Grinding lathe jaws

[camm-1]

Hi!
I have notice that my works dont will be centered then I make them in my cheap crap
China late BV 25-L
Did a slitsawholder and it wobbles a little.
I moved the mill colletchuck with th slitsawholder to the lathe and then it wobbles too.
I think the jaws needs to be grinded.
Thinking about to fit the dremel or another airtool in the topslide and tight the jaws inside
and grind then the lathe is on full speed.
Tip anyone ???

 

[rake60]

I usually take a light dust cut out of the inside of the jaws with a boring bar
to true them up.

I chuck out on the ID of a a narrow ring then touch off the ID of the jaws with a
boring bar that is long enough to reach the whole way through the jaws.
Move out .005" from first touch and bore through.

I have only ever had that fail on one lathe.
The scroll in the chuck was not consistent with the pressure going out in that
one case.

Be sure to check the jaws for burrs after skinning them!

Rick

 

[Ken I]

Following Rick's advice....

First check the runout with differing sizes of groundbar - if the run out is consistent and at the same vector this method will work.

If not you may have a slightly off-centre scroll gear which can usually be fixed by boring and sleeving - but difficult to get right.

If the scroll gear is badly worn or "drunken" then there is not much that will fix that other than replacement.

Ken

 

[camm-1]

Thanks for your tips boys!
I will try that.

 

[Niels Abildgaard]

On 25 of february on The hobby-machinist a member 8nsane has described a very smart way to true jaws.
In my next life I will have to learn to link

 

[lensman57]

On 25 of february on The hobby-machinist a member 8nsane has described a very smart way to true jaws.
In my next life I will have to learn to link

Hi,

The method described is almost exactly the same as I or any other Taig lathe owner have to do to true the soft jaws as supplied from the factory,
With the hardened jaws the use of a grinder or a diamond burr is necessary unless the jaws are not hard or are made of mild steel or aluminium in which case a very stiff and sharp boring bar is acceptable. The washer as described in the article also has to be perfectly flat on both sides and perfectly round, it must sit totally flushed against the chuck body for concentricity.

Here are a couple of links for two different methods:

http://www.hobby-machinist.com/archive/index.php/t-659.html

[ame]http://www.youtube.com/watch?v=cASpZUQ80pI[/ame]

Regards,

A.G

 

[Ned Ludd]

Hi Guys,
Even hard jaws can be turned, with the right tips of course. Just recently I finally got around to taking out the bell mouthing of my Colchester 3 jaw chuck. The procedure I followed was 1, tighten a thick washer as far into the jaws as possible, remembering to tighten using the "prime" keyhole. 2, bore using a Titanium Carbide tipped boring bar, 3,once the jaws were cleaned enough, use a permanent marker to check, take out the jaws and gently grind away the small ridge where the washer was held.

In my case I had two benefits, first, it is now true to a thou or so even at 4-5 inches from the chuck. The second benefit was much easier parting, the bell mouth had prevented the chuck holding securely enough to part cleanly. I had wondered why I could part perfectly well using my 4 jaw but not the 3, doh.
Ned

 

[Blogwitch]

Just a little thing you need to be aware of when you regrind jaws in a chuck.

I have done this thing a fair few times now, and what you will end up with is a jaw that looks like the one in the top left hand corner of this C-o-C. Normally, new jaws are flat on the tip, not concave.

The problem with leaving it in this state is that it grips material in the chuck like shown in the top right hand sketch.

If left like this, you just might find that to stop the work rotating (because it is only held by six small tips) the job will normally be very badly marked or scored as it tries to rotate in the jaws.

The ideal would be to flatten the ends off, but then you run the risk of losing concentricity again.

I normally mount the jaws in the mill and remove as much as possible from either side, to leave just a very narrow concave tip. This will also allow you to hold small parts again, because if left as is after grinding, you will find that holding very small parts become impossible because the jaws 'bottom' against each other on the tapered side lands before the tips have got close enough together.

Doing this isn't as difficult as it looks, by using a good angle gauge (UK English) and a tipped or solid carbide cutter, the jaws are easily reduced in size.

BTW, the wider you can have the jaws whilst grinding will leave you with a shallower concave, and so much better.


John

 

[Ned Ludd]

Hi John,
Agreed, but all things being equal though, I would rather have a true chuck and loose the ability to hold very small items, which I would normally use a collet to hold anyway.
In my case I only took off about 4-5 thou, so width not greatly enlarged. As for the curve left, as I used a 1 1/4" washer, this was not that great and if found to be a problem a permanent marker and a Diamond file would soon dress them back to flat without loosing any appreciable concentricity. On thinking about it, anything less than 1 1/4" dia. held in the chuck would still only have a single line contact per jaw and anything much larger I would use the 4 jaw, but your point is still well made.

I don't thing the jaws had been touched in their fifty odd year lifespan, so they should see me out before needing to be done again, hopefully.

