Chatter!

[Oldbrock]

In an emergency such as that I often pick up a hammer and rest it on the shaft changing the harmonics. Works sometimes, but to leave hands free make up a roller that mounts on the toolpost so that it rests on the work could kill the vibration. Also use lots of top rake on the tool to minimise tool pressure. A chunk of lead wired to the toolpost so that it rides on the work could help. I'ts all about changing harmonics. I have lead slugs in a rubber strap that I fasten around auto rotors and drums on my brake lathes and you better not forget it or you get chatter every time. Good luck, Peter

[Arthur.Marks]

As my recent experiences may attest, I suggest a possible culprit is that he's turning 4140: http://bbs.homeshopmachinist.net/showthread.php?t=53257 4130, 4140, same practical difference. Try turning the rpm up. It may be a simply difficult situation as I found 41XX steels like comparatively higher speed and higher feed and a fair DOC, but you're working with a long length/diameter ratio.

Notes on steady rest - loosen fingers and dab the work and finger tip with a hi-temp grease. Don't press the fingers into the work when setting position. Just lightly come to rest on the work; then tighten. Every time you stop the lathe, dab a little more grease if it looks like it has thinned. To be honest, I have extremely little experience with follower rests, but would suggest the same basic procedure as above. It is not meant to really press against the work so much as "steady" the work when it tries to deflect from cutting forces.

[Forrest Addy]

Yeah, try the hammer trick. The follow rest should control chatter. Set it so the jaws are bearing just after the tool.

If you still get chatter and it happens only at certain places I wonder if the problem isn't a type of resonance other than tool induced work/tool mass/elasticity excitation - chatter. Is your lathe single phase powered? Does the machine hum or emit a constant note while running. Do tools laying on the machine or in the chip pan sometimes buzz or vibrate?

If you get any yesses, look at vibration originating with the single phase motor. A machine tool is a closed mechanical system. If the motor vibrates, the vibration propagates through the lathe cancelling and reinforcing depending on localized dynamics. Machine borne vibration may be recorded by the tool onto the finish leaving an appearance of the grooved surface of a phonograph record. It's very difficult to make an efficient compact single phase motor without them amplifying the motor's tendancy to vibrate.

If you wish to pursue the motor question, take a spare week-end day and experiment. Loosen the motor bolts and insert isolate the motor with hunks of hose from the lathe itself. Then loosen the belt so it can slip somewhat while still delivering enough oomph for light cuts. Run a few operations past experience has shown to promote chatter. If the problem diminishes but returns when you restore the motor to its original state you can make a good case that there may be a problem with the motor itself.

The cure is a smoother running motor. Many machine tool owners have reported smoother finishes and less chatter when they replaced their single phase motors with DC motors and electronic drive or AC three phase motors and VFD.

BTW I refer to "vibration" knowing confusion may emerge. Vibration comes from many sources. The first that comes to mind is mechanical unbalance of rotating parts. I'm not referring to this; most motors come from the factory in excellent mechanical balance. In an AC motor magnetostrictive vibration is possible. The alternating magnetic fields in the motor causes the iron to very slightly change volume emitting a 60 (or 50) cycle musical hum. This is not the my assertion either. The vibration I refer to is what I have come to call "cogging". A single phase motor has no rotating magnetic field. It's an alternating field that the armature rotated in. When lightly or partly loaded the magnetic field snatches at the armature accellerating to to near sync RPM or maybe over-speeding slightly. The next cycle may even DEcellerate the armature. The result is a semi-chaotic tortional vibration part in the stator and part in the armature kinda like a gasoline engine with an intermittant miss. The RPM may average constant but might err +/-5% for part of a revolution. DC and three phase motor do not have this "cogging" or tortional vibration. The DC motor is smoothly commutated and has armature slots slightly skewed to make its rotation even smoother. A three phase motor has a smoothly rotating magnetic field.

Don't get lost in theoretical considerations or tracking the faults of the motor. If your machine tool motor causes a big enough problem for you, that's enough all by itself. Replace it with a motor featuring smoother power delivery.

Finally, Old Tiffie is absolutely positively forbidden to contribute to this thread. Not one single colored sketch or pages of alleged math. Got that?

[rohart]

Chatter is a word the describes any kind of vibration whereby the tool and workpiece come together and move apart regularly at some frequency.

If you take note of the frequency, it can be a great help in deciding which components are flexing.

You are machining a fairly heavy chunk of steel. Unless the chatter is of a pretty low frequency, you might find that what is flexing is the tool/compound.

Have you ever heard squeal ? Squeal is a high pitched vibration - 1khz or higher - and is something very light vibrating. Usually it's the tool itself. It can be a thin workpiece.

Keep an open mind. When you get the chatter, start wedging, levering against things. If you lean on something and you take up play, or feel vibration, then what you're leaning on may be the problem. Some solve tool chatter by anchoring a heavy lump of steel or lead as near the tool as possible.

Just some thoughts.

[Doozer]

If you are using a ball bearing center, try using a dead center and see if chatter goes away.
--Doozer