Noob question: joining birch plywood

[VirtuaLogic]

Woodworking newby here.

some 15 odd year ago I had a desk made for me from 30mm birch plywood. It was too big and heavy to be kept in one piece so it was cut in two pieces. Joining was done with steel profiles, threaded rods and nuts, and biscuits for alignment. I have decided that I want to make a smaller one-piece desktop out of this material.

I have cut away all pieces that I do not want to use. I'm left with two pieces of desktop roughly sized 600mm by 1200mm. The cuts were made with the TS55 and a home-made MFT top acting as a square, so I'm quite confident the pieces are square in both planes.

What is the best way to join these pieces and achieve a strength comparable to an uncut board? I think a simple butt-joint might not be enough. I do not have a domino and cannot afford one at the moment. I do have a cheapo biscuit joiner, but I doubt the biscuits will add any structural strength.

My idea would be to take the router and cut a mortise and tenon over almost the whole length of the surface to be joined, maybe 5cm deep. Will this add to the strength, or is it not going to give additional strength over a butt-joint?

Other suggestions are very welcome.

Cheers,  Alex.

 

[ear3]

A hardwood spline as you suggest would work, just don't make it too thick and compromise the strength of the tongues that are left.  I'm not sure if one that is just 5mm on either side would give you all that much more strength than biscuits however -- if you go that route I would make groove 10-12mm deep on either side.  If the desk top is well supported and you won't be standing on top of it , I wouldn't even worry about it using biscuits.

An additional security measure would be once it is joined to add a hardwood cap along the entire edge of the assembly, or at least perpendicular to the joint. 

Also take care when clamping it that you use cross pieces/cauls perpendicular to the joint to keep an even plane -- when gluing up large panels like this the clamping pressure can easily cause the pieces to fold up or down along the joint. 

 

[VirtuaLogic]

Hi Edward, thanks for the reply.

It seems I was not clear enough about my plans; I'm contemplating making a groove in one side and a tonge in the other, not using hardwood in two grooves. Also, the depth of the tongue and groove I suggested 5cm, that equals 50mm. I agree 5mm would not do much for strength.

Is a hardwood floating tenon like you suggest desirable over a tongue from the same plywood?

Cheers,  Alex.

 

[ear3]

I've never done tongue and groove with plywood -- but since the ply you're using is so thick and is presumably good quality (thin plys) theoretically I don't know why it wouldn't work.  Though I don't think it would be as strong as a hardwood spline.

The concern I would have is about the precision you would need to cut the mating pieces for an exact fit.  You would have to ensure that the groove was perfectly centered, and then have to change the depth settings to do the tongue also similarly perfectly centered on that groove (or vice versa).  If you have scrap pieces and digital calipers then it is doable, but it will be nerve racking. With tongue and groove on hardwood you can correct inelegant seams fairly easily with sanding.  But with plywood those imperfections are a lot more evident and difficult to correct.

The advantage of doing a hardwood spline is that it doesn't rely on perfect measurement, just repeatability.  You set the depth once and use the same setting for cutting the groove on both pieces.     

Hi Edward, thanks for the reply.

It seems I was not clear enough about my plans; I'm contemplating making a groove in one side and a tonge in the other, not using hardwood in two grooves. Also, the depth of the tongue and groove I suggested 5cm, that equals 50mm. I agree 5mm would not do much for strength.

Is a hardwood floating tenon like you suggest desirable over a tongue from the same plywood?

Cheers,  Alex.

 

[tiralie]

My idea would be to take the router and cut a mortise and tenon over almost the whole length of the surface to be joined, maybe 5cm deep. Will this add to the strength, or is it not going to give additional strength over a butt-joint?

Tongue and groove or if you have enough material a wide half lap will work. It will never be as strong as an uncut board.
Tim

 

[Holmz]

If it needs to be strong then you would need a new piece of plywood.

Almost all the bending become compression one side and tension on the other, so a biscuit in the mddle does not help a lot.

Grain aligned 90 to the joint with a dado on each side would work.
The half lap is also strong enough if no one is dancing on the desk.

You will probably need a router.