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GREEN TURNING BOWL WALL THICKNESS/EVENNESS

B

Bernie Kopfer

Joined
Mar 8, 2021
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Location
Wenatchee, WA
The standard advice about wall thickness is 1/10 bowl diameter, which I’ve used for years with good success. The other standard advice about wall thickness is that it should be uniform everywhere on the green wood bowl, purportedly to reduce cracking by minimizing internal stresses in the wood. My question is, does uneven drying (due to nonuniform wall thickness) create retained internal stresses that upon second turning results in cracks showing up? I realize this question is going to be answered largely based on personal experiences. But that is a purpose of this forum to provide us with a place to share experiences and observations that provide answers to life’s vexing turning questions !
 
I don’t know the answer but I’ve also read, in addition to 10% wall thickness, rounding the edges is important also. So many factors determine cracking—type of wood, the humidity of environment, moisture content of the wood, what part of the tree the blank came from, type of sealant, other factors I’m not mentioning or aware of.
 
Alan Weinberg said:
I don’t know the answer but I’ve also read, in addition to 10% wall thickness, rounding the edges is important also. So many factors determine cracking—type of wood, the humidity of environment, moisture content of the wood, what part of the tree the blank came from, type of sealant, other factors I’m not mentioning or aware of.
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And then there's luck.
 
Bernie Kopfer said:
My question is, does uneven drying (due to nonuniform wall thickness) create retained internal stresses that upon second turning results in cracks showing up?
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I don’t recall ever having a bowl crack after drying but then I got into trying for even walls early on to include the tenon so the tenon counts in the thickness of the bottom.

More problematic is trying to turn a bowl a second time that was way off center on the grain so that it warps catywompus and you can’t get a second turning with a 2/4” wall.
 
Wood types can vary widely on their axial to radial shrinkage rates, so no one formula applies universally.

With green turning cross grain bowls I turn thinner towards the base as that is the area that has the least differential in shrinkage and it matters less about the warping in that area.

In the woods that I turn the lip area of bowls are the most vulnerable area for splitting while drying, so I only turn them as thick as the warping will allow if I'm going to 2nd turn them. Thinner cross sections allow for more warping without splitting, but getting to know your wood species avoids not leaving enough wood to 2nd turn.

I've found that ripping down the pith line of the log almost immediately the tree has come down and cut into lengths is the most critical step in avoiding cracks forming at what will be the lip of bowls and platters. I then immediately bag in plastic to slow down shrinkage until I can green turn. I no longer use sealers.
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Well, cracks happen to relieve stress. Stress areas are created by uneven wall thickness, and sharp edges. ALWAYS round over the rims, inside and outside, and the tenon. Some woods like madrone and sycamore have such high water contents that they can not be dried prior to turning the second time. With sycamore, at least it is predictable. With madrone, you never know how it will move.... "It looks like a potato chip!" Boiling is an option for some woods, but too much work for me, and I like the warped shapes of madrone.

robo hippy
 
robo hippy said:
Well, cracks happen to relieve stress. Stress areas are created by uneven wall thickness, and sharp edges. ALWAYS round over the rims, inside and outside, and the tenon. Some woods like madrone and sycamore have such high water contents that they can not be dried prior to turning the second time. With sycamore, at least it is predictable. With madrone, you never know how it will move.... "It looks like a potato chip!" Boiling is an option for some woods, but too much work for me, and I like the warped shapes of madrone.

robo hippy
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This is interesting. Do you round these over before turning the rest, or is this more of just a "final steps" sort of thing? I'm turning a piece of holly right now. It started out fine, but as I'm turning more, I have noticed three thin cracks on one side. I'm just on the outside right now. It has a right angle corner at the rim right now, although it curves down at the bottom. I don't know if the cracks were already there, and just inside the wood, entirely possible. I have some very light colored "Goodfilla" wood filler that should at least fill these three cracks on the outside. I would like to avoid any more issues once I flip to the inside, though.
 
hockenbery said:
I don’t recall ever having a bowl crack after drying but then I got into trying for even walls early on to include the tenon so the tenon counts in the thickness of the bottom.

More problematic is trying to turn a bowl a second time that was way off center on the grain so that it warps catywompus and you can’t get a second turning with a 2/4” wall.
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I watched a Glenn Lucas video where he measured his once-turned bowl and found out his prior center point was now off to one side by 1/4" or so. I had never considered that. He found the new center and returned the tenon.
 
Alan Weinberg said:
I don’t know the answer but I’ve also read, in addition to 10% wall thickness, rounding the edges is important also. So many factors determine cracking—type of wood, the humidity of environment, moisture content of the wood, what part of the tree the blank came from, type of sealant, other factors I’m not mentioning or aware of.
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Agree on the rounded/chamfered edges.
 
Gotta say, I only chamfer edges maybe 10% of the time when I green turn bowls and I see no difference in the rate of cracking. I agree getting a consistent wall thickness is key, and the points made by others of knowing your timber is critical. I turned a bunch of very wet American Elm and did the normal 10% thickness, only to find that it moved so much that a number of them really pushed the ability to get them round for a second turning.
 
