You're not going to believe me but instead of clamping your bar down super tight, try loosening it until all you're doing is keeping it from pulling out of your holder. I ran Mazaks in Tulsa for quite awhile and I'd have NEVER believed this would work except we'd do this all the time on extremely long overhangs of 12:1 plus... It goes against popular convention but it works like a charm with a little experience. Just keep the RPM's low if doing this and we regularly dulled the insert ever so slightly before use. Trying this technique is dangerous if done wrong so please keep your door closed when running and DO NOT EVER show inexperienced operators this trick, IT IS POTENTIALLY DANGEROUS IF NOT DONE WITH CARE!!!!!!!!!!!!
On one extremely long bore we completely loosened our bar until it would literally pull out by hand, so you have to keep your feeds and speeds reasonable. Also, doing this runs the risk of chips catching inside your I.D. and sliding your tool out and wrecking everything on a tool change. USE CAUTION!
Hope this helps... BTW, try it rigid first, this is a last resort for the 'impossible' bores you cannot do ANY other way~