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oxford
06-07-2013, 10:54 PM
I have to do a little more checking but I think my chuck jaws are worn on the 3 jaw chuck I have and are in need of a re-grind. Is there a certain diameter that is best to hold the jaws at? I know they are only going to be perfect at that diameter. I have removable top jaws, what is the best way to hold the jaws in place when grinding? I was thinking a ring on the outsides that picked up one of the jaw bolts. I think I have seen them used before, but is a Dremel ok to do this with and what stones do you use? Thanks.

J. R. Williams
06-07-2013, 11:12 PM
Check out the attached video location. It is the grinding the jaws in my chuck using a simple fixture to load the jaws and a low cost pneumatic grinder.
http://homemetalshopclub.org/hints/videos/video.php?file=http://www.homemetalshopclub.org/hints/videos/grinding_chuck_jaws.mp4&caption=Grinding+Chuck+Jaws

Paul Alciatore
06-08-2013, 12:13 AM
Before grinding the jaws, you should be sure to eliminate all other possible sources of inaccuracy. The most obvious would be the mounting of the chuck. If it is not mounted properly on the back plate, grinding the jaws will only make things worse. And of course check the threads if it is a thread mounted chuck and any other mating surfaces. Even a small ding can raise the metal and destroy the original accuracy of the mount. Also check the threads and mounting surfaces on the spindle. Most dings can be filed off or taken off with a small stone. You should limit the metal removal to the local area of the ding: it is better to make a slight depression at the site of the ding than to wear away all of the surrounding metal that forms the original mating surface. Keep as much of the original surface as possible when removing dings.

I dare say, most problems in an older three jaw are not due to wear on the jaws. Even if it is properly mounted, it is much more likely that there is damage to the scroll or the fingers that engage the scroll than to the chuck jaws. The points of contact between the fingers on the jaws and the scroll are local high stress areas and can easily wear. Unfortunately, there is little that you can do to repair these problems.

Only if you are completely convinced that the problem is with the jaws should you think about grinding them. This is a last and irreversible measure.

The jaw tips should be flat: that is the factory condition. Grinding them WILL produce a concave surface; there is no avoiding it. You can only minimize it by grinding with the jaws open to the widest position possible. However, the stone you use must fit all the way to the back of the jaws so they must be inside the chuck's bore. This usually provides only a small range of diameters at which you can grind them. So they will be noticeably concave. Don't be tempted to "correct" this after grinding as you will only destroy any accuracy you may have created. Just use then in the concave condition.

The jaws should be tightened toward the center while grinding. That is how they will be tightened when in use and will be the most accurate. So you need some pins or protruding screws. In my opinion the best position for the ring that you will tighten against is as close to the face of the chuck as possible. This will place the force from tightening it as close to the mid point (in the direction of the lathe axis) of the jaws as possible. So they will be cocked as little as possible in their slots which are probably also worn if the jaws are worn.

Yes, a Dremel tool will do it. I used a small abrasive wheel that was part of a Dremel assortment - no special properties except that it fit in the chuck bore. I did dress it with a diamond tool after mounting it in the Dremel and before grinding the chuck jaws.

You do not need a fancy, super sturdy mount for the Dremel. You will be taking very light cuts and the forces are not large. I simply strapped it to a tool holder in a lantern style tool post with a couple of hose clamps. There was some folded rags between the Dremel and the tool holder to fill the voids. This worked just fine. Here are a couple of photos of my adventure into jaw grinding:

http://img.photobucket.com/albums/v55/EPAIII/ChuckGrind1R.jpg (http://smg.photobucket.com/user/EPAIII/media/ChuckGrind1R.jpg.html)
The setup

http://img.photobucket.com/albums/v55/EPAIII/ChuckGrind9R.jpg (http://smg.photobucket.com/user/EPAIII/media/ChuckGrind9R.jpg.html)
The fun part

Oh, I did have to make an extension holder for the stone because it's stem was too short to reach to the rear of the jaws. You want to have it pass completely past the rear edge of the jaws to insure all of the jaw's length is ground to the same diameter. I used an extra Dremel collet and collet nut.

