Loud resonant frequency requires hearing protection. How can I dampen

[Herrmann]

I have a process running on a CNC turning center that produces a loud enough harmonic vibration that hearing protection is required. A grooving tool is used to plunge a 3/8" wide slot at a 3.0 DIA. The cut is made inside a hub about 4.0" deep. I know there are devibe tools available, but at this point they are cost prohibitive. I have alternated the spindle speed manually and it works; however, my controller does not have the capability to accept an alternating spindle speed in automatic mode. Could somebody please provide a method by which I may solve this problem in house by modifying the current tooling? I have a tool and die department and may be able to get somebody to hook up an accelerometer and O-scope. Thanks.

 

Have you tried dampening the workpiece? I know it sounds kind of silly, but depending on your application it might work. Depending on where the harmonics are coming from (i.e. tooling or workpiece), puting a brass ring or non-ferrous material (kind of like play dough) around circumference of the nearest diameter might help. However, it may not be practical if the rpm is too fast or if they are high volume parts.

Write back if you would like to give more details on your problem. I once had a chance to machine a bronze bell for a potential customer, the bell did exactly like it was supposed to - it made noise. Thus, I learned a little about how to control harmonics.

 

[butelja]

It might be possible to dampen the hub as Machining Dude has mentioned. I'm picturing something along the lines of a layer of damping material (rubber) bonded to the outside of the hub. On the outside of the damping layer, another ring of metal is bonded to the rubber. This should give you constrained layer damping, which will maximize the effectiveness of the rubber. Figuring out the optimum thickness for the rubber and the outer constraining ring is not easy, but you may want to just try trial and error. The inverse of this problem (ring inside) has been used successfully to quiet noisy gear trains.

As an alternative to what is mentioned above, you could try to install a squeeze film damper on the outside of the hub. An outer ring with properly dimensioned internal O-ring grooves at each end would be fit over the outside of the hub. The space between the O-rings is filled with grease. The ring-hub fit is of the same order as what would be used for a hydrodynamic journal bearing of similar size (maybe ~0.004" diametral clearance). The O-rings act as radial springs as well as sealing the grease in. The grease is displaced and sheared as the outer ring moves, adding damping. A similar device is used (with circulating oil rather than grease) in the roller element bearing supports in many gas turbines. Of course, this approach would require machining the outside of the hub to a tight tolerance for the O-rings to seal.

The dampened tools that I have seen (which are very few) have a mass in an internal cavity filled with oil so that the oil shearing adds damping. It may be possible to modify the tool to utilize constrained layer elastomer damping as I have mentioned for the hub.

To optimize any of the above ideas would take a detailed FEA analysis coupled with actual vibration spectrums of the existing setup. This would not be cheap. Hopefully, you can take one or more of the ideas and do some trials to come up with a "good enough" rather than an optimum solution.