What is causing this problem?

LatheLizard · Sep 27, 2021

Hey guys. Brand new Hyundai CNC lathe. Im turning a simple, straightforward part with some tight runout tol .0003 o.d. to I.d turning everything at once and parting off.
Rigid set up, 1/2 bar dia, 36 long scaled .700 out of Royal collet chuck, with correct spindle liner installed. Long story short, I thought cutoff was distorting the part. After I cut it, but before partoff, I put indicator on it.and o.d. reads less than .0001 but I.d. runs out .0008 - junk. I usually dont run very close to max rpm, but surface finish required it on this part. Its running about 3500rpm on o.d. And 4500 on I.d.
Turning down the rpm on the internal work solved the runout to less than .0001. I didnt have time to troubleshoot everything today, I know I can eliminate more possibilities, but Im thinking it has to be balance and or vibration in chuck at the higher rpm right? Like the long ass bar behind headstock? Any other ideas? Thanks in advance

Ox · Sep 27, 2021

Have your ID and OD roughed before dooing any finishing.
Then keep RPM the same during both finish opps w/o stopping.

BTW - what causes all the "A's" in your post?
There are more there than you would need - even if you were from North Ontario.

Hey, what happened to Michigan Buck's post?
Or was it the other Mich fella?
Well, either way - there was another post ahead of me.
???

---------------------

Think Snow Eh!
Ox

wmpy · Sep 27, 2021

You can try cutting your bar shorter so there's nothing whipping around in the spindle just as a test to see if that's your problem.

Are you using the Quick-Grip collets? Max clamping pressure? Is your barstock round to begin with?

LatheLizard · Sep 27, 2021

Not sure why there are A’s everywhere. Sorry!
I was going to cut a small piece of stock to eliminate the bar whip as a cause, just curious about your opinions.

LatheLizard · Sep 27, 2021

The stock is rigid, round, and clamped well. Turning the rpm down fixed the issue. I guess when I cut the smaller piece tomorrow it’ll answer the question of bar causing it. Didn’t mean to post a stupid question, just never ever seen two diameters not concentric by that much in a free state before cutoff.

LatheLizard · Sep 27, 2021

By never, I mean diameters with stout wall thickness and reading .00015 roundness like this case.

DrEsgard · Oct 15, 2021

Sounds like maybe the ID tool is getting coolant starved, heating up and causing out of roundness on the ID.

LatheLizard · Nov 26, 2021

So I got to the bottom of my question and heres the result in case it may help the next guy. Im decent at troubleshooting and it wasn’t being deformed, and a short piece of stock didn’t resolve it. I videotaped my testing of the machine bearings, backlash and such (my dad is a retired tech and machinist so i had some help) Anyway, the mfr tech flew in and, surprisingly to me, all he ended up doing was indicating in the chuck cylinder housing, and indicated/adjusted the pulley that drives. The machine wasnt “vibrating” much at all before he did this and didnt seem to vibrate any less afterwards. But at max rpm, the runout on a light finish cut is now .0002 instead of .0009. I rarely run at max, so at 80% the indicator shows almost nothing. Maybe .00005
So lesson learned. Turning features in same workhold doesn’t guarantee they’re concentric.

M. Moore · Nov 28, 2021

Lathelizard,
Thanks for posting and following up with the solution. However this info will be very hard for anyone to find in the future given your thread title.

“OD and ID not concentric “ would have been much better.

Do a search and see if you can find your own thread.

Scruffy887 · Nov 28, 2021

M. Moore said:
Lathelizard,
Thanks for posting and following up with the solution. However this info will be very hard for anyone to find in the future given your thread title.

“OD and ID not concentric “ would have been much better.

Do a search and see if you can find your own thread.

Very nice way of putting it. Canadians really are not super polite, they are simply not assholes.

metalmadness · Nov 29, 2021

LatheLizard said:
So I got to the bottom of my question and heres the result in case it may help the next guy. Im decent at troubleshooting and it wasn’t being deformed, and a short piece of stock didn’t resolve it. I videotaped my testing of the machine bearings, backlash and such (my dad is a retired tech and machinist so i had some help) Anyway, the mfr tech flew in and, surprisingly to me, all he ended up doing was indicating in the chuck cylinder housing, and indicated/adjusted the pulley that drives. The machine wasnt “vibrating” much at all before he did this and didnt seem to vibrate any less afterwards. But at max rpm, the runout on a light finish cut is now .0002 instead of .0009. I rarely run at max, so at 80% the indicator shows almost nothing. Maybe .00005
So lesson learned. Turning features in same workhold doesn’t guarantee they’re concentric.

Interesting. Is 0.0002" runout considered acceptable on a brand new lathe? Just wondering because I was able to hold really tight tolerance last week and was getting runout in the bar until I dialed in the process. Never in my mind did i expect the lathe might be the issue, so I am curious about this.

Thanks

michiganbuck · Nov 29, 2021

RE OX; [Hey, what happened to Michigan Buck's post?]
I'm not the CNC lathe guy so likely I thought to let all the sharper lathe guys tell first.

ID and OD can have cutting forces come to the part from different angles/directions. Sometimes changing the side you push to can take up any machine slack a different way for a couple of tenths.

Cutting forces can also change as a result of the geometry of the cutting edge side cutting edge angle, sharpness rake angles, clearances.

For example, think of a manual lathe the OD turning pushes the part away, and common ID pulls the part towards. Sometimes one may need to reverse the spindle to ID with the same way cutting forces(push).

And thin parts can distort during the cutting action...rake angle and sharpness can help control cutting forces on thin parts.

Some aluminum intended inserts can be more brittle and sharper and so may not last as long but do/may have lower cutting forces (less push away).

RE:[However this info will be very hard for anyone to find in the future given your thread title.]

so the thread a waste of time/words.

LatheLizard · Dec 1, 2021

Oh man that’s a really good point, and the entire reason I followed up. Ugh, I’m learning. Thanks for the tip

LatheLizard · Dec 1, 2021

It’s debatable depending on the machine and class of bearings I suppose. .0002 at max rpm, at 80% the indicator is almost .0000

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