Plywood question...

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Forum topic by woodcox posted 06-29-2015 06:01 PM 1009 views 0 times favorited 8 replies Add to Favorites Watch
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2026 posts in 2005 days

06-29-2015 06:01 PM

I am building a couple cases from this 3/4”-7 core ply(5-solid wood core, 2-veneer). Plans call for five hard maple drawer runners dadoed in at a 1/4” depth. Upon machining the 27” tall, 20” deep side panels they developed an outside bow around a 1/16” as seen in the pic. Runners have been glued in at this point and there will be hardwood edging applied to the front with a 1/4” ply back rabbeted in flush. I have a couple ideas to try and correct this bow to bring them back to flat if possible.

1. Make two 1/4” deep saw kerfs at about 9” and 18” on the OUTSIDE of the cases to try and relieve the core stress and fill them back in with solid wood.

2. Same as above except 1/2” dados to be filled back in flush to the outside of the panels.

3. Remove the rabbets for the back panel to have a 3/4” ply back not flush with back of the cases. I have doubt in this one as it should bring the back flat but the front would only have the hard wood edging trying to keep the front sides flat.

4. Assemble as is. Build drawers to possibly find room inside between the drawers near the center for one or two horizontal braces to push the side panels back flat.

This is a shop storage project and if I go with #1 or #2, I really don’t care if the filled in kerfs or dados are seen. This is my first time machining ply to this extent. I also realize I should have selected a higher quality ply for this project. Any ideas or comments would be greatly appreciated.

-- "My god has more wood than your god" ... G. Carlin.

8 replies so far

View rwe2156's profile


2916 posts in 1474 days

#1 posted 06-29-2015 07:02 PM

Personally I would try to get the bow out if the drawers are flush.
If the drawers are overlay, I wouldn’t worry about it.

Here are my thoughts:

Were the runners kind of tight? If so, they cumulatively could be causing the bow.
With this in mind, you could try relieving the tension by ripping the ply along the bottoms of the middle 3 runners maybe 5/8” into the ply see if that works. You can always fill in the saw kerf if you’re worried about strength.
We I see it you’ve got nothing to lose and the groove will be covered by the edge banding.

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

View HokieKen's profile


4955 posts in 1132 days

#2 posted 06-29-2015 08:03 PM

If you can give up the space at the rear of the cabinet, you might try using 3/4 ply rabbeted in. The rabbet will give you extra surface area for glue. As long as the sides of the back piece are straight and parallel, you can clamp it up and know it’s bringing the sides square. Of course, that may still leave some bow at the front as you noted in #1.

Concerning #2 & #3, I’d be leery of making any relief cuts on the cupped side of the boards. Chances are the runners are causing the bow, either due to dadoes being too tight or humidity changes causing swelling. Relief cuts on the outside of the case will most likely only allow it to bow further in that direction. I’d keep any relief cuts inside the box.

As rew2156 suggested, if you cut your dadoes too tight for the drawer runners, relief cuts through the runners and around 1/4” into the ply below may help as well. It could also be that you machined the runners and cut the dadoes in low humidiity and increased moisture in the air has caused the wood to swell. Relief cuts through the runners will help now and in the future in that situation.

I wish you the best of luck fixing this issue but the good news is that it’s a cabinet and doesn’t have to be perfectly square to function properly. Be careful to allow some “wiggle” room for humidity changes when you build your drawer boxes and make sure those dadoes have room to accommodate changes in the runners without locking up.

-- Kenny, SW VA, Go Hokies!!!

View nkawtg's profile


275 posts in 1245 days

#3 posted 06-29-2015 08:27 PM

How tight are the runners in the DADO? They might be imparting some pressure causing the bow.

View woodcox's profile (online now)


2026 posts in 2005 days

#4 posted 06-29-2015 11:37 PM

Dados were ok, runners just fell in. I think I need to point out that the bow started immediately after cutting the dados and actually got a little better after gluing the runners in. Panels sat machined for a few days under weight before I glued the runners in. The bow look like this… ) ( .

Thank you guys for the input so far.

-- "My god has more wood than your god" ... G. Carlin.

View rwe2156's profile


2916 posts in 1474 days

#5 posted 06-30-2015 11:52 AM

Thought about it some more and I would go with option 4.

If you do overlay drawers you won’t see it.

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

View BinghamtonEd's profile


2298 posts in 2363 days

#6 posted 06-30-2015 01:18 PM

Where’s this plywood from? If its from a big box store, you could probably bring in the pieces and argue for a replacement or your money back. They seem to be pretty non-chalant about it at some places. I know this doesn’t help you now, but I just recently started getting my plywood from a local lumber place, it is a much more stable domestic plywood that runs me about $10 more per sheet.

-- - The mightiest oak in the forest is just a little nut that held its ground.

View Daruc's profile


460 posts in 1126 days

#7 posted 06-30-2015 01:49 PM

Just like your saying, put the hardwood on the front, being sure to turn any bowed pcs (if there are any) to help straighten the sides.
The 1/4” back rabbeted in and glued should be fine. Be sure to clamp the sides tight before nailing off the back.
And then, #4, if you can put some stretchers in between the drawers, you should be fine.
(Might not even need the stretchers)
I wouldn’t make the drawers until your boxes are made in case you have to adjust the drawer box sizes.
Your only 27” high, I don’t know how wide your face pcs are but the back and face should take the bow out.
You may want to clamp your side pcs down flat and apply your edging while it’s clamped down and let dry. If your only using 3/4” edging it may not help as much.

-- -

View woodcox's profile (online now)


2026 posts in 2005 days

#8 posted 07-02-2015 04:36 AM

So far so good…

I made a couple 5/16” deep kerfs on just two of the four side panels. I had an ‘oh yeah’ moment when I realized that two of the panels are going to be next to the bench frame. When installed the screws would bring the other side of the cases flat against the legs. Here is the frame they are going into…

I still need to fill them in but at least the kerf sides will be on the inside and not as visible. I also couldn’t believe I do not have a full kerf blade, I have all kinds of 1/8” scraps I could have used here. Dry fitting one of the cases…

-- "My god has more wood than your god" ... G. Carlin.

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