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Truing jaws on 3 jaw - what am I doing wrong??

Mram10

Cast Iron
Joined
Dec 17, 2017
I started from scratch on my lathe by
1. leveling with my machinist level.
2. realign the headstock and verified the cut of the rod to be within .0005.
3. Removed the jaws and ground out the rear 2 teeth.
4. Placed a 1.25” piece of rod in the rear of the chuck and tightened the jaws down.
5. I used a carbide insert boring bar and 1200 RPM with the carriage feed toward the tailstock and did a number of finish passes until the jaws were nice and shiny.
But when I put an indicator at the headstock on the .750” drill rod I used for indicating, it is +/- .001(.002 difference)and 12” toward the tailstock it is +/-.006(.012 difference)

What am I missing and why is it wobbling?
Thanks in advance
 
The scroll in your chuck could be worn differently at various points along the travel of the jaws. Or your jaws could be slightly loose in their slides. When you have the 1-1/4 stock in the back of the chuck with the very back of the jaws gripping it, the forces on the jaws and scroll are different than when you are holding a workpiece out on the end of the jaws. If you are going to hang a piece out 6-12” or more, you should be using a tailstock or steady rest anyway. Either one of these will take care of your +/-.006 runout far easier than trying to dial in a set of jaws. It will also help deal with chatter
 
what you are doing wrong is expecting an old 3 jaw to runout less than a thou. as Spruewell said, can be any one of a number of factors, I would only add that your chuck "will be" worn differently at various...

even a brand new quality scroll chuck isn't going to be under a thou.
 
You need to preload the jaws near the tips. Preloading as you did it does not force the jaw tips outward and results in bellmouthing. If you have a worn scroll or a scroll that can shift inside the chuck body your chuck will never be repeatable.
 
You preloaded the jaws by closing on a piece that was back inside the jaws. But that does not load them he way they will be loaded in use, quite.

Your other problem was expecting to fix off-center that way. Grind to fix bellmouthing, or gnarled up jaws. DO NOT grind to fix concentricity, it's a losing battle. 3 jaw chucks are always off center, so deal with it.

I have two things I will do to a chuck that has bell mouthing or is inconsistent in chucking.

1) I drill holes for pins in the jaw tips, insert pins, clamp them on a ring, and then grind the jaws until they are all sparked out. That fixes bell-mouthing pretty well.

2) If the scroll is loose on the center bearing, I may shim that.

If those do not make the thing decent, then out it goes, or maybe to the drill press for holding round stuff to be drilled.
 
12" of 3/4" drill rod???

that's a bit of an issue right there.

don't know if it's the actual problem but could certainly be a contributor.
 
What?

Chasing tail is what.

A three jaw chuck has nothing to offer in achieving the goals you are looking for.

Add shims, or an adjustable back plate mounting along with a brass hammer, and you might get where you want to go.
 
My first thought is, why do you care ?

And the second is, if you plan to flip parts end-for-end and need concentricity, buy a Buck adjust-tru. Very quick to center a part, combines the ease of use of a three jaw with the centerability of a four-jaw.

No normal three-jaw runs out less than a thousandth. If you need that, plan to pay good money.
 
Couple things to add. Mount the chuck on the lathe in the same orientation relative to the spindle always. When you regrind, or re bore the jaws, tighten them with the SAME pinion, you plan to use. I usually use the one above the label. The Pratt Burnered chucks state repeatability in conjunction with using the same pinion. I believe how you place a bar and hold it while tightening the chuck makes a difference to. You could try rotating the chuck by hand In between tightening up the jaws gradually to allow the material to settle.

I used a clapped out 3 jaw once. I bored the jaws true to a pinion it ran within .002”. Use the other pinions you could get up to .020”. If you put a dial on the bar close to the chuck, when you find the low point of the runout, nip up the pinion that is uppermost. It will pull the workpiece, truer. That’s if the chuck is quite worn. But you can get quick and good results. 12 inches from the chuck you are probably always going to have to tap it true or use a steady or tail stock, however.
 
I don’t have a grinder, so I’ve used a carbide insert. Works great at high rpm. I’ll try a ring in the center of the jaws and cut it as I go.
 
You preloaded the jaws by closing on a piece that was back inside the jaws. But that does not load them he way they will be loaded in use, quite.

Your other problem was expecting to fix off-center that way. Grind to fix bellmouthing, or gnarled up jaws. DO NOT grind to fix concentricity, it's a losing battle. 3 jaw chucks are always off center, so deal with it.

I have two things I will do to a chuck that has bell mouthing or is inconsistent in chucking.

