Rio et al,
I tried to look at what would be considered the resonant frequency of the system on my Pm1340 lathe, and also compare different chucks to see if it is a balance issue. I dialed up some 1" stock in the 8" QMT 4J independent chuck, measured the chuck run out and ran it carefully form 10-90 Hz, I also repeated it with my Bison 8" combo 4J which is about twice the weight. I had previously mentioned that the new Taiwanese QMT 4J was a significant improvement, and when I tried to measure the TIR of the chuck body with it mounted on the lathe it was under 0.0001", I actually did a double take because the dial indicator looked like it wasn't moving. Much lower TIR on the body than any of my other chucks. I then ran the lathe and could get some small vibration at ~620 RPM (~45.2 Hz in high gear low speed belt ratio) and around 1230 RPM. I reran the lathe with the Bison, and I encountered the same two resonant points within a few RPM. It was independent of gearing and also if I had the feed engaged or not. It did not change. The total resonant band was maybe 20-30 RPM, on either side this machine was absolutely smooth. Without the chucks, I really could not see or feel any resonant frequency no matter what I tried. So these are most likely the resonant frequency nodes for my lathe with the additional weight of the chucks, and normally one would not be seen these in the non-VFD version because they do not align with any of the 60Hz speed ranges. It was also interesting that it did not change with different chucks. The vibration nodes may not be the lathe per se, but the resonant point of the cabinets and flexing where it is mounted to the top of the cabinets or between the cabinets.
There is one other possibility which has to do with an oscillation between the belt vibration and the mass of the chuck. Looking at your video, you can see the belt walking back and forth (wobble) on the pulleys, and up and down in the grove. The pulleys look like they are running without any wobble, and the TIR reported was fine, which would indicate that the pulleys are running true.
Having a more rigid mounting base, should go a long way to preventing any flex, and it might be a bit more noticeable on your lathe because of the additional motor mass hanging off the back. But it did not appear to be chuck, gear or feed dependent on my machine. I think it is worthwhile to note that there is always resonant frequencies, nodes that mechanical systems will vibrate, this is a big issue with VFD drive systems. Regarding the new Taiwanese QMT 4J, I am impressed with its quality, the casting/finish, jaws and mechanics are top notch (world of difference from my Chinese one). I do not feel this is a factor in this issue, but try different chucks. Look forward to your update.
Agree, with you on Royal supply shipping costs, and I had to do a second order for the larger size belts
. The belts are cheap relative to the packing/shipping costs of most vendors, so you do need to shop around.
Mark