Chuck Problems

[drillbit]

I have a Jet 1014 (midi/mini) lathe and an 80mm axminster chuck with these jaws which grip down to about 40mm.

I was trying to turn a goblet from a piece of square soft pine (practice) about 100mm wide x 200mm long. So I roughed it into a cylinder, and cut a spigot on one end to fit the jaws of the chuck.

First problem - as soon as I have chucked the piece up, I then need to cut another centimer or so off the cylinder to get it balanced again. Is that normal? I technically have a perfect cylinder, with a perfectly cylindrical spigot on one end, and the jaws of the chuck should technically push it into a central position when gripping it, so I would expect it to be pretty much in the same position and pretty much balanced.

Second problem - when trying to hollow the unsupported end, the piece keeps suddenly shifting, or worse, flying off. No wonder they call it a 'chuck'. I have it well tightened, the spigot is as deep as it can be for this size of jaw, so I can't see how to make it any more secure. I've seen plenty of youtube videos of people doing it this way, so I am a bit stumped. The pine is quite soft, but then, I would have thought this would mean the jaws grip tighter if anything.

Any light anyone can shed on what I might be doing wrong, I'd be very grateful.

 

[Wood spoiler]

My take on your questions is:

When you put into the chuck are you using the tailstock to centre?

If this doesn't make sense, what I mean is if you loosely place in the chuck and bring the tailstock up and bring into the centre, then tighten your chuck and you should be centred perfectly.

It is useful to keep the tailsock up as long as you are able on whatever project you are working on.

As to the flying off it is usually down to cutting technique and trying to take too big a cut or you are using pulling cuts which is not good physics. You should be cutting from outside inwards. Smaller cuts keeping the bevel rubbing.

 

[jumps]

Agree Colin's points 100% and would add -

1. the soft nature of the wood can definitely cause, or contribute to, the resultant 'off-centre' because each jaw can indent the spigot to varying degrees due to variation in this softness.
2. how are you arriving at your perfectly centered spigot/cylinder? Does your lathe have a moving headstock (rotating) and is this absolutely aligned to the tailstock?

My money would be on No 1 being the issue.

 

[mikec]

Third option:

Unlikely, but have you taken the jaws out of the scroll. If they are back in the wrong order they may be offcentre.

The jaws should move to the centre equally. Is the offset greater at the tailstock than the chuck?


Hope you find a solution

mikec

 

[DaveyP]

And don't forget that the jaws only form a perfect circle to match your 'perfect circle' at only one exact circumference, any more or any less will cause the problem.

Sure the jaws move together.. but the curve of the jaws is fixed and its only an issue if changing the method of clamping.

So just rough a spigot first and then transfer to the chuck for the rest of the steps.

 

[drillbit]

Thanks for all the ideas. Lots of things to explore here.

@woodspoiler - that's a great idea about using the tailstock to ensure the spigot is in 'square'. I was just trying to ensure the shoulders were flush with the jaws, which doesn't seem to work. I will try this tonight.

On cutting from outside inwards. I read that while I was trying to find something about this on google. But the weird thing is, I was finding that doing it this way was knocking the piece skewed when I tried it, meaning I had to loosen, reposition and re-tighten. As long as I stayed in the middle, it was ok, but as soon as I applied any pressure to the outside of the piece, the jaws would lose their grip. Perhaps it's my tools not being sharp enough? Although I think I am getting them quite sharp now.

Jumps - I have checked my tailstock / headstock and looks good. But what you say about the jaws gripping unevenly makes a lot of sense. Would I be better off with a screw chuck? I have one to fit the jaws, but I haven't had any success with it so far - just seems to unscrew when I try turning with it....

Mikec - I am pretty sure I have the jaws in correctly but I will double check. How do you mean about the offset?

Thanks again all.

 

[CHJ]

Also make sure that your spigot is sized correctly in depth to provide maximum support by having the correct clearance to allow the jaws to pull the work piece up against the Jaw Face.

If you are trying to use soft wood and needing maximum wood stability of grip try soaking the spigot or recess in thin CA glue to harden it before clamping.

Alternate method:-
Turn a parallel spigot on your workpiece with a good right angled shoulder.
Mount some scrap hard wood in your chuck, turn a suitable straight sided socket in the hard wood to take the spigot.
Glue your piece into the hard wood socket with CA using the tailstock to ensure alignment and apply pressure whilst mounting.

 

[jumps]

Jumps - I have checked my tailstock / headstock and looks good. But what you say about the jaws gripping unevenly makes a lot of sense. Would I be better off with a screw chuck? I have one to fit the jaws, but I haven't had any success with it so far - just seems to unscrew when I try turning with it....

personally I am all for a simple life so I go the route of DaveyP - square an end between centres, carefully cut a spigot as near to the optimum diameter for the jaws as possible ie where they will grip as a circle, and with a matching bevel if they have one then switch to chuck and tail support for all the cutting. If I have to remove the piece from the chuck at all I mark the No1 jaw on the piece to re-locate to the same point. However, I try to avoid removing it with soft wood because it's impossible to re-pressure the same (but having the maximum chuck face in contact with the wood reduces this issue)

 

[drillbit]

Ahh ok Chas. Thanks for that picture. I have been trying to get the spigot the exact size of the depth of the jaws, and if anything, erring towards being larger than smaller, thinking the bigger the spigot the better the grip. Looks like I couldn't be more wrong. This could make a big difference.

Jumps that is a good tip about marking the first jaw on the piece. I will remember that, ta.