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  1. #21
    Hi Ross,

    I'll keep this brief because we're deviating a bit from George's post, and I think he's lost interest anyway!

    The reason things pass vibration along their structure is due to the level of damping. Highly damped materials such as wood, don't 'ring on' or let the vibration travel very far through them because the small movement of the fibres creates heat and the wave quickly runs out of energy. Lowly damped materials, such as a steel bar, will pass this vibration through the structure without much loss. But if you add external damping to the steel, it will be just as stiff, but not vibrate so much. So stiffness and vibration can be seperate, but a highly damped structure will always vibrate much less because damping is the controlling factor for vibration propagation.

    I think that the bottom line is though, getting back to George's post, is that we're all a bit worried about the use of perspex for a structural item, which is exposed to repeated vibration inputs from the cutter etc. I don't know enough about perspex specifically to comment on the stiffness/damping/cracking question, but wish George well with the idea . . .
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  2. #22
    Hi Guys.
    Most of you are talking about vibration being a major factor here when building a CNC in certain acoustic material.

    For the life of me the only area I can see vibration emmitting is on the spindle Y axis? if we were to dampen the spindle were it is clamped to the Y axis with rubber seal then vibration would be kept to a minimum.
    Any type of bearing/s should glide with precision and if it rattles then its poorly made.
    We could also dampen the steppers with a piece of rubber matting at the face of where they're mounted.

  3. #23
    Whats the pros and cons over a fixed gantry and a moving X axis?

    Since I'm going to tackle the CNC in perspex I thought it might be wise to have the gantry static rather than moving.

  4. The up side of a fixed gantry, moving table means, for smaller work pieces, smaller drive mechanisms as (generally) you're not throwing so much weight around and a more rigid structure. The downside of a fixed gantry is a larger footprint for the same work area. A 500mm long area needs the table to move 1m, whereas with a moving gantry it traverses 500mm plus the 1/2 the gantry depth in the X-direction (often the bearing spacing) so potentially as little as 600mm.

  5. #25
    Quote Originally Posted by GeorgeD View Post
    For the life of me the only area I can see vibration emmitting is on the spindle Y axis? if we were to dampen the spindle were it is clamped to the Y axis with rubber seal then vibration would be kept to a minimum.
    If you rubber mount the spindle, it will flex and reduce accuracy. Basically, for accuracy, you want maximum rigidity all the way from the tool tip to the work piece which means using rigid materials. Any form of damping that relies on movement will reduce accuracy, the only thing you can really do to control vibration is to increase the mass so that the energy going into the system is dissipated by moving the whole machine a very small distance. If the forces you are putting in are small, say a low power well balanced spindle cutting foam, or a laser then you do not need so much mass or stiffness.

  6. #26
    Hi FatFreddie.

    The rubber I was on about would be of 1mm thickness and the clamping of the spindle would then give it a tight secure grip. :-)

  7. #27
    Quote Originally Posted by GeorgeD View Post
    The rubber I was on about would be of 1mm thickness and the clamping of the spindle would then give it a tight secure grip. :-)
    Hi George,

    It all depends on the level of accuracy you want. If you can accept that amount of flex (and believe me, in some applications that would be A LOT of flex) then you should consider wood as a material. It's cheap, plentiful and as already mentioned, has good damping qualities. I've built a few things from perspex and it's not a great material either to work with or structurally.

  8. Quote Originally Posted by GeorgeD View Post
    Hi FatFreddie.

    The rubber I was on about would be of 1mm thickness and the clamping of the spindle would then give it a tight secure grip. :-)
    The modulus of elasticity of rubber is about 0.1GPa. A crude estimate of the flex at the tip of a typical router 50mm below the mount if this was used to mount a router in a 43mm mount 30mm deep would be about .25mm per 10N of applied cutting force... doesnt make for an accurate cut!

  9. #29
    Hi guys.

    This is in no way meant to be rude or a stab at your vast knowledge which is appreciated by all reading and listening to the knowledgable,but this is alight in theory most of what has been put forward but has most of it been put into actuall practice?

    What I see is a lot of maths being worked out and I've alays said that the practice can sometimes flaw the theory.

    Again I am not doubting or thwarting your input and its great that you take the time to help those not knowledgable about these things.

  10. Quote Originally Posted by GeorgeD View Post
    Hi guys....the practice can sometimes flaw the theory.
    ...the practice can sometimes flaw the FAULTY theory... However, the theory is well established in this field, and irving2008 (and all the others that tried to help) explained everything in layman terms (or as layman is possible). If you need to go all the way up, then go and please let us know of the result - that's why we are here.

    Good luck,
    Florin

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