Re: [CAD_CAM_EDM_DRO] Re: Re: Al Extrusion Vibration Damping

Jon Elson wrote:

>
> Rob Sarten wrote:
>
> >Hi All,
> >
> >Have come up with another couple of ideas.
> >
> >One idea is to use candle wax. It's a pretty dense dead material. Another is silicon sealer.
> >
> >Correct me if i'm wrong but anything hollow is going to have a low to mid resonant frequency and if its solid it will be in the mid high frequency range.
> >
> >
> Not necessarily true. A solid bar will generally have a LOWER natural
> frequency than a
> hollow pipe of the same OD. It is just like a pendulum or a tuning
> fork. Adding mass will
> lower the rate of the vibrations. If you made the bar or pipe stiffer
> to bending, the natural
> frequency will go up. But, adding mass in the center has little effect
> on stiffness.
>
> Jon

*****************************

Hello Jon,

As I recall from making wind chimes, you are correct. Machining methods
to fine tune a wind chime are:

Shorten the length or increase the inside diameter to increase
frequency.
Decrease the outside diameter to decrease frequency.

Two websites for wind chime or tubular bell making are listed below. The
second reference includes a serious derivation of the formula if anybody
wants inspiration to come up with a formula for square or rectangular
wind chimes. The formula is probably somewhere out there already but
just using the round tube formula will give you some numbers to ponder.

http://home.fuse.net/engineering/Chimes.htm

http://www1.iwvisp.com/cllsj/windchimes/

Sorry, but I don't have a formula for cement filled aluminum wind
chimes. ;)

BTW, these equations use density and elastic modulus but do not use
speed of sound. However, the speed of sound can be computed from the
ratio of density to elastic modulus. Steel has about triple the density
of aluminum and triple the elastic modulus too. So steel has about the
same speed of sound as aluminum. That means that steel wind chimes and
aluminum wind chimes with the same ID, OD, and length will have about
the same frequency.

However, I think (not sure of this) that steel, having a higher elastic
modulus than aluminum, will ring louder and longer than aluminum. I
would guess that is not good for a frame.

Tom Hubin
thubin@...