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PostPosted: Wed Dec 14, 2005 8:25 pm 
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Joined: Thu Dec 02, 2004 11:29 pm
Posts: 84
Location: NC Mountains
The jaws in my 3jaw lathe chuck seem to have a wopple when a piece of shaft is chucked in them. the shaft doesn't seem to be centered in the jaws. What harm would it do to just run the boring bar thru the open jaws and try to true up the center. I dont do precision turning or sale my work. I just use the machine for making a few short length shafts for hinges and pins. Lathe is 14 in Monarch. The jaws look like something has turned inside them and boogered up the jaws and I am just wantint to take off enough material clean them up. Is this possible or do I need to just buy new jaws.


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 Post subject: Boring Bar
PostPosted: Thu Dec 15, 2005 8:31 am 
By itself, the boring bar MIGHT be able to true the chuck, but the jaws need to be firmly anchored against something, or they are going to wobble all over the place as the bar 'cuts.

Hence the clamp piece that is around the same diameter that you want the jaws to be true at.

Without it, well things just aren't likely to be quite what you expect them to be.


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PostPosted: Thu Dec 15, 2005 2:35 pm 
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Joined: Fri Dec 20, 2002 11:02 pm
Posts: 13761
Location: Onalaska, WA USA
If you're familiar with the use of soft jaws, you understand that the jaws need to be loaded in order for the corrective action to yield acceptable results.

You can restore the jaws to quite respectable condition, but I'd suggest you do it this way:

Open the jaws to a given diameter, large enough for a grinding wheel to enter the opening. It need not be a large wheel, a mounted point (3/4" or so) will suffice. When you have an opening that is acceptable, use either a washer that is an acceptable size, or make a thin disc of the proper diameter that can be chucked at the very deepest part of the jaws. It should be round, not irregular in shape. Make certain that the disk runs true (no flutter side to side) when you tighten the jaws, so the pressure is at right angles to the spindle. Allowing it to lean will generally yield a poor job because the jaws will not be loaded properly against the slides. You'll grind up to the disc, then kill the miniscule area after grinding.

An older chuck has usually experienced considerable springing, so the jaws grip only at the rear. By loading the jaws as suggested, the jaws will spread to the sprung condition, leaving the jaws out of parallel. Your grinding operation will then make them parallel, so they grip front to back in use. You load the jaws as suggested so they aren't free to move about while you're grinding them. The disc will load the jaws such that they will be held captive, generally in the general position they occupy in use. That provides for a correction of eccentricity as well as correcting the angle of the jaws.

It is recommended that you seek the best of the three holes in your chuck, then mark it. By using that hole when tightening the chuck, you'll minimize the amount of error in chucking. Don't expect an old chuck to repeat within anything less than .003" as you move through a range of sizes, but it should grip squarely after you've ground the jaws.

I keep talking about grinding, avoiding your talk of turning. It's a pretty tall order to expect tooling to stand up to the rigors of the interrupted cut and the hardened condition of the jaws. Even if you could get the jaws to machine, you'd be hard pressed to get them machined from front to back without considerable grief. You'll be far better served to try grinding them. If you don't have access to a tool post grinder, you might investigate the possibility of using an air or electric grinder firmly clamped to the toolpost by some means. I've done that with reasonable results, even though they don't have high precision bearings. As long as you're not trying to hold tenths, you might be pleasantly surprised at the results.

Good luck with the correction. Why don't you give us a report when you get the job done, and tell us how you did it.

Harold


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 Post subject: Deja_vu
PostPosted: Thu Dec 15, 2005 3:26 pm 
All over again.....


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 Post subject:
PostPosted: Thu Dec 15, 2005 4:58 pm 
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Joined: Thu Dec 02, 2004 11:29 pm
Posts: 84
Location: NC Mountains
I had considered the grinding wheel but had planned on chucking it in the tailstop and letting the lathe turn the three jaw as I fed the wheel into the jaws. Will that work? I hadnt condidered that I might should place somesort of band around the outside of the jaws to hold them stationary. I suspect that a band around the outside of the jaws would spring them inwards and leave a taper. The washer chucked just at the furthermost inside of the jaws is probably a better solution to the springing. This chuck is old and I aint sure of the make, I dont see any markings to reference off of. I am sure it is older than me.LOL For information, the number one jaw seems to be the one that is out of wack, when tightened on a piece of shaft you can almost see daylight between the jaw and shaft material. The chuck has been taken apart but each jaw is stamped and is in the correct position. I also checked to see that the jaws where installed in the correct order. something at one time or other has slipped in the jaws and left grooves in the jaws. Hopefully just grinding the burrs off will cure the problem. I didnt really pay much attention to the wobble until i tried to drill a hole in the end of a shaft, most of what i do just isnt that critical. I guess when I get the wobble fixed i will find something else that needs to be improved just a little more than the way it is. LOL


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 Post subject: Grinding
PostPosted: Thu Dec 15, 2005 5:03 pm 
Never thought about the grinder in the tailstock.

I have always put the grinder in a location where I can adjust the wheel position so that it is easy to grind off what is needed....hence a toolpost grinder or some 'equivalent' that you can rig up to do the same thing.

You can actually purchase these truing rings from people like MSC, but I see little point when most of us have more than enough scrap around.

Keep in mind that chucks don't always wear evenly...often it is the scroll that wears as well as the jaw, so truing a chuck at one diameter is no guarantee that it will be true at another...

Food for thought, even if it is cat food...


