Bowl wobble after bottom recess

[Kirwilli]

Hope someone can help me with this problem. After turning a recess and finishing the base of a bowl, when I fit the bowl to my nova chuck I often get a bad wobble. I have checked the spindle of my MC900 and all seems in order. I am trying to work with a minimum recess eg 4mm for a 250mm diam bowl. Any ideas.

Thank, Kirwilli

[cedar n silky]

Kirwilli, the problem could be a number of things. I have rarely found things to go back "dead perfect" once reversed. Have you tried "tweeking" it a few times to try and seat it square? Just set your tool rest nice and close, switch on the lathe slowly and just tuch a pencil on the "high" spot. Then loosen the jaws a touch, bump the high spot, and nip up the jaws and check again. You might have to do this a few times until it's pretty close.
Another option might be to check your jaws and ensure they are sitting flat on the chuck, and set up according to the instructions that came with the chuck.

[DJ’s Timber]

One thing I alway do when mounting bowls, is to take the chuck off the lathe and sit the bowl upside down on the bench, then put the chuck into the recess and push down and then tighten. 99% of the time it will be right.

[Red neck]

Hello Kirwilli,

I agree with Cedar n Silky. These problems are usually associated with re-chucking and not errors in the geometry of the lathe. Of course if you turned the outside on a faceplate and then chucked your workpiece on the Nova chuck, any differences between the faceplate and the chuck may show up in the process but this would be an unlikely scenario.

I would check to ensure that your dovetail jaws are attached in accordance with the instructions. One or two sets I have are numbered incorrectly (reverse order) and Teknatool issued a correction to this. More recently they have stopped using numbers and now use dots to indicate the jaw number and jaw slide position. I feel this is a backward step – maybe it is NZ literacy thing.

When attaching the jaws it is important to ensure they are located properly in the jaw slides to ensure the chuck functions accurately. Check there is no light between jaw slides and the add-on jaw segments or that one or more segments is/are not higher or lower than the rest.

Do you turn a foot or a recess? I have a preference for a foot as it offers more accurate re-chucking than a recess. It may also be turned ‘off’ using Cole jaws or a vacuum chuck or at least minimised to be less obtrusive..

If the foot is too deep you may find that there is a gap between the jaw face and bottom of the bowl. The foot needs to be cut accurately so that the face of the jaws rest against the bottom of the bowl providing an accurate register. I found the standard 50mm jaws that came with my chucks ineffective for bowl turning and bought a set of stepped jaws (among others). I also turn the foot just a few millimetres larger in diameter than the closed jaw diameter to avoid ‘grip’ marks if I leave the foot on.

[Hickory]

One thing I alway do when mounting bowls, is to take the chuck off the lathe and sit the bowl upside down on the bench, then put the chuck into the recess and push down and then tighten. 99% of the time it will be right.

My approach and it seems to center the bowl more easily, for no matter how you hold it, there seems to be a slight crook or slant.

As Cedar (in the Sky) has said. Tweeking is often necessary.

I usually allow for some material removal to regain "round" after rechucking.

[Kirwilli]

Thanks to everyone for their suggestions. The chuck in brand new out of the package a Supernova 2 and all is well. I believe that the suggestion of placing the bowel face down and fitting the chuck to the recess was the answer. I also rechucked and turned a deeper recess. It's fine now. Thanks again.

Kirwilli

[Alastair]

I have used the Nova chuck, (both types) for some years. The problem is usually due to not seating the recess fully when tightening.
Make sure your recess is cleanly cut, and at the right (70deg) angle. Don't oversand, as the more resistant endgrain will allow you to sand the recess oval.
Finally, on seating the recess on the jaws, I tighten lightly, and then give the piece a couple of firm thumps with the handle of my trusty bowl gouge, (or whatever else is blunt, heavy and to hand), while tightening up to the final tension. As you do this, the chuck will "give" with each thump, as the jaws seat properly in the recess, until snug.

You might be left with some marginal runout, usually from sanding, but the piece should be secure and running true.

A

[joe greiner]

All good suggestions. But I usually re-mount on a tenon, not a socket, because expansion mode may crack the bowl. For pressure while centering, I use the tailstock against the not-yet-excavated interior. Like Hickory said, allowance for re-turning is advisable, especially if there's any delay in the entire process. This can happen even after re-mounting and partial excavation, as warping continues and internal stresses are relieved by excavation.

Joe