Any advice - Resawn board curling up

[jay gill]

So I'm having some issues resawing some 10 & 13 inch wide ash. It is curling up in both directions. The rough lumber was 6/4. I tried resawing it 2 different ways - after jointing and planing down to about 1 1/8 inch and resawing a rough piece. My ultimate goal is to plane it down to 3/8 of inch for ship lapped back panels of a train display case that will be 36" tall. I had this same thing happen a little over a year ago with some red oak for a similar project.

I've attached pictures and was wondering if anyone could tell me what I am doing wrong.

227592

[Danny Hamsley]

The wood is likely higher in moisture content in the core versus the shell. Exposing the wetter interior versus the dryer exterior sets up a cupping stress.

[Jim Matthews]

It will be near 80 this week, up your way.

If you put these out in the yard in the morning, cupped side up, perhaps the top face will dry while the bottom face is damp.
If you try this, I would run a pressure treated sticker down the middle, under the board and put a couple of (known) dry boards along the top edges of the "U" to encourage the desired shape.

I wish I could say my resawing looks better - but this happens with my stash, too.

jim
wpt, ma

[Myk Rian]

Let it stabilize a day or 3.

[J.R. Rutter]

The wood is likely higher in moisture content in the core versus the shell. Exposing the wetter interior versus the dryer exterior sets up a cupping stress.



If you put these out in the yard in the morning, cupped side up, perhaps the top face will dry while the bottom face is damp.

In my experience, the concave face has less moisture than the convex face. You can see the same thing when putting a dry kitchen sponge into a puddle of water: moisture swells one face.

One other note. I think that kiln dried lumber is more likely to have a dry core (at least where I live) vs air dried lumber, which is more likely to have a wetter core relative to the outside layer.

[Jim Foster]

When I see this happen I always wonder how often the issue is a result of incorrect kiln drying.

[Tony Joyce]

If this is kiln-dried lumber it sounds like it is not run through a conditioning cycle, at the end of the kiln run. Some kilns skip this step to save time. I resaw a couple hundred feet a month of different species. I rarely have this problem especially with oaks. Stick it and weight it for a few days if it doesn't flatten after a week or so it probably won't stay flat.

Tony

[Kent Chasson]

If it's dry wood and has acclimated to your shop, it's probably case hardening from kiln drying. If that's the case, it will not flatten out. If you force it by wetting the convex side, it will just cup again as it dries.

If you are unsure whether it has acclimated to your shop, let it sit for a few days and see what happens but I'm guessing that if you flatten it on the jointer, it will stay flat (if you have enough thickness to do that).

[J.R. Rutter]

Hey, another Bellinghamster ^^^

[James White]

You can try planing some more material off of the outside of the board in order to eliminate the case hardened material. Other than that believe it or not you can re wet the outside and when it dries this time it may dry in a less tensioned state. The outside dried while the inside was still wet. Therefore the outside dried in a stretch out state (case hardened).

James

[Kent Chasson]

Hey, another Bellinghamster ^^^

Yep. Celebrating the first day of spring with wind and hail. Summer's only 4 and a half months away.... if we get it this year.

[Scott T Smith]

Hi Jay. For starters, the movement in your resawn board is not a result of anything that you're doing wrong.

Fellow SMC'ers: There are some interesting aspects to Jay's photo that tell the story of the root cause of his problem. It is not case hardening (I'll explain why below). Most likely there was about a 2-3% difference between the core and shell moisture contents on his board before resawing, with the core being slightly higher than the shell. The lumber could have come out of the kiln at around 9%, and then the shell dried down to 6% in your shop, accounting for the difference. Or, as Tony Joyce suggests it could have come out of the kiln with a small delta between core and shell due to a lack of conditioning.

Here is why this is not caused by case hardening. If you study the photos closely, you will note that the only portion of the board that is cupped is near the pith, where the juvenile wood / cathedral grain exists. As you move a couple of inches from the pith towards the outside of the board, i.e. into the rift / quartersawn section, the board appears to be relatively flat.

If the root cause was case hardening, then the entire board would be curved in an arch across its face. Jay's board appears to be cupped in only one spot - the portion closest to the pith, so it was not case hardened.

By the same token, there was not a large difference between the core and shell MC%, because only one portion of the board appears to be cupped. Did you have the wood stored someplace other than your shop until recently? If so, that could account for the small difference between MC% on the shell versus the core, as the outside of the board would have dried down more than the core after being brought into your shop.

Wood movement is influenced not only by the orientation of the wood cells in the board (radial versus tangential), it is also influenced by the ratio of early versus latewood cells, and especially by the presence of juvenile wood (such as near the pith in the log).

If you try rewetting the board and laying it outside, only rewet the portion near the pith that is curved; otherwise your entire board may move in some unexpected directions.

To better prevent this from occurring in the future, use a pin type moisture meter to measure both the surface and core MC% before resawing. It's best to use insulated pins when measuring the core MC%, so that the readings are accurate.

[Jim Matthews]

On closer reading of the OP, if the boards are ripped in length to make shiplap, can the cup be minimized to the point of inevidence?

If you rip each board into ship laps more like 4" wide, would you still see the cup?

jim
wpt, ma

[James White]

Good eye, Scott! To be honest I hadn't noticed that there was pith in the board. Would this also explain the bow in the length of the two boards?

Not to change the subject too much. But when sawing on your mill for flat sawn material. How far away do you stay from the pith? Are you looking for color change? Or is there some kind of ratio in relation to the log diameter? I had asked this on the forestry forum a few years ago but no one responded except for someone who said the just make a railroad tie from every log center.

Jay,

Is the bottom board in photo two as flat (no cup) as it looks in the photo?


