What on earth am I doing/is wrong?

[LancsRick]

Right, I refuse to believe that there's a patch of my garage where the laws of physics don't apply (although it would explain a lot of my projects!), so there's an issue somewhere that I'm failing to track down...

Coronet Major lathe with an Axminster Super Precision chuck on with O'Donnell jaws.

14" x 2" circular piece mounted via dovetail recess.

I keep getting wobble and a spiral pattern on the front face.

- Checked the bearing is snug on the lathe shaft
- Check the chuck isn't cross threaded
- Removed all chuck jaws, cleaned, refitted to ensure everything is true
- Chuck is absolutely tight to the workpiece, even with both hands I can't move the thing

and yet...

I seem to have a perpetual wobble that I can't get rid of. Gives a typical eccentric cut on the outside edge, and the front face demonstrates about 5 instances of a spiral ridge heading in to the centre of the workpiece. If I take extremely shallow cuts then I'm ok, but anything more and the wobble comes back. I'm therefore assuming that something is moving (physics!), but for the life of me I can't figure out what!

Any help appreciated.

Yours truly,

Frustrated in Lancashire

 

[Random Orbital Bob]

I'd check concentricity from the spindle outwards ie get the chuck off and test the spindle (at different speeds), then mount the chuck and repeat the tests so you can eliminate each variable as you progress. It sounds like it's moving in the jaws, is there a split in the wood that's difficult to spot around the chucking point???

Mount a second piece and see if it reoccurs which would point towards the lathe or chuck/jaws being the origin of the problem.

 

[chipmunk]

Have you been able to true-up the blank at all?
I assume you're using the 2" O'D's and not the 1" ;-)
Could there be a dense or hollow part in the blank making it off-balance?

Another possibility problem is your chucking point. Is the dovetail recess full depth - in which case the surrounding face around the dovetail needs to be flat.

Jon

 

[CHJ]

You say you are using O'Donnell jaws, what diameter socket are you using? and is it sized correctly for maximum support.

For something 14" diameter I would expect a socket in the order of 50mm to be the minimum and then in a firm wood and subjected to light cuts.

The leverage on the 14" X 2" thick blank may well be causing deflection on the jaw/socket unless it has a good rear shoulder providing lateral support.

 

[buzzby]

Have you made the recess too deep? The wood should not bottom on the jaws but rather the front face of the dovetails jaws should sit snug on the bottom of the recess.
Is the recess definitely flat?

I had a similar issue with a small chuck i was using via a thread adaptor. After ages of trying I noticed that the chuck was not seating properly and therefore was never truly flat against the spindle. Do you have a thread saver or something in between the chuck and spindle thread?

 

[CHJ]

Have you made the recess too deep? The wood should not bottom on the jaws but rather the front face of the dovetails jaws should sit snug on the bottom of the recess.
Is the recess definitely flat?

..

The above may be appropriate for extended nose jaws such as the O'Donnell jaws, but the complete opposite for standard dovetail jaws such as the Axminster C jaws where the socket needs to be deeper than the jaw spigot so that the work piece has maximum diameter lateral support of the jaw body.

O'Donnell jaws are not the best support for large diameter work that will be subjected to large lateral loads.

Above image from these large diameter platters using 50mm socket.

 

[Paul Hannaby]

The spiral patter points towards tool technique - you are probably putting too much pressure on the wood when you try to take bigger cuts.

 

[graduate_owner]

Obvious point, and probably not the cause, but are the chuck jaws correctly fitted? It is easy to get one not gripping when replacing them in the chuck body, then it will be out of alignment when you tighten up. Worth checkimg?

K

 

[buzzby]

Thanks for the correction Chas. I was getting it mixed up.

 

[woodpig]

I wouldn't use O'Donnell jaws for a bowl blank that big. "C" jaws would be more appropriate.

 

[LancsRick]

Wow, so many replies, thanks guys. To address all the ideas raised...

- No visible split but I'll mount something else to at least rule out the whole chuck assembly, it was getting late and I'd had enough by that point!
- I'm using the 2" OD's, and both the recess and surrounding area have been flattened to avoid that very issue
- With them being OD's, there's no great shoulder on the piece - I've got some C-Types on order with Axminster (any excuse! )
- Chuck is definitely mounted correctly on the spindle
- The recess is deeper than the OD dovetail yes, but I have to say that's the first time I've heard of that being an issue, I can't wrap my head around that one.
- I've confirmed the chuck jaws are correctly fitted, I've even taken them off their mounts and cleaned them!

Thanks again for the wealth of suggestions in here - given what I know about me/my setup and the comments in here I'm going to put money on it being a compounded effect of a) too small a chuck for such a large blank, and b) taking "normal" cuts when I should be treating something this size with a bit more of a delicate touch.

I'll investigate at the weekend, but given I've already turned the base (started off with a glue chuck and large gripper jaws), and started on the top surface I don't have anything to mount to now to change the base recess to take the C-Types when they come, so I think I'll just have to have a gentle touch to finish it off - as fate would have it this is going to be a cheeseboard and I'm sticking a 10" slate disc in the top so it's not the end of the world (but as ever, I'LL KNOW it's not a good finish underneath ).

Cheers guys .

 

[CHJ]

......
I'll investigate at the weekend, but given I've already turned the base (started off with a glue chuck and large gripper jaws), and started on the top surface I don't have anything to mount to now to change the base recess to take the C-Types when they come, so I think I'll just have to have a gentle touch to finish it off - as fate would have it this is going to be a cheeseboard and I'm sticking a 10" slate disc in the top so it's not the end of the world (but as ever, I'LL KNOW it's not a good finish underneath ).

Cheers guys .

This WIP might give you some ideas on holding such items note that in this case a holding socket to facilitate the cleaning up of the base is hidden under the tile.

 

[LancsRick]

Found my culprit!!

The mounts for the chuck jaws are detachable, and the markings are worn - I had the number 1 mount on the number 4 jaw, so even though the jaws were mounted correctly, they were slightly off centre.

Sanity restored!

(and new mounts ordered so I don't have to keep swapping the damn things around).

Yours faithfully,

Foolish in Lancashire.