Relationship between bowl diameter/height, and wall thickness

[Robert Marshall]

I've only been turning for a bit more than 3 years, so I'll readily admit I've got a lot to learn. Here's something I've been wondering about, lately.

Context: I've been turning some larger bowls lately, using some larger wood pieces that I'd been saving until I acquired a bowl coring tool. I've been coring large maple, box elder and camphor pieces, getting 3 to 4 bowls per log-half. And now I'm beginning to work on some of those rough core-outs. The bowls range from 18" (rough size), down through the teens.

At the same time, I've been trying to make my wall thickness (for the final turn) thinner than I've been doing up to now. I'm trying to get a good surface at 1/4" wall thickness, but having some problems.

What I've found is that I do better getting thin walls with the smaller (diameter and height) bowls. I think that is due to having greater vibration problems, as the bowl gets further out from the centerline of the lathe, and further out in front of the chuck. I've tried applying a gloved hand to the outside of the bowl while I'm shaving the inside wall thickness down, but one-handed tool control (the other being outside the bowl) is a challenge. I end up getting little "divots" along the inside surface of the bowl, particularly out close to the rim.

I think part of this is due to impatience. I'm thinning the walls down, working from rim to bottom of the bowl, in increments of an inch or two. That way, the thin portion being worked on is not extended far out from the remaining thick wall, and is thus more stable, as I work in. But sometimes I move on to the next inch or two before getting a really smooth final cut on the part just thinned, meaning I have to come back later, at a point that vibration can be really bad because the whole wall, rim to bottom, is thin.

Anyway, here are my questions:

1. Have others also experienced this problem, or is there something I should be doing to avoid it?
2. What thickness of wall do you try to achieve, for bowls of, say, 17" diameter, and is that different from wall thickness for a 12" bowl?

Thanks.

Robert

[Barry McFadden]

Try using a bowl steady to eliminate the vibrations when turning bowls ....especially the larger ones... http://www.leevalley.com/en/Wood/pag...330,49238&ap=1

[Leo Van Der Loo]

Yes to a bowl steady, and never go back to the thinner part, make sure the initial part is good before going on to the next part, plus make thicker walls on large bowls ⅜ to1/2” is not too thick for a large bowl IMO

[Peter Blair]

I agree with what has been said above and as said, would caution that a 'thin' large bowl is likely not going to last in use, the way a thicker walled one might! You might check Robo's bowls (video's) to get a good idea of what an ideal thickness is in relation to the diameter.

[Reed Gray]

I guess part of this depends on what you mean by thin. My larger bowls, I seldom turn over about 14 inches, I go down to maybe 1/4 to 5/16 thick. You can get vibration on a cored bowl because there is so little mass by the rim. Most of the time, I will turn the walls down to final thickness in steps of about 1 to 2 inches at a time, and then when I do the next step down, I blend in the start/stop point. I am pretty sure that higher speeds play a part in the warping/going oval because of wood grain orientation where the bowl wants to flex in the same direction that it warps, so if you go a little slower, Another part is putting too much pressure on the bevel as you go down. "The bevel should rub the wood, but the wood should not know it!" I generally use my hand instead of a steady rest, which I learned to do before I even knew what a steady rest was. Tool pressure = hand pressure, and if your hand is getting hot then you are pushing too hard. I am still trying to figure out how light my pressure needs to be, but on good days, I can start at the top of a bowl and go all the way to the bottom in one pass. Easy on 10 to 12 inch bowls, more difficult on bowls bigger than that. The hand you keep on the shaft does nothing other than just lay there, it does no pushing at all....

robo hippy

[Pat Scott]

Something that I learned from Glenn Lucas is to use a 3/8" gouge with 45 degree bevel for cutting the top of a bowl. Using a bigger gouge and/or with a blunter bevel (like 55 degrees) on the top of the bowl can cause chatter. With a 55 degree bevel the tool handle is farther away from your body when you start the cut, and you are more likely to push against the side wall in order to maintain bevel contact. A sharper bevel cuts cleaner as well. Once I went to a 3/8" 45-degree my problems went away.

I agree with others that a 1/4" wall thickness on a 15" bowl is not only hard to do but impractical. A lot depends on the density of the wood. For dense heavy woods you might need to shoot for 3/8" walls, and for lighter woods a 1/2" or little less still feels nice in the hands.

Yes you should size wall thickness depending on the size of the bowl. On a 17" bowl a 1/2" wall thickness is fine. People will see a big bowl and expect it to weigh more than a smaller bowl. On a 12" bowl a 3/8" wall thickness is fine. I make my 6" bowls with a 1/4" wall thickness. Don't get too hung up on wall thickness or thinking that you must make a bowl a certain thickness. The two most important things to me are a smooth flowing curve, and how does it feel (weight) in your hands. My wall thickness is not constant either - the rim is thicker and undercut, the wall thickness is thinner as the curve flows down the side and then gets a bit heavier as the curve flows into the base. If you haven't yet, watch a Glenn Lucas or Mike Mahoney video. I do make bowls with a constant wall thickness from rim to base, but a wider rim is easier to grab and pick up.

[Reed Gray]

I hope to get some play time with Glenn next March as I believe he is one of the demonstrators at the Oregon Woodturning Symposium. Stuart Batty was the first one I heard mention that the more acute nose, his is 40/40, takes less effort to push through the wood. After hearing that, I started paying attention to it and it seems to be true. However, for me, when doing finish cuts, it doesn't seem to matter as long as you can 'feel' the cut and not push too hard, which is trying to cut the wood faster than you should. This does cause vibration. I do like the 45/45 for the outside wall, and same for the inside, then a BOB tool for the transition and across the bottom. I have tried rim to bottom with 50 and 60 degree gouges, and it can be done, but patience is very important. Pushing too hard on the gouge and the bevel both contribute to vibration.

For my once turned bowls, especially madrone, a 1/4 inch thick wall is mandatory to prevent cracking and allows some nice warping...

robo hippy

[Frank Drew]

More important than thickness (within reason) is, IMO, ending up with a consistent wall thickness; nothing feels clunkier than a bowl that gets noticeably thicker towards the bottom.

I've never uses a bowl steady but frequent sharpenings can help prevent some vibration because you don't have to apply as much pressure as you would with a less sharp gouge.

[Frank Drew]

IMO, more important than thickness (within reason) is ending up with a consistent wall thickness; nothing feels clunkier than a bowl that gets noticeably thicker towards the bottom.

I've never uses a bowl steady but frequent sharpenings can help prevent some vibration because you don't have to apply as much pressure as you would with a less sharp gouge.