Something change and now I get chatter.

[Doug H]

I have been practicing on my lathe (11" Logan) and had found some settings that the machine really liked, particularly with a couple of the tools I had been using for roughing. as an example, 2" diameter cold rolled, with WNMG 431 insert 20-30 thousandths cut with a moderate feed rate, and 125-150 RPM. Up until the last couple of days, it always left a great finish, then all of a sudden, nothing but chatter. It will start the cut fine, next to the tailstock, but always starts chattering as it nears the headstock. I have tried a number of different tools, and have been all over the settings, but I can not get rid of the chatter. I have locked the compound, adjusted the cross, pulled the QCTP and made sure there was nothing preventing it from sitting flat. In short, I have checked everything I know to check. My gut tells me the bearings in the headstock, since it always chatters on that end, even when it starts out well, but it runs smooth as silk, and I can detect no movement. I pulled the chuck, cleaned the taper (L00) and re-installed.

I am at a loss where else to look. I am beyond frustrated at this point.

HELP

 

[hvnlymachining]

Put an indicator on the spindle and bump lightly with a short prybar to check for play. I'd try that first.

 

[Orange Vise]

Did you flip the insert?

With a 431 insert you can reduce depth of cut to about .010" without affecting the finish.

 

[michiganbuck]

Put an indicator on the spindle and bump lightly with a short prybar to check for play. I'd try that first.

Enough pry to lift the chick so not that much of a pry, looking for a quick indicator movement of .001- .005 to suggest your pre-load adjustment is loose a tad.
A cold machine you may get .002 and that is near OK.
Yes, you might check the rear of the spindle also.
Run the spindle for 15 minutes at 500 RPM the run-out lift test should be zero.
Good to know that for mild steel the cutting tool edge is fingernail shaving sharp, there is some positive rake angle, the tool is about the center of the part, there is clearance under the cutting edge, and going to a nose radius often gives a better finish.
A loupe inspection of the cutting tool can be an asset.
The compound pretty much centered on the saddle can often off set a rocking chair attitude,

 

[trevj]

Just started? No other changes to the set-up?

Have you had a good look at your insert cutting edge? A chip on the edge may have changed the geometry enough to set up a harmonic that wasn't there before, which may be a source of trouble.

Per above, check the headstock bearings preload. Should be a pair of rings at the outer end of the spindle, one that is the actual nut, and the other acts as a lock. Mark them with a sharpie or similar, so you can see how far you have moved. I would try (if it did not really feel loose) tightening it an eighth of a turn, maybe less if snug. Otherwise, ttighten until you can feel a little friction on the bearings, with no belts and the spindle free, and a chuck on, you should be able to flick the chuck for a spin, and have it come to a halt from the friction in about a turn. Kinda a feel thing. Monitor in use when you run the lathe, to see if the bearings get warm. Warm is OK, hot isn't. Again, harder to describe than see firsthand.

Is your compound pointing straight at the chuck? If so, try setting it so it is pointing across the lathe, so the load is against the way surfaces, rather than allowing the dovetail to try to carry it all.

Is your chuck a bit bell-mouthed?

Same Material? Different bars have sometimes changed the whole picture, at times.

 

[Ralph_P]

Your rpm is way too low. Probably ruined the insert.

 

[JohnMartin]

If the ways are worn, it is likely that the heaviest wear is near the headstock…

 

[Doug H]

When I said I tried everything I could think of, I covered pretty much everything advised here. Several different pieces of stock, from 3/4" tubing, to the aforementioned bar. Every turning tool I have, which includes some CCMT and DCMT, as well as a few pieces of HSS. Have tried compound parallel to the cross, and perpendicular, as well as 29 degrees. No joy on bearing pre-load. Bearings are sealed, not taper. I did add a little more preload, but no change that I can tell. No wear on the ways.

This is not a question of setup, tho I have been able to make improvements with some changes. It isn't the insert, or the speed, or the ways or even the stock. I have changed everything at least once. About the only thing that eliminates it altogether is making slow, very light cuts with HSS.

The thing was running as I described for weeks, and then it started chattering.

