Flattening a cupped board

[Todd Burch]

If a board, counter top or tabletop won't stay flat and wants to cup, and I need it flat on its own merit, I'll kerf the board lengthwise on my tablesaw with the grain, on the bottom or backside, push/clamp it down flat, and glue in a strip of matching wood. I've attached a photo-essay of the process. While I show the kerf running all the way out the end of the wood for illustration purposes, on pieces that the end-grain will show, I stop short of the end so that you can not see the glued in strip(s).

I use this technique to flatten both lumber and plywood. If I'm flattening plywood, like for a counter top, I won't go through the hassle of glueing in a wedged spline, 'cuz you'll never see it. The technique is the same technique as kerf-bending, but the objective and results are just opposite (curved to flat instead of flat to curved).

I'll make the splines rectangular, the same width as the fattest part of the wedge, and then either block plane or sand the spline to fit. It can be full length or pieced in.

Depending on the piece and the amount of cup, I will choose a depth of cut for the kerf. For a large solid wood tabletop, I will kerf only as far as I feel I need to to get the cup out. On utility items, I generally kerf deeper for speed's sake.

Todd

(Drawings done in Sketchup!)

(How many people wanna put odds on this technique showing up in a woodworking rag within the next 2 months? ;) )

[Chris DiCiaccio]

If a board, counter top or tabletop won't stay flat and wants to cup, and I need it flat on its own merit, I'll kerf the board lengthwise on my tablesaw with the grain, on the bottom or backside, push/clamp it down flat, and glue in a strip of matching wood. I've attached a photo-essay of the process. While I show the kerf running all the way out the end of the wood for illustration purposes, on pieces that the end-grain will show, I stop short of the end so that you can not see the glued in strip(s).

I use this technique to flatten both lumber and plywood. If I'm flattening plywood, like for a counter top, I won't go through the hassle of glueing in a wedged spline, 'cuz you'll never see it. The technique is the same technique as kerf-bending, but the objective and results are just opposite (curved to flat instead of flat to curved).

I'll make the splines rectangular, the same width as the fattest part of the wedge, and then either block plane or sand the spline to fit. It can be full length or pieced in.

Depending on the piece and the amount of cup, I will choose a depth of cut for the kerf. For a large solid wood tabletop, I will kerf only as far as I feel I need to to get the cup out. On utility items, I generally kerf deeper for speed's sake.

Todd

(Drawings done in Sketchup!)

(How many people wanna put odds on this technique showing up in a woodworking rag within the next 2 months? ;) )

Thanks! Now that is information worth keeping. Do you use a formula for depth of cut or trial and error? Also, when you need to stop short of the ends, do you just stop your table saw blade short or do you use a router, etc? Thanks for sharing, and have a great Christmas.

[Todd Burch]

Chris,

No formula, just trial and error, until that turned into experience! The way I showed the cup in this illustration would be difficult to cut on the tablesaw - since it's cupped! In this case, I would use a circular saw (now that I have a Festool). A router would work, but that would be a lot of traveling with a 1/8" router bit. I break all my 1/8" bits on first use typically. (The board I showed is 40" wide and 10' long - to scale). When I have to bend a board flat and the kerf wedges closed instead of wedging open (like I showed), I just make the spline rectangluar in section (simpler) and use glue to fill the gap down in the wood where it is hidden and will never be seen. Todd.

[Chris Padilla]

Nice...thanks for the tip, Todd. Are you preparing for that chunk 'o bubinga, maybe??? :p

I need to start playing with my trial version of Sketch-up...interestingly, I think LOML will get a trial version, too! :p

another Chris

[John Miliunas]

Hey, now that IS information worth keeping! Todd, many thanks for that super helpful hint. You should send that in to one of the WW rags. I'd wager it would earn you a top spot and "prize"! The added exposure wouldn't hurt, either! Thanks again. Great info. :cool:

HAPPY HO-HO!