1) I drill holes for pins in the jaw tips, insert pins, clamp them on a ring, and then grind the jaws until they are all sparked out. That fixes bell-mouthing pretty well.

2) If the scroll is loose on the center bearing, I may shim that.

If those do not make the thing decent, then out it goes, or maybe to the drill press for holding round stuff to be drilled.

Won’t preloading in the front give the opposite effect as the back? How does that help? I must be missing something
 
Most drill rod is neither straight or round.

And 12" of 3/4" diameter sticking out will flop around like a duck's member.

If you aligned your headstock with it it is probably still wrong.

But as noted by others you can only really hope to remove bell mouth and such by regrinding old chucks.
Make the chuck "tap true"
 
All good advice..along with being quick to go to tail center for long jobs and most any chuck..Steady can be handy also.

Even a fair/decent 3 or 4jaw may be off .006 at 12" out.

But center drilling can be a chore with a less than perfect machines and not having a perfect drill point sharpening.. so if having that problem one can hold the part very short in the head stock and indicate the OD at near the very end, then nick the part end at center with a tool bit, then follow with the center drill...this very good if not having any/much turn-to-true stock on the part.

It is not uncommon for a good lathe hand to make near perfect parts with a near clunker machine.
 
All good advice..along with being quick to go to tail center for long jobs and most any chuck..Steady can be handy also.

Even a fair/decent 3 or 4jaw may be off .006 at 12" out.

But center drilling can be a chore with a less than perfect machines and not having a perfect drill point sharpening.. so if having that problem one can hold the part very short in the head stock and indicate the OD at near the very end, then nick the part end at center with a tool bit, then follow with the center drill...this very good if not having any/much turn-to-true stock on the part.

It is not uncommon for a good lathe hand to make near perfect parts with a near clunker machine.

10/10 for your last sentence. I used to have a regular customer who had a 1970's " Colchester " Mastiff. I was forever in and out of his place tweaking things because the operator thought a 40 year old lathe should turn like a new lathe.

One day I called into the shop on another job and I saw a new guy was working on the " Colchester ". Very nervously I asked him " Everything all right with the lathe matey ? "
He said " Yeah no problem, it's seen better days but so have I "

I never had to touch that lathe again.

When I was a green kid I saw a bloke turning a long shaft in big engine lathe. I noticed about every ten inches he had chalk marks on the ways. I asked him " What are the chalk marks all about ? "

He replied " The thing is son this lathe turns tapered when the tailstock is right down the far end of the bed. I can't be bothered adjusting that huge tailstock for one job so every time I get to a chalk mark I knock on a 0.0005" cut ".

Regards Tyrone.
 
When I was a green kid I saw a bloke turning a long shaft in big engine lathe. I noticed about every ten inches he had chalk marks on the ways. I asked him " What are the chalk marks all about ? "

He replied " The thing is son this lathe turns tapered when the tailstock is right down the far end of the bed. I can't be bothered adjusting that huge tailstock for one job so every time I get to a chalk mark I knock on a 0.0005" cut ".

Regards Tyrone.

Lol. You ain't seen nothing yet buddy. One of the machines I used to have the pleasure of running, I used contrasting colors to make the marks (always on the work, never mark the ways, that philistine!). That way I knew whether to tap IN or OUT! The longer stuff I needed to switch colors more than once! And that was only to keep within .001" over about 8' length.

Some of the pickier jobs I had to take two semifinish cuts - one to map the curve and another because there was such a variance that the changing tool pressure from bumping would affect size! I normally worked in .0002" bumps on that machine since that was what resolution I had available on the readouts - some other machines I'd use a dial indicator set against the tool. It finished in an S-curve as I recall - if that particular machine started at nominal near the tailstock - and with the tailstock set to produce minimal total variance without bumping - size would decrease about .006" in a nonlinear curve to within a foot to foot and a half from the headstock then curve back out to nominal then another curve back in. Eventually we finally reground the ways on that poor machine and it would cut within .001" over the length of the bed. How much nicer that was!
 
Some chuck jaws are too hard to drill.. with/for grinding jaws and turning slow perhaps 2 or 300 RPM one can attach small C clamps to the front most part of the jaws.. run a stack of rubber bands from each jaw to a holding position, and then with a mounted grinder grind the jaw's ID..Best from the inside and traveling out-way, with a very light skim grind per pass.
I know this sounds Holkie but in a pinch most anything has been tried..No I have not tried this but have done some very odd things in grinding.

Yes the chuck need be cleaned very well no matter how you true it. And for any lathe grinding the lathe covered with you wife's best towels..
 








 
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