Resawing and cup
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I have trouble producing 1/4″ to 3/8″ thick material when resawing because of cupping. A common situation is to start with 4/4 rough and try to produce two 3/8 thick pieces. Right now, I need to make a pair of pieces 3/8″ thick that are 10″ x 20″ by resawing some 6/4 walnut. The material is plain sawn with some rift on the edges.
In the past, I’d hand plane a surface flat and then resaw. Since the cupping comes from both moisture and tension, I’m wondering if I should try something different and plane *both* faces some amount before resawing. That way, all 4 faces are fresh rather than producing one board with two fresh faces (newly jointed plus newly resawn faces) and one board with one fresh face (from reseawing) and one original rough face. Would that help? Since both halves will cup, usually, I suspect this isn’t going to help much.
Usually, I take as little off of the thickness as possible before resawing to give myself material to flatten after the cupping. Is that a mistake? Should I be taking a heavy amount off of each face trying to get rid of the faces exposed during drying?
Any suggestions / tricks? I’ve done the “cut fat and sneak up on it over days” trick many times. For 1/4″ and 3/8″ final, it hasn’t been reliable. Getting wide quatersawn isn’t really an option.
Ed, this is a really interesting topic. It’s one of the reasons wood is such a frustrating material to work with. The cupping, I believe can be due to the moisture in the wood responding to your resawing and due to tension in the wood. I recently tried to resaw some mahogany and the resulting board bowed at least an inch or so, making the wood almost useless for what I was doing. That was clearly the result of tension.
My first inclination is you should just joint one face roughly. You don’t need it perfect for a good resaw but you don’t want any twist either, then joint one edge square to the first face. You might have to resaw off a thicker board to account for any new wood movement, in which case you might not get 2 equally thick pieces out of the board.
In my experience, really dry open grain wood like oak and ash do well resawing. Maybe really well-seasoned pine and black cherry also do well.
I’m interested in what other readers say.
Harvey, what you describe is how I have approached it so far, but with only so-so success, including in oak. As you say, this is the battle of moisture and tension. What I find interesting is that producing very thin material, like 1/8″ or less, is easy because cupping usually doesn’t matter and usually isn’t very much. For 1/2″ and thicker, the cupping is usually minor, I think because the material is rigid enough (thick enough) to resist the changes in tension. But, at 1/4 to 3/8″, roughly, it seems quite challenging. My theory is that the material is thick enough to build up a fair bit of tension but not thick enough to resist.
I have a 14″ wide piece of 6/4 walnut from which I must obtain 10″ wide by 20″ 3/8″ stock for the sides of a clock. This material is flat sawn. Buying quarter sawn to yield 10″ in walnut isn’t an option right now, patly because of cost and partly lack of supply.
Yes, I think you’re right! The thinner pieces don’t have enough strength.
I would seriously consider making walnut veneers about 1/16 to 3/32” thick IF you have a well tuned bandsaw. Then glueing those to a lumber core you have made from strips of quartered yellow poplar ( for us yanks that is actually a magnolia, not poplar) or pine.
I recently did this for the top of a small demilune table.
You also don’t need a vacuum bag, you can hammer veneer it to the core with hide glue.
I’d love to hear how it progresses, Ed.
Best of luck
I reduced a little bit of thickness on each face of the 6/4, then resawed down the middle giving two halves in the ballpark of 5/8. I’ll let them sit for at least a day now. The wood made a popping sound as the blade came through and the two halves sprung apart. I had to release the fence and shut off the saw to remove tension and complete the final 1/4″ of the cut. The cupping follows the standard, “growth rings want to straighten.”
My (limited) experience is that warping that happens from tension being released happens fast. On a table saw it can cause sudden kickback as the wood suddenly distorts and binds. Cupping that occurs from uneven moisture does not (again, in my limited experience) happen so fast. I have resawn pieces that looked fine only to find an hour, or a day, later they were cupped. The cupping that occurs from uneven moisture can sometimes be corrected to some extent by moistening the dry concave side and drying the moist convex side. This is a trick sometimes used for warped plywood sheets, especially if the warp is in just one direction. It works less well if the plywood is all twisty, as often happens with thin pieces. A simple approach to the plywood is to just place it concave side down on grass with the convex side facing the sun. Maybe some moisture sprayed on the concave side will help before laying it down. There are a number of variations on this theme for plywood. One danger is it can be so effective that the board quickly warps the other way, especially if it is thin plywood!
