There have been a few threads on how to control vibration in machines and I have posted some thoughts on the topic.

I was reading a recent issue of New Scientist and came across the subject of "tuned mass dampers" or "dynamic vibration absorbers". For a light weight machine that has vibration problems these may be much more suitable than simply adding lots of weight to control vibration. These do add weight but it is a small amount in the correct place.

The first link is interesting. I think you could extend this to experiments on a vibrating machine that would just use a C-clamp and a weight on the end of a bar. Clamp it on in different locations with different lengths sticking out and then hit the machine with a hammer and see how it rings.

http://www.tinyurl.com/3cvzkt

http://www.newscientist.com/backpage.ns?id=mg19626273.000

http://www2.eng.cam.ac.uk/~hemh/tmd.htm

Interesting topic, however the applicability isn't obvious,compared to other known methods, wheere the added weight is often placed in the machine's cavities.

Best regards

Bruno

Geof,
Dynamic absorbers work well... if the frequency of the vibration is constant. They must be tuned so that the absorber vibrates and causes a counteracting force that reduces the vibration of the machine. I had an issue where an 800 HP motor was on a steel structure about 12ft tall that just happened to be naturally resonant very near the operating speed of the motor. A 6 foot piece of 1" by 4" steel attached vertically to the top of the support structure with a 20 lb weight about 8" from the top made the structure vibration almost stop. The weight was in motion about 2 inches, but less than .003" at the motor.

Steve

I have used a number of them and all but 2 worked great.
2 of them were applied in places where the vibration was not at a constant and the result was still very positive but the 1x4 steel broke after we tuned it to suit the best we could. A other areas have proven very effective. Never tried them in an application like a router table. Let me know how it works.