Boring Issues

[RustedBolt]

Hi guys, new to the forum. Quick question regarding boring on a cnc lathe. I am boring out the middle of a tube. SFM is 800, Feed is .005 ipr. I am getting an ok finish, but the problem is that on my final pass, and only my final pass, about an inch into the bore my lathe "rumbles" and I get a very rough and ugly finish for about 2 inches. After that, everything is fine again. Coolant is constantly being flooded into the tube during this. I am not a great lathe mind so I am hoping someone can help me out. Also, my chips are very stringy and resemble that of a miniature saw blade.

[sdopp]

Sounds like harmonic vibration, how is it supported?

[RustedBolt]

The bar stock is chucked in a three jaw chuck. the boring bar has an 1 1/8" shank and is supported in a sleeve with 2 set screws. The boring bar does overhang around 4 inches which may be a problem as well.

[Geof]

It does sound like you are hitting a harmonic vibration on either the work or tool or both. Sometimes this will happen with a finish cut because it is normally a light cut and does not load the tool enough to stabilize it.

A quick (maybe) solution is take a heavier finish cut.

Or slow the rpm for the finish cut.

Or increase the feed.

Try damping any vibrations on the work by wrapping it in duct tape if that is possible; several layers.

If you have the clearance around the boring bar shank wrap it tightly in solder, one layer then cover with duct tape.

Sometimes boring tube can be a real pain because it can ring so efficiently.

[RustedBolt]

Thanks Geof,

You were right. I slowed the RPM way down and that did the trick with the rough finish problems. I am wondering, however, why the long stringy chips. Is there something I can do to better my chip breaking. The insert I am using is a Kennametal Kenloc MWNG 432MW with a grade of KC9110 and the tube is 1018 steel. I am under the impression that this should be a sufficient chip breaking insert. Like I said, I am not a great CNC lathe mind, so feel free to knock some sense into me.

[Geof]

Long stringy chips all the time or just on the finish pass?

I do not do a lot of boring in steel these days but I have found that the DOC and feed greatly affects how well the chips break; increase either or both if chips are not breaking during roughing. Long stringy chips from the finish pass are almost impossible to deal with I find.

[RustedBolt]

All the time. I will take your advise and increase my DOC. I will let you know the results. Thanks for your insight Geof.

[Thawk53]

Rusted,
The insert you are using is a wiper insert. A wiper has basically another radius following the leading radius. These are mainly used for finishing where part geometry allows. You can double the feed rate with these inserts and maintain the required surface finish. If it were me, I would try an MP style chipbreaker. You can also try doubling your feed rate.

Rick