Your statement that grinding only produces an accurate position at one diameter is quite correct. For accuracy at any other diameter, you are relying on the scroll. Now, a change of 0.001" in the OD of the work being held will produce a rotation of about 1.5 to 2 degrees of the scroll. At a diameter of 3" on the scroll, that translates to a linear distance of about 0.025" along the scroll. This will be a distinctly different position so even parts that are nominally the same OD can easily be gripped differently. This is why the condition of the scroll is much more important than the condition of the jaw tips.

BTW, the chuck you see in the above photos was very worn. It came with my used lathe. The damage proved to be more in the scroll than in the jaws and the grinding operation, although entertaining with all those sparks, did not improve the accuracy in the least. Hence my contention that grinding the jaws is the absolute last thing you should consider. I would guess that 90% of the time it is not going to make any difference. Only if there is obvious damage to the tips of one or two of the jaws AND the rest of the chuck is in good condition would I expect grinding to help.

J Tiers
06-08-2013, 12:43 AM
Before grinding the jaws, you should be sure to eliminate all other possible sources of inaccuracy. The most obvious would be the mounting of the chuck. .................

I dare say, most problems in an older three jaw are not due to wear on the jaws. Even if it is properly mounted, it is much more likely that there is damage to the scroll or the fingers that engage the scroll than to the chuck jaws. The points of contact between the fingers on the jaws and the scroll are local high stress areas and can easily wear. Unfortunately, there is little that you can do to repair these problems.

Only if you are completely convinced that the problem is with the jaws should you think about grinding them. This is a last and irreversible measure.

The jaw tips should be flat: that is the factory condition. Grinding them WILL produce a concave surface; there is no avoiding it. You can only minimize it by grinding with the jaws open to the widest position possible. ................

Your statement that grinding only produces an accurate position at one diameter is quite correct. For accuracy at any other diameter, you are relying on the scroll..............

BTW, the chuck you see in the above photos was very worn. It came with my used lathe. The damage proved to be more in the scroll than in the jaws and the grinding operation, although entertaining with all those sparks, did not improve the accuracy in the least. Hence my contention that grinding the jaws is the absolute last thing you should consider. I would guess that 90% of the time it is not going to make any difference. Only if there is obvious damage to the tips of one or two of the jaws AND the rest of the chuck is in good condition would I expect grinding to help.

I'm going to disagree with a lot of the above.

1)
If you want to grind to fix inaccuracy, STOP RIGHT NOW AND PUT DOWN THE GRINDER... Grinding IS NOT FOR IMPROVING ACCURACY. A 3 jaw chuck is NOT accurate, and a worn one is even less so..... There is no reason to try to make it more accurate with crude equipment.

Grinding is to fix bell-mouthed jaws. ONLY

2)
I have, and have had, a number of chucks. All but one have been used. NOT A ONE OF THEM has had anything wrong with the scroll spiral or jaw fingers that was in any way a problem. I expect all were worn a bit, so what?

Many have had bell-mouthed jaws, I ground them, with good results. Some have had wear in the "jaw ways" that showed up like bell-mouthed jaws, and I ground those also with good results.

One had a worn scroll center pivot, for that one I shimmed the center pivot with good results.

3)
You should be able to grind flat. If you hold the chuck steady with jaw at the bottom, you can feed the grinder in as well as move it with crosslide, and it should grind the jaw flat. With all 3 ground the same, it should work, after you have "circle ground" them.

I have not done that, I didn't care that much, and I wasn't confident I'd get the jaws accurately at the same bottom point.

4)
Again, grinding is not to improve accuracy, it is to eliminate bell-mouth. If you do it for any other reason than bell-mouth, you are fooling yourself.

