Truing lathe chuck jaws

[steamboat willie]

Hello!
I have a 12" Pratt Bernurd chuck that is showing signs of its age on my equally elderly Nuttall lathe (bed date 1.4.59). Its a great lathe.
I was wondering whether anyone has had any experience or knowledge about regrinding lathe jaws in situ to bring them back to true to facilitate holding a piece of finished size stock bar.
Any advice will be most welcomes.
Thankyou.
Bill.

 

[Charles Lamont]

I can recommend the method described towards the bottom of this page:

http://www.homemetalshopclub.org/news/aug04/aug04.html

 

[steamboat willie]

Charles,
Many thanks for that! That is the way that I had thought to do it, but just wanted to hear whether or not there were any major pitfalls in utilizing this method, or if the obvious way was fraught with hidden problems!
Thanks again!
Bill.

 

[Tin Falcon]

The first thing to think about is disassemble cleaning and inspection of the chuck. This could solve a lot of problems.

Do not expect to make the chuck perfect just reduce error.

also if the scroll is worn in one area and causing problems grinding can increase error where the scroll is not worn or worn as much.
Try to find the source of the problem before fixing it.
Tin

 

[Charles Lamont]

... just wanted to hear whether or not there were any major pitfalls in utilizing this method ...

As in the article, I used a air die-grinder. It will do the job, if properly mounted, but it is not the same as a proper toolpost grinder. You will need to dress the grinding point in situ, and that really means you ought to strip the lathe down after the job is done. Take vey light cuts, and repeat the last pass over and over until you get virtually no spark - "spark out" - to compensate for the light construction and ordinary bearings in the grinder.

 

[rklopp]

I have trued hard jaws using light cuts with a new sharp insert on a carbide boring bar. I preloaded the jaws tight to, in effect, spring the ends open so they grip slightly harder at the ends in normal use. I cured some significant bell mouth in an otherwise good chuck. The secret is very light cuts and moderate surface speed.

 

[steamboat willie]

Gentlemen,
Thankyou for your helpful advice. Tin, I will do what you suggest first and hope that the problem you mentioned does not occur. I could well imagine it exacerbating the problem - thanks for the warning.
I will proceed with more caution than usual.
Many thanks to you all.
Bill.

 

[goldstar31]

This question emerges literally several times a year and on several forums.- and seems to be like the Phoenix! Sorry but it does.
The correct answer is that unless one has very sophisticated equipment, you cannot correct things like worn scrolls and worn chuck bodies. If such things are the reason for poor performance, all that can be done is to grind jaws to accurately hold ONE diameter. No more- sorry folks- don't blame me as it is a fact of machining. The only reasonably successful thing that can be achieved is to remove such lesser problems as bell mouthing. Other contributors here have pointed to 'how they did it' and there is nothing really to add apart from adding my own couple of suggestions which are to hold the jaws firmly with some jig or other. You can use an outside ring or an inside ring or three bits of bar or make a fancy jig-- but you will still arrive at ONE dimension if the rest is worn. My own favourite way to grind is to use a cheap wood router. I have a very ancient Bosch POF 45 which seems to have started life as a tool post grinder and after I read the included one page instruction book on how to sharpen an axe, it went back into a 43mm collar thing on my lathe saddle as a tool post grinder. Without such a gimmick, a piece of parallel rod in the tailstock drill chuck and car valve grinding paste is as good as anything. One has to be very careful not to adjust the setting as the rod goes in and out but that is about it.

So one can do what most of us do with an old chuck and that is make a set of soft jaws to fit over the worn ones and cut them to fit MORE diameters.

I hope that I haven't upset too many folks.- but these are the grim truths.

 

[Tin Falcon]

we know goldstar. The old worn out ones can be hard to correct.
Tin

 

[jgedde]

There's a question in here... But first some background info...

My old lathe chuck has jaws that were slightly bell mouthed and resulted in inconsistent chucking. I was able to correct them fairly well by loading the jaws with a shop made fixture then running them through with a carbide boring bar and very light cuts.

The good news is that the chuck repeats very well now, the bad news is the faces of the jaws now have a radius.

So, when I chuck up things that are larger in diameter than what was used when the chuck was cleaned up, the jaws tend to mark up the chucked piece. I lightly stoned the edges which has made it better....

But, what I'd really like to do is to regrind them so as to have a flat surface like they had when they were new. I have a surface grinder so maybe this can be done?

How do the chuck manufacturers do this? Is it something we can do in the home shop having a surface grinder?

At some point, I'd like to buy a new chuck with the single reversable jaw type setup vs the inside/outside separate setup I have now. So, if I screw it up trying to regrind the jaws flat, it's just incentive for me to get another chuck - one that's easy to make soft jaws for.

