Vibration Hollowing Bowl

========================= Barry, I'd say it's not too unusual. you probably have some distortion from the drying it has already done. That, combined with the normal tendency for a bowl wall to vibrate when cutting near the rim, is not abnormal, and may be more or less pronounced depending on wood species, size and shape of the turning. If you can put your left hand against the back side of the bowl opposite the cut, (Or have someone else do it for you, the vibration should either go away or be greatly damped. If not, I'd start looking for a crack. FWIW

Ken Moon Webberville, TX.

"Barry N. Turner" wrote: (clip) Any ideas would be appreciated. Thanks. ^^^^^^^^^^^^^^^^^ All the ideas that have been offered seem valid. But no one has mentioned speed. Have you tried raising or lowering the speed, in order to get away from possible resonances?

Another comment, though. The wall thickness you have may be necessary after the bowl is dry, to restore roundness. I think it would be better to bag it and wait for slow drying to take place. As the bowl dries and distorts, it will be inevitable that you will experience uneven cutting. If you do this more than onece during the drying process, you will probably have the problem each time, so why not just wait, and suffer only once?

That's a pretty small tenon for a 10" bowl. Either Mike Mahoney or Stuart Batty (I don't recall which), at the AAW Symposium in Pasadena (2004) (where they recorded the Two Ways to Make a Bowl video), says that the tenon should be about 40% of the bowl diameter. That would be 4" on a 10" bowl.

Matt Heffron

a few things

- reverse the bowl against your chuck holding it in place with the tailstock.

- True up the tenon or cut a new larger one. I bet you have enough wood with the thickness but if all you have is the 50mm jaws on the super nova you may not. With larger bowls I use the power grip jaws.

- While it is reversed also round of the outside of the bowl so when you put the tenon back on the chuck it will be more balanced.

- Look for cracks. I find that little cracks in the wood can cause more vibration resonance. You can almost tell that something is not right when your bowl startes to sing. You can expect something to come flying off

- How fast are turning the bowl? Always start slow.

- Test your nova chuck by spinning it without anything mounted.

I've recently started putt> I put my roughed-out box elder bowl back on the lathe yesterday. Its about

Barry - it's not the chuck. several others have posted valid ideas as to cause, but you will almost always get resonance at some RPM with a bowl because you are making a bell and gee, bells ring.

I disagree with teh 40% rule - I've turned a platter that was about 24 inches across (asymetric, that's the long dimension) using a tennon that was a lot less than 10 inches (I'd say about 3 inches), no problems with the tennon, though I was unusually careful to not have the wings smack me - I'm not particularly fond of splattering my own blood on my nice black lathe.

As Leo suggested, you can be patient and let it dry. Or you can do what I do and cut to finished dimensions in one pass and then deal with any distortion bycalling it a "feature" - in that case, turn thin (1/4 inch or so) and declare victory.

To cut and avoid the vibrati>a few things

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will iam_ b_ No ble at msn daught com

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"Barry N. Turner" wrote in message news:IpkTf.1107$ snipped-for-privacy@bignews6.bellsouth.net...

OK, as others have mentioned, you're not supporting the piece in a way calculated to keep it from swaying. Gripping a tenon that has certainly distorted from drying, especially without a true reference face at right angles to the tenon, won't do it. If you buy a set of jaws that chew the tenon as you tighten you have all the problems above plus artifact from the original torn/crushed fibers to contend with. Derek has found a way to get around some problems of drying distortion. He re-trues the tenon and face to get full support and hold.

I do the same, but with the pin chuck, I just run a 1" Forstner down the narrowed (cross-grain drying distortion) hole I used when roughing the wet blank, put on the lathe, and do the outside. Easy, extremely close to the original turning axis, and when I'm ready to reverse, I have a circular mortise and a flat bottom inside for my expansion dovetail. BTW, that safety talk from the manufacturers is probably pretty well padded for legal purposes, so use the size mortise or tenon that brings success. It may get smaller as your proficiency improves. You might even do things that look dumb to prove you can do it at all.

