Warpage

[Ray Bell]

Hi Folks, most of the bowls I turn I make glass lids for, which for this question is no different than wood lids. Occasionally the bowl top will warp into an oval where the lid will no longer fit. This generally happens while still turning, but has also happened long after the bowl and lid have been fitted. I have always wondered the cause of this MC, type of wood, bowl top design, or maybe a combination of all of the above. Any answers? any and all answers are greatly appreciated, thank you.

[Mike Cruz]

Ray, warpage, to my understanding, is caused by moisture content either increasing or decreasing. So, either when you finish turn the wood, it is not totally dry...and it dries over time, making it shrink in one direction more than another. Or, you are turning very very dry wood, that over time, absorbs moisture from the air (high humidity climates), swelling the lid in one direction more that the other.

If the wood you are finish turning has "stabilized" (moisture content wise), then it should no long "warp/move". That said, wood moves, it pretty much always will depending on the moisture in the air. Wood will expand and contract up to about 1/4" per 12" across the grain in flat work scenarios. Over time, the wood tends to stabilize and not move as much. But as any flat worker knows, you MUST account for movement in your construction, because NO method of "restricting" the wood from moving will work. Wood movement will sheer the heads off bolts, buckle the wood, or split the wood...something's gotta give. It is like water flow outside...you can't "stop" water flow, but you can direct it.

[Ray Bell]

Thanks Mike, sounds reasonable.

[Dennis Ford]

In addition to moisture content, internal stresses in the wood can cause you problems. I would suggest that you avoid limb wood for lidded bowls and use wood from the trunk section.

[Fred Belknap]

Mike pretty much hit the nail on the head about warpage. I have considered making lids out of segmented pieces to try to control the warpage, it might work but the bowl/hollow form will warp. I don't do very many lids over two or three inches anymore. I suspect that some internal stress can be relieved by boiling.

[Prashun Patel]

I also find trunk stock to be more stable than limb stock.

Another huge factor is grain orientation. Bowls in a QS orientation tend to hold their form for me much better then pith out or pith in orientations. Even green turned bowls do well.

[Reed Gray]

Wood moves! Like a sponge, when humidity goes up, the moisture content in the wood goes up, and it swells. When humidity goes down, moisture content in the wood goes down and it shrinks. The only way to stabilize any wood is that polymer impregnation method which is expensive. About the only way to get around this type of lid/container movement is to make both out of the same chunk of wood and matching grain orientation so when they move, hopefully them move at the same rate, and while you may not be able to spin the lid, it will still go off and on.

robo hippy

[Ray Bell]

Thanks all, good ideas, and answers from everybody. Since the wood is probably
going to move some no matter what maybe one good idea is not to fit the lid real tight.
Leave a little wiggle room.

[Jeff Nicol]

Ray, Mike laid it out as well as anyone and just like old hardwood floors will buckle in the summer and shrink to having gaps in the winter, wood never dies it just stops growing rings.

Rough out in the winter, finish in the summer so lid is at its snuggest and when the humidity drops it will loosen.........Sounds good to me!

Take care,

Jeff

[Ray Bell]

Thank you Jeff, and just to let you know, none of the warped bowls are from blanks you sent.

[Reed Gray]

Funny thing out here, summer is our dry season, and winter is the wet season, so expand in the winter, and shrink in the summer.

robo hippy

[Mike Cruz]

Isn't is always wet in Oregon, Reed? :)