Cupped Wooden Slabs, How to Flatten and Join

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Forum topic by natzem posted 12-31-2016 02:49 PM 3130 views 0 times favorited 5 replies Add to Favorites Watch
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2 posts in 1521 days

12-31-2016 02:49 PM

Topic tags/keywords: router jig jointer joining wood slab flatten planer cupped wood sugi wood

Two Book Matched Pieces:
1.375” Thickness, 18” Width, ~90” Long

There have been many conflicting opinions I have read online about how to approach some of the problems I am having with a pair of book-matched wooden slabs. Currently each slab is cupped enough such that joining them would produce something similar to the picture below (under current status) I also can’t joint the edge to get a straight line since I wouldn’t be able to get a clean line since the cupping changes slightly throughout the slab. I did try running the mating edges through a panel saw to get a clean line, but some warping in the wood made it such that the cut twisted a bit. Unfortunately when I had bought the slabs they were flat, however, the change in humidity level from where we got them to where they are now means that the wood doesn’t stay flat (as I had seen it when I bought it).

So far my intuition tells me 1) to take each individual half and remove material from the top of the rainbow by using a router sled (image for reference). 2) Then use a planer to flatten the other side. And then 3) use a panel saw to get a straight edge.

Current Status:

Step 1:

Step 2:

Step 3:

The boards themselves!

Left Slab:

Right Slab:

5 replies so far

View Ger21's profile


1100 posts in 4140 days

#1 posted 12-31-2016 02:57 PM

How did you store them after you bought them?
They should have been stickered so that air could circulate all around them.

What you propose is really the only option. But if they aren’t completely dry, they may warp again after you flatten them.

-- Gerry,

View avsmusic1's profile


680 posts in 1694 days

#2 posted 12-31-2016 04:17 PM

If you flip the slabs will they flatten out a bit? Probably only helpful if they weren’t stickered while acclimating initially

One other quick option if u haven’t already built the router sled – you could build a jointing sled for the planet (assuming your planet can accommodate the 18” per your plan).

View Logan Windram's profile

Logan Windram

347 posts in 3471 days

#3 posted 12-31-2016 04:20 PM

First, get a moisture meter to make sure these slabs are at ideal workable dryness. I’d use a handplane to knockdown and flatten the concave sides… find a shop with a timesaver and call it done…

View natzem's profile


2 posts in 1521 days

#4 posted 12-31-2016 05:36 PM

When I initially stored them I laid them on their sides (lengthwise against the wall) with spacers in between. The I noticed the cupping issue so I tried laying them down such that the legs of the rainbow were facing up. Then I used a wet sponge for a few days to try and expand the fibers on the drier side. I did that for about three days but it doesn’t seem to have done much. I am now storing them upright but they each have their own spot on the wall.

I was told they were kiln dried so I assumed they were dried properly, but I do think it would be a good idea to get a moisture meter to see the difference between both sides. Any recommendations for good ones on the market?

First, get a moisture meter to make sure these slabs are at ideal workable dryness. I d use a handplane to knockdown and flatten the concave sides… find a shop with a timesaver and call it done…

- Logan Windram

How would I go about doing this hand planing operation precisely if I do?

View JBrow's profile


1368 posts in 1929 days

#5 posted 12-31-2016 08:47 PM


The router sled idea could achieve the flat slabs you are after; as could other methods. However, a great deal of thickness could be lost leaving slabs a little too thin.

Another option, after the slabs have fully acclimated, would be to rip the 18”wide right slab to narrower widths and flatten and then plane the ripped slabs. Narrower the ripped pieces would result in less material removal for flattening. After jointing the ripped edges, the slabs could be glued back to re-form wide slabs, although a bit narrower than the originals. Unfortunately if internal stresses exist in the ripped slab, a fair amount of material would have to be removed from the edges to produce straight glue lines. I know of no way to determine whether any internal stresses are present in the right slab.

The left slab does not look too bad from the photo.

I can offer no advice on moisture meters and since I am not proficient with hand planes, I will remain silent on hand planning techniques.

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