How to attach a divider in a plywood box for strength ? stupid question #147

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Forum topic by OldBull posted 06-13-2021 06:52 PM 725 views 0 times favorited 5 replies Add to Favorites Watch
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541 posts in 585 days

06-13-2021 06:52 PM

Good afternoon, I am not a kreg joint person,

If I am building a simple birch plywood box and I want a divider to divide the inside into 2 compartments, what method should I use to join them so the divider strengthens the box. My (perceived) problem is the birch divider will be on edge everywhere it is glued (3 sides), will edge glued be strong enough ? I have made few boxes from plywood and in my mind can’t see what joinery would be strong. Brads might hold till the glue dries but adds little strength, screws would seem weak in plywood into edgegrain. I am sure I am wrong somewhere so I thought I would ask. Does a plywood box need a frame ?? I may be making the mistake of not having a dado or some other joint, just edge grain? Even just the box itself needs a rabbet joint right ? I did make an open box from plywood but rabbeting plywood was very difficult.

Sorry for what feels like a stupid question.


5 replies so far

View Bill White's profile

Bill White

5362 posts in 5250 days

#1 posted 06-13-2021 07:00 PM

Ply has end grain and long grain. Gluing will attach the ply to the long grain face of the plywood carcass, so it will be strong enough for a divider if ya use a GOOD glue like Titebond Three.
Brads will add some strength, and not split the divider ply if they are “centered” on the divider.
Give it a shot. I’ll bet it will work good enough.

-- [email protected]

View Phil32's profile


1613 posts in 1193 days

#2 posted 06-13-2021 11:09 PM

The primary strength will be the corner joints of the box. If your divider is butt jointed to the sides the main weakness will be sheer forces (parallel to the sides.) The best solution would be box corners that are rabbeted or box jointed & dado joints for the divider.

-- You know, this site doesn't require woodworking skills, but you should know how to write.

View SMP's profile


4974 posts in 1195 days

#3 posted 06-13-2021 11:18 PM

Kind of depends on a lot of things. How big is the box? How thick os the ply? Does the divider go vertically or horizontal? If horizontal how much weight will be on it?

Without knowing any detail:

Drywall screws into 3/4” ply is plenty strong.

If a lot of weight on a horizontal shelf and you don’t dado, can always add a cleat under each side screwed inyo the side and back.

View Madmark2's profile


3167 posts in 1878 days

#4 posted 06-14-2021 12:39 AM

Dado the dividers into the sides. Once glued they will act as a stressed web members and be hugely strong.

Don’t miter plywood. Rabbet or dado much preferred.

Each ply’s grain runs differently than the adjacent plys. Butt joint will work for most light to medium duty tasks.

Joining ply like for a carcass is a good application for biscuits in a butt join. A couple-three biscuits in the edges hold things in place while gluing. Glue the biscuits into one piece and let dry. Then you can test fit and the pieces will hold in place. When glue is added to the edges it should have a “drop in” fit and not get squirmy while wrestling with clamps.

This literally turns final assembly an “Insert tab A into slot B.” operation.

-- The hump with the stump and the pump!

View Jimothy's profile


85 posts in 2229 days

#5 posted 06-16-2021 06:16 AM

Considering the layers of plywood alternate in grain direction, all edges are a combination of side grain and end grain. Just put a little more glue than usual considering it is partially end grain and let it soak in for a bit and put more if needed. Honestly, plywood is kind of convenient like that. Dont really need to do any reinforcements most of the time, especially for a divider and a box. If you are concerned about getting the divider in the right spot, you could always make a dado to fit the divider.

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