As we described in earlier posts, there what is called Process Damping (PD). The PD range starts at 0 RPM up to a speed where the geometry and edge prep of a tooth, "rubs-away" or dissipates vibration before the next tooth arrives. With some tools, this region has a very low max RPM, some are very high.
Here are two stability lobe diagrams of the same 1/2" 4 flute endmill in the same toolholder in the same machine. Figure 1 used the standard process damping wavelength for alloy steel. The PD region is the area from 0 to 6000 RPM. Figure 2, the PD wavelength was updated experimentally through guided cutting tests. That raised the PD almost two fold to 11,500 RPM.
The green area is the max SFM for this tool in this material (the pink area is the power curve of the spindle). In other words, this endmill could be run at the SFM limit (7500 RPM) at full depth and full width. It couldn't reach this speed in the figure 1 diagram. So if you make or use carbide endmills, the actual PD wavelength is good thing to know especially in low speed, tougher materials. Of course, it is not that easy. Even with an updated PD wavelength you still must tap-test the assembly in the machine to determine the PD range on the lobe diagram.
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