excentric?

Re: excentric?

Dennis in Southern Oregon

Unless you re-chucked it between the fiinal turnings of the inside and outside of the lip, equipment should not be a problem. However, chuck jaws can get out alignment such that the center of their grip is not the same as the spin axes of the lathe. Small errors generally are not a problem if you are careful to re-chuck such that the work piece is rotated the same way relative to the jaw numbers. However, given the non-uniformity of wood, small differences in how you re-tighten the chuck each time may cause a problem with when dealing with thin-walled items. My suggestion is to plan the do the last bit of shaping in the vicinity of the lip at the very last without any re-chucking.

Re: excentric?

john lucas

Could also be the chuck is slipping slightly. Tighten it often. Also it may be gripping unevenly. Make your tenon short enough that the blank sits on top of the jaws as well as gripping from the sides. this will keep it from rocking and possibly coming loose. another good idea is to locate the turning in the jaws so that there is an equal amount of side grain and end grain on each jaw. An easy way to do this is to draw a line directly across the side grain and then one that goes from end grain to end grain. Then put the points of this X on the gap between the jaws. This will give you equal wood compression on each jaw and won't pull it off center as much.

Re: excentric?

jorge

Dennis,

I did re-chuck once to try to figure what was causing the uneven thickness. Normally I don't. I was using the Oneway and one of the jaws looked a little out of place, had to take the jaws out and then the slides, completely out and remount them, they always ended with one or two in a different place until I found a sequence where all came in evenly, then remounted the jaws, it looked even, then back to the goblet and finished. Alas, did not mark it because the Oneway jaws have no numbers, I always use the edge of #1. Yes I must re-tighten often.

Thank you!

Re: excentric?

jorge

John, for the second I used the Supernova 2 chuck with the tower jaws, the opposite train of thought than your suggestion of shortening the tenon, and yes seated properly in the lips of the jaws, never bottoming, the rationale was that a longer tenon should be less prone to shift.. I never tried to position the jaws according with the side grain and end grain, I will do that in the next.

This second one came a lot more even, But still thicker in one side, I will remember to tighten often. I am hollowing with just a round and a flat scrapers, as always did before, and finished with a clean up pass with the Hunter No 5, my favorite finishing tool, alas! arthritis got my hands very coarse and now if I try the No 5, inevitably I get a catch, sometimes too deep, I cannot have catches in a goblet, so now I am doing the clean up by sanding. And yes I must re tighten often but not over tight.

Thank you!

Re: excentric?

Gary clarke

If all else fails use a glue block.

Re: excentric?

Peter Hyde

First up use dovetail jaws not the grain crushing Oneway serrated jaws. Try putting the jaws on so that there are no gaps when the jaws are closed as already suggested. By a cheap engraving tool and number the jaws to match the numbers on the slides. Open up the jaws to the point where they make a perfect circle. This is the maximum grip position. Turn a disk to fit that is thinner than the depth of the jaws. Then with the disk gripped firmly use a fine file to dress the face and gripping bevel sides of the jaws using a slow lathe speed. This will true the whole assembly and make sure everything is concentric. Sometimes there is slack in the slides and that can only be fixed with new slides.Always mark the jaw positions on the tenon so you can get back to the exact same place if you need to re-chuck at any time.

This effect can also be the result of wood with tension in it. So if it's a branch then the tension and compression is already there. So turn the outside and inside at the same time. Remove wood from outside, remove wood from inside, hopefully this will relieve the tension. The same thing will happen if the wood is not equally dry. So turn the outside and apply a finish or sealer before you do the inside.

Tightening the chuck and stuff

John K Jordan

Dry wood, I hope? From the same batch? Reaction wood? If there are internal stresses even dry wood can warp while turning, especially after hollowing. One way to test is to hollow to a thick wall, let sit for a few hours or overnight, then finish turn. (I always do this for boxes.) However, it seems to me stresses would have to be extreme for such eccentricity.

Beside that, it might help if you list exactly the steps you are using to turn the goblet. Do you usually rechuck?

