rgdaniel Posted January 31, 2012 Report Share Posted January 31, 2012 Hi folks, I recently re-sawed a 4/4 walnut board in half, for use in small box making (grain wrapping) which I've done a few times with some success. Today, though, the two half-inch boards both cupped significantly along their whole length (about 28 inches) as soon as they left the bandsaw blade. I immediately reversed the faces so the re-sawn faces were on the outside, and clamped the boards back together. Like with a caul, kind of, the boards flattened against each other in the clamps. After a few hours I experimentally loosened one clamp, but that corner sprang back open as before, so I tightened it up again. Lots of clamps. Any clues on how long I need to leave the boards clamped, or if that will even work to flatten them? Or any other tips? Thanks in advance, cheers. Quote Link to comment Share on other sites More sharing options...
Boatworks Today Posted January 31, 2012 Report Share Posted January 31, 2012 This may sound a little odd, but a little trick I use for cupping actually involves a heat gun. If you take the gun and heat the top, center 2/3 of the board down it's length (the side that's peaked) it will actually tighten the cellulose in the grains and pull the cup out. Set the gun so it is just shy of being hot enough to burn the wood and heat as uniformly as possible JUST (and I can't stress this enough) until you start to see the slightent movement. As the wood cools it will continue to pull and flatten the board (which will take about an hour). I've actually taken a cupped board and as an experiment pulled the cup the other way!! By the time the board starts to move you'll have a hard time holding onto it with bare hands... It gets hot. When it's fairly cool, clamp it flat and let it cool overnight. I won't say that it work 100% of the time, but I've had about 90% success doing this. If you have any cutoff's of the same board (or any that you can sacrifice) I would suggest experimenting on those before the main board itself... Good luck! Quote Link to comment Share on other sites More sharing options...
PaulMarcel Posted January 31, 2012 Report Share Posted January 31, 2012 (edited) Take a sponge and get all the faces wet then clamp cup-to-cup. Cover with a plastic garbage bag or so. This happens a lot to me and this trick always works. If the boards have been out for while (and it sounds like they might be), it may not completely take it out. For me, I've had boards show 1/4" of cup and they came out basically flat. In one of my podcasts, I talked about it using really advanced video illustrations (ahem, a dry-erase board) to explain the problem and how this solution seems to work. That particular podcast episode has a bunch of random things about the build that would bore you to tears so this link includes the timecode to start where that discussion starts: http://www.youtube.com/watch?v=S-VbFzXPtTY&t=602s Edited January 31, 2012 by Paul-Marcel URL got rewritten; changed to a bit.ly Quote Link to comment Share on other sites More sharing options...
Brian VanVreede Posted January 31, 2012 Report Share Posted January 31, 2012 Never havinig to deal with this (no band saw) I like the heat gun idea. @ boatworks, what was the thickness of the wood you tested this on? After doing that, whatever cup was still there I would attempt my handplane to finish flattening. Quote Link to comment Share on other sites More sharing options...
Boatworks Today Posted January 31, 2012 Report Share Posted January 31, 2012 I've done this on stock as thin as 1/4" up to 3/4" on Sapele and birch. I know it sounds a little 'out of the norm' but I've pulled some severe cups (just like what was described above due to re-sawing) out; and if I wasn't careful actually over did it and created a cup on the opposite side Quote Link to comment Share on other sites More sharing options...
rgdaniel Posted January 31, 2012 Author Report Share Posted January 31, 2012 Thanks folks! As I don't have a heat gun, I think I'll try Paul-Marcel's trick tomorrow. And by the way, this just confirms my opinion that dry-erase whiteboards are a seriously efficient and effective communication tool. Cheers! Quote Link to comment Share on other sites More sharing options...
dwacker Posted January 31, 2012 Report Share Posted January 31, 2012 Its not a moisture issue its the relieving of the wood. Wait until you do a real tall thick board, something like african mahogany and it sounds like a gun shot just as you finish the cut. Don Quote Link to comment Share on other sites More sharing options...
rgdaniel Posted January 31, 2012 Author Report Share Posted January 31, 2012 So the trick with tenting the wetted boards, clamped together crown to crown, did not really help too much. I tried resawing another 4/4 board from the same lot, and it did the same thing. I got that one wetted down and into the tent right away, so we'll see if getting it in quickly like that makes the difference. Meanwhile, I just talked to the wood dealer, and he said they see that often when resawing kiln-dried boards. He said it's not a moisture differential NOW -- that would have equalized by now -- but that it's a result of the kiln drying process, where stresses are imparted into the wood by the moisture differential at THAT time. If you kiln dry too fast, you get case hardening, which I don't think I'm seeing here. Air drying is best, in this regard, but of course is slower. He said they expect to get two 1/4 finished boards from resawing 4/4 stock. (I need 3/8) He also suggested that applying heat, as Boatworks suggested, might also help, although he suggested leaving them under a radiant heater, crowns up, and keep an eye on them. Quote Link to comment Share on other sites More sharing options...
