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Thread: How to true lathe chuck jaws?
12-12-2008, 01:53 AM #1
How to true lathe chuck jaws?
I have a large Monarch manual lathe and the chuck jaws need to be trued. How is this done? I imagine mounting a ring to the outside steps, then tightening the jaws to the ring and then boring out the inner faces of the jaws. I just wonder of this will create an accurate facing. It seems like a more accurate job would be done if the jaws could be clamped internally instead.
12-12-2008, 02:07 AM #2
I am assuming you are talking about a 3 jaw . If they are the hard serrated jaws , then you need an internal grinder . Whenever we bore out soft jaws , I have chucking rings that I put inside the chuck , deeper in than the jaws . There is usually a part of the master jaw that you can chuck on . Actually I don't use a ring at all . I have a few spiders I made . Just a thick wall "doughnut" with 3 holes tapped equally spaced around the circumference . Then I adjust some long set screws out so that the jaws are approx the same size as the job I run .
The spider does not even have to run true , just as long as it provides a place to tension the jaws against . If you do this right , you can bore the jaws to almost the same size as the part . Be sure to mark the chuck hole with a paint stick , and always use the same hole to tighten the part .
12-12-2008, 07:32 AM #3
Spin Doctor, I have tried what you are attempting with limited success. You do need to "preload" the jaws to take up slop ,but the scroll is really the problem. It is usually worn unevenly. If you grind the jaws at one diameter then it will be off at another. I did have one that was way whacked out and grinding brought it back to some degree of usefulness. All things being said, the three jaw is less then stellar for accuracy anyway unless you have a set-tru. But if you did then you wouldn't need to grind the jaws. If you have a very nice three jaw chuck that you are in love with and you have some extra time on your hands, you can make it into a poor mans set-tru by machining a cup shaped back plate with four screws in the perimeter bearing on the chuck. You loosen the bolts holding the chuck to the back plate, adjust the four screw to dial it in and then CAREFULLY tighten the clamping bolts. I have done this on a few three jaw chuck and it works very nicely. My real advice would be to not use the three jaw where accuracy is at a premium.
12-12-2008, 08:43 AM #4
Another problem is that chucking a ring at the back of the jaws tends to splay the tips inward. When you grind the jaws and chuck a part, now it wants to grab at the back and the tips don't exert any force. The chuck has to be in really good condition for this to do much good. I did it on a fairly worn chuck and it looks nice but doesn't work any better.
12-12-2008, 09:14 AM #5
I find that eventually three jaw chucks need to be replaced. There's a point in time where you will have to fight with it to get it to run true in some situations. I recently purchased an adjustable 3 jaw for one of my manuals (Bison made in Poland). It really did not cost all that much, and it is fairly decent quality, somewhere between a Buck and a Chinese knockoff.
12-12-2008, 10:42 AM #6
Thanks for all the helpful comments! I guess my best bet is to use what I have and for accurate work I do have a collet type chuck.
12-12-2008, 05:51 PM #7
12-12-2008, 05:58 PM #8
Last edited by David Carlisi; 12-12-2008 at 06:02 PM. Reason: after thought
12-13-2008, 01:41 AM #9
At work we anneal the front of the jaws then reinstall them then machine a grove in the front of them so that we can put a ring in their and tighten down on them so that they are not tilted out. It is alot of work and I dont think it is worth doin but the boss is so cheap. We even will sleeve the scroll when it is loose and regrind the whole body of the chuck so it looks new. I would just go buy a new chuck even a chinese one is better than a worn out one.
12-13-2008, 04:16 AM #10
Keith, another option is to buy a set of soft jaw tops to fit your chuck's master jaws, and whenever you need something to run really true you can stick the soft tops on, skim them like you would the ones in your cnc lathe, and be good to go for the particular part. Sources like US Shop Tools have soft jaws for standard tongue and groove master jaws and they're pretty cheap.
12-13-2008, 04:41 AM #11
Anyone ever make over-sized top jaws to take up the slop of a worn chuck body?
I've got a Rohm that's clapped out real bad (was a spare that came with the lathe along with a decent Bison) and have been wondering if that might be a way to salvage it. The jaws splay outward when clamping on the OD, so if I prevented the jaw from being able to rotate down into the scroll it would potentially resolve the issue.
I do some thinner parts with steps in them that could benefit from such a jaw style (especially increased clamp area), but I don't want to be a guinea pig if someone already knows it won't work. For me it seems faster to pull the chuck than to change the jaws out (camlock spindle). Even more so, if I could leave the jaws true to the work diameter.
If it's a bad idea, I can always sell the thing to a welder for a positioner and keep the back plate for another chuck.
12-13-2008, 05:21 AM #12
The master jaws are hardened. The wear is usually in the "tongues" in the chuck body that the grooves in the sides of the master jaws ride on. The wear typically isn't even, so you'll find the jaws sit tighter in some places than in others. Anything's repairable if time is no object, but that's one of those things that would be impractical unless it was a big chuck worth a lot of money.
I saw a chuck one time where the owner had taken a cold chisel and put a pattern of X's on the tongues and then worked the surfaces back to the proper clearance with a file. Same effect as knurling. The master jaws did seem to be nice and snug in their slots, but I'm sure it took a lot of time.
12-13-2008, 05:41 AM #13
You're probably right that it's more effort than it's worth in the end.
I like the idea of having a chuck ready to go when making the stuff and not having to unscrew jaws, clean chips out of them, and install new ones - but I suppose the time saved would be more than expended in having to rework several aspects of the worn unit.
12-13-2008, 06:03 AM #14
Scroll chucks are a sorta pain in the butt for repeatability regardless of condition or the treatment they get. I used to do quite a bit of repair work for a company here that makes jet engine parts. They use scroll chucks on almost all of their turning centers because so many of the parts they run fall in the "thin ring" category and they don't want to risk smashing them out of shape with a power chuck. Each particular part has its own set of custom made full pie jaws, which you'd think would keep the chips out front and away from the chuck itself. But about every time I ever went into their maintenance dept they'd have one or more chucks torn apart for cleaning. It was such a regular occurrence that they had spare chucks just so they could change them out for cleaning and not have downtime on big turning centers that cost half a million apiece.
12-13-2008, 08:10 AM #15
the 30 or so times i've been directed to 'fix' the jaws of a chuck, it worked great, for that one diameter of part being machined.
then it went to crap as soon as a new diameter of part was to be turned. after some time and discussion with the old-timers, i've decided 3 jaw chucks are for use when new, or when worn out and in a welding rig. otherwise, it's 4 jaw or bust.
wether it has to do with something i've missed in the machining (beit push/pull or worm wear, i don't know or care anymore) or the change in part diameter doesn't matter to me anymore. i just like 4 jaws, steadies, and goood centers.
12-13-2008, 12:38 PM #16
Collets. Don't forget collets!
12-13-2008, 02:26 PM #17
12-13-2008, 02:41 PM #18
all ya gotta do...
12-13-2008, 04:07 PM #19
Truing Chuck Jaws
The best chance you have is:
On a three Jaw chuck, clamp on a piece of round stock large enough so the backs of the jaws stick out a little farther than the outside diameter of the chuck.
Tool post grind the backs of the jaws to true them. Those surfaces will then be true to the inside flank of the scroll.
Unwind the jaws from the chuck and set them up in a surface grinder with the contact surfaces (opposite end of the jaws) facing up to the grinding wheel. Grind all three at the same time. A clean up cut will then make them true to the back sides and hence to the clamping flank of the scroll.