I have a project that requires a cylinder of 6061 T651 aluminum 1.1" long, 1.7" OD and 1.35" ID, then at the end of the cylinder the ID goes down to 1.30" for the last 0.120" and for the last 0.020" it goes down to 1.25".
I am trying to make this on my Okuma lathe. I am running the spindle at 1500rpm, I know higher would be better, but it's got a 12" chuck and it already sounds like an airplane about to take off at 1,500 so I don't dare go higher! I am cutting right up against the chuck, the stock only sticks out about 1.5"
Anyway, I start by drilling a 1.00" hole, then I bore at 0.100 depth of cut, 0.002 IPR at the aforementioned 1500rpm. The boring bar is 0.375 dia with a CCMT 21.51 positive rake insert and the bar extends about 2" out of the holder.
I've tried varying the DOC between 0.050 and 0.100 and varying the feed from 0.001IPR to 0.008IPR. The finish is horrendous every time. I've made sure the orientation of the bar is good and the insert is cutting at the correct position, etc. I've tried taking a 5-10 thou clean up pass and it just squeals like a pig during the cut and the finish is as bad as ever (I am using flood coolant aimed pretty well at the insert and not being blocked when cutting)
Now I know it would be ideal if I could run a larger size boring bar, but I have other jobs that require the smaller bar, so I am really hoping I can get this finish issue nailed down so I don't have to buy another boring bar and swap out the tool just for this one part.
Can anyone suggest what I am doing wrong? I may be waaay off on feed and speed, I've just been messing with it after my original calculations for feed and DOC produced a horrible finish.
And by horrible finish, I mean a diamond scalloped pattern that must be a good 15 thou deep. Incidentally, if I run the exact same program in a piece of brass, it is smooth as glass, inside and out.
What should feed and depth of cut be for this setup?
If I am in the ballpark with feed/speed and it's the flexing of the bar and I need a larger one, would a different insert geometry produce a better finish? I do need to get right into the corners with the boring bar because I have shoulders on the ID, but I am open to a different tool setup if it would help and if this problem can't be solved by adjusting my current setup.