Cherry wood - cupping


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Old 06-15-17, 04:55 AM
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Question Cherry wood - cupping

I am face jointing and planing some 4x4 cherry down to 1/2" boards, 6.5" in width. After doing this and checking that all are flat, 24 hours later I find that the boards have developed a cup across grain. The wood had been setting in my garage for a couple of weeks. Why does this happen and is there anyway to "uncup" these boards?
 
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Old 06-15-17, 05:38 AM
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AFAIK, all wood has some cup to it & you're supposed to match the cupping before you join the wood. I remember that from building decks. I don't know if steam can unify the wood now that it's all joined together.
 
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Old 06-15-17, 06:10 AM
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Its due to a couple reasons. First, it's a tree... so you can't predict what it will do when it's cut. There is also a difference in plainsawn and quartersawn wood when it comes to stability. And the wood was likely never kiln dried.

Plainsawn wood will tend to want to wrap around the center of the tree when it is wide and cut thin, it is a cross section of a few rings. With plainsawn wood, a large percentage of the wood will want to cup.

Quartersawn wood is more stable because when the wood was cut up into sections to be milled, it all radiates from the center of the tree to the edge, its wide edge is perpendicular to ring growth and as a result it contains more rings.

But wood can also cup due to uneven drying. When one side dries faster than the other... or maybe one side gets exposed to more humidity than the other... one side will expand more than the other causing the wood to cup. Stacked wood is usually placed on "stickers", (a spacer), and in a cool dry place with even temperature and humidity to ensure even air flow. A small amount of wood would need to be clamped to help resist warping as it dries.

Its also a general rule that if you have a large chunk of wood, that the wood will have varying moisture content... dryer on the edges, moist in the center. So when the wood is milled, it is often rough cut to size, then either left to dry, or kiln dried. Kiln drying heats the wood and hardens the lignin (like a natural glue in the wood) so that it is less likely to bend. Only then should you plane it to its final thickness.

And generally carpenters know that wood has limits to what it can do. It's why a flat tabletop is made up of lots of narrow boards that have all been placed side by side, glued together and planed, rather than just a few wide boards (which are more likely to cup).

So the answer to your question is likely no, there is not much you can do about it at this point.
 
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Old 06-15-17, 08:11 AM
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Kiln Drying

Thanks for the info. I will start with another board and rough cut to size. If I want to kiln dry the parts in the oven, what would be a good temperature and time for a piece that is 7" x 20" x 5/8".

Thanks,
 
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Old 06-15-17, 08:51 AM
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Hard to say. I don't kiln dry lumber for a living. Not sure what kind of results you would get trying to dry one board in an oven.

It depends a lot on what the existing moisture content is (need a meter). If I had to guess, I would say cherry would best be dried at a low temp like 150 for an extended period of time, maybe 4-6 hours or more. Cherry is very susceptible to darkening due to heat and/or sunlight.

Also keep in mind that your wood can end check as it dries, so you want your pieces to be as much as a foot longer than needed because the ends dry faster and split. You might need to cut 6" of waste off each end, you just don't know.

Kiln drying isn't necessarily about applying heat or speed. Yes, heating the wood cures the lignin, but trying to speed up the process (simply because we are too impatient to wait) can be part of the problem. The heat on an unclamped board can actually make cupping worse by drying the surface too quickly. Kind of like when you spill a bunch of water on an oak floor... the top dries but the bottom doesnt, ruining the floor.

Also, you do realize that if your finish size is 1/2" that starting with 5/8" rough barely allows for 1/16" of cup, right? Because you have to plane both sides multiple times to take the cup out. (Plane/flip/plane/flip/plane/flip, etc) Starting with 1" rough cut is more realistic, imo. The wider the piece, the thicker it will need to start out.

In an oven, there may not be enough air movement to produce even drying, and moisture may be trapped inside if the door is closed. So I wouldn't say that the oven in your kitchen is automatically the "best place" to dry wood. Although I admit I did it once with a batch of mulberry that I was experimenting with.

Air drying a large quantity of wood where it has been stickered and clamped, in a controlled environment where air can slowly pass over the wood, dehumidifying it, is generally a preferred method of drying, but it takes a lot of time... months.
 
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Old 06-15-17, 09:11 AM
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A storm knocked down a cherry tree and a client of mine had the main stump rough sawn to I think 2 to 2 1/4" thick and then let it dry for 2 YEARS. He then had it milled down to 1 1/2" and I made a couple of mantles out of them and then did stonework around it. I half lapped them (Kind of like Lincoln Logs) and made the mantle look more substantial. LOL, I was given a picture out of a log home magazine and told to build this.

Name:  Fireplace stone final.JPG
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Old 06-15-17, 09:46 AM
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Thanks for the info. It was very informative.
 
 

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