Are you sure that the turret isn't out of line or that maybe the boring bar isn't bent or otherwise not properly oriented to the piece? THere is no way that you should have to choke the RPM back that far; I'm surprised that the insert doesn't chip going that slow. As far as the positive / negative argument goes, the insert geometry is at least as important as the actual boring bar geometry. CNM inserts designed for aluminum are in the boring bar negative [for security and durability] but are ground or molded strongly positive for a good free cutting action. If you are using a CNM insert designed for aluminum now, going positive is unlikely to make a HUGE difference in cutting forces, nor are you likely to be able to run the DOC or feedrate that you want for very long without edge failure. If you choke the boring bar up and only bore about 1" deep with the same setup and parameters what does it do? If it is still loud or shows vibration I'd say you definitely have a cutting tool or machine tool prob... C