John

 

[goldstar31]

John

If you have access to a surface grinder- or a tool and cutter one, I would heave the jaws out and do what many of us did- and that was to hard face the gripping tips up to their old width. Of course, it needs jig of sorts to do both the sides and the edges. It is perfectly feasible as the late Kenneth Cleeve Hart writing as Martin Cleeve wrote it all up in Model Engineer. This is where the use of grinding paste etc was culled from.
Moving on to another gem from yesteryear, the late Tom D Walshaw , writing yet again as 'Tubal Cain' had three jaw problems and had a single dedicated adjuster in the chuck which gave the best accuracy. Professor Dennis Chaddock writing in his Book of the Quorn suggested that he pushed the chuck around in the chuck body and 'Martin Cleeve' did something similar. It emulates the Tru Grip type of chuck.

Neil Hemingway altered the system of adjustment to a spring- or at least sold it in his firm called Hemingwaykits.

Of course it is not unknown to put a chuck within a chuck. It is not 'me' but I've done it- plus a lot of funny variations.

So I can only wish you success in your endeavours.

Norman- writing as Goldstar31

 

[jgedde]

John

If you have access to a surface grinder- or a tool and cutter one, I would heave the jaws out and do what many of us did- and that was to hard face the gripping tips up to their old width. Of course, it needs jig of sorts to do both the sides and the edges. It is perfectly feasible as the late Kenneth Cleeve Hart writing as Martin Cleeve wrote it all up in Model Engineer. This is where the use of grinding paste etc was culled from.
Moving on to another gem from yesteryear, the late Tom D Walshaw , writing yet again as 'Tubal Cain' had three jaw problems and had a single dedicated adjuster in the chuck which gave the best accuracy. Professor Dennis Chaddock writing in his Book of the Quorn suggested that he pushed the chuck around in the chuck body and 'Martin Cleeve' did something similar. It emulates the Tru Grip type of chuck.

Neil Hemingway altered the system of adjustment to a spring- or at least sold it in his firm called Hemingwaykits.

Of course it is not unknown to put a chuck within a chuck. It is not 'me' but I've done it- plus a lot of funny variations.

So I can only wish you success in your endeavours.

Norman- writing as Goldstar31

Thanks for the detailed info Norman! I'm not comprehending what you mean by heave the old jaws and hard face the gripping tips. Maybe if I had access to the article or a synopsis I would understand better.

Just to be clear on what I was talking about... After I ran the boring bar through the jaws, it repeats and centers rather well at different clamping diameters. So, at least that's a plus.

I see no evidence that, apart from those initial jaw problems the chuck was worn out. I did buy it used from a robotics hobbyist, but it was made in 2003 and is still fairly new.

My gripping tips have a radius cut into them as a result of truing with the boring bar, the sides of the gripping tips have a tendency to mar whatever is being chucked up if it's large in diameter. That's my beef with having bored the chuck in situ.

Since I do have a surface grinder, I could, at least in theory, grind the gripping tips flat again. But, unless I know how to accurately fixture them for the grinding operation while out of the chuck, I'm certain they would cause significant runout on chucked workpieces after I ground them. In other words, I presume the setup would have to use the jaw's scroll teeth as a datum - somehow...

I presume they were ground this way when they were made since they used to be flat albeit bell mouthed.

Cheers!
John

 

[Swifty]

If you have access to a surface grinder, you could hold the jaw in a vice and indicate the face that you ground in the lathe until it was parallel and then surface grind the slight radius out until there is barely a witness left.

Paul.

 

[goldstar31]

John
Apologies- I'm an ancient geezer and well- a bit- you know!

So let me take it a little differently. When you ground/bored the jaws you created a wider land than when the chuck was new. I therefore assumed that you wanted to bring it back to that. As far as I can recall there are only two simple(?) methods. The first is to grind the flanks of the jaws to reduce the width again. You might need a jig on your grinder- as I said. The second is to build up the removed metal with a hard facing- or even a softer job with a mig/mag welder. This will raise rough edges and you will have to grind away excess weld on the sides and then top the jaws by grinding.

I've done both in my time and it is not too difficult. Harder to describe- eh?

As for the articles in themselves is quite another matter. Some years ago, a brother mason and myself set off to put these snippets on the net----- for you younger generation to benefit. We ran into 'copyright' threats and the published stuff was taken down.
Jim Early is dead, I'm left as an oldie with far more problems and family responsibilities than to do the same articles or near enough- again. Maybe someone else still has the articles which may guide you further. I'd feel that Jim and I would appreciate that.

Drop me a note, if you feel that it would help.

Norman

Apologies Swifty but I have always been a poor typist- and now have two problem hands.

 

[goldstar31]

Just had a look at Model Engineering Clearing House forum. Some guy in Kuala Lumpur has just won a Myford soft jaw chuck- and is raising questions. Maybe some of you might be interested in what looks a lot of information.

Me? No, not really. Someone seems to have some of Jim Early's stuff. Talk about voices from the dead.

Regards

Norman