Plywood Dado & Glue

[Marty Tippin]

Building a cabinet-type box, 3/4" birch veneer plywood. Wanted the floor of the box to sit in a dado in the sides, so grabbed my Whiteside undersized plywood router bit and cut a nice dado. Floor fits snugly into the dado. All is right with the world.

Until I went to glue it up.

I glued the dado - sides and back -- and a little glue on the sides of the floor that was to go into the dado. Before I could get them together, they swelled up and made it all but impossible to join the two. I ended up frantically pulling them back apart, trying to clean out the glue, waiting for it to dry and hitting both pieces with some sandpaper to try to make the fit a little less tight. Eventually got them glued up, but had to pound a lot harder with a mallet and block of wood than I think I should have.

What did I do wrong? Should I have put glue only in the bottom of the dado and edge of the floor piece?

[John Schweikert]

Clean fit instead of a snug fit might be better. Gluing all the interior of the dado is good, more glue surface, stronger joint. There is no proper standard anymore when it comes to plywood thickness unfortunately. I think we have reached the point of laziness from plywood manufacturers of the world. But any wood product does naturally vary in size due to moisture. So the undersized router bit was just a tad undersized to that specific undersized ply. Making your own dado jig (many simple designs are out there) is the best approach to routing a dado. The jig is set by the thickness of the ply, then just rout out that space with the jig clamped in place.

[Alan Bienlein]

Just a bead down the center is all I ever do. Never had a problem doing it this way and is plenty strong!

[Roger Rayburn]

I agree with Alan. A bead down the bottom of he groove will force some squeeze out into the sides of the groove when you clamp it closed. If you have a good fit that then swells a bit, good clamps will force it home. If you have a little slop in the fit and adding glue swells it to a tight fit, when the glue dries the glue is going to have a stress you'd rather not have.

[Joe Scharle]

Make an adjustable dado jig and forget the special, undersized bits. This one has a built-in clamp, but yours wont have to be as complicated.

http://www.ncwoodworker.net/pp/data/1582/Dado_Jig.JPG

[Rod Sheridan]

Hi Marty, any joint needs a gap to allow the glue to be in there, in addition to any swelling.

Joints should fit nicely together, no straining or pounding allowed..............Regards, Rod.

[Curt Harms]

Make an adjustable dado jig and forget the special, undersized bits. This one has a built-in clamp, but yours wont have to be as complicated.

http://www.ncwoodworker.net/pp/data/1582/Dado_Jig.JPG

I agree with the jig. You use the ply in question to set the width of the dado. Nice fit every time. I made my jig with 3/4" sides and use a 1/2" pattern bit. Clamp one jaw, insert a piece of the subject plywood, set the other jaw snug and remove the plywood sample. Run the bearing against each jaw and perfect fit, no having to calculate bushing offsets. Downside is that it only works for 1/2" and wider dados.

[Thomas love]

Do not let the ply govern the size of the dado, I dado my parts less than the thickness of ply , the width of the dado dose not matter as long as it is less than the thickness of ply. When its time to assemble I route a shallow rabbit to fit the dado on the part I want to go into dado. you can use a straight bit in router table or rabbeting bit in hand held . I have had instances where I plowed all my dados in the morning and they fit perfectly only to have a swing upwards in the humidity by time of assembly and nothing fit. The thickness of the ply should never matter,, just think like a woodworker.

[glenn bradley]

I do as Thomas describes but, if you want to dado in the whole thickness a self adjusting jig as shown earlier is the ticket. In the immortal words of Sam Maloof "leave some room for the glue!"