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Walnut slab cupping after flattening?!

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Forum topic by Tishman posted 08-30-2016 04:42 PM 761 views 0 times favorited 20 replies Add to Favorites Watch
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Tishman

6 posts in 269 days


08-30-2016 04:42 PM

Topic tags/keywords: slab flattening cupping

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!


20 replies so far

View RRBOU's profile

RRBOU

136 posts in 1752 days


#1 posted 08-30-2016 04:50 PM

I am guessing that you laid them on the bench, the under side did not get any air and this is why thy cupped. It is very important to after removing thickness to let air get to both sides.

-- If guns cause crime all of mine are defective Randy

View Aj2's profile

Aj2

684 posts in 1257 days


#2 posted 08-30-2016 05:02 PM

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

View gargey's profile

gargey

457 posts in 235 days


#3 posted 08-30-2016 05:03 PM

Higher than usual neutrino shower last few days could have contributed.

View soob's profile

soob

223 posts in 668 days


#4 posted 08-30-2016 05:53 PM

2” lumber that shows 7% on the outside might be 20% in the center. Thick slabs take a long time to dry.

View rwe2156's profile

rwe2156

2187 posts in 940 days


#5 posted 08-30-2016 06:05 PM

Big checks and splits indicate internal stress.

or what he ^ said.

-- Everything is a prototype thats why its one of a kind!!

View Tishman's profile

Tishman

6 posts in 269 days


#6 posted 08-30-2016 06:13 PM

Thanks, everyone!

@RRBOU you are correct. I flipped it and it seems to be flattening back out. If it flattens back out, can I continue working on it (belt sanding, finishing, etc.) and just leave it on its edge when not working it?

View WDHLT15's profile

WDHLT15

1571 posts in 1935 days


#7 posted 08-31-2016 12:51 AM

Yes, you need to let it get air on all sides. I suspect that the slab is not as dry as you think it is, as a dry slab would not cup.

-- Danny Located in Perry, GA. Forester. Wood-Mizer LT40HD35 Sawmill. Nyle L53 Dehumidification Kiln. hamsleyhardwood.com

View mahdee's profile

mahdee

3547 posts in 1227 days


#8 posted 08-31-2016 10:44 AM

Instead of putting it on its edge, put some 2×4s under it.

-- earthartandfoods.com

View dbray45's profile

dbray45

3178 posts in 2236 days


#9 posted 08-31-2016 12:43 PM

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.

-- David in Damascus, MD

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dbray45

3178 posts in 2236 days


#10 posted 08-31-2016 12:49 PM

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.

-- David in Damascus, MD

View Tishman's profile

Tishman

6 posts in 269 days


#11 posted 08-31-2016 03:02 PM

Thanks everyone! Slab has started to flatten back out. Will keep it on 2X4s on the bench while working on it, installing bowties, sanding, etc. Appreciate all the advice!!

View ki7hy's profile

ki7hy

493 posts in 199 days


#12 posted 08-31-2016 03:24 PM



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! ;)

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dbray45

3178 posts in 2236 days


#13 posted 08-31-2016 07:21 PM

Yep – setting it on the ground in the sun will scorch it.

-- David in Damascus, MD

View Tishman's profile

Tishman

6 posts in 269 days


#14 posted 09-05-2016 07:11 PM

Thanks again, everyone. I reflattened both sides. It’s at about 13% moisture content. Should I put it on 2×4s and clamp it to the workbench so it won’t cup or what? Please advise!

View Logboy's profile

Logboy

43 posts in 2689 days


#15 posted 09-10-2016 03:54 AM

You need to stop working on it and wait until the dang thing is dry or you’ll just be chasing your tail. It will continue to just cup and warp until it finally reaches 6-8% moisture.

-- No log is too big to saw! www.logboy.com

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