Optics Curmudgeon
06-08-2013, 12:57 AM
I'm going to agree with Mr. Tiers here. I've ground jaws on two 3 chucks, one for a bellmouth condition and the other to fit a set of outside jaws I found online (yes, you can find jaws if you look long enough). Every time I have found a three jaw with excessive runout the cause was crap in the scroll, a thorough cleaning fixed it. Some of this "stuff" is tough to get out, so disassembly is the real answer. BTW, Dremel bearings are mounted on rubber O rings, so rigidly mounting the tool is a waste of effort.

oxford
06-08-2013, 09:24 AM
Thanks for the replies. I was not attempting to grind these for lathe accuracy. I haven't used the chuck that much but I threw a brass drift into it the other day to clean up the end and I noticed that the front of the jaws were not gripping the material when tightened. I need to check into this more to see if it was indeed the jaws that was the problem. It could have very well been the material or it may have been that way the other couple of times I used it and just didn't notice it.

dian
06-08-2013, 09:46 AM
consider hard turning them.

Jaakko Fagerlund
06-09-2013, 05:29 AM
Most chucks I've seen have a little edge behind the jaws on the inside, this makes for a wonderful place to put in a ring to tighten the jaws against, as it clears the jaw surfaces and allows grinding the whole length of the jaws.

What comes to the statements about 3-jaw accuracy, sure, it is not the best, but with proper working techniques one can get to 0.01 mm or less TIR if the jaws are in good condition. When you tighten the chuck, don't just use the "master" key slot, as using only one tends to cock the scroll ad this affects how the jaws center. What I've found out in the chuck I use most frequently is that when my DTI shows the highest position, I tighten a little from the key slot nearest to it and then rotate the chuck again to check it.

I would not do gun barrel work in a 3-jaw, but 99.9 % of all work can be done with it. For anything more accurate, there is a 4-jaw, a faace plate and a cylindrical grinder.

dian
06-09-2013, 12:49 PM
thats exactly how i did mine, but i feel the preload back in that groove is not realistic, so i turned a taper of 0.01 mm into the jaws.

J Tiers
06-09-2013, 09:35 PM
You can use that step, but it is usually at the back of the jaws. Not the right set of forces for loading the jaws as they are with work in them. I drilled holes in teh jaw tips, and put in pins (loose) to close on the ring. That pries them open the way the work will, and so it is most like the way you will use the chuck.

http://img.photobucket.com/albums/0803/jstanley/chuckgrind.jpg (http://smg.photobucket.com/user/jstanley/media/chuckgrind.jpg.html)

macona
06-10-2013, 03:14 AM
I did the same as Jaakko.

In the case of my near new Buck 6-jaw there was not an issue with bell mouth, the jaws were not coming together on the stock at the same point, in fact one jaw would not even touch the work. Ground them and now the chuck is happy.

http://i65.photobucket.com/albums/h228/macona/Monarch%2010EE/DSC03790.jpg (http://s65.photobucket.com/user/macona/media/Monarch%2010EE/DSC03790.jpg.html)

http://i65.photobucket.com/albums/h228/macona/Monarch%2010EE/DSC03795.jpg (http://s65.photobucket.com/user/macona/media/Monarch%2010EE/DSC03795.jpg.html)

George Seal
06-10-2013, 05:17 AM
Since the statement was made that the faces should be flat. Could you place all three jaws in a vise and surface grind them flat????????

sides and outer jaws would have minimal wear since I very seldom clamp internal peices.

MrFluffy
06-10-2013, 06:12 AM
I tried that george, and it didnt work for me.But my chuck was very very bad, and now its sort of parallel but when you tighten down the jaw, you see one visibly rock.

J Tiers
06-10-2013, 08:29 AM
Since the statement was made that the faces should be flat. Could you place all three jaws in a vise and surface grind them flat????????

sides and outer jaws would have minimal wear since I very seldom clamp internal peices.

Grinding in the actual chuck as an assembly compensates for all errors, wear, etc that may be present. The manufacturers no doubt grind them in some manner similar to what you suggest, but they know how to compensate for "spring", and all the parts are new then.


I did the same as Jaakko.

