Twice turned bowls. Here's a theory.....what do you think?

[Odie]

For many years, I've been taking bowl blocks that have <14% MC, and finish turning them in one operation. This is very successfully done, especially if one pursues basic shapes. However, the degree of warp directly effects any successfully sculpted fine details.....the less warp there is, the better those fine details will present. Now, for the past 5 or so years, I've been twice turning everything.....even those with a very low 10% or 12% MC. I've found that low MC roughed bowls will still lose a small amount of weight during the seasoning process, and the overall time element for these bowls over their higher MC counterparts prior to stabilization, is significantly less because of it.

I can't say for certain if this is true, but it seems to me that those bowls with low MC, when twice turned, will warp less in the second turn than if I had finish turned them in one sitting. For someone like myself, who specializes in many turned details in my bowls, this is a great advantage. Even if the warp is less by a very small percentage, it's a great benefit, as well as a preference, when considering these desired multiple details.

There is nothing scientific about my conclusions here, but I'm interested to know if anyone else has come to the same conclusions that I have......?

-o-

 

[Larry Mercier]

Odie, I wish I had wisdom to share. But, I'm very interested in this thread. I am frequently impressed with the threads you start.

 

[Chris Lawrence]

It could also be movement from removing tension in the wood. Like when you resaw a board that is straight and after it is cut it starts to bow.

 

[Odie]

It could also be movement from removing tension in the wood. Like when you resaw a board that is straight and after it is cut it starts to bow.

Yes, there is always some amount of movement no matter what the process.

What I'm concerned with is the difference in degree of movement for a low MC roughed and seasoned bowl during the second turn, over the amount of warp in a similar, and respectively low MC piece of wood that doesn't go through the normal twice turned process. Is there an overall difference in the warp between the two? I'm beginning to think there is, even though the MC is close to equal between the two bowls in the final turning process. That difference is likely to be very small. Even so, that small difference is probably a significant consideration, when the bowl has many finely executed details.

-o-

 

[Tim Hunter]

The twice-turned bowl blank (ready to be finished-turned) will have less residual stress than an equally dry block.

Tim

 

[hockenbery]

The balls, platters, and goblets I have turned from dry wood do not tend to move out of round to the eye
If kept in a stable environment 40-60 Relative Humidity.

Balls, natural edge bowls, hollowforms, and goblets I have turned from wet wood move a lot as they dry.

 

[Dean Center]

Odie, here in the arid northern Rockies, 13% moisture content is not as dry as the wood is going to get, even over in the green side of the state. I've had kiln dried lumber move, and I believe it's dried down to 6%. Also, I've had blanks that have sat around for years move when turned. Every block of wood is different, of course, and it probably makes a difference if it's summer or winter.

With the precision you impart in your work, it's no surprise to me that giving wood time to rest, and maybe dry a little more, makes for a more stable canvas on which to work.

 

[Leo Van Der Loo]

Odie I never turn kiln dried blanks, it is always fresh green wood turned twice but for a few that do get finished turned in one go.

Now for most of my twice turned bowls that have sat for 10 or more years, the small amount that they do warp after twice turning does not make any difference, and yes they do change some.

Where I have done extra work to keep them from warping was/is on platters, I hate the wobble you get when these do warp, so I have turned some 3 times, with longer times between turning.

I got a couple pictures here from a Black Walnut crotch platter that is flat, turned originally from green wood and left drying for several years (longer than 10 years) and returned, left to relax as that is what I would call it, then glued a smaller piece of dry wood on it as a tenon after a couple years, then returned/trued that platter and put PTO on it, letting the PTO polymerize and then polish it, it is flat, or at least as flat as the surfaces it has sat on.

So yes dry wood returned will still move some, if that is important, you need to let it relax, as it is already dry, but there is still tension in it and given some time that will be reduced, I don't think there is a way around it then give it time.

Here is the trice turned platter, top side finished and the tenon is going to be removed here.


With the tenon removed the bottom is finish sanded and couple lines turned on edge and center.


The finished platter

 

[Gerald Lawrence]

Not sure what all the fuss is about.#1 "WOOD MOVES". Change environment check rule #1. Weather changes check rule #1. Look at it wrong check rule #1. Even metal an glass move but it is such a small amount the human eye will never sew it.

Now as to wood moving unless it is a machine part What is the big deal? As Leo said as long as the bowl does not rock Who cares? As long as the object does not move so much it jumps off the shelf Who cares?

Not saying that it is not a good thing to know but it really only needs a few precautionary cautions. Oh I did enjoy seeing the extent other turner go to because Wood Moves. Oh yeah I do some of that stuff.

 

[hughie]

It makes sense as you give it time to move and lose the stresses of machining the blank and what's left is the small seasonal movements that all wood species have. I alternately do it a little differently, choosing where possible stable species not known to move much and using well-seasoned dry blanks. Its been along time since I have turned and green wood.

 

[Doug Freeman]

I can't say for certain if this is true, but it seems to me that those bowls with low MC, when twice turned, will warp less in the second turn than if I had finish turned them in one sitting.

