Question about Turning wet wood using the 10% rule

[Brian Brown]

If you are turning wet wood to make a hollow form, does the 10% rule work just as good as with bowls and open forms? Do you turn a consistent 10% of the largest diameter, or work for 10% of the diameter of the area where you are cutting? In other words, if the largest area is 7 inches, and the smallest is 3 inches, do you turn the largest diameter area to .75" and the smaller diameter to .3'? I would think the consistent diameter would work better, but I have never tried this. What do you think.

[Fred Belknap]

Brian if I understand your question correctly. I do a lot of natural edge bowls that are longer than they are wide, the area that shrinks the most is the side grain and very little across the endgrain. I use the width across the block (sidegrain) to estimate the thickness. Depending on the type of wood and how dry it is also is a factor in guessing the wall thickness. The thicker they are the more prone to cracking. I have lost more rough out from cracking than from not being able to turn it because it is warped to much to round up. Sometimes they are a little thinner than I planned.

[Reed Gray]

Since hollow forms are usually turned end grain, you can get away with a lot less thickness, say if you want 1/4 inch thick walls, then leave them about 1/2 inch plus a little. If you are turning a side/bowl grain hollow form, then you have to use the 10% rule, which works for most woods.

robo hippy

[Steve Peterson]

I have always wondered the same thing. The 10% rule can only be an approximation at best. The part that bugs me is that the 10% rule does not account for wall thickness or grain direction. If you turn the 3" diameter portion to 0.3", then there is little margin for error if you want a 0.25" wall thickness.

Most woods shrink around 4-5% radial and 7-8% tangential, but some species can shrink as much as 12% tangential. A hollow form cut from the edge of the log (hollowing end grain) will have to deal with the difference between radial and tangential shrinkage. A natural edge bowl will have to deal with up to 12% tangential shrinkage in one direction and 0% shrinkage in the other direction. A 10" NE bowl could become a 10" by 8.8" oval before the second turning. It would need at least a 1.5" think wall to be made round again and have a 0.25" wall thickness.

Steve

[Dennis Ford]

A couple of points:
* When using the "10% rule"; I make the thickness 10% of the max diameter for the entire piece (sometimes slightly thinner in the bottom).
* For pieces 5" diameter or less, 10% does not leave much for final thickness; may need to add a little.
* Making the thickness much more than 10% is fine for stable woods but they do not need it, the extra thickness with unstable woods is likely to cause cracking problems.
* Careful drying (how careful depends on species and weather) is still necessary to prevent cracking issues.

End grain hollow forms centered on the pith will warp very little except around knots, thickness can be quite a bit less than 10%, this will reduce cracking problems.
Side grain hollow forms with relatively small openings will warp less than open bowl forms as the shape resists distortion, thickness can be less than 10% to reduce chances of cracking.

Why would you twice turn natural edge bowls? They do not look round anyway, what is wrong with an oval shape?

[robert baccus]

Very good advice in a nutshell.

[Fred Belknap]

Why would you twice turn natural edge bowls? They do not look round anyway, what is wrong with an oval shape?

I like my natural edge bowls to set on a flat surface without rocking in the wind. Some wood with thick bark such as walnut the bark will shrink more than the wood causing problems if turned thin when green. It may be a personal preference also.

[Dennis Ford]

I like my natural edge bowls to set on a flat surface without rocking in the wind. Some wood with thick bark such as walnut the bark will shrink more than the wood causing problems if turned thin when green. It may be a personal preference also.

That makes sense to me although I turn them green. I usually hollow inside the foot a little so that the rocking issue can be fixed with some sanding. Do you have trouble keeping the bark attached after drying?

[Fred Belknap]

Dennis
I don't usually have any problem keeping the bark on a blank. If it is loose when it is wet it won't stay on when dry. I cut my own trees and don't let them lay on the ground very long. When I turn walnut from wet/green to final thickness the bark sometimes comes loose on one end of the bowl. The bark shrinks at a different rate than the wood.