How would you do this? (Chuck fitting)

Hello Tim,

How the *&%$£ do you lift it on and off the lathe? I thought my 8" was getting heavy ;-) Happy New Year.

GeoffH (The Pirate) Norfolk - UK not VA

Fraid not sir. All you've achieved is to machine the taper true to some redundant part of the body and that's not how they're made. The body is fairly irrelevant as long as all the important bits (taper, scroll etc) are machined true to each other off one datum.

What you want to do is put a big 4 jaw onto the lathe, clamp some large diameter bar (the biggest that will go down the spout of both chucks for the full length of the jaws) and indicate that dead true (or even better take a light cut over the bit that sticks out) and then clamp the chuck to be machined onto that with its own jaws. Then you know you're machining exactly true to how the chuck will hold something anyway plus you've already compensated for any wear error in the jaws if it's a 3 jaw. Now that's easy with a 3 jaw you want to machine, clamp and go. With a 4 jaw you have to clamp it on the bar and then further indicate the body true so you know the jaws are centred. Still, that's only another couple of minutes work and you're good to go.

If you're careful you should be able to resurrect the one you got a bit wrong by redoing it as above. It'll sit a bit further back on its taper but if you had a bit of clearance anyway, or can create some by taking a gnat's c*ck off the back, you'll be ok.

Simple really innit :) FWDITOT

-- Dave Baker Puma Race Engines

Actually the face of the body shouldn't be redundant, since it's quite normal to hold work against it, rather than always against the jaws. The outer rim of the body isn't important, to within a few thou, other than for maintaining balance but on a 4-jaw it's as good a datum as any for concentricity.

Er.... Read my post again. I don't have a 4-jaw to fit the lathe, all I have is a 52-year-old 3-jaw. I want to adapt this 4-jaw which I do have so that it does fit the lathe, to a better standard than I managed last time on a different lathe. What you suggest sounds good, superficially, if I were in a position to do it that way but I suspect that the play of the jaws on their tenons might mess it up in practice, because of the weight of the chuck. How would I check that, other than by indicating the body? It would probably be much better on a vertical axis where the weight is having the same efect on all four jaws.

Not neccessarily, see above.

No, I don't want to resurrect it, I want to make it fit a different lathe.

It might be if I were starting from a different point ;-)

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

With some difficulty, I can do it unaided but it's not a clever thing to do to my back. In due course when I get the lathe to its final destination I'll be setting up some lifting tackle.

And you.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Then use the 3 jaw and take a cut over the bar stock before putting the 4 jaw on it. Or use a different lathe. It really doesn't matter what chuck you use to hold the bar with.

Not a chance. The clamping and centralising power of jaws on 4 jaw chucks will be massively more than the weight of the chuck itself. You wouldn't be able to hold anything heavy on any lathe if moderate weight pulled the jaws out of alignment. If it bugs you then support some of the chuck's weight while you clamp it up.

-- Dave Baker Puma Race Engines

Might I suggest a couple of centres and a bit of bar?

Mark Rand TYGM

That's fair enough

No other lathe around here that'll take a 16" dia chuck. My 'little' CVA is 6 1/2" CH & no gap.

Well that was the only explanation I could come up with for my previous efforts being less than wonderful. Other than incompetence on my part, of course.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

I'm open to all sorts of suggestions. There might be a problem getting the boring bar in next to the central bar/arbor if it's big enough dia to support the weight of the chuck without flexing.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

I've played this game before when I wanted to adapt my 3 jaw chuck for holding work on the mill. I needed to mount the chuck on the mill so it was dead true. When I started looking into things carefully I found that none of the external surfaces of the chuck were really dead true to its axis on the lathe about its own taper. Of course they're not really needed to be. The o/d certainly not although you'd hope the front face wouldn't be too far out. The back face which I wanted to mount off is of course not even relevant to normal lathe use. Even if the factory did a pretty good job to start with things get dropped, pranged, move about and wear and tapers get rust and crap in them. In my case I could take the jaws out and put a very light cut over the various surfaces so they became true to the taper as it operated in situ. In your case the chuck doesn't even fit a lathe yet so your only true datum is the jaws themselves and that's why you have to use this method.

-- Dave Baker Puma Race Engines

On or around Sat, 30 Dec 2006 14:27:39 +0000, Tim Leech enlightened us thusly:

You've probably seen this and related pages...

If you don't mind slightly ruining the bit of bar, you can always turn it down a bit where you need clearance for the boring bar. I had assumed that the taper was about 3 1/2" to 4" diameter. a 2 1/2" bar should be more than stiff enough if no more than 2' long, shouldn't it?

If the three jaw really is too shagged to hold anything steady, what about turning up a 12" faceplate for the DSG on the CVA, then bolting a bit of bar on to it and turning it true on the DSG. A lot more work, but might be useful in the future.

Just thinking...

Mark Rand RTFM

No, it has a 2 1/4" parallel nose, about 1" long, behind which is a flange with 4 stud holes on 6 3/4" PCD. Other versions of the same lahe had standard D1-8 Camlock noses. I have seriously wondered about 'converting' it to the C8 spec. This is the same dimensions as D1-8 but with 4 studs instead of 6 camlock pins. The stud holes are identical to those for C8, the main work would be in shrinking a sleeve onto the parallel nose and machining/grinding a taper onto it. The big advantage to me would be it makes chucks much easier to change. Sliding maybe 50 kg of chuck onto a parallel shaft with a close fit is a bit tedious & time consuming. The centring of the chuck ought in theory to be more certain with a taper. Also in theory I could buy chuck adapter plates off the shelf, but the price they are at that size, I wouldn't be buying many :-(

What I'm doing now won't stop me changing to C8 spec in the future, it'll just mean boring the chucks out a second time.

Cheers Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

No, the 3-jaw isn't totally shagged, the jaws are quite good, it just needs to be tweaked to get the work centred - just a matter of trial & error on the three tightening points.

It suddenly hit me that, because of the nature of the spindle nose, I could clamp the chuck directly to that - which is exactly what I've done. Set it up with lots of pressure from the tailstock to make sure the jaw faces were all in good contact with the spindle end, then removed the tailstock & got the job done - carefully, otherwise there could be a risk of the hard jaws slipping on the hard spindle. Job done, now I need to get it onto the mill table & drill and tap for the studs. Not tonight, though!

Thanks to all for suggestions. I'll report back once the studs are in, as to how successful it has been.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

A result!

Mark Rand (spent the day making some 4" mandrels) RTFM

Yeah, but I don't yet know whether it's a good result

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Course it will be, negative thoughts Moriarity.

-- Regards,

John Stevenson Nottingham, England.

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