In the case of my near new Buck 6-jaw there was not an issue with bell mouth, the jaws were not coming together on the stock at the same point, in fact one jaw would not even touch the work. Ground them and now the chuck is happy.....


macona is showing a 2-piece jaw, 6 jaw chuck. A different animal entirely.

With the screw counterbores holes already present, it might be easy to load them up using plugs, but there is no guarantee all 6 are on one circle. If they are not, loading would be wrong, and the grinding worthless or harmful. About your only chance is to use the faces of the bottom pieces, and hope those non-critical faces were made concentric.

Alternately, you could use the mounting reference surfaces to line them up, and grind with a surface grinder. In that case you are using surfaces which should be true and in position. If they are not, then you could never replace those jaws and hope to have an accurate chuck. But they are made to be replaced.

With a 3 jaw chuck, it is of no consequence if the holes are drilled perfectly on one circle or not, three define a circle, and will always fit and tighten evenly.

The two cases are not at all the same.

becksmachine
06-10-2013, 10:08 AM
macona is showing a 2-piece jaw, 6 jaw chuck. A different animal entirely.

With the screw counterbores holes already present, it might be easy to load them up using plugs, but there is no guarantee all 6 are on one circle. If they are not, loading would be wrong, and the grinding worthless or harmful. About your only chance is to use the faces of the bottom pieces, and hope those non-critical faces were made concentric.

The two cases are not at all the same.

Thus the conundrum of a 6 jaw chuck. :p

If in fact perfectly circular reference can be found on the jaws somewhere, and it can be utilized to provide preload to the jaws, the problem then becomes one of finding a perfectly circular object to apply to these surfaces and it must be something that does not obstruct access to the surfaces to be refinished.

Alternatively, much patience will need to be expended finding a proper assortment of shims, which will no doubt need to be recovered from the chip pan numerous times before a proper lash up is achieved.

:)

In practice it may be preferable to use something that is not so rigid that it will not distort somewhat to allow it to conform to the inaccuracies of the mounting surfaces while still providing adequate pressure to preload the jaws sufficiently.

Dave

Jaakko Fagerlund
06-10-2013, 12:30 PM
macona has a 3-jaw, not a 6-jaw.

J Tiers
06-10-2013, 07:58 PM
macona has a 3-jaw, not a 6-jaw.


That's not what HE said...........




I did the same as Jaakko.

In the case of my near new Buck 6-jaw there was not an issue with bell mouth, the jaws were not coming together on the stock at the same point, in fact one jaw would not even touch the work. Ground them and now the chuck is happy.

Jaakko Fagerlund
06-10-2013, 11:56 PM
The picture is of a 3-jaw, that is what I meant.

Paul Alciatore
06-11-2013, 04:25 AM
J, I believe we are saying much the same thing. My use of the word "accuracy" was a reflection of the OP's language which lead me to believe he was seeking that. Perhaps I did not understand him correctly. I was trying to discourage grinding except as a last resort if he was having accuracy problems. Or even any kind of problems with it.

Yes, a three jaw is not particularly accurate. But they are not necessarily overly bad either. The one in my photos had over 0.018" run-out in some positions of the scroll and in others it was within 0.003". I would expect a new one to be within 0.003" or better. And it did have bell mouth. Severe bell mouth. The grinding did improve the bell mouth. It did nothing for the runout or the differences in run-out at different scroll positions. I believe I said that near the end of my post. It was a used chuck and obviously had endured more than one crash. These crashes had distorted the scroll at various points. As I said, there is little you can do about that. I guess this differed from your experience with used chucks.

I do believe it is possible to improve the accuracy with grinding in some cases where the wear on the jaws is not even. This could also happen with crashes as one of the jaws is likely to absorb more of the force than the others.

The most important thing with a worn chuck is to properly evaluate where the damage is and then decide what, if any, corrective may be appropriate. In the case of the chuck shown in my pictures I had already purchased a new three jaw and I wanted to see just what would happen with grinding. I had already determined that there was uneven wear on the scroll so it was probably a hopeless case. The result was what I expected, no improvement in accuracy, but it was fun trying.