 

[michiganbuck]

If the pry test showed .002 or less then your headstock bearing is likely good/ok...yes your chuck if bell-mouthed may be a problem(?)
Just that it ran Ok does not rule out an insert error. You may have been on the verge of problems when it ran well.
1. Check your insert with the fingernail-shaving test
2. With the insert at the part center line bring the insert cutting edge to just touch a flat steel held verticle at the part, you should see 7 or 8 degrees clearance right up to the edge, (just looking at the insert touch to the part does not work) The n insert may be zero clearance.
3 500 RPM might be better.

positive top rake, very sharp, and proper tool edge clearance make for best cutting.. increasing the nose radius good for surface finish up to where it begins to stress the rigidity of the part or the lathe.

https://littlemachineshop.com/images/gallery/instructions/grindingtoolbits.pdf

Here s a tool bit/insert comparison:

*Pretty easy to see all the examples for turning mild steel have positive clearance below the cutting edge.

 

[michiganbuck]

: Re: Just imagine how this insert shape would not have the needed clearance at the cutting edge,

wnmg 431 insert photo - Bing

Intelligent search from Bing makes it easier to quickly find what you’re looking for and rewards you.

www.bing.com

A very good way to visualize part-to-cutter contact is to draw a part at 10 x scale, with the intended insert also drawn
So the 1" dia part would be drawn a 10" circle..to see how the insert .187 draw at 1.87" has or does not have proper clearance.

A 5-minute drawing exercise can show a lot.

Every machine shop class student should be given this assignment.

note: Careful drawing with a sharp pence at 10X can show .001

Yes, one might calculate how much drop to see on an I" diameter at a 7* clearance angle, or eyeball loupe look at the flank wear after a light cut.

 

[mike44]

I have been practicing on my lathe (11" Logan) and had found some settings that the machine really liked, particularly with a couple of the tools I had been using for roughing. as an example, 2" diameter cold rolled, with WNMG 431 insert 20-30 thousandths cut with a moderate feed rate, and 125-150 RPM. Up until the last couple of days, it always left a great finish, then all of a sudden, nothing but chatter. It will start the cut fine, next to the tailstock, but always starts chattering as it nears the headstock. I have tried a number of different tools, and have been all over the settings, but I can not get rid of the chatter. I have locked the compound, adjusted the cross, pulled the QCTP and made sure there was nothing preventing it from sitting flat. In short, I have checked everything I know to check. My gut tells me the bearings in the headstock, since it always chatters on that end, even when it starts out well, but it runs smooth as silk, and I can detect no movement. I pulled the chuck, cleaned the taper (L00) and re-installed.

I am at a loss where else to look. I am beyond frustrated at this point.

HELP

Last week i had a similar problem. I was turning 6061 aluminum and the finish was lousy. I use HSS tooling. Ground the cutter with a 1/16"radius, still had a poor finish.
I removed the piece and saw that half the the round was smooth and the other half was rough. I realized the chuck might be the culprit. Removed the chuck which has a threaded back plate. One bolt looked like it was not tight. I tightened all the bolts and then turned the same piece. Results were a smooth finish.
I had removed the back plate to mount the chuck on a rotary table. Apparently when the chuck was installed back on the lathe I did not properly tighten the bolts.
One mistake that I won't make again.

 

[Doug H]

Thought I should re-visit this thread and share my observations. First, While I can't say for sure that it was the issue, once I resolved it, the chatter has gone away.

I have only had the lathe a couple of months and have been trying to get it as true and reliable as possible. One of the first checks I made was to verify tailstock alignment, and I made several cuts withe varying lengths and diameters of random steel drops I had acquired, and there was no measurable taper(within my limited means to measure). Last Friday I was turning a piece to make a 1/2 hold-down stud from, and all of a sudden I had .011 taper when I measured. I cut another pass and measured again, and same issue, so I started with the most obvious, and chucked my drive center, cut a new taper onto it, and then slid my tailstock up. I could tell visually that it was off center, so I started to loosen off the center adjust on the tailstock, only to find that it was not even snug. Re-centered everything and ensured the lock screws were tight this time. Maybe I should have checked them previously, but when I bought the thing it was set to center, and it never occured to me that someone would take the time to do that and not lock them down. Live and learn, I guess.

Anyways, I don't know if that contributed to the chatter, but I suspect it may have. For the most part, I am sticking with HSS in the future anyways. I find I can touch up a tool about as quickly as I can change an insert, so the indexed tooling won't see much use going forward. I actually enjoy the process more, knowing that I am involved in every step.

Will post up some results when I get a chance. I did finally get my stud completed, and I didn't run into any chatter when turning, or threading.