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Walnut slab cupping after flattening?!

8K views 26 replies 20 participants last post by  therealSteveN 
#1 · (Edited by Moderator)
Hi all,

I got some great walnut slabs from a friend and am working on them for some coffee tables. Pretty big stuff, maybe 60" long, 24" wide, and 2"+ thick. Some big checks and splits, but working around those with some bow-tie keys/epoxy/etc. I built a router flattening jig and got both sides nice and flat. A couple days later there is not a pretty noticeable cupping in the center of the piece. I checked and moisture level is around 7%, so this seems strange that this should be happening. Anyone have any advice/thoughts? Help!
 
#3 ·
It could be several reasons.like RRBOU mentions they didn't acclimate evenly.
Or it could be the slab has tension in it.If you look at the sides can you see the way the grain runs.Does it match the cupping.
Or is there a oval cathedral grain on the surface that matches the cupping.
These are the most common one that I find.

Aj
 
#10 · (Edited by Moderator)
One of the problems with walnut is the stress. I have had blades stop from the stress locking on the blade and pieces fly off when cutting. Watch the grain carefully.

I had a local mill (he moved away) that kiln dried everything. For the big stuff, he would let it sit for 2 years - covered from the elements and stickered to dry before putting it in the kiln. Since he moved away, all wood that I get, I let it sit in my basement for 2 months with a dehumidifier running 24×7. This has worked well.
 
#11 ·
If it doesn't flatten out, and you are not in AZ, lay it on the ground, cup side down for a few hours. It will pull moisture from the ground on that side.

If it is white oak the rules are different. This wood can take up to 4 times longer for slabs to dry.
 
#13 ·
If it doesn t flatten out, and you are not in AZ, lay it on the ground, cup side down for a few hours. It will pull moisture from the ground on that side.

If it is white oak the rules are different. This wood can take up to 4 times longer for slabs to dry.

- dbray45
Cause in AZ our wood is dry 10 minutes off the tree. Instant kiln dried wood here! ;)
 
#19 · (Edited by Moderator)
One of the problems with walnut is the stress. I have had blades stop from the stress locking on the blade and pieces fly off when cutting. Watch the grain carefully.

I had a local mill (he moved away) that kiln dried everything. For the big stuff, he would let it sit for 2 years - covered from the elements and stickered to dry before putting it in the kiln. Since he moved away, all wood that I get, I let it sit in my basement for 2 months with a dehumidifier running 24×7. This has worked well.- dbray45

This is worth repeating, I have been working some really nice local walnut. I have had some incidents where pieces have broken off, and kicked back out. Best not to stand in front of the blade!
 
#22 ·
New to woodworking so any help appreciated. I'm making cutting boards and charcuterie boards and having issues with cupping. I am using slabs that have been dried but that's all I know. They were 2" and I took them back and had them re sawn to 1" give or take. What do I need to be aware of. I am using black walnut, cherry, and rainbow poplar. Any help appreciated!
 
#23 · (Edited by Moderator)
New to woodworking so any help appreciated. I m making cutting boards and charcuterie boards and having issues with cupping. I am using slabs that have been dried but that s all I know. They were 2" and I took them back and had them re sawn to 1" give or take. What do I need to be aware of. I am using black walnut, cherry, and rainbow poplar. Any help appreciated!

- Papa3
When wood is cut it dries quickest at it's ends, then it follows across the side, and face grain. Last to go is the interior wood which even if done in a kiln, may not dry as completely as the exterior. Air dried is even slower to dry, unless evidently you live in Arizona :)

So when you cut a board open, especially if you open it lengthwise you are getting to moister wood in most cases. Probably not dripping wet, but often wet enough to actually feel it if touched right after you resaw it for thickness.

Because of this, if you can, it's best to remove wood from both sides in an attempt to "balance" this moisture. Then right after cutting, to lay it out on some stickers so air can flow all around the wood. The OP had an issue where he laid it flat, and the side down didn't get air, and the board cupped. Fortunately the fix is flip the board, and allow air over the entire board to balance the drying. Allowing time for the wood to acclimate is the next step.

Some folks put a piece of wood, sticker it, another piece of wood, sticker that, and build a pile, then clamp the entire pile to try to keep any movement from occurring. I do not, I just sticker, and most of the time end just fine.

For your boards, it's a fact smaller pieces dry faster, and sometimes more completely. If you cut your strips from the resawn slabs, cut them oversized, so you have stock left OVER what you plan for final size. Allow that wood to dry for several days, to a week, stickered is good. Then when you are close to glue up, joint, plane and cut to final size, and glue right after you prep. Cutting them more, and allowing them to sit, and you will likely see more minute movement.
 
#24 · (Edited by Moderator)
Never in my 76 years have I seen a piece of wood @ 6-8 %. Unless of course it just came out of a kiln. I have wood in my shop that's 20 years old and none of it gets to 6-8% .

Wood is hydroponics kind of like a sponge. It will adjust (in time) to the environment its store in. Right now the humidity is 95 percent outside and 43% inside.

About the best I've seen in my shop it 10-12%. I build with that all the time. One last note, maybe my moisture meter is off.
 
#26 ·
Never in my 76 years have I seen a piece of wood @ 6-8 %. Unless of course it just came out of a kiln. I have wood in my shop that s 20 years old and none of it gets to 6-8% .
- AlaskaGuy
Location is everything.
Have 7% RH this afternoon in Arizona. Single digit RH is normal here. Was 22% RH this morning. Most of my stored wood measures 4-6% or less 9 months of year. Only during monsoon season, or Dec/Jan cloudy/gloomy season will it climb higher. Have some air dried slabs I checked a couple weeks ago by cutting off a slice; measured 5% on outside and 9% on inside.

Most sawyers in AZ keep fresh slabs in shade for 6 months before using kiln to equalize the moisture levels and kill bugs, as kilns get too hot to fast. Don't dare put fresh cut oak in AZ sunshine kiln, it cracks/splits and surface hardens; making it firewood in week or two.

Reference materials that might help:
https://www.wood-database.com/wood-articles/wood-and-moisture/

Chapter 4 of USDA lumber handbook: https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_04.pdf

Entire Wood Handbook if you want all 15MB of PDF file:

Cheers!
 
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