I have also tried this with some success for thin pieces of solid resawn wood that had cupped I have sprayed the concave side with water and placed that side down on my bench (on some plastic to protect the bench) and then directed some heat at the convex side. I have some of those ceiling mounted radiant heaters that worked okay for this. Again, if the pieces are thin the cupping can quickly go the other way. Ooops! And if the added moisture or heat is uneven other distortions can happen. I have not tried this for pieces longer than a foot or two. That might be harder since it is harder to heat a large piece evenly.
I have no idea how often this works for solid wood or whether it is likely to work well for somewhat thicker or longer pieces. I had enough success to figure it was at least worth mentioning.
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Hey Ed! Sorry to hear about your troubles. IDK if you listen to the Fine Woodworking podcast, but Mike Peckovich recently addressed this exact issue, and his answer sounded to me a lot like your thoughts.
My recollection is that he said re-sawing 4/4 material is a train wreck of cup/twist/bow/etc, and it’s simply a non-starter for him. He acknowledged with some exceptionally well-behaved stock it is absolutely possible, or with less-than-perfectly-behaved stock there are some tricks with moisture and stickering and weights and etc, etc, that can be employed, but on average, he doesn’t look to 4/4 material.
He said to get thin slices that don’t go crazy, start with properly-dried, 8/4 stock (or thicker), and take your slices from opposing sides. Obviously the outsides will be much closer to (or at) equilibrium moisture content, and in his experience, there is much greater stability in the resulting boards.
It was, I think, in one of the most recent 2 episodes. I’d start with episode STL268 (I won’t include the Youtube link because this site often mis-identifies replies as containing spam if you simply link to other woodworking content, but just go to the Fine Woodworking channel on YT and search for STL268) and if it’s not there, try STL269. Get the info straight from the source in case I’m mis-remembering something.
Good luck!
Sanford- The cup happened instantly this time (and other times, if I recall correctly), so my assumption was internal stress rather than moisture, as you suggested. It’s possible the moisture issues could cause further movement, so I’m letting it sit for a bit. Since the movement arose from tension, I don’t think I’m going to try the moisture tricks because I think the piece will return to equilibrium and (reverse) cup if I do this, at least in this case. I realize you were suggesting this only for when the cupping came from moisture, not from stress.
Edmund- That’s interesting. Does he try to go right to the desired thickness or close to it? I suppose that would minimize the difference in stress across the thickness and reduce the amount of strain (movement). On the other hand, if there is movement, you have nothing to work with. I resawed down the middle of 6/4 with the idea of having plenty to work with to get back to dimension. Maybe what I will do is focus on removing material from just the exterior surface with the hope of reducing the stress difference between the two faces. Maybe that will help it relax a bit and, at the same time, I’ll be taking away the hump. I’ll measure the sagitta of the concave side and see if it lessens when I remove material from the hump.
I’ll bet the lesson here is to cough up the cash and buy quartersawn, but walnut is scarce these days and quartersawn that will yield my needed 9 inches is rarer still. Another lesson is to just avoid this in the design, if possible. Unfortunately, I’m building a reproduction, so there aren’t many options. (Harvey- That’s why veneer isn’t an option, partly).
Ed, IIRC, he resaws it fat. As you noted, going directly to the final thickness off a resaw is asking for trouble. My impression was that resawing off opposite sides of a thicker board is a recipe for more manageable results than simply resawing thinner stock in half. He still would take proper care with the resulting boards, bring them down to final dimension in stages, all that good stuff.
And since this is still wood, I’m not sure how quartersawn is going to help you. If the problem here is immediate movement after resawing thinner boards from thicker stock, then moisture and tension can still exist regardless of the grain orientation, so I’d be concerned that you’d be spending money for no real gain. If instead your concern was wood movement due to seasonal changes, then sure, quartersawn boards will move much less than flat-sawn boards. Hopefully I’m not misunderstanding!
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