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 !
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GREEN TURNING BOWL WALL THICKNESS/EVENNESS
- Thread starter Bernie Kopfer
- Start date
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.
And then there's luck.
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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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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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The thicker areas are more likely to crack, not due to retained stresses but due to the stress caused by shrinkage during the drying process.
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
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.
I've heard the round the corners advice, but sometimes I forget. I haven't had splits in those. (yet)
I'm thinking some pieces of wood just don't want to become bowls.
I'm thinking some pieces of wood just don't want to become bowls.