It’s fact, not theory, @odie. Even if the rough does not lose moisture, the large amount of material removed from blank to rough will cause movement as the rough sits for a day or more. How long (and how much) the “dry” rough will take to “stress relieve” is piece dependent - each one has different grain structure. Some will still move in the 2nd turn due to stress relief.

 

[Odie]

The twice-turned bowl blank (ready to be finished-turned) will have less residual stress than an equally dry block.

Tim

Yes.....this is a rule, Tim.....and, this is the main point of this thread. The object here, from my perspective, is to minimize the amount of warp because of the many details I include in my turnings. The less movement there is, the better these details present to the eye.....(whether, or not, the observer knows exactly why their senses are tweaked by less crisp details, vs more crisp details.)

Odie, here in the arid northern Rockies, 13% moisture content is not as dry as the wood is going to get

This is correct, Dean. To some degree, climate in the location you reside does have some amount of influence......but, not to be construed as the entire picture!

Now for most of my twice turned bowls that have sat for 10 or more years, the small amount that they do warp after twice turning does not make any difference,

Leo.... I can't speak for your experiences, but even a very small amount of warp can make a big difference in the results I am able to achieve.

As Leo said as long as the bowl does not rock Who cares?

I do Gerald....very much so!

Some will still move in the 2nd turn due to stress relief.

Hi Doug.....From my POV, ALL of them move to some degree on the second turning. The point of this thread is to minimize that.....even with KD or long term shelf life that have low residual MC from the gitgo.....they all benefit from twice turned results. It all depends on the turning style applied, and my turning style, with many finely executed details benefit more for me, than most other turners do for theirs.....

-o-

 

[Dave Bunge]

When you turn a bowl from a "dry" blank, I suspect that most of the movement of the resulting bowl is due to stress relief as others have commented.

However, I suspect some of the movement is caused by the blank not having uniform moisture content. Probably the most common situation is that the inside of the blank has higher moisture content than the outside (the blank has not finished drying). So Odie measures a blank at 14% moisture content. That means it is 14% at the point where the pins of the moisture meter were located. But the exact center is wetter and the very outside is drier. So when you cut your bowl, the outside walls of the bowl are slightly drier than the inside walls of the bowl. As the bowl sits and comes to equilibrium with the air around it, the inside shrinks a bit and distorts the bowl.

Changing humidity where you store the blank could also be a factor, even for very old blanks which "should" be dry. My basement shop is about 30% RH in the winter and about 55% RH in the summer. At the end of winter, the blank will be as dry as it gets. When spring comes, it starts to gain moisture again, from the outside in. So in spring and early summer, the outside of the blank will be wetter than the inside. At the end of summer, the process reverses and the blank starts to dry out again, so the outside becomes drier than the inside. .

This could even come into play for twice turned bowls, if they are turned from wet wood. When the rough turned bowl dries, it distorts. When you re-turn it, you cut more wood away from the end grain on the outside and more wood from the side grain on the inside. So if the moisture content was not completely uniform from inside to outside, you have the same situation again, where some of the newly cut surface is wetter and will shrink more.

I think this is minor factor, and honestly don't notice it in the stuff I make. But if what I wrote is actually a factor, the thing to do if you wanted to go directly from a dry blank to a stable bowl would be to keep the blank for a long time at a very consistent relative humidity. Or be pragmatic and twice (or thrice) turn it.

 

[Jonathan Silwones]

Oldie, I see better explanations than I can give, but over the last 20 years I have found that if I want a stable blank, I twice turn it, no matter how dry it seems to be. Thanks for bringing this up.

 

[robo hippy]

Well, any wood, other than petrified or 'stabilized' with polymers, is "Hydrotropic' which basically means it is a sponge, and will continue to move with changing humidity levels. A most interesting demonstration I saw at the Oregon Woodturning Symposium was Eric Lofstrom. If you ever get a chance, do see him, and I hope he will be at the nest AAW Symposium in Portland next year. His demonstration was on his boxes. He used to call them cloud boxes, and he did change the name, but I forgot it..... Anyway, the boxes are from straight grain wood, with a 1 inch tenon and recess. The recess in the lid is perfectly straight, and the tenon has a VERY gentle curve to it, with the middle of it matching perfectly with the recess diameter. If you pick the boxes up, there is a 3 to 5 second delay as the bottom drops off. His process for turning the box blanks is about a 6 month process. He rough turns the box blanks up to 6 times before finish turning them. We all know that wood moves, besides the humidity changes, as any bulk is removed. By taking this method, by the time he finish turns it, the wood is pretty much stable. I have yet to try this method, but that does seem to make a lot of sense. There was one box he had that he didn't pass around for us because 'just the temperature changes from handling' would change the fit of the lid'.