[Mark Singer]

If a board, counter top or tabletop won't stay flat and wants to cup, and I need it flat on its own merit, I'll kerf the board lengthwise on my tablesaw with the grain, on the bottom or backside, push/clamp it down flat, and glue in a strip of matching wood. I've attached a photo-essay of the process. While I show the kerf running all the way out the end of the wood for illustration purposes, on pieces that the end-grain will show, I stop short of the end so that you can not see the glued in strip(s).

I use this technique to flatten both lumber and plywood. If I'm flattening plywood, like for a counter top, I won't go through the hassle of glueing in a wedged spline, 'cuz you'll never see it. The technique is the same technique as kerf-bending, but the objective and results are just opposite (curved to flat instead of flat to curved).

I'll make the splines rectangular, the same width as the fattest part of the wedge, and then either block plane or sand the spline to fit. It can be full length or pieced in.

Depending on the piece and the amount of cup, I will choose a depth of cut for the kerf. For a large solid wood tabletop, I will kerf only as far as I feel I need to to get the cup out. On utility items, I generally kerf deeper for speed's sake.

Todd

(Drawings done in Sketchup!)

(How many people wanna put odds on this technique showing up in a woodworking rag within the next 2 months? ;) )

Todd,
Excellent.... What is Sketch-up, I think I was using it incorrectly on french fries.. Where do you get that program
Mark

I went to the "sketchup" website...it looks good. Did you pay around $475?
mark

[Todd Burch]

Yes, www.sketchup.com, Free to download an 8 hour trial, $475 to get a permanent key.
Todd.

[Mark Singer]

Yes, www.sketchup.com, Free to download an 8 hour trial, $475 to get a permanent key.
Todd.

Todd,
Do you like it? Merry Christmas!
Mark

[Todd Burch]

Do I like it?

Do I like it?

DO I LIKE IT???

Does a bear poop in the woods? Is the Pope Catholic? Yes, I like it. (Understatement of the year award goes to Todd)

:D

[Mark Singer]

Do I like it?

Do I like it?

DO I LIKE IT???

Does a bear poop in the woods? Is the Pope Catholic? Yes, I like it. (Understatement of the year award goes to Todd)

:D


Enjoy the day !!!!
mark

[Todd Burch]

Keith, please make into an article. Todd

[Martin Lutz]

Todd, great tip.

I am curious if you worry about the amount of moisture the glue adds to one side of the board. do you hold it flat for a certain amount of time? also have you ever tried it on burl? I have a large (30" x 60") maple burl 1" thick that I need to flatten. I have heard advice to lay it out in the sun over some damp grass. Any advice from the ranks of the creekers would be appreciated.

[Gene Collison]

Todd,

Really cool idea which I will remember and use, ideal for the Festool saw too. Thanks for posting....

Gene

[Chris Padilla]

Martin,

I would think the moisture unbalance to be a very temporary problem and not one that should cause the piece to curl back to where it was. MC should be back to "even" in a few days.

Flattening something that thick (1") using moisture, heat, weight, and TIME takes some patience but it can be done. Todd's method is quite speedy. Also, I think the reverse of Todd's method could be done to the other side of the cup: only this time the kerf cut would close up: If someone made a router bit or even saw blade that would make a very steeply-angled V-cut, it should work to flatten the board. This way, you don't have to glue in extra wood...just close up the board on itself.

[Martin Lutz]

Chris,

Thanks for the reply. I am not sure how to get that flat. I dont want to cut into it because of the burl, but it would make a great desk top if I can flatten it. the moisture, pressure and time route may be the best.

From your location it looks like you are in Colorado. Welcome to God's Country. The only better place is on the western slope of Colorado, but no prejudice here.

[Chris Padilla]

Martin...I am from Colorado but have lived in California since 1998 so I enjoyed the first Super Bowl win by the Broncos while still in town but watched the second SB win from here in California.... :)

You might rent some time with a pro woodshop, Martin, that has a wide enough belt sander and just go for it. Otherwise, you could get quite a work out with an appropriate hand plane to shave down either the sides or the middle.