Rechucking: I have never once rechucked a goblet. I make goblets and similar end-grain things like this:

- chuck a short dovetailed tenon at the base,

- shape and finish turn the outside of the cup

- hollow and finish-turn the inside, smooth/sand,

- turn the stem, transition, and base, smooth/sand

- part off if needed,

- reverse and jam chuck with tailstock support to finish-turn the bottom

Chuck tightening: I generally don't retighten during turning or need to (using good, dry wood.)

I think the need to retighten may be from backlash in the gears due to normal manufacturing tolerances: when you tighten the first pinion gear, friction at the teeth, axle pin, and race holds its teeth tight against the teeth of the larger gear on the scroll plate in the "tightening" direction. But the second pinion is left with the teeth in contact on the wrong side, in the "loosening" direction (since the large gear is driving the second pinion instead of the second pinion driving the large gear.) This play can cause the large gear on the scroll plate (and the jaws) to quickly loosen a bit.

Instead, I tighten a different way than some:

- Tighten the first socket moderately (not with a lot of force).

- Immediately rotate to the opposing socket and tighten again with the same amount of force. You will almost always find it will tighten a little more.

- Then rotate on around to the first socket and tighten again.

- Repeat.

- After several cycles you will feel that no more tightening is even possible without increasing the force dramatically. This will happen quickly on hard, dense woods such as ebony and may take a few turns on softer wood such as cherry. At this point I usually go around at least once more with a little more force just to check.

I heard a wise man once say "tighten the chuck in all six places." And a surgeon told me she was taught in medical school to tighten exactly like that when chucking drills and cutters. I use the same method now on drill press, mill, metal lathe, etc.

Unless the wood is punky, soft, or wet, it should be as tight from the start as it will be later. Try it.

JKJ

Thanks Peter!

Re: Tightening the chuck and stuff

jorge

Thank you John,

It is bone dry Maine cherry, left from a bed I build in 2009, the owner did not like it, colorless she said, so I bought some from Pennsylvania and she was happy.

It is extremely hard and does not make shavings, it just crumbles, I will buy a new batch.

Love your method of tightening, I will remember and use it !

The second goblet came out nice and even, so the cause was the Stronghold chuck with a slide that did not closed all the way, now is fixed. I did use the Supernova II chuck with the tower jaws because it closed very smooth and tight, and I thought the longer tenon would help t prevent shift. The new goblet is fine,

The bowl is more like the original she likes, but it is hard to sand the inside; I will go with a wider and shorter bowl and a longer stem to make holding it more comfortable.

Re: Tightening the chuck and stuff

John K Jordan

>>>It is extremely hard and does not make shavings, it just crumbles,...

Yikes, even on the outside, cutting with a sharp tool? That sounds more like a sharpness problem. Or scraping instead of cutting.

I'm glad they are coming out concentric!

JKJ

Re: Tightening the chuck and stuff

Jim Barbour

John, when I read your chuck tightening approach I had to chuckle. This morning I finally got fed up with the bit of run-out in my drill press. Dismounted the chick from the taper and the taper from the spindle. Checked the spindle, cleaned and inserted the taper; checked the taper, cleaned and attached the chuck; checked the chuck - no run-out anywhere. Chucked in a bit and there it was again. Tried several bits with the same result. Then I remembered my high school shop teacher admonishing us to tighten one jaw a bit, then the next a bit, then the next, etc., never putting much additional pressure on each successive jaw. Tried it and the run-out was gone. Two hours later I read your note. The Gods must be speaking.

chuck and chuckle

John K Jordan

Ooo, when they speak, we shall heed!

My chucking experiences have been improved with this method so I try to remember to show it at demos. Seems so obvious once you do it once and feel the effect, especially with wood.

BTW, smiled when I read your note! I have a tiny chuck that came with a little bitty Unimat metal lathe bought used. The runout was horrible making drilling with small bits impossible (the reason I wanted to use it). Tightening as described didn't help so I suspected internal wear. Like you, I finally got fed up with it and disassembled the entire chuck, removed and cleaned the jaws, springs, bearing surfaces, everything. Polished it inside and out, lubed and reassembled. Yea! It is now perfect! This chuck has a cylindrical shaft for mounting that I can grip with the larger chuck in the drill press or lathe. I used it immediately to some .026" holes into ebony end grain.

Life is good.

JKJ