Every one knows about keeping an even wall thickness. So, what happens when you get to the rim of the bowl. A sharp edge, besides wanting to slice you to the bone, will offer a weak point in the wall thickness area. I would put at least a 1/8 inch radius on the edges. I am pretty sure that I came up with the stretch film wrap for bowl rims. I use it on most of my once turned bowls. This can also help on most bowls, except for any that have a lot of sugar in them like maple. It will mold under the plastic. I doubt it would do much for twice turned bowls which are generally sealed before setting aside for drying. I would still round over the edges.

robo hippy
 
Paul Paukstelis said:
only chamfer edges maybe 10% of the time when
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I chamfer the edge of the rim and teach students to do it as a safety precaution.
That sharp edge will cut even when the wood isn’t spinning.
Takes a fraction of a second and becomes second nature.

You would never leave a cutting edge knifelike on a finished utility bowl why leave it on a rough turned bowl you need to handle.
 
hockenbery said:
I chamfer the edge of the rim and teach students to do it as a safety precaution.
That sharp edge will cut even when the wood isn’t spinning.
Takes a fraction of a second and becomes second nature.

You would never leave a cutting edge knifelike on a finished utility bowl why leave it on a rough turned bowl you need to handle.
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Everyone is welcome to their own ways of doing things, and depending on what I am going for, I'll remove a knife edge one way or another.
My point is that I think, from my observations, that rounding over or putting on an intentional chamfer is not a determinant if a once turned bowl will crack.
 
Paul Paukstelis said:
Everyone is welcome to their own ways of doing things, and depending on what I am going for, I'll remove a knife edge one way or another.
My point is that I think, from my observations, that rounding over or putting on an intentional chamfer is not a determinant if a once turned bowl will crack.
Click to expand...

It would have to do with the tensions in the wood, right? I think it was back in 2020, I watched a video by Kent Weakly, where he explained how to turn the inside of a bowl from the outside in. Up to that point, I often turned away all the wood in the middle, then started turning near the wall. The problem I always had, was once you started getting into the bowl a ways, I couldn't go back up towards the rim to turn any more, as the shape had changed, and was no longer true. Weakly's approach was to turn near that rim first and leave a bulk of wood near the center, and MAKE SURE you turned it to exactly the thickness you wanted, and made sure you turned away any ridges or valleys, before continuing on. Only once you reached deeper into the bowl, would some of the bulk of wood in the center be turned down a bit, but not entirely. He would progressively remove small amounts of that bulk of wood in the center, until he had turned the walls of the bowl to his optimal preferences, making sure they were the right thickness through and through, and only working on a section at a time, never returning closer to the rim after completing that...band, you could say. As he removed more and more of the bulk of the wood in the center, the tension in the wood was released, which is why the previously turned walls of the bowl could, and often would, change slightly, preventing any chance of returning to those sections later to correct any previously uncorrected imperfections.

He turns certain kinds of wood, and I don't know if the kind of tension that exists in the woods he turns most often, are always present to the same degree, in every wood. However, I could see how tensions in the wood, could lead to issues during drying, if they are not managed properly, and perhaps for some woods, rounding over corners could release tensions that could lead to cracking. It may also not necessarily be the type of wood, but maybe the cut of wood? There are many different kinds of cuts of wood, right? I know of plain, quarter, rift, and live sawn wood cuts, and I suspect that each of those would have different kinds of tension in the wood. So maybe chamfering works for some cuts, and not for others.

In any case, I try to take into account the tensions in the wood when turning these days, and try to make sure I shape things optimally as I go, so I can avoid having to return to previously turned wood later, as you never know if you'll be able to or not (at least IME.)
 
I think the term is a “stress riser.” The sharp edge, being just a few wood fibers wide isn’t strong. It’s very easy to start a minuscule crack. Maybe those few fibers, multiplied by the number of these fibers around the rim, are just a bit weak or have a natural separation already. It could be microscopic but there, none the less. Those imperfections will run, just like nylons or a sheet of paper, especially once adding the stress of drying. One might stop the run/crack with ca glue (the ladies used to use nail polish to try to stop the run). Either way, you’re now into crack remediation instead of prevention. A simple blunting of that edge is really all it takes to lessen the load. Those few fibers are now a hundred or more. Stronger by orders of magnitude.
 
I have donated a fair amount of blood over my years as a woodworker. By far, the biggest cuts have been from sharp edges of wood. This includes hand planed edges, drum sander edges, and bowl edges. I have learned to "avoid" those most of the time now days.

robo hippy
 
I tend to leave the bowl rim a little thicker than the base of the bowl.....so, why is it a "rule" to keep the thickness even throughout the bowl? My theory is the moisture travels from the base to the rim before expelling, so it takes less time to stabilize if there is less volume of wood at the base.

1/10 the diameter is a good starting place for a beginner, but I usually go a little thicker. This means the seasoning process will take longer, but overall, the warping is less. I probably have around 50 bowls seasoning at any one time, so adding some time to the seasoning process is not a big deal for me...because I always have at least that many bowls ready for the 2nd turn.

Always take the sharp edges off the corners of intersecting planes, but sometimes I forget to do this. Even so, I've never seen a bowl crack because of the sharp edge. For safety's sake it should be done.

=o=
 
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