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Discussion Starter · #1 ·
Hi All,
Planning on making a series of plywood boxes that are going to double as bench seating. The dimensions are going to be about 30" long x 20" wide x 13" high with 3/4 plywood. The bottom will also be 3/4" ply. I'm thinking of using rabbet joint with glue and screws. Should the rabbet go on the 30" or the 20" side? How deep does the rabbet need to be? Thanks
 

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Structurally, it does not matter which way you go, but the appearance will be different when viewed from the front. It will look neater if the joint is not visible from the front : ie. if the rabbet is cut in the piece that becomes the front panel.

As for depth, I would go for 3/4 of the thickness. For a tidy cut you might like to consider a depth of around that much, but fine tuned to correspond with one of the plys so as not to have thin fragments left.
 

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Ordinarily for ¾" material or thckness, I rabbet ¼" deep. For this box, a double rabbet might work out better. The front and sides and floor, can be machined like the picture below.
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Wood Product Hardwood Floor Plywood

For the two front vertical corners, a rabbet done like the below picture would take care of the plywood edge. A spline can be machined for additional strength. The joinery should be clamped up with glue in two directions.
Line Parallel Diagram





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That second illustration at the bottom of Cabinetman's post was what I had in mind.

If you plan to use screws then you will need a deep rabbet like that to have room for them. The spline is neater of course....
 

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Discussion Starter · #5 ·
Thanks for the suggestions. I'm working up the plans now.

The plywood I have has five ply can I cut through the center of a ply?

Would the glue be enough strength without the screws?
 

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Personally I would not risk glue without reinforcement. Splines would give you acres of glue surface for strength and look neater too, but screws would do.

Make sure you drill the right size for the core diameter of the screw, cos you risk the wedge action of the screw forcing the plys apart.
 

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No, there are other options. Router and slot cutter bit would be fine, a dedicated biscuit cutter (essentially the same thing with different guides and dedicated to the job) would be good too. Another option is to use the table saw.

Set the fence for one edge of the slot, set the blade to the desired depth and cut. Then move the fence (or use a spacer) to widen the slot with a second cut. If you use biscuits, they are typically 4mm (here in Europe) so with a 3mm blade width, 1mm movement of the fence and a second pass and you have a 4mm slot. If you dont want the slot to extend all the way to the edges, you can use stop blocks on the saw fence to limit the travel. With the work against the stop block, lower it onto the turning blade - not for the faint hearted perhaps ! Turn off the saw before removing the work after each cut though for safety.

Same principle, but different tools would be to use dowels in place of splines. Then you can do it all with a drill but accuracy is tricky without a dedicated jig. That's what I did with these drawers, solid wood not ply, but the same principles apply. I made mine visible 'cos I like the effect, but you can work from the "inside" faces and have nothing visible just like splines.

 

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Discussion Starter · #9 ·
Hi All,
I'm about to start this project and would like a opinion on the dimensions for joint spline. Is a 5/16" x 3/4" spline large enough? See the attached screenshots. Thanks
 

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