Fighting severe chatter/harmonics

[cdhknives]

I am trying to profile a rifle barrel blank. 21", turning between centers with brass pins in the bore for the centers. Picture attached. Since my follow rest doesn't fit the aftermarket cross slide I have ot install a steady near the center. Green Mountain 416R steel blank.

Problem: While cutting near the live center end It started chattering/howling like crazy and cutting a herringbone pattern in the stock. It started when I was part way down the diameter and has steadily gotten worse as the stock thinned. The close to the steady the cut gets the chatter fades out. When I say chatter, it is vibrating my machine enough to move loose items on the bed.

I tried: Changes cut depth, feed rate, RPM, tool profile and type HSS vs carbide insert. Sound changed, end result still the same.

Redneck solution found: Hold a 3-4 lb bar of brass held vertically and firmly pressed into the body of the tailstock live center.

Better solution? Help!

 

[Moderatemixed]

I had the exact same problem. The metal looked as though I had gnawed it off like a beaver vs turn it. Most of the problem went away when I tightened the spindle nut. I would say that I almost “overtightened” it but my surfaces have improved exponentially. For reference I have exactly the same lathe, Atlas QC10F.


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[Moderatemixed]

Mine vibrated so badly that one of the gears in the drive train vibrated it’s retaining bolt loose. In aluminum, I was able to vibrate the four jaw chuck on the solid maple bench beside the lathe move.


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[Moderatemixed]

I also mounted it to a 44” x 10” x 1 1/4 “ piece of U channel to rule out rigidity. Gussets were welded into the U channel to endure rigidity. With these changes I am getting .0005 runout at 7 1/2 inches from the spindle nose.


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[cdhknives]

Thanks. I'll look at my spindle nut but I went past the recommended (2 teeth past snug) tightening last time plus the usual 'indicator on the spindle nose' tests are in the green. It is odd that it only occurs near the live center end, and since this is a new (Shars extended point 2MT) center I wonder....

My bench is a welded up steel frame 'desk' with a 1/4" plate for a top...the drawers are 1x lumber and *full*. It is as steady as I am going to make it.

 

[cdhknives]

I should probably add that I started with HSS at 418 RPM, .048"/rev and .005" cut depth, dark hi sulfur cutting oil. By the end I was using coated carbide at 685 RPM, .06"/rev, .010 advance, dry. It really didn't change all that much.

 

[derf]

First of all, get rid of the brass centers......they are not doing you any favors, and yield to less rigidity. Use a pointed tool, like this ^ and set it above center, high as you can get it and still cut, carbide is fine. Set your feed rate at .015" per rev and take .015 depth of cut.

 

[cdhknives]

I don't think my light lathe has a chance in hell of taking that much metal per pass unless I take the RPMs way down...but we'll give it a try. My luck with heavy slow turning has been poor in the past but that was mostly threading.

I have flipped the bar and am back to working down the other end tonight.

 

[mattthemuppet2]

All I could add is that that's a very long live center and it's probably allowing the end of your work to flex away from the cutter. Do you have room to use a shorter nose live center (if you have one)? I'd even give a dead center a go at those rpms and see if that fixes the problem.

 

[cdhknives]

All I could add is that that's a very long live center and it's probably allowing the end of your work to flex away from the cutter. Do you have room to use a shorter nose live center (if you have one)? I'd even give a dead center a go at those rpms and see if that fixes the problem.

I just washed the evening's oil off my hands...and you are spot on. I eventually need to cut the barrel tenon so I got the long nose center for workspace. When I changed to an old stubby live center most of the problem went away. None of the feed and speed changes did more than change the frequency of the chatter to a much slower/deeper harmonic. I tried a carbide tip solid center and it got stinking hot fast. I will go back to the solid center for threading (at much lower RPM) since I will need the room, will thread between centers to help with concentricity.

If I ever try to profile a barrel again I will switch back to the factory cross slide so I can use a follower rest!