly put it beyond my understand the
    principle/empirical/experimental approach though.
    The CF comes out looking pretty good there but they cherry picked the other items in the comparison. Yes it did better than steel but steel rings like a bell. I think the conclusions of a damping comparison to grey cast iron or E/G wouldn't look so good.

    That said, I have often wondered how a machine with an E/G bed and a CF gantry would perform. The bed's weight and damping would keep the machine stable and the light gantry could be accelerated at insane rates. I suspect it would be something like a race car though. It would work very well within its design parameters but be a pain to use outside of them.

    bob



  8. #20
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    Quote Originally Posted by CaptainVee View Post

    When the first boat was proposed to be made from steel the designer was laughed at because everyone knew that steel does not float.
    I hope this isn't true...as it was well known for a long time before the introduction of the steel or iron hull that the weight of the boat only need be less than the weight of the volume of water that the hull shape will displace. Successful metal hull building was I believe more a matter of construction methods than an understanding of hydrostatic principles.

    Chris



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    How about a hollow CF structure filled with mercury?

    That will give you looks, weight, damping, and added thrill of having a couple hundred gallons of mercury on hand.

    Matt



  10. #22
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    One of the reasons for the popularity of cast iron is that it is self damping, so vibrations are rapidly removed by the material.

    The science behind "why this happens" is more complicated than this explanation, but basically it comes down to the fact that cast iron is not one, homogenous material, it is a mix of hard clumps and softer adhesive areas. The difference between these materials and how each behaves in relationship to the sound / vibration wave is what makes it damp out quickly.

    Epoxy granite mixes share this general concept, which is why they often damp very well.

    Al is completely different than cast iron in this way, which is why it rings like a bell. Some Al castings contain so much sand that they start to act a little like cast iron, but most Al material does not.

    Steel is somewhere in between, so it depends more on total mass to dampen, then dissipation inside the material. This is why you can fill a steel tube with sand, rubber, or other polymer coatings and it helps more than you might think.

    Ger21 tries to make the best of this mixed materials dampening effect by bonding Al and wood laminates together in his builds. He is more patient than most builders though and it appears that he spends at least 1/2 of this time building up layers. That isn't a criticism, just an observation that this approach is labor intensive.

    In theory, a composite fiberglass or carbon fiber could be made very stiff, and might actually be able to damp out vibrations very well. One interesting example is the new airbus 380. I have not flown it yet, but my friends that have claim it is MUCH quieter than the Al planes they are used to flying on. That is an indicator of good vibration dampening in a light structure. If you call a many ton vehicle a "light structure".

    There is a honeycomb polymer material used in high end boat building known for its vibration dampening. I don't remember the details though, but it is commonly sold down in the Florida area.



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    I made this beam as a support for solar panels on the boat i am building. It is 16mm endgrain balsa with 600 gram epoxy glass. The edges have been rebated and filled with 20 strands of 50K carbon tow. The round is only a couple of layers of glass over sone pvc tube and is only for decoration. When i made it i was surprised just how stiff it is and my thought at the time was it would make a good beam for a router. It s 2.6mtr long and only 90mm high. I tried to deflect it and twist it and it seems to be a lot better than a 100x50x4mm aluminium box section that i have but without putting a clock on it that is a bit subjective. I am only playing with a 6040 chinese machine at the moment and the boat takes priority but i will surely give it a try when i get to a bigger machine. Reading this thred i went and experimented a bit more and i am sure it would dampen a lot better than the ally section, i think the balsa, carbon and regular glass together would work a lot better than just solid carbon section.



    Mike



  12. #24
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    Default Since you're a millionaire....

    I say go for it. I am in the aircraft industry. Carbon fiber is a wonderful material. And since you'll do this when you're a millionair what do you have to lose but money? And for those pointing out vibration and resonance they have a point (the first all carbon fiber airplane has tuning forks all over to handle resonance issues). But who knows. Perhaps a composite lay up of alternaiting layers of carbon fiber, aramid, fiberglass, and, even aluminum. There is so much you can accomplish. You can design around resonance and vibration issues with an experienced composites engineer. A bit expensive but what the heck, you'll be a millionaire when you start this project

    Other ideas I would have for the best CNC ever... Mind reading capability would be nice. Definately pre tool crash shut down. Maybe there's a way to measure stresses inside of a cutting tool and send that feedback to the controller...auto feedback for speeds and feeds. Air bearings for linear travel. Some kind of ozone radiation based purifier for coolant systems. I HATE the smell of rotton coolant. I think a carbon fiber / composite gantry is quite possible with proper engineering.

    You wouldn't be fighting the inertial loads you would have with steel.

    Fun fun.

    Quote Originally Posted by flojor10 View Post
    So I was thinking if I ever became a millionare one thing I would do would be to build the best Cnc machine ever! So I started thinking about characteristics I wanted and of course speed, what my current machine lacks. So would a carbon fiber moving gantry be something to consider. I mean the material is amazingly stiff and lightweight. I also thought it might contribute to resonance, and weight could help, how much weight is desirable on the gantry to have a sturdy machine? And if anyone else has any ideas to make the best Cnc machine ever we'd love to hear them.




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