Other than that, I don't think you would want to spend 6 months for turning and returning your bowls. I do avoid any 'kill' dried wood. Air dried, vacuum kiln dried, or solar kiln dried woods all are much more stable. While properly kiln dried woods can function well, they are more interested in quantity rather than quality. Without exception, every kiln dried wood I have ripped for book matched panels will cup, twist and spring in all sorts of directions. Kiln dried wood, if you rip it on the table saw, you get dust. With the other methods, you get shavings.

robo hippy

 

[Brian Gustin]

Well, any wood, other than petrified or 'stabilized' with polymers, is "Hydrotropic' which basically means it is a sponge, and will continue to move with changing humidity levels.

Think the word you're looking for is Hygroscopic Just hadda pick on ya - first time I learned the term was at Ford factory school when they were discussing brake fluid (Which is also Hygroscopic) way back in the 1990's

 

[Gerald Lawrence]

Think the word you're looking for is Hygroscopic Just hadda pick on ya - first time I learned the term was at Ford factory school when they were discussing brake fluid (Which is also Hygroscopic) way back in the 1990's

not quite as descriptive but Hydrophilic works too.

 

[Odie]

When you turn a bowl from a "dry" blank, I suspect that most of the movement of the resulting bowl is due to stress relief as others have commented.

However, I suspect some of the movement is caused by the blank not having uniform moisture content. Probably the most common situation is that the inside of the blank has higher moisture content than the outside (the blank has not finished drying). So Odie measures a blank at 14% moisture content. That means it is 14% at the point where the pins of the moisture meter were located. But the exact center is wetter and the very outside is drier. So when you cut your bowl, the outside walls of the bowl are slightly drier than the inside walls of the bowl. As the bowl sits and comes to equilibrium with the air around it, the inside shrinks a bit and distorts the bowl.

Changing humidity where you store the blank could also be a factor, even for very old blanks which "should" be dry. My basement shop is about 30% RH in the winter and about 55% RH in the summer. At the end of winter, the blank will be as dry as it gets. When spring comes, it starts to gain moisture again, from the outside in. So in spring and early summer, the outside of the blank will be wetter than the inside. At the end of summer, the process reverses and the blank starts to dry out again, so the outside becomes drier than the inside. .

This could even come into play for twice turned bowls, if they are turned from wet wood. When the rough turned bowl dries, it distorts. When you re-turn it, you cut more wood away from the end grain on the outside and more wood from the side grain on the inside. So if the moisture content was not completely uniform from inside to outside, you have the same situation again, where some of the newly cut surface is wetter and will shrink more.

I think this is minor factor, and honestly don't notice it in the stuff I make. But if what I wrote is actually a factor, the thing to do if you wanted to go directly from a dry blank to a stable bowl would be to keep the blank for a long time at a very consistent relative humidity. Or be pragmatic and twice (or thrice) turn it.

Very good information to ponder here, Dave......and thank you very much for responding to this thread.

(@robo hippy , too! )

-o-

 

[Odie]

Odie, I wish I had wisdom to share. But, I'm very interested in this thread. I am frequently impressed with the threads you start.

Well Larry.......I think you and I both are benefitting from the useful information we've seen in this thread.....from several other's POV!

Keep the comments coming, guys.......you are making me put my "thinking cap" on!!!!!

-o-

 

[hughie]

I also find that species and where in the tree its cut from play a part. I have some white Cedar crotch blanks that is not fully seasoned and they wont be on the lathe for some time.
Where I live, fresh cut Redgum is considered to be volatile in its movement and so many avoid it, it just continually move far more than many other species whether thin or chunky. Some time ago I collected a stump of Redgum that had been cut down maybe 40 or 50 years previous and left sitting in the field and too my great delight its so well seasoned that I have carved a filigree piece from it and its held together intact for a couple of years, so age must be a strong factor as well.

 

[William Rogers]

Seems to be a couple of things that cause wood movement. Moisture and stress. With moisture the wood can move either by loosing moisture or gaining moisture. At some point the wood reaches an equilibrium with moisture and little can be done to change the small gains or loss. When that happens is difficult to determine. Stress is different although moisture may contribute to more or less movement. I have always been told for twice turned bowl to turn the wall thickness to 10% of the diameter. I think this was developed over time as to where the wood would dry without much cracking. Mainly to reduce the moisture, but not be of benefit to stress. Robo’s account of Eric Lofstrom’s process is certainly interesting to me. Maybe for the best shot at stability would be to do more than twice turned.

 

[Rusty Nesmith]

Not sure what all the fuss is about.#1 "WOOD MOVES". Change environment check rule #1. Weather changes check rule #1. Look at it wrong check rule #1. Even metal an glass move but it is such a small amount the human eye will never sew it.

Now as to wood moving unless it is a machine part What is the big deal? As Leo said as long as the bowl does not rock Who cares? As long as the object does not move so much it jumps off the shelf Who cares?

Not saying that it is not a good thing to know but it really only needs a few precautionary cautions. Oh I did enjoy seeing the extent other turner go to because Wood Moves. Oh yeah I do some of that stuff.

I care. I like everything I make to stay round and not oval or twisted.