BuckeyeDennis wrote:One issue that I’ve never seen discussed in the context of woodworking is mechanical resonances. All structures, be they machine components or workpieces, have various frequencies at which they mechanically resonate, or “ring” in layman’s terms. The wooden bars on a xylophone are perfect examples.
When you cut wood with a typical rotary tool, the cutting action is discontinuous. There’s an impact every time a cutting tooth engages the workpiece. This creates a driving frequency that can excite those mechanical resonances. The driving frequency, in Hertz, can be calculated as Fd = RPM/60 * N, where N is the number of cutting teeth in one revolution. At 25000 rpm with a two-flute bit, for example, the driving frequency is 25000/60 * 2 = 833 Hz. If the driving frequency happens to be close to one of the mechanical resonant frequencies, you excite the resonance, everything starts vibrating, and the resulting “chatter” causes a rough cut.
In metal machining, this is a well-understood phenomenon. On a lathe, for example, if you get chatter, you either change your spindle speed to move the driving frequency, or maybe try to stiffen your tool mount to move the resonant frequency. I’ve personally seen a bell-shaped brass faucet component be machined with a beautiful finish at one spindle speed, yet come out looking like a pleated tin-foil Christmas ornament at a different speed.
For high-end metal machining, you can buy instruments that measure all of the resonant frequencies in the system. Then the spindle speed is calculated to place the driving frequency in between resonant frequencies, with no trial-and-error tuning.
So how does this relate to router speed?
1) At higher driving frequencies, you’re more likely to be above the fundamental resonant frequency of your workpiece.
2) With a variable-speed router, if you get chatter, you can simply change the router speed.
You can see the results of chatter as a rough finish. But you can also hear it while it’s happening. On a metal lathe, chatter typically sounds like a high-pitched shriek. When working wood, if the cutting action sounds unusually loud or harsh, you’re probably getting chatter.
This sounds a lot like a marriage ... pitch, chatter, high-pitched shrieks, rough surfaces ... et al.
I personally am, in my humble opinion, getting good results on the shopsmith machines (what do I know LOL). Yes, I have a plunge router and it does an amazing job, but I still do a lot of raised panel doors exclusively on the SS. I have even done some of the center raised panels on a 10ER (large cutter) just for the hell of it and got outstanding results.
My question really stems from how much speed do you really need to get the job done, and are the router manufacturers just padding their stats with RPM.
Good info Dennis and Ed, stuff to ponder. Thanks