Bell mouthed chuck

[Brian Rupnow]

The 3 jaw chuck on my lathe has gone bell mouthed. That's not surprising, because the lathe gets almost daily use, and the parts I make are short, they do not extend full length of the chuck jaws. A new 6" chuck will cost me $350. I can not buy replacement jaws, so must buy an entire new chuck. Is there any GOOD way of fixing these bell mouthed chuck jaws? I don't think there is, but just in case there is, I'd love to hear about it. I know about locking the jaws in place and regrinding them, but as I understand it, the chuck is then only "accurate" at whatever diameter it was locked at before regrinding.---Brian

 

[Charles Lamont]

You have two different issues to separate here.

If the scroll has been damaged by overtightening (you would not have done that!) then you will get variable levels of accuracy in different positions, whether the jaws are reground or not. Actually, a brand new good quality chuck will show some differences in runout at different diameters.

Correcting bell-mouthing by light internal grinding will have no effect on the scroll.

I have used this method with the expander shims very successfully, using an air die grinder mounted on the toolpost:
http://www.homemetalshopclub.org/news/aug04/aug04.html#truing
Your 4-jaw should be used for rough or heavy work such as gripping anything cast, and the 3-jaw kept for accurate work. For short work I often use soft jaws.

 

[Brian Rupnow]

Thank you Charles. I may go that route, as I have really nothing to lose. If it works, that's just great. If it doesn't, I was going to have to buy a new chuck anyways. I'm pretty sure that another small design job will find it's way to me sooner or later, and when it does, a day and a half's work will buy me a new chuck..

 

[Richard Hed]

Thank you Charles. I may go that route, as I have really nothing to lose. If it works, that's just great. If it doesn't, I was going to have to buy a new chuck anyways. I'm pretty sure that another small design job will find it's way to me sooner or later, and when it does, a day and a half's work will buy me a new chuck..

However, if you regrind it, which is what I would at least try, you hve to be careful that the jaws are tightened up against something to hold them against the spiral lest you have loose jaws. I was told about this but I forgot the exact technique. It's trixy. Someone on the forum should know more about it than me. I have never done it but would if I needed it.

 

[MIKE4444]

Hi ex site , well worth a visit , many thanks , go well and be safe to ALL, MIKE

 

[johwen]

Hi Brian if you go and buy a new chuck buy a 4 jaw self centering you won't regret it. And before someone says you cannot hold Hex material you can and very accurately too. square is obvious as well. They are generally more accurate as well. I use one all the time and with truly round material will chuck to an accuracy of .002 thou. Polish brand are very high quality and priced reasonably for a 5 inch size. Investigate if you are having to resort to a new one. John

 

[methuselah1]

As Charles has said, you cannot remedy a concentricity problem. (By the way, Charles, I loved your Seagull build!) The regrinding of jaws is hard to explain in text, but you'll find it under "clover leaf" in the files section of the drummondlathe.io newsgroup. It's public. The method is self explanatory. And it's not hard to do if you've got a dremel drill or a die grinder that'll fit in your toolpost.

Two other dodges spring to mind. A piece of thin card, folded double at the front, and wrapped around the work will compensate for bell-mouthing. As Professor D.H.Chaddock noted, for concentricity, turn 1/32" off your chuck backplate, slack the chuck mounting bolts, tap everything true, retighten, then off you go.

Andrew UK

 

[clockworkcheval]

Most of my equipment is of good lineage but has seen more than half a century work. Like on myself this leaves some marks, so the internal grinding of the jaws of chucks is not new to me. To tighten up the jaws I first followed the bad advice to put a ring at the backside of the jaws. Then I thought to be smart and I had put 4 mm diameter holes in the jaws by EDM. This allows for tightening the jaws up to a ring against 4 mm pins you put in those holes. Finally I came across a much more experienced machinist than me. He showed me the quite intelligent solution of simply putting some blocks between the parallel end-sides of the jaws and tightening up against those blocks. See the enclosed pictures of the jaws with the pins, a ring clamped between the pins and the final solution of simple blocks between the jaws.

 

[ajoeiam]

snip
I have used this method with the expander shims very successfully, using an air die grinder mounted on the toolpost:
http://www.homemetalshopclub.org/news/aug04/aug04.html#truingsnip

I would think that keeping those newsletters would give you a whole pile of useful tips!!!

 

[almega]

My 3-jaw chuck is a 1948 vintage and had developed similar wear. I used the final solution with some aluminum blocks as shown by clockworkcheval and it worked quite well. Take light cuts with the grinder and be patient. Perhaps one slight down side is that the jaw faces are now concaved with a concentric curvature to the headstock axis rather than flat but that has not been a problem to this point. The entire process took me less than an hour to complete, but you may be able to do it more quickly with your experience and skill.

