I need to fix solid wood, ash board 5cm thick to a plywood, two boards glued together, total thickness 42mm.

Here are the parts, grey is ash tabletop enter image description here

Can I glue it? Do I need fingers, or dado joint cuts? What should be the finger size?

  • What forces are going on this joint? What is it? – Oli Feb 12 '18 at 13:53
  • In this case, given there is 20 cm hang — there would be no more than 200N normal to the connection plane – Gleb Feb 12 '18 at 13:56
  • C-table , overbed table – Gleb Feb 12 '18 at 14:01

Before I ramble on, if this is for personal use, you could just wang some glue on it and it might be fine. We tend to over-engineer things because many of us are surrounded and inspired by furniture and techniques that have lasted generations.

I think you're under-accounting the forces here. If the overhang is 200mm on three sides —which seems perfectly reasonable for a bed table— 2kg applied to (eg) the near edge will pivot around the near side of the joint and exert ~10kg across it. Glue should handle that but repositioning in bed means more like 40-50kg down, ~200kg up. It needs a strong joint.

Wood glue is strong, but really only when long-grain fibres meld. You're not going to get that in a butt joint from plywood end-grain .

Cutting a shallow dado will provide much more glue surface, as well as a mechanical advantage.

You can use dowel and loose tenons in plywood but they can lever apart the plies. A capped end (eg inside a dado) would help prevent that but I don't think it'd be any stronger.

But what about a brace along the leg side? It will change the design slightly but it'll provide a lot of stability, even at the far end. I expect this would be against the grain so I'd attach the brace to the surface with flat-head screws (eg pocket hole screws) and oversized holes in the brace, and not glue. This will allow the surface to swell and contract with the environment, something you don't have to worry too much about with your long joint.

Some combination of that and I expect you'd have something moderately solid, but again, if you have the ability to test any of this without making serious commitments, do that first. I would certainly consider a brace before routing out a dado slot.

And by extension, I would not consider a box/finger joint as a way for attaching the legs onto the surface. It's more complicated than a dado with no benefit.

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