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Flattening (reasonably) curly cherry board4/16/14
I am using a beautifully curled cherry board in the top of a 42" high countertop. The board is 3/4" thick, 7-1/2" wide by 74" long. Unfortunately it has a severe twist.
I don't have enough thickness to remove much wood in order to get rid of the twist. I only need to get it close to flat since I can attach it to the wall structure below to help pull it flat. I am thinking that a series of closely spaced, cross-cut kerfs across the bottom of the board will weaken the board enough to let the board be pulled flat during attachment. Good idea? If so, how deep should I saw the kerfs?
If I was you, I would find another use for this board. A board that is bowed relative to the edge or face can sometimes be fastened down flat. A "severely twisted" board will be hard to fasten down flat. You could join the board flat then re-saw or plane the board to uniform thickness and laminate it to a piece of mdf. You would have to laminate a same thickness and type (cherry) board to the other side of the mdf or it will cup
I cut all my own veneer for table tops and desk tops and often from really badly twisted boards. In nearly 30 years I've never had a failure and my clients are in awe of the amazing grain patterns and not in awe of how their furniture twisted itself to pieces! Have fun with it and if you don't want the wood drop me a note maybe we can work a deal.
Only place I'd disagree is with veneering it onto mdf. I wouldn't recommend it for a c-top that could likely see liquid spills over time. Not to mention it's more work than needed. No I'd re-saw the curly into thick veneers….maybe 1/8" or just a bit thinner. Then I'd glue it in a vac press to both sides of a nice flat piece of plain cherry, preferably closer to 1-1/4" thick for my taste.
Other than that I agree, trying to force a piece of wood into a position is a very risky procedure.
Well, Monte, I guess I'm going to be the contrarian this time. I think you can do what you want, but not "cross-grain" as you state.
If the board it twisted, it probably also has a spiraling grain as well as other figure, which leads to the twisting as it dried.
What I would do, is to make the kerf run diagonally along the bottom down it's length parallel to the grain, from one corner to the opposite corner at the other end.
Cupping is a weak force, so if you have plenty of wood on either side to draw it down, that shouldn't be that hard to do. However, you do need to allow it to move due to the changing MC of the seasons.
I could be wrong since I can't see the actual board and situation. Just keep in mind, that if you make it too tight, and it tries to shrink, it will likely split up from the weakest of one of those kerfs.
If you kerf the board then try force it flat it will most likely crack.
The good is, you will have some nice cherry to roast hot dogs this summer.
Good luck, I hope you are successful.
Gentlemen, thank you. I assume all of you are speaking from experience.
I was about to commit to jointing and resawing it into a veneer when Keith gave me hope that my first thought to kerf the bottom may work. When I say "severe twist" I mean opposite corners are about 5/8" out of plane...over a 74" span. The convex face would be the show face. I am able to clamp it flat onto a bench which made me think that I could relieve some stress by kerfing it.
The board will be edge joined to other boards to form a 19" x 72" top for a freestanding wall 42" high to divide an office.
Keith, how deep would you cut those diagonal kerfs?
Not over half, but before you even do that, I would try this first.
If you have any thicker lumber around, like a 2x8 SYP or something that you can clamp it down to. Then apply some heat to that convex face, drying only that side, may get it back to flat before you do anything else.
It is easier to hold something flat than to try bending it. Also, you will need a straight edge square to the face before you can get a good glue line to join the boards next to it. This is not easy while it is twisted.
If you had it flat earlier, but now it is twisted, it may have been on top of a stack, with the bottom up, which lost moisture, or face up gained moisture. But your goal now it to dry the face. You may be able to place it out in the Sun, or use a heat-gun or blow dryer. If you have a small space heater, you can make a little tent with a tarp blowing under into it. But try to have it only heating the convex face, and check it fairly often.
I'd put a clamp on the high corner of one end, then see how much pressure it takes to push the opposite corner down. This will give you an idea of how much it will be pulling later against your fasteners. You may decide that it doesn't need any kerning if it is easy enough to hold out.
get another board. all the messing around with 1 junk board, you could have bought 10
Twist does indicate diagonal grain or spiral grain. Such pieces are shrinking or swelling in all three directions--it is a three dimensional situation. Cutting a diagonal kerf can help in two directions, but not in all three, so it is likely that you will still have lengthwise shrinkage and undesired warping with seasonal changes in moisture.
So, if you can straighten it now and you make something, how upset will you be and how much time and effort will be lost if it still misbehaves? I suspect enough so that this board should be discarded for now.
Note that it is impossible to bend a piece flat unless you bend it further than flat. If you bend this 5/8" on 74" flat, when you take off the force, it will go back to 5/8", as it has a memory (or some would say that wood is elastic with small bends). Wood has spring to it, so you need to go beyond flat...quite a bit more. Second, the piece will still have its memory of wanting to twist. That is, bending does not cause the diagonal or spiral grain to go away. Third, dry wood is so strong that to bend it when dry is very difficult; you will need to go back to about 20% MC to get a piece that will bend with a reasonable amount of effort. As mentioned, once you bend it beyond flat, then you have to hold it in that position while it dries. The big trick is estimating how far beyond flat you need to go...you are upset about 5/8" in a 74" span which is not a lot, but certainly is unacceptable. What you need is better than that, so we need more than just a rough estimate.
The forces that make lumber twist and cup are quite large. The proof of this is when you look at a stack of lumber that has been dried and see cup and twist within a stack (sometimes even near the bottom) that has been well stacked. As more proof, try and stand on a piece of lumber that is cupped or twisted and make it go straight with several hundred pounds...won't happen.
I went ahead with the twisted, but beautiful board. If I need to revisit this top in the future, I need to revisit it. I used twelve buttons from below for mehanical strength along the length of the center board, the one in question. I kerfed the bottom and edge-glued it to straight boards on the outside.
Two pics attached.
If it fails, I will let you all know that I should have taken advice to use a different board.
I can see why you went to that effort - that is a stunning board.
Hope it holds up.