New three jaw chuck

I am looking at replacing my Pratt Burnerd 160mm D 1 4 mount chuck.

The jaws are bell mouthed and it does not run true without a lot of fiddling and tapping things true. Rotagrip quoted around £170 for new jaws, they would cure the bell mouth problem but may still not run true.

Looking at new integral camlock mount chucks they range from £140 at Warco to around £200 for a TOS chuck. Any recommendations ?

Chris,

A good cure is to fit a section of silver steel in the tailstock chuck, put a little valve grinding paste on it and use it as a gentle grinding aid, moving it in and out of the 3J chuck jaws like a hone as you do so.

Fit an O ring round the outside of the jaws before you start to keep up inward pressure and readjust the light grip on the silver steel as the jaw gripping surfaces take up their new shape.

Naturally, take care to clean everything afterwards with scrupulous care. I have done this twice now with first class results. It might save you buying a new chuck

Regards Brian

Chuck jaws should be clamped as they would be when holding a part when they are being ground to correct centering errors. Therefore if you are grinding the inner faces of the jaws they should be pressed against the same side of the chuck spiral as they would be when holding a piece of round bar otherwise they may not centered correctly.

A special plate as shown below is used to hold the jaws in their correct working position for grinding.

Paul.

Or you could use something like this and use a grinder to true the jaws

If the three 'H' slots in the chuck body are worn, allowing the jaws to move sideways , then the chuck will never be accurate .

When you say it doesn't run true it's worth asking how much run out there is on the od of the chuck. Often work run out problems are just down to how some one mounts the work in the chuck but they are not collets so there is always likely to be some.

The jaws in the chuck can appear to be bell mouthed due to wear on their fit into the chuck. The fit should be pretty firm on a new chuck - no play when they are pressed down onto the scroll. If these are loose there may be no point in buying new hardened jaws.

As an alternative all soft jaws I have bought from Rotagrip are always far to tight and need fitting. It takes maybe an hour or so to do but some care is needed to see where they need relieving. The slot usually does need widening and may well need deepening as well. The jaws are usually a touch too thick.

Unless the machine can turn dead parallel machining the jaws on the lathe just leaves them bell mouthed in what ever direction the lathe happens to turn.

In terms of sorting the Burnerd I don't think that the usual TOS for instance really compare. Some makes come made in various materials and I would suspect this is why. From memory Gloster Tooling offer them made in various materials. The prices vary.

John

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How much of a runout are we talking about here? one must always be wary that an unsupported bar more than 3 or 4 times it's length in diameter is always going to have a degree of runout, it's impossible to be dead true in this fashion. A runout of about 0.1 is probably normal for a self centre mine is about 0.02 and i'm happy with that (and chuffed for being lucky with an asian chuck). 

I keep telling people i learned to turn on a colchester that had a runout about 0.2 on it's 3 jaw. I'd say anything greater than that becomes a nuisance.

Michael W

Edited By Michael Walters on 17/07/2016 12:05:11

I never expect a 3 jaw chuck to run true, if I need a job to run true I reach for the 4 jaw independent or collet chuck.

Folks may disagree with my method but, I've always ground the jaws off lathe, vertically on the flank of my 8" white wheel grinder, I also adjust the rest to give a slight backward angle, to grind more off the heel than the tip of the jaw, to reduce bell mouthing.

I grip a ground bar in the suspect jaws and take readings and mark the jaws, doing this several times does get tedious but the results are excellent.

The reason being, on lathe grinding/lapping leaves a curved surface on the gripping part of the jaw, which will only corresponds to one diameter, leaving horrible grip lines on others.

In the end I converted my stock Burnerd 3 jaw to a Griptru one, takes but a few minutes to get less than 1/2 thou tir.

P.S. should anyone be interested  in how I converted said chuck, I can post a YouTube link.

Bernard.

Edited By Bernard Wright on 17/07/2016 13:33:59

Edited By Bernard Wright on 17/07/2016 13:34:20

Yes please Bernard

Hi Paul, it is a collage of photos, as it was done over several days, when I used to go to work...

**LINK**

Bernard

My recommendation? Get your Pratt Burnerd repaired or overhauled. Or think of more money to replace with an item ofequivalent quality.

Hi Chris,

If you want to replace the old 3-jaw, my recommendation would be TOS or Bison. I have a small TOS 3-jaw and I am very satisfied with it.

Thor

Thanks for the input. I am well aware of the limitations of a three jaw but I get around 0.012"/0.015" TIR.

Jaws are a good fit in the body. If I could get 0.003"/0.004" TIR I would accept that. 80% of my work is done in a 5C collet chuck the three jaw only used when I go above 1" or hold hex bar. I don't mind spending the money on a quality chuck, it may be the last one I buy!

may be one of these **LINK** plus a suitable back plate.

Not a great lover of integrated fitting chucks. A backplate gives more room for adjustment if needed.

I was very satisfied with the brand new TOS I got for my SouthBend 9" model A. But that was 20 or more years back so things may have changed.

When I lucked into a very nice Southbend Heavy 10 D1-4 spindle toolroom lathe with a well used chuck I decided it was worth getting a decent chuck for it and spent almost as much as the lathe cost on a 160 mm Pratt Burnerd precision, backplate mounting, type. Which worked out well as the same chuck now lives happily on my Smart & Brown 1024. Being a precision chuck the jaw tips are narrower than those in the more common run of 3 jaw chucks which is great for smaller work. It does mean that total grip is reduced so I need to be bit circumspect when roughing out larger jobs with heavy cuts. Trade off is worth it in my view, especially as I prefer to avoid mass swarf generation. I have a Taylor 3 jaw about the place if I really need grip.

Although the chuck is excellent the current incarnation of Pratt Burnerd make it such a total pain to get information, let alone order anything that I hesitate to recommend getting one.

The NOS Bison I got for my Pratt & Whitney a few years ago is excellent albeit bit chunky in the jaw. However if information on American forums is to be believed Bison went through a fairly rough patch not that long ago so maybe some quality checking is in order. As I understand it Bison own label some chucks from other makers and some of those weren't really up to scratch. I believe many Bison lble chucks are made by TOS.

Clive.

Bernard, that's an excellent piece of work! To return to Chris' original problems, having ground the jaws back using one of the methods described, it's quite possible that the chuck won't now grip truly concentric. Don't forget the simple dodge of turning a few thou off the chuck backplate register diameter and then bopping the chuck to run true with a mallet - a cheap and cheerful alternative to Bernard's Griptru solution.

I am going to try some soft jaws I have to see how they perform. Looking more closely at the chuck holding a ground 1 1/4" bar it only grips on two jaws until the lazy jaw is wacked with a small copper mallet.