Hogger,
You have my empathy. I've experienced exactly the same thing. My manual lathes use Klopfer style 40 position toolposts. I can bore as much as 12" deep with a 2" bar, just a plain homemade steel bar with a bolt on insert head.
My 1980 vintage American Tool slant bed, 20" swing machine, has an old style turret, with 1.5" holes to accept various bars. The turret block is 2.5" thick, so if I want to index the turret, I have to cut bars off so they don't hang up on the back end. Anyways, this turret is very susceptible to chatter. I figure it is just a matter of the turret being too thin to dampen the bar properly. Although the turret looks huge, the actual cross section between the hirth coupling and the bar holes is not as heavy as a first glance seems to indicate.
I've tried many things to try to adapt a good tight fitting bushing to the hole, even shrinking the adapter bushing onto the bar, then turning it precisely to the 1.5"+ to match the hole in the turret, but it really makes no difference, the bars still want to vibrate. I've added extra setscrews to the bar, two below and two at 90 degrees, with barely any improvement.
I tried a 1.25" devibe bar one time, and I had to cut it off of course, because it came in at about 12" long. It did not work on this machine. Of course, the Kennametal people told me I had effectively 'detuned it' by shortening it, but, what alternative was there? Run it with 9.5" sticking out? Not a hope.
I have not conquered the problem, but I feel that a new turret block about 5" thick would be the solution.
In the meantime, if there is plenty of room in a large hole to work with, I have occassionally rigged up a brace from an adjacent tool slot. I learned that holding a 2 lb hammer about half way out on the bar would usually kill vibration quite effectively, however, that is dangerous as heck to do. Be better to rig a brace to emulate that action. I do not know if it is better to fasten the brace rigidly to the bar/turret, or to have a parallel 'helper' bar projecting out, above the boring bar, and have a push screw coming from the helper bar, to push against the boring bar. This permits some degree of devibing the bar by adjusting the pressure of the push screw.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)