I'm going to disagree with a lot of the above.

1)
If you want to grind to fix inaccuracy, STOP RIGHT NOW AND PUT DOWN THE GRINDER... Grinding IS NOT FOR IMPROVING ACCURACY. A 3 jaw chuck is NOT accurate, and a worn one is even less so..... There is no reason to try to make it more accurate with crude equipment.

Grinding is to fix bell-mouthed jaws. ONLY

2)
I have, and have had, a number of chucks. All but one have been used. NOT A ONE OF THEM has had anything wrong with the scroll spiral or jaw fingers that was in any way a problem. I expect all were worn a bit, so what?

Many have had bell-mouthed jaws, I ground them, with good results. Some have had wear in the "jaw ways" that showed up like bell-mouthed jaws, and I ground those also with good results.

One had a worn scroll center pivot, for that one I shimmed the center pivot with good results.

3)
You should be able to grind flat. If you hold the chuck steady with jaw at the bottom, you can feed the grinder in as well as move it with crosslide, and it should grind the jaw flat. With all 3 ground the same, it should work, after you have "circle ground" them.

I have not done that, I didn't care that much, and I wasn't confident I'd get the jaws accurately at the same bottom point.

4)
Again, grinding is not to improve accuracy, it is to eliminate bell-mouth. If you do it for any other reason than bell-mouth, you are fooling yourself.

J Tiers
06-11-2013, 08:18 AM
OK, no issues.

The "accuracy" thing comes up enough to make the distinction important.

I suppose my point isn't that 3 jaw are trash, but that one probably ought to assume that a 3 jaw will be "out" by some amount that could be significant for the part. So "countermeasures" when using one are in order.

And, grinding probably isn't going to solve an accuracy issue, although it probably will help to some unknown degree.

oxford
06-11-2013, 10:28 AM
Would this be the preferred method to hold the jaws? I have removable top jaws and could use the mounting bolts instead of drilling holes for pins.

http://img.photobucket.com/albums/0803/jstanley/chuckgrind.jpg (http://smg.photobucket.com/user/jstanley/media/chuckgrind.jpg.html)

Earl Sigurd
06-11-2013, 03:17 PM
Is this an easier way to load the jaws for grinding? I should perhaps have cut a slot in each part so that they could yield a bit and even up the jaw loading.http://i1297.photobucket.com/albums/ag34/sigurd3/11Jun2013018_zpsdcd73cfb.jpg (http://s1297.photobucket.com/user/sigurd3/media/11Jun2013018_zpsdcd73cfb.jpg.html)

becksmachine
06-11-2013, 04:33 PM
In my opinion, one of the best ways.

The trick here is to get the ring to stay put for the whole operation as the pins want to "bell mouth" and squirt the ring out at the most inopportune moments.

One of the advantages of this method is that the preloading force is exactly where it should be. Consequently you don't have to tighten the chuck so tight to assure that all of the clearances have been shoved in the proper direction.

It also allows access to the full length of the jaw gripping surface, thus no undercuts or steps are needed.

When doing this with a master/top jaw set up, you can sometimes use dowel pins in the hex socket of the socket head cap screw that retains the jaw.

Dave

J Tiers
06-11-2013, 09:49 PM
Would this be the preferred method to hold the jaws? I have removable top jaws and could use the mounting bolts instead of drilling holes for pins.



Obviously I think so, as that is my picture. It wasn't my idea, though. The original plan is due to Rich Carlstedt, who introduced the idea to me in a posting somewhere many years ago.

I have yet to experience the ring being squirted out like a watermelon seed.... Possibly this is because I drill the holes fairly deep, and to a fairly tight fit, as tight as I can do without making them a press fit. Even roll pins are no fun to pull out with pliers, and it makes re-use difficult.

if you have removable top jaws, it may be possible to use the counterbores, or cap screw recesses. The only thing I see against it is that the screws might be on the next step down, and have less leverage. That means more tightening, and so your pins may need to be a pretty darn good fit.