Lathe chatter/pattern

[Captain K]

I am getting this pattern on a piece I'm machining on the lathe. Have seen it on this machine before but never tracked it down. Workpiece is way out of balance so only turning at 150 RPM. 1-1/4" diameter. 0.010 depth of cut. I am using an 8" long, 1" square tool holder with hss tool. I don't think the extreme stickout is the main issue as I've seen similar patterns when using minimal stickout. Lathe is a summit 20x80, 4000 lbs so not a weight issue either. I can not move the chuck vertically at all with a 5' bar. Anybody have a suggestion?

[J Tiers]

First rule.................... fix what you know is wrong.

The extreme stickout is clearly not helping. Change that first, see if it changes anything. Same for the work, support it with T/S or a steady if it sticks out too far

Change speed.... if the machine is single phase, that may not do anything, but if 3 phase, it ought to make a change.

Change feed and/or DOC.

Check the carriage for "rocking horse motion", tipping back and forth, usually corner to corner, but sometimes left and right as you look at it. Another test is to reverse feed without changing the cut. If it then cuts more, it may be shifting/twisting relative to the ways and have the "rocking horse" problem.

Look for swarf under the toolpost, loose toolpost, loose compound, etc.

[old mart]

You should look at tooling with carbide inserts for that size of lathe. If I got those patterns, I would change inserts to another grade, or manufacturer. Check the gibs on the crossslide and compound.

[redlee]

IMHO your tooling is the issue.
On center ?
Sharpened properly ?
Using a live centre ?
How big of a tool nose rad ?
Can you counter balance and up the RPM ?

[Captain K]

Thanks guys I'll look in to some of your suggestions. I'll post a pic of the setup here tonight.

[boslab]

Motor problem?, belt bearing or mains, very regular pattern for chatter,
mark

[Bented]

Is your location north or south of the equator?
Mercury is in Scorpio, (this always causes chatter) at the moment, it enters Sagittarius on 12/1/2020 as seen in the northern hemisphere.
The chatter problems should be over by December 5th in the north and a few days later in the south.

[The Metal Butcher]

Motor problem?, belt bearing or mains, very regular pattern for chatter,
mark

Chatter is by definition "regular". Constructive interference of a harmonic frequency... is going to be regular. Doesn't have to be caused by the motor, etc.

[PStechPaul]

That works out to 49 FPM, which may be too slow for good results. It might also be torque ripple, especially if it has a single phase motor, or it could be a wonky gear tooth in the power train. Does lube help?

[Ringo]

My vote is spindle bearing preload. just because you cannot 'feel' loose preload doesn't mean its not there, you should put a indicator on it at the least
check it both end play and side loaded

[genea]

That works out to 49 FPM, which may be too slow for good results. It might also be torque ripple, especially if it has a single phase motor, . . .

When I've experienced chatter ( I'm tempted to say every single time, but maybe I'm forgetting something ) I've managed to eliminate it without changing from a single phase to a three phase motor.

[The Metal Butcher]

When I've experienced chatter ( I'm tempted to say every single time, but maybe I'm forgetting something ) I've managed to eliminate it without changing from a single phase to a three phase motor.

Lol. It's brought up every time there is a thread about chatter. Yet somehow folks think that a 20x80 lathe with more drivetrain mass than their entire lathe, has chatter induced by minute motor speed fluctuations. Cracks me up every time.

Somehow methinks that if a 20x80 lathe can't take 10 thou without chattering, a major issue is afoot, either with the machine or setup.

[Captain K]

I tried several suggestions from this thread. Nothing made much difference until I took 1 of the 2 belts off. Finish is pretty good with dcmt insert now at 220 rpm. It seems that 1 belt is slightly longer than the other. See attached photo for setup. The shaft I am trying to repair is offset from center on the disc. The disc is almost touching the tool post in the pic, can’t get in with a shorter tool.

[boslab]

Have you changed the belts?, ( I must admit I had a problem recently due to a motor bearing on mine, lumpy running I suppose) and yes as MB points out it doesn’t have to be motor etc, but after a few decades messing things up it either loose blunt or shaking, my go to list I suppose, ( I cringe at some of the tool /work overhangs or stick outs I see on the utube, that’s definately looking to get the things resonating)
the lathe should be able to take most people well past where they feel comfortable!
mark

[Captain K]

No I haven’t changed the belts, just took one off. I am using the minimum stick out I can in this case. It’s a one inch square bar and 0.010 depth of cut. I don’t believe the stick out to the main issue. Originally a 10hp 3 phase machine it is currently running a 5hp single phase baldor motor. It is fine on lower speeds but doesn’t have the power to get rolling at max 1500 rpm