Cupping after resawing

[Robert Hartmann]

I've done some resawing on my bandsaw in the past, but only on narrower stock (I haven't had the saw very long). I had some 5/4 cherry that was about 9 1/2 inches wide I'm breaking down for a small jewelry chest. I typically clean up one side with planes and then run it through a planer, which I did this time. It was dead flat on both sides. I then took it to the bandsaw. It cut well, but for some reason it immediately warped. Fortunately I cut it thick enough to clean up and still be usable. We'll see if it stays flat over the next couple days. I had two sections each about 13 inches and it did it on both. I could possibly understand if it did it over a few days, but it was pretty much immediately after.

So, did I do something wrong to make it warp? Maybe it needed more time to acclimate to my shop. I have a dehumidifier in the shop set at 50, so a constant humidity level.

thanks

[Scott T Smith]

This is almost always due to a delta in moisture content between the shell and the core of the lumber. Did it cup away from the center? If so the shell picked up moisture after kiln drying. If it cupped away from the center, then the core had a higher MC% than the shell.

Sometimes this is also a sign of case hardening in the lumber.

If it was a warp instead of a cup, then they’re could be some inherent tension in the board. Do the growth rings match up from end to end?

[roger wiegand]

It happens, either due to differential moisture inside and outside the board, or simply a release of tension in the board. Immediate movement suggests the latter. I like to equilibrate boards for a number of weeks in the shop (check moisture if you have a meter, if it's higher than equilibrium warpage is more likely. If boards do warp let them sit for a few days, they may warp back to near flat as interior moisture is released. You pretty much always have to cut a little thick and then re-flatten after they settle down, something almost always moves.

[Derek Cohen]

Robert, as others have mentioned, inevitably there is movement as either moisture or inner tensions have their say. Left stickered, boards relax and generally return closer to their original dimensions. Always leave a little to remove to get back to the exact dimension. Some boards do not return and continue to move. If, possible, this is a reason to have a little extra on hand.

Many boards can be seen moving as the are being cut by the bandsaw. I use a short fence on the bandsaw to accommodate to this, as a widening kerf will push away from the fence and the cut will be skewed.

After boards are cut, and especially after panels are glued up, I use cauls over night and in the time between sessions to hold the boards and prevent movement. I works quite well. Here is an example ....

The following photo shows the lower section at the rear. What I wanted to show is the way boards are stored. Since I shall not get back to this build until next weekend, all boards are stickered and clamped using steel square sections.



The steel sections are inexpensive galvanised mild steel. These are covered in vinyl duct tape to prevent any marks on the wood and ease in removing glue ...



Done for the day ...



Enough for the case (top/bottom and sides), which will be through dovetailed with mitred corners, the stock for 4 legs (yet to be turned), and rails for the legs (the legs will be staked mortice-and-tenon) and attached with a sliding dovetail.

Regards from Perth

Derek

[Warren Mickley]

I think the most likely explanation for your cupping is case hardening, a common defect caused by drying too fast. Case hardening is not a great term, but that is what we call it.

Here is the gist of the problem: When drying, the outside dries more quickly and wants to shrink, while the inside is still wet and resists. The outside gets a bit stretched and the inside gets compressed. Then when the inside dries, it wants to shrink more, but it is held in tension by the outside. When you resaw the board, the inside is able to shrink and will cup the board.

Case hardening also shows up when you plane too much off one side of a board, so some are taught to take equal amounts from each side.

I would avoid truing up a side and planing before resawing, so after sawing you have as much thickness to play with as possible.

[Robert Hartmann]

Thanks for all the great info. I think it was a combination of case hardening and not letting it acclimate in the shop long enough. Fortunately I only needed two panels (one 1/2 in the other 1/4) out of each section. Not being confident in my resawing I didn't try to get the most out of it, so plenty of thickness to correct the cupping. I probably should have waited a few days to correct them. They are still flat, so maybe I'm ok. I'll be more aware the next time.

I've got so much to learn

[Robert Hartmann]

Derek, is this something you do for individual boards as well or just glued panels? I've never had issues with individual boards in the past. Great idea for panels, thanks.

[Tony Shea]

I always sticker wood when I'm done for the day. Sometimes the wood behaves and other times it does move a bit. I'm curious if actually clamping it down does anything to help this. Wouldn't the wood behave the same whether it is clamped or not? I assume if it was going to cup it would cup as soon as you pulled the clamps off.

I have also heard of people shrink wrapping or placing the freshly planed wood inside plastic bags then clamping it all up so the moisture doesn't come and go as easily. I've never tried this so cannot comment on whether or not it works. I personally just deal with wood movement and if it becomes too bad I shouldn't be using that piece anyways.

[Derek Cohen]

Derek, is this something you do for individual boards as well or just glued panels? I've never had issues with individual boards in the past. Great idea for panels, thanks.

Robert, mainly with panels ...





Individual boards generally are just stickered, under their own weight ...



Regards from Perth

Derek

[David Eisenhauer]

Same for me Robert. I always clamp re sawn panel material up with spacers between each piece after sawing and when leaving it until I am ready to work with it. Ideally, the panel material gets a 2-3-4 days to settle down before I use it. For whatever reason, lots of times I re saw late in the working day and just clamp the stack (panel pieces plus spacers between each) down to a far end of one of my work benches before I shut down for the night. I may or may not move it the next day or two before unclamping it for good to use it. Derek's second photo looks a lot like my stacks, only usually placed horizontally on a bench top and clamped to the thickness of the bench. I have seen 1/4" thick re saw panels curl like a potato chip if left un clamped.

[roger wiegand]

I've never been much of one for trying to force wood to be flat. It seems that if I clamp it flat prior to assembly there is a good chance that it will try to move later, with potentially disastrous results. Letting it assume whatever shape it really wants to take after being cut to close to final size and then making it flat after it's well equilibrated seems to give good long term results. Do things clamped flat actually stay that way? Not so much in my experience. Planning construction for ongoing movement as humidity changes over the life of the piece also seems prudent. I restore lots of old woodwork and in general it was well designed from that point of view, however almost inevitably when someone tried to force the wood it ends up with big cracks; the forces involved seem quite large.

[Derek Cohen]

Roger, that has not been my experience, but then we live in different climates.

Regards from Perth

Derek

[brian zawatsky]

My experience mirrors Roger’s as stated above, but then again we are both in the Northeast part of the US so seasonal humidity changes are similar for both of us.

Once I mill stock to final dimension, I do not sticker it. I pile it up on a flat surface and cover it completely with a piece of plywood or some other sheet good. I’ve left milled maple sitting like this for a week or more with zero movement issues.