warping after resawing

[Greg Book]

I recently needed to resaw some poplar and white oak from 5/4 into 1/2" thick boards. Immediately after resawing on the bandsaw, I put them through the planer. They looked great, flat as a pancake. When I went back after a couple days to cut the pieces to their final size, they had cupped. Some cupping was worse than others. But none of the pieces were flat anymore.

I read recently in FWW that you need to let resawn boards sit for a few days, and now I see why. But... if I'm resawing an 8" wide piece of 5/4 and I ultimately want two 1/2" boards out of it, there's a good chance that the cupping will be severe enough that my board will be thinner than 1/2" after planing out the bandsaw marks and cupping.

What are some ways to mitigate the cupping or handle the effects of wood movement after resawing like this in the future?

[Gerry Grzadzinski]

Start with 6/4, so that you can joint and plane after it cups.

[Andrew Hughes]

Sometimes we can get away with resawing tall stuff.Really depends on the wood.If there is any question ripping them down the middle resawing and glueing back together works.If your jointer is set up properly its not a big deal at all.Leave them long so the grain can be matched back.None will be the wiser.

[Terry Hatfield]

I sticker the resawn boards with concrete blocks on them for weight for several days before machining them further. Has worked for me but I don't do a lot of resawing so certainly not an expert.

[Frank Drackman]

The first question that comes to mind is how long was the wood in your shop before you cut it? The better questions is what was the moisture content before cutting, but that would require a moisture meter. I have found that if the material isn't dry that when you resaw you get a lot of wood movement.

[Greg Book]

The wood was kiln dried and had been sitting in my shop since August. It's still winter up here, so its not humid. I'm thinking it might have just been the internal tension in the wood.

[Peter Aeschliman]

The wood was kiln dried and had been sitting in my shop since August. It's still winter up here, so its not humid. I'm thinking it might have just been the internal tension in the wood.

That may be, but in my experience, internal tension reveals itself almost immediately after resawing (and in some cases, while I'm actually cutting).

Did you sticker the pieces? Only way to really know if it was moisture vs tension would have been to use a moisture meter to measure it before resawing, after resawing (comparing the moisture of the freshly sawn surface to the previously exposed surface), and after acclimating the next day (to see if the moisture content changed).

Another observation that would help- which direction did it cup? Is the apex of the cup on the freshly sawn face, or on the other face?

[John TenEyck]

My experience is the same as Peter's. If it's internal tension, it cups right away. Yours sounds more like it happened due to non-uniform moisture content, regardless of how long it was in your shop. If the wood was dense stacked this could easily have been the case. Or perhaps did you leave the boards face down on your work bench after you planed them? Anyway, if it was due to non-uniform moisture content the boards will often flatten back out in a day or two if you leave them standing on end or on edge so the air can circulate around all sides.

John

[Chris Padilla]

Wood is a fickle medium! It could be some combination of what everyone else above has talked about.

But my own experience is what Peter and John have said: moisture imbalance.

[Terry Hatfield]

Wood is a fickle medium!

Oh sure, now you tell me. :D

[John K Jordan]

I often use air dried wood from my sawmill around the farm (for outdoor use) and it is not always completely dry. If it cups after sitting a bit I flip it over and put the concave side down on the ground and expose the convex side to the air and/or sun. This will often straighten the boards, sometimes after a few cycles of flipping.

The same thing in the shop - if one side dries faster or more than the other a board will cup. Same thing if the wood is very dry and an exposed side absorbs moisture from the air.

If given the choice between two boards, of course, resaw the one that is closer to quartersawn!

JKJ

[Chris Fournier]

You have to be an optimist to think that you can get two flat 1/2" boards out of 5/4 with any certainty. Air dried may well improve your chances but I agree that 6/4 is the better starting point.

[Brian Henderson]

Sounds to me like uneven moisture content. Internal moisture may be quite different than surface moisture and if it is, once you make the inside the outside, as you did in resawing, you can see some pretty decent wood movement. You really need to allow more wood for machining to take out the cupping. I bet if you took a moisture meter and checked both sides of the newly cut board, they'd be different.

[Chris Fournier]

Sounds to me like uneven moisture content. Internal moisture may be quite different than surface moisture and if it is, once you make the inside the outside, as you did in resawing, you can see some pretty decent wood movement. You really need to allow more wood for machining to take out the cupping. I bet if you took a moisture meter and checked both sides of the newly cut board, they'd be different.

Kiln dried lumber can have an even MC from face to face but still move radically when resawn. The stresses from a poor drying regimen "case hardening" cause this. Wiki this and all will be revealed!

[Cary Falk]

I sticker the pile and clamp it together(because I don't have any cinder blocks to weigh them down). I let it sit for about a week. I never have any problems.

[Brian Henderson]

Kiln dried lumber can have an even MC from face to face but still move radically when resawn. The stresses from a poor drying regimen "case hardening" cause this. Wiki this and all will be revealed!

Exterior face to face yes, but once you resaw, you reveal a new face that may have a different MC. Lots of kiln driers take this into account in the drying process, some do not. As you said, it often comes from a poor drying regimen, just having the wood in a kiln isn't a magic formula for success.

[glenn bradley]

The wood was kiln dried and had been sitting in my shop since August. It's still winter up here, so its not humid. I'm thinking it might have just been the internal tension in the wood.

As mentioned, start with thicker stock for the yield you are after. Your stock was acclimated so it was probably just internal tension. We all get this from time to time. If you get it consistently, change suppliers. Although it meets with questionable result I too weight or clamp stickered resawn stock while it re-acclimates.