Mark the chuck and the part with a Sharpie where its chattering, then rotate part 45 degrees in chuck and turn, did the chatter follow the part or stay put on the chuck. This should confirm or eliminate hardness in the stock as the problem. I used to turn cast parts on a huge Mori lathe, we had our own foundry on site. Occasionally castings would come through which had 'chilled' sections that were hard as a rock. Even with flood coolant it was a fireworks show of carbide insert sparks destroying themselves. Typically we had to slow way down and would burn through 4x the inserts as usual turning those hardened castings.
Also how much stock do you have extending into the spindle? Are you using a bar feeder, are you using urethane spindle liners to keep the stock centered?