James

[Larry Fox]

Based on my experience resawing I would say that you did nothing wrong, happens to just about every board that I resaw into two equal thickness pieces. Approach I take is to figure it in and cut the boards well in advance of when you need them. I resawed a bunch of 8/4 cherry to make bookmatched panels for a kitchen project I was doing. I resawed them, stickered them and put them in clamps for > a month. I slowly loosened up the clamps until at the end there was no pressure and the boards behaved very well after. I just chalk it up to the release of internal stress.

Having said all that - wherever there may be differences between what I say and what Scott says, believe whatever Scott says. He clearly has forgotten more than I will ever know on the topic.

[Scott T Smith]

Good eye, Scott! To be honest I hadn't noticed that there was pith in the board. Would this also explain the bow in the length of the two boards?

James


James, re the bow I would hazard a guess that there was a slight bend in the log at that location. Look closely at the edge grain on the top board above the bow. Notice how the angle of the grain relative to the face of the resaw cut changes just slightly in the vicinity of the bow versus at the left and right sides of the photo? Ill bet that there is a greater presence of cathedral grain where the bow occured, indicating more movement as the center of the board dried after resawing.

[Scott T Smith]

Not to change the subject too much. But when sawing on your mill for flat sawn material. How far away do you stay from the pith? Are you looking for color change? Or is there some kind of ratio in relation to the log diameter? Jay,

James

James, I don't mill much flat sawn, just quarter and rift sawn, and I try to leave at least 2 - 3" between the center of the pith and the edge of all of my lumber. QS boards that are milled too close to the pith tend to crook while drying. Usually I mill at least a 4 x 4 if not a 6 x 6 from the center of every log, and set it aside to use as either cribbing (dunnage) or farm wood.

I also tend to look at the consistency in thickness of the growth rings. If the center of the log has noticibly larger growth rings in it for the first few inches, I'll try to keep that out of my lumber.

[Scott T Smith]

Having said all that - wherever there may be differences between what I say and what Scott says, believe whatever Scott says. He clearly has forgotten more than I will ever know on the topic.

<grin> Larry, you just made my morning! I seriously doubt that I've forgotten more than you know - but thanks anyway for the compliment! I like your approach to resawing and clamping; very practical solution.

[jay gill]

Scott,
Thanks for the great info. Very perceptive on the curl occurring around the pith. It definitely helps me understand what is going on and how to avoid or plan for these potential issues in the future. The lumber has been in my basement shop since last August, admittedly stored standing on end. The original pieces were ~9' and were cut down to ~36 inches each prior to bringing them into the shop, but it does make sense that there has been additional drying on the outsides based on how the board cupped/curled in all directions.

Jim,
The goal of the wide boards as the back of the case was to to book match the catherdral of the grain across the 6' width, so I was hoping to not have to resaw down to ~6-7" pieces if at all possible thus giving fewer joints. Luckily, my design has the final thickness of the boards at 3/8", but could get away with 1/4" and still include a 1/16" dado to provide some mechanical support to the shelves along their length to prevent sagging. I think I might be able still joint and plane them into usable pieces.

I also have a feeling a moisture meter is in my future.

[Danny Hamsley]

James,

The pith is a necessary evil. Some species are worse than others. Oak and cherry have very bad piths that are prone to splitting and cracking. Walnut behaves much better. I just leave it in the wide boards, captured in the center when I am sawing the boards. After drying, when I get ready to use the board, I rip it out. Splitting a board thru the pith leaving the juvenile wood on the pith side will cause the green board to dry with a pronounced side bend. The juvenile wood on the pith side shrinks a little longitudinally while the mature wood on the bark side does not, causing the side bend. That is why I leave the pith in the center of the board until it dries.

[Myk Rian]

Are you sure the mature wood is under the bark?
When a tree grows, the new wood is on the outside. (Sapwood)
The oldest wood is in the center.

[Scott T Smith]

Are you sure the mature wood is under the bark?
When a tree grows, the new wood is on the outside. (Sapwood)
The oldest wood is in the center.

Myk, during the first several years of a trees growth, it produces a type of wood that the forest products industry terms "juvenile wood". During these years the characteristics of the wood produced each year varies widely from growth ring to growth ring. Usually when the tree reaches between 5 - 20 years of age the cellular structure becomes more consistent, as do the boards ultimately produced from it. This later wood is called "mature wood" by the industry.

All trees have juvenile wood, but in older growth forests it does not have as much of an impact due to the close proximity of the early growth rings. Trees harvested from modern forests are more problematic when the boards contain juvenile wood.

Technical differences between juvenile and mature wood include a different cellular structure, lower strength, higher lignin content, more compression wood, higher longitudinal shrinkage, etc. These cellular differences causes it to react differently when sawn or resawn, which contributed to Jay's problem.

By the same token, during each year's growing cycle the tree adds cells that are referred to as "early wood" and "late wood". Late wood cells are more dense than early wood cells, and the ratio of early to late wood makes a difference in the strength of the lumber. The color differences between the two (latewood cells are usually darker than earlywood cells) is what makes the growth rings stand out on on wood.

[Danny Hamsley]

Are you sure the mature wood is under the bark?
When a tree grows, the new wood is on the outside. (Sapwood)
The oldest wood is in the center.

No, that is not how it works. You would think that the oldest wood near the center of the tree would be mature wood, but it is not. Juvenile wood is produced at the apical meristem as the plant grows. This forms the pith. Then, the first ten years or so of growth in the young stem is juvenile wood. As the tree gets older, the wood that is laid down after this juvenile core has different cell wall configurations and contains less lignin. This is mature wood. If you look at the cross-section of a tree, the mature wood is to the outside, and the juvenile wood is associated with the pith.