Red oak, and you can bet the bottom is heavy to hold it from tipping. Last, the bowl has distorted, and until you get it back to circular, a task which will be difficult if not impossible without proper chucking, you'll be shaving the side grain and missing the end, which makes the balance worse, of course, as it shifts the center of mass outward. Your tool hits twice each revolution, but the effect diminishes toward the bottom of the bowl, because shrink is in proportion. You are also putting much less strain on the piece, as the force varies with the square of velocity, and your force is therefore much less the closer to center you go. Which is why, of course you're rotating as close to slow as your patience can support, else you exacerbate all the out-of-balance problems.

Speculation of the walls being too thin is just that. They're not going to move much until they get below 1/2" green or 3/8" dry and have some reasonable force placed on them by spinning too fast or "riding the bevel" too hard. . When you get close to that, it's time to resharpen or rehone, your preference, and peel easy to desired thickness.

Oh yes, boxelder has approximately the same drying characteristics as soft maple, so you can get to 20% MC from green in 160 above-freezing days according to FPL experimental data on _planks_ 1" thick. Means one tenth of that time through the end grain. Under winter rules, if you've got that blank open to the warm air, you're probably talking 4-6 weeks to ambient 6% , tops.

I think everyone's missing the real problem here. I'm guessing that by using a gouge with an ellsworth grind, you're using it much like a scraper. When you do this, you put a force on the edge of the bowl that is radial (outward). This complicates rounding because every time you cut, you're distorting the shape of the bowl. The vibration won't go away once it's round. It may even get worse because it's thinner and flexes more.

You need to use a tool in a way that puts the force axial (parallel to the axis) which will drastically reduce vibration near the edge of the bowl.

2 ways to do that, off the top of my head:

1. Use a regular gouge with a different grind in a cutting mode .
2. Use a scraper with a sharp corner to get the cut started near the edge and define the round circle. Do not push the tool in a radial direction, align it so you're cutting on the top edge of the bowl, cutting straight back toward the headstock, just as if your tool is a drill and you're going to push the drill into the bowl to make a perfectly round hole. Once you have a round rabbet half an inch or an inch deep (o two inches deep) you can use a gouge in a cutting mode (not the ellsworth grind in a scraping mode) to move deeper, rubbing the bevel on the already cut surface.

I have a couple gouges with a straight grind on them which can be used to do both cuts... the first cut I do with the very corner edge of the grind as if it were a skew, with the tool parallel to the lathe axis. Works like a charm. You get rounder circles and cleaner edges on the bowl, especially if it's a natural edge.

Once it's round, you can use the ellsworth grind to clean up the surface as long as you use a feather-light touch., but it may sometimes chatter, and you'll need to clean it up with sandpaper.

I think you're mis-characterizing the ellsworth grind here. As with any grind on any gouge (or any tool) you can scrape with the cutting edge, but the ellsworth gouge is definitely used in a cutting mode (riding the bevel) the majority of the time.

Jr.

Seems an Ellsworth grind allows cutting at many places on its edge depending on the presentation angle. Wouldn't be my choice, even if I could afford one however, because I'm a devotee of flatter forged gouges. They can be held firmly on the rest and present the same edge to the piece that a badly supported deep gouge cutting with its ears can do.

Some pictures of the action and the result at

for feeding your thoughts. #4 - Bevel Reference In shows a hogging-depth cut used to establish round for a finer pare in progress. Remember, the bevel doesn't guide on the surface as it was, but the surface as created. Make your entry cut under firm control, muscles locked and shift your weight as you roll in. Won't to a bit of good if the piece is allowed to move at will, of course.

Sometimes if you turn your gouge slightly steeper down into the bowl than you would normally for bevel rubbing, you can alleviate vibration of this type. This helps keep the bevel from bumping.

I personally grind a second bevel to slightly reduce this problem. Additionally, the gouge does a better job following tight radii on the inside of small bowls.

Derek

I think you are right on. I did just that tonight with another bowl, this time about 12" in diameter. I took the bowl walls down incrementally an inch or so at a time and had no problems with vibration at all. I took the walls to about 3/8". I'm still a little timid on thin walls. I generally turn slow, probably 700 to 900 RPM.

Barry

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