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 Post subject:
PostPosted: Thu Dec 15, 2005 5:38 pm 
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Joined: Fri Dec 20, 2002 11:02 pm
Posts: 13761
Location: Onalaska, WA USA
Yeah, what Bill said. The wear on the chuck alone will be cause for mixed results, which is why I suggested that you shouldn't expect any better than maybe .003" concentricity, assuming you do your job well. Even new, universal chucks aren't perfect. The scroll is typically machined before heat treat, so it's subject to minor deviations from that process alone. Still, there's no reason you shouldn't get reasonable results, assuming you do the grind properly.

The idea of a ring on the jaws is fine as long as you have some kind of fixturing that holds the jaws as if you had clamped on a piece of stock. You can imagine some kind of bridge device that goes over the end of the jaws and supports them from the tapered portion just past the gripping surface. Truth be told, that would be the best way to do the job, because it loads the ends of the jaws and expands them fully, which is what you're supposed to do in order to make them grip parallel. The ring in question would require considerable more setup time than a simple disc does, and the disc, applied properly, will do a very good job.

I suggest to you that you do not use a ring, where you expand the jaws to restrain them. If you do, you'll compound your problem. What that does is cause the ends of the jaws to lean in instead of out, then when you grind them, the taper will have more or less doubled. It's very important that you follow the instructions in principle if you want to improve the chuck, even if you find a way around one or two of the steps.

Grinding from the tailstock would work fine, assuming you have a means of feeding the wheel. As Bill said, it's better if you have complete control over how it feeds, and how fast you can move it while grinding. You're quite limited as to speed with a tailstock, and feeding the wheel could prove difficult unless you have some kind of slide mechanism you can install in the quill. If not, I'd strongly recommend you concentrate on the compound, maybe not even using the toolpost. It really is the best of all worlds for the task at hand.

Harold


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 Post subject: Deja_vu
PostPosted: Thu Dec 15, 2005 5:45 pm 
all over again....


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 Post subject:
PostPosted: Thu Dec 15, 2005 9:08 pm 
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Joined: Tue Dec 09, 2003 11:41 pm
Posts: 38
Location: Sequim, Wa.
I had the same problem on my lathe when I first got it. I tried using a grinding stone w/ 1/4" shaft mounted to the tool post, It helped true up the jaws a little but, ultimately, I found that the register on the backing plate for the chuck was too loose (.005" undersize), and the face of the backing plate needed to be trued. I eventually ended up replacing the chuck (I really wanted one with 2 piece jaws) and making a new backing plate from a plate of round stock. Then I found that it really pays to use a torque wrench when you bolt up your chuck to the backing plate!


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 Post subject:
PostPosted: Thu Dec 15, 2005 9:28 pm 
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Joined: Sat Jan 04, 2003 10:29 pm
Posts: 4090
Location: Tennessee, Obion County, Town of Troy
I made myself a clamping device with 6 pieces of keystock welded to an iron ring. This made in effect, three slots that the jaws fit into and I put in setscrews to clamp against the jaws. Result is that when you tighten the chuck slightly, the loading on the jaws is almost the same as if it had something chucked in it. I then use a heavy duty hand grinder mounted in the toolpost to true up the jaws. Of course as has been mentioned, just because you have it running true at one diameter, doesn't mean it is true at another one. Most of my work is done at less than 1" diameter and the chuck will hold small work and run within .001" concentricity after grinding.

Unka(I could post a picture of the set-up, but have to make one first)Jesse

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"The same hammer that breaks the glass, forges the steel" Russian proverb


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 Post subject:
PostPosted: Fri Dec 16, 2005 12:54 am 
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Joined: Wed Jan 14, 2004 12:59 am
Posts: 195
Location: Southern BC, Canada
Ok...I have one for you guys.
I bought a 5" Buck three jaw from Fleabay. It threaded right on my old SB and of course it is worn and has about 4 thou of run out.
I want to put it on my 6" R/T for my mill.
I was going to turn an adapter for it that threaded in the back and slips into the MT3 center hole in the R/T.
How can I grind the jaws true in this application?
Ha...I no longer have the old SB to do it with.
I thought of mounting it in the 4 jaw on my 14" lathe but that won't work...how can you indicate it? What would you indicate? I don't see that working.
Only thing I can think of is to make the adapter...screw it in the chuck and do the truing before I turn the MT3 "spiggot" to slip into the R/T.
Then I'm not sure how to grab the adapter to do this(to turn the short MT)...it will have threads where it needs to be grabbed.
Could make another adapter for it to screw into so I could hold it but this all sounds like the possibilities for error are pretty big.
Any ideas?
Thanks!
Russ


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 Post subject:
PostPosted: Fri Dec 16, 2005 1:00 am 
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Joined: Fri Dec 20, 2002 11:02 pm
Posts: 13761
Location: Onalaska, WA USA
UnkaJesse wrote:
I made myself a clamping device with 6 pieces of keystock welded to an iron ring.


That particular setup is great, but it must be properly indexed. If the interval between supports isn't identical to the chuck, you risk tilting a jaw sideways when it's tightened. Not a big deal if the chuck is fairly tight, but if it's quite sloppy--it can be. Even then, you'd improve the chuck, though. The real benefit of grinding the jaws this way is to improve jaw to material contact. Chucks that grip only at the rear are a real PITA, causing excessive chatter and making it difficult to keep parts round, as they oscillate in and out under the pressure of the cut as the material flexes between support points.

I was going to comment earlier about the suggestion that one jaw appeared to be all the trouble. While it's a long shot, it's possible the chuck has been through a crash, where the part exited the chuck by forcing one jaw out, which would surely lead to some kind of damage. The description of the chuck tends to support the idea. Before attempting any kind of repair, it might be a good idea to remove all the jaws and examine the slides to see if there's any substantial differences in them. Even though a chuck might be improved, it's possible that any given chuck has simply outlived its useful life and you won't get the benefits expected. Just a thought.

Harold


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