3-jaw repeating, or lack thereof.

I have a fairly new TMX Set-tru on a TL2, that since it was very new (one other machinist used it for 2 months before me) has failed to repeat at all. I can dial in a precision ground pin, then loosen and re-tighten the jaws (moving nothing) and have it run-out up to .010". So, for the last five years I've had to indicate in virtually every part not turned out of oversized stock. Is this normal? My boss is considering replacing the chuck, but if any replacement would be just as bad why waste the money.

Previously I spent 95% of my time on mills, and what little lathe work I did was using 4-jaw or collets. Thus my lack of experience with these dubious 3-jaw chucks.

A 3 jaw chuck is a very week holding devise, a fool can ruin one in one try just over-tighten with a cheater. So the fact it is new doesn't count..Old 3 jaws get bent out of shape, or the scroll gets worn. You should just replace the chuck, or better yet buy a 4 jaw and dial in its easy...Phil

QT: [better yet buy a 4 jaw and dial in its easy...Phil]
Agree and
I have advised young guys to leave a little finish stock and be quick to go between centers when you need close.
Once in the habit 5 or 10 minuets can often get a near perfect part.
Going from a 3 jaw to centers is also good.
A soft/dressed center in any chuck is quick

G Bohannon said:

I have a fairly new TMX Set-tru on a TL2, that since it was very new (one other machinist used it for 2 months before me) has failed to repeat at all. I can dial in a precision ground pin, then loosen and re-tighten the jaws (moving nothing) and have it run-out up to .010". So, for the last five years I've had to indicate in virtually every part not turned out of oversized stock. Is this normal? My boss is considering replacing the chuck, but if any replacement would be just as bad why waste the money.

Previously I spent 95% of my time on mills, and what little lathe work I did was using 4-jaw or collets. Thus my lack of experience with these dubious 3-jaw chucks.

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when did you clean scroll last ? dried grease and chips on scroll and it will vary. some will spray in non residue electrical contact cleaner till dripping solvent / cleaner working scroll back and forth to flush chips, grease, dirt out, obviously any liquid when lathe rotating will come out (dont stand in front of it if you dont want to be sprayed when first turning on)
.....if a manual chuck it often needs periodic cleaning. if hard to adjust chuck it usually dried grease and chips, many chucks are precision clearances are tight and they get very hard to adjust when stuff gets inside. grease is soap and oil and after some time the oil is gone and dried grease acts like glue
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also if jaws are 2 piece each if numbered ones not put in correct order runout changes.
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obviously mounting a chuck make sure mounting surfaces are clean and no burrs.
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and some steel is wavy and not perfectly round. that is chucking a ground bar rather than hot rolled not straight bar obviously get different runout
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some scrolls are damaged, you might get zero runout chucking 1" dia and a lot of runout chucking 2" dia. usually though scroll is dirty needs cleaning 1st. just saying scrolls usually not perfect if old, never saw where 3 jaw would have <.002" runout for very long

You can always use soft jaws and bore them to size. This might be a consideration for you.

Advice mentioned earlier should be tried first. Tom gives very practical points to be done now. The recommendations are perfect. I would not buy a new chuck until I took steps to solve the issue firstly.

Are you chucking and unchucking using the same "master " jaw as was dialed in with?

I agree with all the responders to this thread before mine, but there are very accurate scroll chucks that repeat well across their clamping range, but they are rare indeed. I own only two that do that and neither one is an adjust true. They are easy to damage if abused and they need to be cleaned and lubed with only oil from time to time. My advice is to scrap the chuck and buy a good one, it will pay for itself over time. I recently bought a 4 jaw scroll chuck. I couldn't use it. It was that bad. It was Chinese. I threw it in the scrap. It was one week old.

you should have chucked it up on a bar and skimmed the back.

Set Tru chuck was never designed to run on a TL-2 CNC lathe! They are not built heavy enough to handle the cutting forces put out by a CNC lathe. Like the one poster said, probably had a cheater pipe used to tighten the jaws because the part moved on them! Ken

Solid advice, and has been followed. I do all the maintenance, and it gets oiled weekly, cleaned out as needed. Jaws are always used in their respective places (1 with 1, 2 with 2, 3 with 3).

I'm not saying it is a junk chuck, as it will indicate in and make great parts. Just tedious and slow. On a positive note, I'm a dang pro at dialing in parts now...

And as mentioned, I tested the repeatability issue with precision ground pins. Actually it was a new gage pin that failed our calibration inspection. None the less, pin was perfectly round and smooth.

A chuck after having a wreck , a spinning part , cranking hard on a hard part, having been used for a grinding operation , being over tightened, not cleaned or oiled /greased and likely other reasons will no longer be accurate as new.
Usually the face of a chuck should test close to flat,, how does that chuck face and OD run when checked..this to consider the mounting accuracy of the chuck.
An older scroll chuck can have wear at the OD of the scroll and s having changing direction of chuck error. this should not be in the case of a newer chuck unless it was used for grinding.

4GSR said:

Set Tru chuck was never designed to run on a TL-2 CNC lathe! They are not built heavy enough to handle the cutting forces put out by a CNC lathe. Like the one poster said, probably had a cheater pipe used to tighten the jaws because the part moved on them! Ken

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I wonder if the jaw slide area is sprung.