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Thread: Thoughts on adding vibration dampening materials inside a CNC gantry?

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    Default Thoughts on adding vibration dampening materials inside a CNC gantry?

    Goemon,

    If dampening is not dependent on mass, a fiber reinforced composite using SiO2 based micro/nano balloons (silica fume/fly ash, etc.) bound into a syntactic foam with a fiber reinforced resin is going to be hard to beat. The fiber and resin form a 3d space frame around the spheres and associated strength to weight ratios can go through the roof. If a syntactic foam is surrounded by a high tensile strength material, like fiber weaves, metals, plastic, etc. it can form macro composites that further leverage the composite properties.

    A common example of this principal is how foam surrounded by a thin layer of plastic being incredibly strong. I have some experience playing with these kind of composites. Even my mediocre attempts can make a good thermal insulation. Vibratory insulation? I'm hoping to have the free time to figure that out soon.

    Last edited by UrbanForge; 01-06-2018 at 01:46 AM.


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    Default Re: Thoughts on adding vibration dampening materials inside a CNC gantry?

    mass has nothing to do with dampening and dissipating vibration energy.

    it lowers the resonant frequency of the machine, or increases it, and for a given impact the increased mass lowers the resultant velocity of the impacted device, which lowers the amount of audible sound created. but the energy still has to go somewhere.

    The hollow cavity inside a gantry is mostly useless for adding stiffness, because you can only add stiffness according to its share of moment of inertia, which follows the outside dimension of the structure to the 4th power.

    if your cavity is only 3/4" thick x 6" x 36" then i don't think you're going to make any difference regardless what you fill that cavity with. if you meant to say your gantry is 3/4" thick leaving a 6x6" cavity inside, 36 inches long, then you would have a great chance of dampening vibrations. just fill it with a mix of lead shot and sand.


    basically the figure of merit for machine dampening is modulus of elasticity multiplied by dampening coefficient. loose materials which have no stiffness but have incredibly high dampening coefficients dissipate energy by friction between the particles, rather than in the material.


    silicone rubber has great dampening coefficient but low modulus of elasticity.


    if you have an outside surface or two of your gantry free, you could take a tube of silicone or butyl rubber (not sure which is better) and coat the outside surface of the gantry with strips of it like this |||||| leaving air spaces between them so the chemical can cure. use some spacers to control how much the goop gets squished out, and set another sheet of 1/4" by 8" by 48" long steel up against the goop, compressing it slightly but still leaving air gaps between the lines of goop.

    if it doesn't work, you can cut the metal off with a knife/saw through the goop and cut the steel away and try something else.


    two part polyurethane foam is also good at absorbing vibration but its stiffness is relatively low. you might fill your void with 16 pound per cubic foot polyurethane foam.. it only costs 4$ a pound..



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    Default Re: Thoughts on adding vibration dampening materials inside a CNC gantry?

    Hi Goemon, what is your objective for eliminating vibrations? Is it because (1) of imprecission during machining or is it (2) because of the noise generated? If it is 1, then I think you should have a stiff structure, while if it is 2 then you need something which has been discussed in the preceding posts. Have you thought of filling the cavities with Oobleck (https://www.scientificamerican.com/a...-science-home/) under high vibration this acts a damper by turning stiffer while for lower vibrations it remains more of a mushy substance. It is very simple to mix, but you would need to have the cavities water tight.
    Regards
    Marting

    Martin G


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    Talking Re: Thoughts on adding vibration dampening materials inside a CNC gantry?

    Quote Originally Posted by UrbanForge View Post
    Goemon,

    If dampening is not dependent on mass, a fiber reinforced composite using SiO2 based micro/nano balloons (silica fume/fly ash, etc.) bound into a syntactic foam with a fiber reinforced resin is going to be hard to beat. The fiber and resin form a 3d space frame around the spheres and associated strength to weight ratios can go through the roof. If a syntactic foam is surrounded by a high tensile strength material, like fiber weaves, metals, plastic, etc. it can form macro composites that further leverage the composite properties.

    A common example of this principal is how foam surrounded by a thin layer of plastic being incredibly strong. I have some experience playing with these kind of composites. Even my mediocre attempts can make a good thermal insulation. Vibratory insulation? I'm hoping to have the free time to figure that out soon.

    I already have most most of those ingredients in my gantry structure. 3-4" of the 8" (thickness) is carbon fiber, fiberglass and carbon nanotubes. It also has a good number of carbon fiber tubes embeded in the epoxy granite (which makes up most of the rest of the filling). Some of those cf tubes are filled with high density foam. My epoxy granite has glass microspheres in the mix too.

    You are right about the incredible strength you get from woven composite parts filled with rigid foams. I fill my carbon fiber stocks with urethane foam. Leaving the structure hollow is not good for harmonics but I guess reducing vibration is a key part of improving harmonics.

    I never really thought about using urethane foam to reduce vibration in the gantry. The cell structure has natural air pockets built in and apparently air gaps can be good for vibration isolation (or something).

    I bet a high denisty urethane foam would make a good filling for a carbon fiber skin in a low cost gantry build. It might not have the strength of epoxy granite but it would be a lot quicker and cheaper to fill the mold. Epoxy granite costs a fortune and takes foreever when you are making large parts...



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    Default Re: Thoughts on adding vibration dampening materials inside a CNC gantry?

    Quote Originally Posted by Goemon View Post

    I never really thought about using urethane foam to reduce vibration in the gantry. The cell structure has natural air pockets built in and apparently air gaps can be good for vibration isolation (or something).
    Foam is just a convenient method of filling a structure with the density of polyurethane that you want, should be able to get a density on the range of 2 to 20 pounds per cubic foot.

    the air pockets are not going to help with the vibration dampening unless you include the irreversible thermodynamic losses due to compression and decompression of the air. this is negligible for machine dampening because the strains are so low.

    here's a paper on the dampening coefficient of carbon fiber laminate.
    http://www.escm.eu.org/docs/eccm/B072.pdf

    this one is 300 pages, 10mb

    http://www.dtic.mil/dtic/tr/fulltext/u2/a235614.pdf

    anyhow, the dampening coefficient of various glass/carbon fiber materials is highly dependent on the direction of the strain. in the axis of highest stiffness the dampening coeffiicent is close to that of any other metal, and its close to the stiffness of the fiber themselves.

    another paper you might find useful:
    https://faculty.engr.utexas.edu/site...y1993p2395.pdf


    dampening is not dependent on mass as i said earlier, but for a given impact a larger mass will have a lower resultant velocity (and a lower frequency, depending on whether or not that added mass also added stiffness to the structure) the lower velocity and lower frequency combine to produce less noise (this is perceived as dampening but it really isn't)

    some older milling machines have a flywheel on the spindle to increase the rotational inertia of the spindle for similar reasons. this isn't a problem with high or even medium speed spindles as far as i know.


    edit: a stiffer structure will be absorbing less energy from the cutting tool in the first place, this compounds the lower resultant velocity of the impact, lower frequency if mass was increased faster than stiffness.. all these combine to make less noise, which is perceived as better dampening.

    Last edited by Eldon_Joh; 01-13-2018 at 06:20 PM.


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Thoughts on adding vibration dampening materials inside a CNC gantry?

Thoughts on adding vibration dampening materials inside a CNC gantry?