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  1. #37
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    Quote Originally Posted by ihavenofish View Post
    its not complicated at all. its basically a lathe of sorts, winding a spool of carbon tow thats passing through an epoxy bath.

    its really only good for convex tube shapes though. you could make a nice beam or core.
    Sounds easy enough. From experience, some things look deceptively simple to me until I try to build them. I worry about a lot of unforseen variables in building a machine like that.



  2. #38
    Community Moderator ger21's Avatar
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    I checked out the link, pretty cool. They use filament winding.
    It looks like those wound holes actually are a bit more complicated than a lathe of sorts.

    Gerry

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  3. #39
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    Quote Originally Posted by ger21 View Post
    It looks like those wound holes actually are a bit more complicated than a lathe of sorts.
    im pretty sure those arent filament wound. by the looks, its just the outer and inner shells that are wound.

    wotzBotz


  4. #40
    Registered momus_cnc's Avatar
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    A few observations:

    Stiffness- The elastic properties of a material are only part of what contributes to stiffness, the other being how that material is arranged in space- its geometry. A small diameter rod and a large diameter hollow tube can have radically different stiffness with the same amount of material. But even talk of sectional shapes such as tubes or I-beams totally misses the opportunities of composites. Those sectional shapes are the natural result of particular materials and their manufacturing processes, which don't necessarily apply to composites. We can fabricate a much broader spectrum of forms, as well as vary the amount of material that is placed at any particular location. And design processes that utilize form-finding and topology optimization can result in geometric solutions that we couldn't possibly even begin to imagine, yet can be realized with composites.

    Anisotropic- Composites such as carbon fiber are highly directional in their strength, which instantly separates them from most of the other materials being discussed (at least for all practical purposes.) So not only can we explore the wider landscape of forms that are mentioned in the paragraph above, but within a given form we can control placement and direction of fibers, which has an impact on stiffness, ductility, damping, etc.

    Composite- In using “carbon fiber” and “composite” interchangeably we can lose sight of the fact that we are talking about a material that consists of a matrix that holds some other stuff within it. We can put other stuff in there beyond just carbon fibers. We can combine materials that have different engineering properties. Need more damping? Perhaps there is a way of combining a stiff material with one that has inherent damping capabilities. Perhaps we can determine that a material like that only needs to be incorporated in certain areas of the overall component. We are also ignoring that there is a good chance our gantry would be a sandwich construction with a core material. How this material contributes to the overall performance of the part needs to be considered.

    Overall, the three points above mean that we could create a composite gantry that is incredibly stiff, and possibly tuned to deal with resonance issues, while still being very light. And it would probably be some crazy shape that would look quite impressive. The knowledge and fabrication skills to design and fabricate it would be no small undertaking, but the possibilities are huge.

    Damping- I know little about resonant frequencies, especially as they pertain to machine design, so this one is a question: If stiffness can be kept exceptionally high, and therefore amplitude is exceptionally low, do we reach a point where frequency is a moot point?

    Goals- What exactly are the design goals here? “Best” at what? Asking the right questions at the very beginning of the design process would be crucial, otherwise we have no criteria to judge how successful this material might be. Especially since while designing anything we typically need to juggle multiple goals that conflict with each other.
    There isn't one person who doesn't like the look of cf, I need to incorporate cf in the build at least on the outside.
    If the primary goal is to look pretty, you can go with peel and stick vinyl that looks just like carbon fiber, and slash the budget considerably. :-)



  5. #41
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    have you ever heard from the company in Germany www.eew-protec.de ? They build one of the largest lightweight 5-Axis CNC-milling-machine in the world with a carbon-fiber gantry designed as a carbonfiber truss.



  6. #42
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    Default Re: Carbonfiber Gantry

    I know this thread is kinda old but I wanted to add my engineering $.02.

    Vibrations occur at an undamped natural frequency (f) in most CNC cases.
    f=(sqrt(k/m))/2pi where k=stiffness in N/m and m = mass in kg

    example likely not resembling anything in real life except for short gantries:
    K=8740157 N/m, ~50000Lbs/in
    m=88N, ~20Lbs

    f=(squrt(8740157/88))/2pi = 50.157 Hz

    If you were running a 2 fluted bit in a router at 25000 RPM that would equate to 50000 strikes per second or 50000 Hz which is very nearly 1000x your natural frequency which will cause some modal vibrations in your gantry. Now, finding a stiffness is not difficult for a constructed router (pull with 50 pounds and measure the deflection) but measuring the effective mass is. Alternatively if you had the stiffness and could somehow measure the natural frequency when the gantry is struck with a dead blow hammer you could get the effective mass.

    Moving on

    Adding dampening will do multiple things for you:

    1: Shift the natural frequency so that it may be out of a harmonic zone that your machine would operate at.
    2: Reduce the amplitude of any vibrations by converting the vibrations into heat either with a mechanical damper or an elastic damper.
    3: Make the calculations much more difficult (I'm talking about differential equations in which case I am about 4 years out of practice with them)

    The simple way to damp vibrations in large sheets of anything is to add something like Dynamat or similar non-hardening butyl based adhesive backed dampening material.



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    Default Re: Carbonfiber Gantry

    That is the first time that I read something about mass that actually makes sense to me. I always thought that the expression "weight adds dampening" was only part of the story.

    Could it be true that dampening would only be needed if it is needed indeed, ie if you get resonance during operating the mill?

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


  8. #44
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    Default Re: Carbonfiber Gantry

    Dampening is great in most cases unless you make trampolines. You can tune your dampening to cancel out vibrations completely yielding a much quieter machine and less chatter. Damping is not needed in most cases correct and I should add that the dynamat I had suggested will only work for higher frequencies. Lower frequencies require higher dampening coefficients which can be obtained with larger elastomeric dampeners.



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    Default Re: Carbonfiber Gantry

    Quote Originally Posted by slopecarver View Post

    If you were running a 2 fluted bit in a router at 25000 RPM that would equate to 50000 strikes per second or 50000 Hz
    50000 strikes per MINUTE. 833hz. fancy engineering math isn't all that useful if you don't have the right numbers to input

    wotzBotz


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    Default Re: Carbonfiber Gantry

    Quote Originally Posted by ihavenofish View Post
    50000 strikes per MINUTE. 833hz. fancy engineering math isn't all that useful if you don't have the right numbers to input
    Good Catch.



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    Default Re: Carbonfiber Gantry

    I know this thread is kind of old but I think it deserves a revisit. There have been a couple manufacturers now that have started to use carbon composite in CNC machines. There are at least 2 examples in the links in this thread. I was wondering if the costs have started to come down? And if its becoming more feasible for a home built machine? At least just the gantry anyway.

    Since CF is lighter than cast iron/aluminum/steel, using it runs contrary to the conventional wisdom that more mass = better machine. But it seems that higher mass is desirable only due to its ability to dampen resonance. The undesirable property of higher mass is it's greater inertia slows down its movements. (Sidenote: I just built a machine that used servo's and the effects of inertia were demonstrated clearly in the tuning process.) So if Carbon fiber composite fabrication can be done in such a way as to increase its ability to dampen resonance then its lower inertia would make a lighter CNC gantry = lower inertia= faster acceleration.



  12. #48
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    Default Re: Carbonfiber Gantry

    I've read this whole thread. And the thing that keeps coming to mind is that cf is damaged so easy and it's nearly impossible to fix once damaged. So why would you want to use it where a little bump can cause significant damage?
    I've done enough maintenance in machine shops to notice one thing, not a single one looks showroom condition.



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