 

[roncohudd]

The 3 jaw chuck on my lathe has gone bell mouthed. That's not surprising, because the lathe gets almost daily use, and the parts I make are short, they do not extend full length of the chuck jaws. A new 6" chuck will cost me $350. I can not buy replacement jaws, so must buy an entire new chuck. Is there any GOOD way of fixing these bell mouthed chuck jaws? I don't think there is, but just in case there is, I'd love to hear about it. I know about locking the jaws in place and regrinding them, but as I understand it, the chuck is then only "accurate" at whatever diameter it was locked at before regrinding.---Brian

I rebuild m jaws on my Bridgeport. First indicate the vice perfectly then clamp 1 chuck jaw at a time inprotuding out with the clamp erea opposed to mill. This is a time consuming process. The key is to have perfectly aligned setup. Mill each jaw separately, then back to the chuck for indicating. Find the long on and mill again then check again. I did this until I was within. 0005." Took me about 2 hours.

 

[roncohudd]

I rebuild m jaws on my Bridgeport. First indicate the vice perfectly then clamp 1 chuck jaw at a time inprotuding out with the clamp erea opposed to mill. This is a time consuming process. The key is to have perfectly aligned setup. Mill each jaw separately, then back to the chuck for indicating. Find the long on and mill again then check again. I did this until I was within. 0005." Took me about 2 hours.

PS might have to recut the angle.

 

[trlvn]

How-to videos:





Craig

 

[a41capt]

Most of my equipment is of good lineage but has seen more than half a century work. Like on myself this leaves some marks, so the internal grinding of the jaws of chucks is not new to me. To tighten up the jaws I first followed the bad advice to put a ring at the backside of the jaws. Then I thought to be smart and I had put 4 mm diameter holes in the jaws by EDM. This allows for tightening the jaws up to a ring against 4 mm pins you put in those holes. Finally I came across a much more experienced machinist than me. He showed me the quite intelligent solution of simply putting some blocks between the parallel end-sides of the jaws and tightening up against those blocks. See the enclosed pictures of the jaws with the pins, a ring clamped between the pins and the final solution of simple blocks between the jaws.View attachment 133624View attachment 133625View attachment 133626

As a relative newbie to home shop machining, and needing to grind the jaws on an older Cushman chuck to make it usable, I truly appreciate these options. However, could a similar setup using the outside of the jaws with a ring (instead of the holes with pin method) have a similar effect as a stabilizing feature? I thought I’d use a ring that held the jaws at about 1 1/2” opening as that and smaller is my most common stock diameter.

Like I said, I am a relative, and self taught newbie, so don’t think you’ll offend me in your reply. I’ve only built 3 running engines to date and I look to you guys for not only inspiration, but also for guidance! Thanks in advance,

John W

 

[almega]

a41capt - I first tried that method (outside ring) and it didn't work so well. I found out later that you have to take the backlash out of the scroll and put the jaws into compression (as it squeezes the held material)
so the block method turned out better. I don't understand the geometry of it but it did work.

 

[gartof]

a41capt - I first tried that method (outside ring) and it didn't work so well. I found out later that you have to take the backlash out of the scroll and put the jaws into compression (as it squeezes the held material)
so the block method turned out better. I don't understand the geometry of it but it did work.

Does anyone actually check runout when holding a part in a hole expanding the jaws of the chuck? I have just been making a part that way, no idea as to the concentricity and it doesn't matter for this part but certainly uses the other side of the scroll teeth.

 

[almega]

As was touched on before, you may have to regrind the angled cheeks of your jaws afterward because you won't be able to hold materials as small as you could before grinding the jaw faces, if that's a concern.

 

[a41capt]

a41capt - I first tried that method (outside ring) and it didn't work so well. I found out later that you have to take the backlash out of the scroll and put the jaws into compression (as it squeezes the held material)
so the block method turned out better. I don't understand the geometry of it but it did work.

Thanks, makes sense! I’ll cut some nice even blocks and give it a try. Even if it fails I’m no worse off as the chuck is unusable as it sits.

John W

 

[Iampappabear]

On lager chucks you sometimes find holes drilled and tapped in the tips of each jaw, that way you can bolt a what looks like a large washer onto the front of the chuck... When the chuck jaws are tightened, the front very end of the jaws are spread out as far as possible which is what you aiming for. At rest, there will probably be a taper in the jaws with the inside diameter being larger at the back of the chuck. The best lathe man I ever knew seldom took his 4 jaw off, according to him, he could always get something true in a 4 jaw but not necessarily true in a 3 jaw.

One thing to bare in mind is that if the chuck has ever received a heavy blow or similar, the scroll could be damaged and then its almost impossible to get them true.

Colin

 

[almega]

When the chuck jaws are tightened, the front very end of the jaws are spread out as far as possible which is what you aiming for. At rest, there will probably be a taper in the jaws with the inside diameter being larger at the back of the chuck. T

YES! The old brain isn't what it used to be. That is what I was told the reason was for using little blocks between the jaws and tightening rather than using a large washer and tensioning outward. It was so that any taper would result in the rear of the jaws being larger than the front. Tensioning the jaws with an outside washer would result in the belled jaw configuration that Brian, who started this topic (remember Brian?), is trying to eliminate.
Thanks lampappabear for tweaking my dormant brain cells.