Materials used in CNC build projects

[routercnc]

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 . . .

[GeorgeD]

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.

[GeorgeD]

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.

[FatFreddie]

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.