dwacker Posted January 31, 2012 Report Share Posted January 31, 2012 Meanwhile, I just talked to the wood dealer, and he said they see that often when resawing kiln-dried boards. He said it's not a moisture differential NOW -- Its a drag when that happens and happens more times than not. You have less risk if you just use a longer board or wider board. Resaw off a slice for use as a veneer on a different project then plane the board down to 3/8. Make sure you rip to close to final width prior to resawing. Don Quote Link to comment Share on other sites More sharing options...
rgdaniel Posted January 31, 2012 Author Report Share Posted January 31, 2012 You have less risk if you just use a longer board or wider board True, but then you don't get the "grain wrapping" at all four corners of a box, which is my goal here. Three corners plus a "close enough" back corner is not a bad compromise, I suppose... Quote Link to comment Share on other sites More sharing options...
dwacker Posted January 31, 2012 Report Share Posted January 31, 2012 A longer board will give you a mismatched corner. If you buy a board twice as wide as you need that went into the kiln wide then you rip it in half youve relieved most of the issues. You will have better luck reaswing. Don 1 Quote Link to comment Share on other sites More sharing options...
rgdaniel Posted February 1, 2012 Author Report Share Posted February 1, 2012 If you buy a board twice as wide as you need that went into the kiln wide then you rip it in half youve relieved most of the issues. You will have better luck reaswing. Interesting! Thanks, good to know! Quote Link to comment Share on other sites More sharing options...
Darnell Hagen Posted February 1, 2012 Report Share Posted February 1, 2012 I've tried all the suggestions above, aside from the heat gun, with so little success I've given up. In my opinion there is only one method that works, resawing the cupped boards again and laminating them together on a flat platen, like a bent lamination only straightening the stock instead. It's so much hassle it's rarely worth it. In the same vein, steaming the stock and "unbending" it against a flat form should work, but I've never tried. Otherwise, the only other option is to joint the stock flat. Contrary to what you may think, in this case it's best to joint crown down. You've removed material and relieved stress from the inside of the board, now you need to remove material from the outside to help pull the stock flat again. Jointing the inside face excessively ruins the bookmatch, and may actually make the cup worse. 1 Quote Link to comment Share on other sites More sharing options...
jhl.verona Posted February 1, 2012 Report Share Posted February 1, 2012 Its not a moisture issue its the relieving of the wood. Wait until you do a real tall thick board, something like african mahogany and it sounds like a gun shot just as you finish the cut. Don Indeed it does - even by hand. John 1 Quote Link to comment Share on other sites More sharing options...
Wood-butcher Posted February 7, 2012 Report Share Posted February 7, 2012 Tell me if I'm wrong. But could you saw your board a little wider and plane out the cup? As long as it is not too severe? Or is this just a waste? Quote Link to comment Share on other sites More sharing options...
dwacker Posted February 7, 2012 Report Share Posted February 7, 2012 Tell me if I'm wrong. But could you saw your board a little wider and plane out the cup? As long as it is not too severe? Or is this just a waste? Sort of. If you buy wider wood it will still have the cup when resawn at full width. If you buy double the width you need then rip and resaw the cup effect will be cut in half by at least 50% but usually all of it will be gone. Just the way kiln dried wood is especially since we no longer really get any old growth. Don 1 Quote Link to comment Share on other sites More sharing options...
rgdaniel Posted February 7, 2012 Author Report Share Posted February 7, 2012 Tell me if I'm wrong. But could you saw your board a little wider and plane out the cup? As long as it is not too severe? Or is this just a waste? The problem is, you don't know in advance that it's going to cup, and I don't want to just assume it will, and cut an 8-inch board in half every time I need 4-inch grain-matched box sides. More often, I don't even buy 8-inch boards because I have a small shop, 6-inch jointer, and tend to work on small-to-medium projects. On the plus side, I could rip a 6-inch board into 4 and 2, and use the 4 for the box and the 2 for the inner tray. With luck. Quote Link to comment Share on other sites More sharing options...
Wood-butcher Posted February 7, 2012 Report Share Posted February 7, 2012 Ok. Obviously, if the cup is too severe, this would not work. But, I have some rough sawn lumber that is cupped and because I am milling it down anyway, I can usually mill the cup out. I know this is apples and oranges. But if you allowed a little for milling after you resaw, could that work? Quote Link to comment Share on other sites More sharing options...
rgdaniel Posted February 7, 2012 Author Report Share Posted February 7, 2012 But if you allowed a little for milling after you resaw, could that work? Oh sure, and that's what I ended up doing. Sometimes though, like with some of the other boards I ran into while I was experimenting with this issue, there just isn't enough material. Quote Link to comment Share on other sites More sharing options...
Wood-butcher Posted February 8, 2012 Report Share Posted February 8, 2012 Got it. Thank you. Quote Link to comment Share on other sites More sharing options...
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