Chatter is driving me nutty on I.D. work, any suggestions?

[Hogger]

I'm running our Daewoo CNC 400 lathe, 21" chuck and very rigid set up, I have to bore pre-drilled 8" dia. couplings from 2.50 dia. up to 3.251/3.252 dia. for a length of 8". These couplings are made out of 1045 steel or 316 SS, I'm starting the boring operation with a plain 2" dia. Sumitomo boring bar with a Sumitomo WNMG432EMU AC2000 insert, this bar is sticking out of the tool block 8.25 in. and I am having tons of trouble with chatter, I've tried every possible depth of cut, speed and feed and nothing works. Funny thing is I can take the same bar to a wore out manual lathe, stick it out 9 or 10" and cut chatter free all day long, any suggestions on dealing with the chatter would be helpful, this is a problem on all our I.D. tooling. Anyone ever use anti-vibration boring bars? Or made any that worked. I've also tried every other kind on insert and boring bar we have from very sharp positive rake to very negitive. this seems to be a never ending problem on every job I do, seems any ratio greater than 3 to 1 causes major problems. Thanks for any tips.

[dfstrasser]

I recently ran some Sandvik anti-vibe bars in a high-temp alloy and they work. Much better than the same sized carbide bar. Yes, they are expensive, but they reduce/eliminate chatter when the L/D ratio increases. I suggest you call your local Sandvik distributor and have them bring in the Sandvik rep. to look at the application and make some recommendations. If you don't like Sandvik, try KM.

[HuFlungDung]

Hogger,
You have my empathy. I've experienced exactly the same thing. My manual lathes use Klopfer style 40 position toolposts. I can bore as much as 12" deep with a 2" bar, just a plain homemade steel bar with a bolt on insert head.

My 1980 vintage American Tool slant bed, 20" swing machine, has an old style turret, with 1.5" holes to accept various bars. The turret block is 2.5" thick, so if I want to index the turret, I have to cut bars off so they don't hang up on the back end. Anyways, this turret is very susceptible to chatter. I figure it is just a matter of the turret being too thin to dampen the bar properly. Although the turret looks huge, the actual cross section between the hirth coupling and the bar holes is not as heavy as a first glance seems to indicate.

I've tried many things to try to adapt a good tight fitting bushing to the hole, even shrinking the adapter bushing onto the bar, then turning it precisely to the 1.5"+ to match the hole in the turret, but it really makes no difference, the bars still want to vibrate. I've added extra setscrews to the bar, two below and two at 90 degrees, with barely any improvement.

I tried a 1.25" devibe bar one time, and I had to cut it off of course, because it came in at about 12" long. It did not work on this machine. Of course, the Kennametal people told me I had effectively 'detuned it' by shortening it, but, what alternative was there? Run it with 9.5" sticking out? Not a hope.

I have not conquered the problem, but I feel that a new turret block about 5" thick would be the solution.

In the meantime, if there is plenty of room in a large hole to work with, I have occassionally rigged up a brace from an adjacent tool slot. I learned that holding a 2 lb hammer about half way out on the bar would usually kill vibration quite effectively, however, that is dangerous as heck to do. Be better to rig a brace to emulate that action. I do not know if it is better to fasten the brace rigidly to the bar/turret, or to have a parallel 'helper' bar projecting out, above the boring bar, and have a push screw coming from the helper bar, to push against the boring bar. This permits some degree of devibing the bar by adjusting the pressure of the push screw.

[One of Many]

I'd start looking deeper into the holder, post, turret or slide. More positive rake would only add to the problem, but 0-negative rakes usually eliminate the bar itself as a contributor to chatter.

I'm wondering if once you get to the funny farm, the answer might be found in one of those ink blot Rorschach tests!

You might hear Dewikislnkanwiglabit over and over in your dreams while you click your heels together!

DC

[Geof]

If the hole goes all the way through and you can tap a thread in the end of the boring bar you may be able to make your own vibration damper. Bolt a lead block on the end of the bar using a bushing and large washer on the bolt so the lead is loose. You want the lead to move independently of the boring bar because the lead and bar will have different resonant frequencies and they work against against each other.

[ctate2000]

Is your turret on center? If not your tool maybe too high or too low relative to centerline. Try 433 insert and push harder.

[Hogger]

Yes turret is on center, and I have tried 433 inserts and higher feed before, sometimes it helps a little, usually not. I have also tried the extra set screws and even saw cut a tool block on one side under the block clamp screws so when the block was tightened down it would clamp the bar on it's entire surface and not just a couple points, that did nothing to help either. That test leads me to believe that CNC turret's are too rigid for their own good and it's not soaking up vibrations in loose ways and gibes like conventional machines do, that's the only reason I can think of why CNC's tend to chatter so bad. I'm thinking of trying to modify a bar by drilling a hole in the end of it so I can add some loose lead weight to the inside of the bar and see if that may help, it would take some trial and error to get the weight of the lead right but it may help.

[ctate2000]

Is it possible you are holding the workpiece in a manner that enhances the vibration.

[HuFlungDung]

I'd stick with a smaller nose radius, not larger, because large radii tend to exaggerate chatter. I would also try a DNMG insert, what is that...55° angle ?in order to reduce the area of contact, effectively making the insert seem 'sharper'. An aggressive feedrate will then probably work fairly well for the rough boring phase.

Perhaps another experiment to try would be to invert the tool, as this might change the harmonics of the system.


Another thing to look at is the front clearance given to the insert by the toolholder. Normally, I suppose the bar is optimized to permit it to enter a minimum bore, which means the front clearance is excessive as you get to diameters larger than the minimum bore. This is difficult to change, unless you were to somehow modify the clamping angle in the turret. Rotating the clamping angle of the bar in the turret will raise the tool point above center.
It is not strictly necessary to have the tool on center when boring out a large hole. Rotate the bar just enough so that the lower heel of the insert just clears the bore.

It is absolutely necessary to prevent a chattery surface from the beginning of the cut, or it will almost certainly replicate itself perpetually. Remove a chattery cut with an extremely coarse feed at lower rpm to give the tool a chance on the next pass.

[jackson]

I have used the D-vib. bars and they work pretty good in some cases in others they dont, i use to run a puma 400 and i would run a 643 insert run it deep and hard, or some times i would have to just use one set screw on the bar "not something i like to do but works some times"

[RICHARD ZASTROW]

Hu said something that reminded me of another vibration solution. This was an o.d. situation. We made an upside down tool and ran the machine backwards. This made the turret clamp into a hydraulic vibration dampener. This only would work if you are having a turret based vibration problem. Don't try this on anything but very light cuts. In this case we were using 3/16" round carbide inserts to finish the bottoms of deep grooves in forming rolls like those used for steel roof decking.

[eject_21]

I had the same problem ,almost exactly. If I used only one set screw to hold the bar in the block, it cured it instantly. Tightening more than one set screw made it chatter badly, looked more like it was knurled instead of bored! Kinda goes against everything I thought i knew about avoiding chatter,but worked beautifully. 17-4 ph, 400sfpm .075"doc F.01 Mit US735 .03125r cnmg insert.