Re: [CAD_CAM_EDM_DRO] Re: Al Extrusion Vibration Damping

David Bloomfield wrote:

>The damping qualities don't come from the mass of the system. All
>that does is change the resonant frequency.
>

Some members perhaps aren't familiar with or have forgotten some of the
physics of vibration, so I thought I'd write something about the basic
ideas. This list being what it is, I suppose many of the list members
are pretty well attuned to things of a mechanical nature and so already
understand this stuff. Feel free to correct or refine, I'm no expert,
this is just what I know from basic physics and audio fields..

The 'Q' of a system describes the tendancy of the system to ring like a
bell. For example, an iron bell or tuning fork has very high 'Q', it's
springy and will continue to vibrate. The same shape in lead has very
low Q, it won't ring at all. Systems with high Q are subject to
'resonant rise' where small impulses at the right times (harmonics of
the resonant frequency) result in constructive addition of energy.
Vibrationally, this means high amplitude movement of the system. If
uncontrolled, this can lead to Bad Things like cutter chatter, or in
more extreme cases, spectacular failures like what happend to the Tacoma
Narrows bridge (look up a video if you haven't seen it, it's some
amazing footage).

If resonant vibrations are an issue in a structure there are couple of
ways to solve the problem. You can damp the system or move the
resonant frequency somewhere it isn't a problem.

Adding damping to lower the Q means that the tendency for the amplitude
of the vibration to increase to problematic levels will be reduced. This
generally means you'll be adding something to the system that has little
rebound when struck, things like lead and damp sand. You can also use
an technique known as 'constrained layer' damping. For this you put a
thin, viscous damping layer between two high-Q layers. As vibrations
cause the two high-Q layers to slide against each other the damping
layer absorbs the energy.

Moving the resonant frequency somewhere else can be accomplished in a
number of ways. The resonant frequency is a function of the mass and
stiffness or 'springyness' of the system. Increasing mass decreases
frequency, increasing stiffness increases frequency. Think of a guitar
string, the mass stays the same, but tension can be increased, making it
stiffer, which increases the resonant frequency, the pitch, of the
string. There are many other factors as well (speed of sound and
propagation charatristics of the material, size and mounting of
material, etc), but try the easy stuff first.

If problem-causing vibrations occur in a particular piece of the machine
under some conditions, it may be that making the part stiffer (moving
the frequency up), or heavier (moving it down) will fix the problem.
This is because whatever is causing the vibration is operating at some
harmonic of the vibration frequency. By changing the frequency of the
piece, it will stop responding to, or ringing, with the vibration. Of
course, if it's still high-Q it might start ringing when the machine
runs at a different speed.

So, damping reduces the problem, adding mass or stiffness moves it.
Damp sand does both, and I'd expect lead shot to do both as well. Sand
is probably cheaper, but not as heavy. Hey, how about both? Fill the
part with buckshot, vibrate fine, dry sand into the gaps, then soak it
with oil :)

Dave