I recently was given a job where I need to bore a 1.25" diamter hole 5" deep in 6061 aluminum. There is a pre-existing 1" hole drilled in the material. Work is being done on a Femco Durga lathe with a 3/4" carbide boring bar sticking out 5.25".

I have been using an A-MWLNR 12-3W Iscar boring bar with WNMG 3-1-PP IC3028 inserts (Iscar recommended insert) and there is no problem going 3" deep with this setup. I was "gifted" an Ultra-Dex E12S MWLNR3 boring bar so I figured it was pretty much the same insert. But I can't even cut .1" deep without serious chatter. I have tried all sorts of feed and speed settings but nothing seems to work. I went to look up the manufacturer's specs on this bar and found they recommend a WNMG332 insert. Could the .031" difference in insert thickness be the culprit? Could this be putting the inseret tip below centerline?

Also, any speed/feed help would also be appreciated since I have never used a carbide bar before.

check the center height with an indicator, or yank it and check on the OC.. also sometimes bore bars have negative pockets with positive inserts, if the pocket angle changes there will be center height issues that could induce chatter problems as well.

I recently was given a job where I need to bore a 1.25" diamter hole 5" deep in 6061 aluminum. There is a pre-existing 1" hole drilled in the material. Work is being done on a Femco Durga lathe with a 3/4" carbide boring bar sticking out 5.25".

I have been using an A-MWLNR 12-3W Iscar boring bar with WNMG 3-1-PP IC3028 inserts (Iscar recommended insert) and there is no problem going 3" deep with this setup. I was "gifted" an Ultra-Dex E12S MWLNR3 boring bar so I figured it was pretty much the same insert. But I can't even cut .1" deep without serious chatter. I have tried all sorts of feed and speed settings but nothing seems to work. I went to look up the manufacturer's specs on this bar and found they recommend a WNMG332 insert. Could the .031" difference in insert thickness be the culprit? Could this be putting the inseret tip below centerline?

Also, any speed/feed help would also be appreciated since I have never used a carbide bar before.

If I understand you correctly, you are using the WNMG 3-1-PP insert in a bar that specifies a WNMG-332 insert. I am not familiar with that size insert...referring to the Iscar (and am not at work to look it up), but if it is .031 thinner than the WNMG-332 insert, then yes...you are below center.

My preferred bar style for boring that deep is a triangle insert with a high positive rake. If you still have chatter with a 1/64R insert, then try a .008R insert of the same style.

Carbide bar has nothing to do with the speed, other than it will let you go faster than one of steel because of its rigidity. The grade of insert determines speed. In your case the RPM will be dictated by the point where you start to get chatter. Our lathes don't run over S5000, and I would be running aluminum at that speed if possible. Not possible on your part.

Feedrate is dictated by the required finish and the size of the nose radius. You definitely don't want to go with a wiper insert on that type of job. Chatter would be atrocious.

Normally I make .005 DOC for a finishing pass. This is not nearly enough if you go by the manufacturer's specifications. Usually works for me, tho. Sometimes a heavier cut will eliminate chatter because of the load you've put on the tool. Only way to know is to try.

Actually, with deep boring aluminum, with a less that ideal bar I have found more favourable results using a larger tip radius. It tends to help load up the tool and keep it in a constant state of flex as opposed to being light and able to flex (vibrate) back and forth.
I am not familiar with Ultra-Dex is it carbide or heavy metal? If not that may be your first problem.

It looks like the insert will be the problem. The fellow who gave me this bar said it took a WNMG 3-2 so that's I used. I knew I should have checked the manufacturer specs first as they do indeed require a WNMG 332 insert. I have some inserts on order and will let you know if this helps the chatter problem. Someone told me to rotate the bar a little to put the insert above centerline, but with the flats on the bar and sleeve, this won't work. So I will have to wait a few more days to find out.

Yes this particular bar is a carbide bar with through coolant hole. I spoke to a tech person at Ultra-Dex and they set me straight on the insert and recommend no more than 7:1 stickout for this bar. Here's hoping all is well after I get the correct insert in it.