Boring a big hole

[69owb]

Hi I'm looking for any advice on how to bore a 6" hole in a Tube weldament that is 14 1/2" deep with out a ton of chatter. we are able to hold the part fairly ridged but we had to make a makeshift bore bar out of a 1 1\2 7" gage cat 50 endmill holder and added a 2 1/2" Dia, 8" long extension on it and milled out a slot for a lathe tool holder to reach that far down the bore, witch is causing the chatter. tried a few different tip radius & style of inserts (TCMT TNMG) speeds from 100 rpm to 500 rpm, tried different depth of cut, adjusted feeds with no luck. I’ve been looking at those twin or triple insert boring bars wondering if something like that would help eliminate the chatter problem. Thanks for any help.

[sti2011]

This may sound a little off the wall, but on much smaller bores I may add, I have wrapped a boring bar or two with elastic bands. They seemingly change the frequency (chatter) of the tool and holder. I have run some pretty impressive L to D ratios this way. Also you may try and vary the RPM as the tool is cutting slightly back and forth to break the frequency that makes the tool chatter. As it does not stay at any one solitary frequency it might not start chattering or at least control it enough to meet your finish requirements.
Also you may try wrapping the part as well especially if you are not truly sure if the tool or the part or both together are the culprits......Anything you can do to dampen vibration will help...Including modeling clay but that is another story

[HuFlungDung]

The double ended boring bar is likely your best bet as you'll get the opposing tools cancelling the deflection of the opposite. Keep the front clearance of the insert to a minimum, as excessive clearance induces chatter.

[69owb]

Thanks for the help I forgot all about the rubber band trick they do that all the time at the part stores when they turn brake rotors. I have some 1"wide rubber bands that come out of 1000 lbs Lincoln welding wire barrels that I'll try. Also have a longer end mill tool holder and a adapter for a 4" criterion boring bar coming so I can get way from using that extension. HuFlungDung are you referring to using a 80 deg Approach angle versus a 90 deg or the rake angle of the tool holder. Thanks again

[HuFlungDung]

I'd stick with the 90 degree approach angle using the T shape insert and as small a nose radius as is practical. The top rake could be positive if the cut is not interrupted. The front clearance would be determined by how you might rotate the bar in the holder. Just enough clearance so the heel of the insert does not rub on the wall.