Ned

 

[mu38&Bg#]

The only other thing I might add is that it would be worth taking the chuck apart and cleaning the scroll and jaws before truing them.

Greg

 

[Ned Ludd]

Hi Greg,
Agreed, but the advantage of turning the jaws, as compared to grinding them, is that the swarf is much less hazardous to the mechanism, so cleaning afterwards is not essential. Well, no more essential than, say, a normal through boring job. If any of you bother to strip after every boring job, I pity you.
Ned

 

[mu38&Bg#]

Ned, I said before. If you have dirt in the scroll, how accurate can you expect it to be after truing?

 

[Blogwitch]

Ned,

The last set I had to do was for a friend, and I had to take out over 0.030" in the grind, the jaws had been grossly overtightened, so splayed out. A thing people should also note, overtightening anything does no good at all, except cause problems. He had used a two foot pipe on the key to tighten up because the part was slipping, no wonder they were splayed, and again no wonder it was slipping. Because so much was removed, not only from the face, but from the sides also, they were reground again (just lightly) to take account of distortion from internal stresses.

As you can imagine, I reckon it is because of overtightening, not wear, is where a lot of these problems occur, as most of these problems are showing up on newish machines, not ones like yours, that have been used for many many years. It is also caused by people using too short a piece part, only holding on the jaw tips.

I think you must have to take into account the people who are regrinding.

Unlike ourselves, who have tooling that do other jobs, maybe they only have the one chuck, and the ability to hold both large and small items is of paramount importance.
Myself, I normally give away my chucks and milling vices to needy people, after say 5 years general use, and buy new ones, a lot of people just can't afford to do such things, and every penny, to them, counts.

So as I said, account needs to be taken of how small an item needs to be held in the finished jaws.


John

 

[Ned Ludd]

Hi John,
Can't disagree with a word you wrote, :bow: but, we all have different experiences which goes to make life interesting.
I hope you pointed out the error of his ways, when using chuck key "extensions".
Ned

 

[camm-1]

Well there was a lot of replys to my problem.
I might have tighten a bit to hard somtime to.
:

 

[Niels Abildgaard]

Hi Guys,
Even hard jaws can be turned, with the right tips of course. Just recently I finally got around to taking out the bell mouthing of my Colchester 3 jaw chuck. The procedure I followed was 1, tighten a thick washer as far into the jaws as possible, remembering to tighten using the "prime" keyhole. 2, bore using a Titanium Carbide tipped boring bar, 3,once the jaws were cleaned enough, use a permanent marker to check, take out the jaws and gently grind away the small ridge where the washer was held.

In my case I had two benefits, first, it is now true to a thou or so even at 4-5 inches from the chuck. The second benefit was much easier parting, the bell mouth had prevented the chuck holding securely enough to part cleanly. I had wondered why I could part perfectly well using my 4 jaw but not the 3, doh.
Ned

Picture please or I buy something stupid?

 

[MachineTom]

Having purchased three chucks brand new, i will disagree with John that the jaws are factory ground flat. Each of those new chucks had arcs ground in the contact area of the jaws. I do have a 4" Burnerd that has the jaws ground flat, the others were 6 and 8 inches, by Bison and Rohm.

I

 

[Ned Ludd]

Hi Niels,
Picture of what, exactly?
Ned

 

[Niels Abildgaard]

Hello Nedd

It will be nice if You show us the boring bar and where to buy, steal or loan the inserts and what type ?
Some of the inserts I have tried ,do not like interrupted cuts in hard material.

 

[Ned Ludd]

Hi Niels,
The boring bar is a Sandvik S20S-CTFPR-11 and the inserts are Iscar TPGX 221-L.
They are both old purchases and, knowing how things have to change, they might not be made these days with the same numbers. I did see some on Fee-pay, but missed them. They are occasionally a few up for sale on the west side of the pond, so I might have to try there.
Strictly speaking the tips are wrong for the holder, as they have a hole in the centre and the bar is a clamp variety, but they still work well together.

Titanium Carbide tips generally give a far superior finish to the Tungsten ones but they are brittle, very brittle, so small cuts that impart small loadings are the order of the day on interrupted cuts. As far as turning and truing chuck jaws, you only expect to do it very infrequently so if it should take five minutes rather than two, what the heck!
From memory, I think I used about 450 (?)RPM and my slowest possible auto feed, having set the auto stop to just cut out before hitting the washer.

I hope this helps, but I fear that you might not be able to reproduce exactly my set up. With this in mind, I shall have to start thinking of what to replace my kit with when I run out of tips. Sumitomo do a useful looking tip, that I already have a holder for, so I shall have to experiment. It's the one at the bottom of the page. http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Indexable-Turning-Tools
Although it says it is for non-ferrous I would not let that stop anyone using it on whatever they like. I have had good results in the past with Sumitomo's Cermet tips so I am hopeful.

Ned
PS The Iscar tips that I have at the moment give a truly amazing finish when used for normal boring, and facing cuts can give a good impression of a polished and chromed finish.