Sounds like a very nice piece...could you post some pics of it? :)

[Steve Jenkins]

Hey Martin isn't about time for a road trip to Dallas? like maybe for the 23rd?.
I have a solid oak dining table that is about 42x96 that was outside and rained on for a summer. It's not mine but I'm trying to flatten it for a client. I got it to flatten by putting the top concave face up and keeping wet towels on it for about three days in the shop. this was without removing the finish. it flattened out but when it re-dried it curved right back again. I've decided I need to try Todd's method. Steve

[Chris Padilla]

Steve,

I wonder if you remove the finish if it will hold? Perhaps some clamps/weight might help??

[Martin Lutz]

Hey Steve, I would love to come to Dallas for the BBQ and see everyone. Its been a long time. I would also like to come poke around your shop and visit with the master. Maybe around Christmas. My wife is from McKinney.

Did you hold the top flat while it dried? I wonder if that would help. The cup is too much for a wide belt sander. Probably 1" - 1 1/4" over the length of the plank.

[Chris Padilla]

That burl is a pretzel, Martin!! I think the slow route is a good one to take for your situation. Perhaps if you can get it flatter the slow route (and not necessarily flat) you could then sand/plane it flatter or perhaps rip it in half and resaw it into veneers. Fun stuff to play with....

[Greg Tatum]

I LOVE THIS SITE!!! Thanks Todd...what a timely post....I have a need for this very info...I was gonna rip down a board to fit the 6" jointer, remove the cup and then reglue but I'll try this technique first...Thanks.

Greg

[Kelly C. Hanna]

Todd does it again!! Great tip for sure and it will help many of us. I had one like that happen once on an Oak panel I had made. I ripped and re-glued it in alternating patterns...your idea woulda been much easier!!

[Prashun Patel]

This is an extremely old post, but I just had to revive it. This technique worked marvelously for me on a blanket chest I'm making now. Thanks, Todd...

[nicholas guenther]

If a board, counter top or tabletop won't stay flat and wants to cup, and I need it flat on its own merit, I'll kerf the board lengthwise on my tablesaw with the grain, on the bottom or backside, push/clamp it down flat, and glue in a strip of matching wood. I've attached a photo-essay of the process. While I show the kerf running all the way out the end of the wood for illustration purposes, on pieces that the end-grain will show, I stop short of the end so that you can not see the glued in strip(s).

I use this technique to flatten both lumber and plywood. If I'm flattening plywood, like for a counter top, I won't go through the hassle of glueing in a wedged spline, 'cuz you'll never see it. The technique is the same technique as kerf-bending, but the objective and results are just opposite (curved to flat instead of flat to curved).

I'll make the splines rectangular, the same width as the fattest part of the wedge, and then either block plane or sand the spline to fit. It can be full length or pieced in.

Depending on the piece and the amount of cup, I will choose a depth of cut for the kerf. For a large solid wood tabletop, I will kerf only as far as I feel I need to to get the cup out. On utility items, I generally kerf deeper for speed's sake.

Todd

(Drawings done in Sketchup!)

(How many people wanna put odds on this technique showing up in a woodworking rag within the next 2 months? ;) )
Todd, Thank you for the useful post! Any idea if I could make this work in reverse? My tabletop is cupped in the opposite way, as in the concave side is the bottom of the table and convex is the top. I"m trying to picture it in my mind...could I make relief cuts in the bottom could I force it to flatten out? In theory it seems like it might work? wondering as to whether or not You've tried it this way or not. Thanks in advance! -Nick

[daryl moses]

Wow, this thread started 14 years ago lol.

[Jerry Miner]

Todd, Thank you for the useful post! Any idea if I could make this work in reverse?... -Nick

Nick-- I don't know if Todd is around any more but the concept should work on your table top. I would make the kerfs about 2/3 the thickness of the top.

[nicholas guenther]

Thank you for the feedback Jerry!

[Warren Lake]

the old guys did this on furniture when they built it. If I looked at any top made by my favourite old guy there were kerfs under the table top. Even an end table the top had a number of kerfs in it, its called breaking the back. Inserting wood here a way to hopefully solve a problem. They were trained how to avoid them.