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By Trevanion
#1267699
I've got a small problem with my 8" 3 jaw chuck where it only grabs stock (Including precision ground) on the very back edge of the jaws, more than likely due to wear from being quite an old chuck (I think, there doesn't seem to be any branding on it). The scroll itself seems to be in great condition with no play at all in the jaws and the motions are very smooth. It's just annoying that I get quite a lot of deflection and chatter from using it like this more than anything, If I want something turned more solidly I usually pull out the 4-jaw or collet chuck, the 3-jaw is just more convenient.

I'm not quite sure what to do so I was hoping someone here of a higher calibre could point me in the right direction in what to do. Would it be possible to get new jaws if I had the correct measurements from the existing jaws? Do I send off my chuck (to where I do not know) to have to jaws reground to be parallel to the stock? Is there some kind of old-school machinist trick that somebody knows to get one working properly?

I don't particularly want to buy a new chuck because I'm certain a chuck under £300 won't live up to the (Unknown) quality of this one, as I said, the scroll seems to work excellently so it would be a shame to not keep it going somehow. I only do a bit of back street mechanicstry on the lathe so I'm not looking for hyper 100th of a thou accuracy.
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By CHJ
#1267715
Are they out because of wear at the front (unlikely) or are the jaws tilting in use?

What are the inner jaw inner faces like for parallel if you expand them into a ring.

If it's internal wear at the front then you could just expand them into a ring and re-bore the inner surface.

If they are tilting due to slideway wear then it's going to be a little more difficult to and time consuming to compensate each jaw accurately for the inner taper.
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By Trevanion
#1267718
CHJ wrote:Are they out because of wear at the front (unlikely) or are the jaws tilting in use?

What are the inner jaw inner faces like for parallel if you expand them into a ring.

If it's internal wear at the front then you could just expand them into a ring and re-bore the inner surface.

If they are tilting due to slideway wear then it's going to be a little more difficult to and time consuming to compensate each jaw accurately for the inner taper.


I'm fairly certain it's the jaw faces have worn, the jaws seem absolutely rock solid in their grooves but I could be wrong. When I get a chance I'll find a bit of ring from somewhere and measure the faces.
By Myfordman
#1268062
The problem is known as Bell Mouthing and happens over many years use. My chucks are coming up for 40 years old and shown some wear leading to chatter under heavy cuts and in extremis the workpiece can walk out of the chuck.
However the method quoted above is a common erroneous method.
The jaws should NOT be opened against a ring as this puts pressure on the jaws and scroll in the wrong direction. Instead spacer blocks need to be inserted between the jaws and the chuck tightened as it if were gripping on a bar.
This video is possibly the easiest way of show the correct method https://www.youtube.com/watch?v=o3NK5BPaTz0
However it omits to mention until the very end, that the chuck should be opened up and scrupulously cleaned before starting to remove accumulated swarf and this process repeated afterwards to remove all grinding debris. I've just successfully done this process on 4 different 3 jaw chucks. The 40 year old grease was soaked out using paraffin. After the first clean, I just used a little light oil, enough to get smooth operation but used grease after the final clean for long term lubrication.
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By Trevanion
#1268071
Myfordman wrote:The problem is known as Bell Mouthing and happens over many years use. My chucks are coming up for 40 years old and shown some wear leading to chatter under heavy cuts and in extremis the workpiece can walk out of the chuck.
However the method quoted above is a common erroneous method.
The jaws should NOT be opened against a ring as this puts pressure on the jaws and scroll in the wrong direction. Instead spacer blocks need to be inserted between the jaws and the chuck tightened as it if were gripping on a bar.
This video is possibly the easiest way of show the correct method https://www.youtube.com/watch?v=o3NK5BPaTz0
However it omits to mention until the very end, that the chuck should be opened up and scrupulously cleaned before starting to remove accumulated swarf and this process repeated afterwards to remove all grinding debris. I've just successfully done this process on 4 different 3 jaw chucks. The 40 year old grease was soaked out using paraffin. After the first clean, I just used a little light oil, enough to get smooth operation but used grease after the final clean for long term lubrication.



Cheers Myfordman, I'll have to keep an eye out for a tool post grinder or eventually make up something with a router attached. Even though I've ran some really daft and dangerous set ups on a Spindle moulder at 6000RPM in the past, the thought of the square bar spacers coming loose is a little unsettling weirdly.
By Myfordman
#1268075
I used slightly thicker blocks than on the video and fitted a jubilee clip around the blocks just in case they decided to escape.

Katsu, the makers of the little routers that folk here rave about also make a die grinder for about £25 which is very good value.

Have a look here http://www.thewoodhaven2.co.uk/viewtopi ... =19&t=3840 where there is a photo of the the grinder and toolpost mount a mate of mine has made. model no 100170
By Rorschach
#1268094
A die grinder makes for a very good tool post grinder. As soon as I find a bit of suitable steel I will be making one for my lathe. Currently I have a dremel clone based tool post grinder which does work well but I want something with more power.