Flush Joining Plywood to Hardwood Edge to Edge

Woodworkers suggest ways to make a plywood-to-hardwood edge joint smooth and flush. June 18, 2009

Question
We have a production need to join plywood and hardwood edge to edge. We have tried biscuits and dowels with varying degrees of success. We can never get a perfectly flush joint without a lot of adjusting. I would be willing to invest in equipment if I could guarantee a flush joint every time.

Forum Responses
(Solid Wood Machining Forum)
From contributor C:
Rabbet the solid piece with the long side under the plywood so there is ample surface to glue. When set up, sand in a wide belt to flush the surfaces.

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From contributor R:
I just completed a job gluing raised panel edges to veneered panels. I set up a three wing cutter in the router table with featherboards to keep everything flat and splined the joints. Worked very well.

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From contributor M:
Take a look at a lipping planer from either Lamello or Hoffmann. You can glue the hardwood edge on oversize in thickness and flush it up with the lipping planer after the glue sets up. The people I know who have owned them like the Lamello the best.

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From contributor L:
I set up the shaper with tongue and groove cutters for this. Coupled with a power feed, and accurate machining of the hardwood edge, I get good results.

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From contributor C:
That works fine if the plywood is thick, but my rabbet suggestion will work with a very thin panel. Since I had no idea what thickness he is using, I suggest the rabbet. Either will work, providing the panel is the correct thickness for the method.

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From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
I agree with contributor C.

Let me add that if the two pieces are at different MCs when assembled, you can make a perfect joint that will be imperfect after a few days as the MC adjusts. So, getting uniform MC (actually getting the two pieces to be at equilibrium at the same RH, as the heat used in plywood manufacturing means that it will be a little lower in MC than solid wood at the same RH) before machining is important. Then, once machined, the pieces should not change MC, as plywood and solid wood do shrink or swell differently.

It is also important to realize that if you use a water-based finishing system, the moisture will cause the plywood to swell more than solid wood and the joint will again be imperfect. (Plywood with a composite core will be worse than all wood veneer plywood.) For this reason, we often put a small groove at the joint (scratch line) so that imperfections will not be so obvious.