Dado’s in plywood of variable thickness
Moosewood
| Posted in General Discussion on
I recently bought cherry plywood for some book cases which I am building. The thickness of the ply varries significantly from sheet to sheet. As such rabbiting and dadoing the case joints is slow and laborious. Does anyone have ideas on how to speed this process? I do not want to use biscuit joints because of the weight of books which must be supported by each shelf. I believe that I once read that using a combination rabbit, dado joint will help deal with varying thickness of wood. As I understand this joint, it is not as strong as a housed dado. I would greatly appreciate any suggestions.
Rick
Replies
It doesn't have to be a large rabbet, just enough that the resulting "tenon" becomes the same thickness on all sheets. The loss of strength would be small but the ease of making the joints dramatically simplified since you need only one dado setting.
Does the depth of the rabbit not have to vary with the thickness of the wood? Sory to be thick headed about this.
Rick
No that's the entire joy of the method. You create the rabbet with a method that references the surface that will remain, not the surface that will have the rabbet cut out of. Doing this does skate a bit around the absolutely safest practices. For example, if using a table saw, you would cut with the remaining tenon between the blade and the fence. Finger boards and an auxilary fence would be needed. On a shaper, the cut would be made with the waste coming from the top. With a router table the cut would be made with something like a slot cutter taking a bit off the top as the board registers on the table.
You need only the smallest shoulder, which would be desirable since making only small cuts with no detached parts (only the saw dust) reduces the feedback risk.
Thank you. It had not occured to me to run the shelves vertically to create the rabbit. That is a great solution.
Rick
The rabbit is a good solution, but be sure that you put the "tenon" on the bottom of the shelf, rather than the top, or you can get separation between the plies when you put books on the shelf. It's a small detail that's easy to overlook while in the throes of building.
Mike D
There are a number of ways to handle dadoes for varying thicknesses. As time goes by, your collection of routers bits will be different sizes due to sharpening. There are under size bits available for plywood but they don't fit half the time. Sometimes getting a bit that's just right is luck. Lately, I've been lucky.
One way is to shoulder the edge of the shelf, partition, etc. You can shoulder both sides or just one. It doesn't have to be much, so it shouldn't bother strength.
I do a lot of casegoods and cut dadoes all the time. I prefer fully housed dadoes and I like to keep it simple. I replace the round base on the router with a "square" one. This gives a good straight edge that will be the same distance from the guide fence, every time. I make each side of the base less than the adjacent one, so it's not actually square. You can vary the difference as needed. Sometimes I may add a strip of tape or two to fine tune. As you turn the router, the bit will be a different measure from the guide fence to account for various increments of thickness.
I normally clamp a guide fence to the left and push the router away from me. Because you will be making two cuts, you either have to return pulling toward you, when cutting the right side of the dado, or cut that side first on the push. This is due to the rotation of the bit. If you turn the base to a side that is farther from the fence, after making the first pass, it will climb cut on the push. You'll understand once you try a sample.
A picture may explain it better. By using a smaller bit you set the base up so two passes, without changing the fence, makes the size dado you need. You can do the same thing with a template guide and a double guide fence, or just a double guide fence. The only thing with that, is there are more things to complicate the measurements.
For rabbets, I just clamp a fence on the router base. Use an over size straight bit and put a cutout in the fence, set it to the size you need.
Here's a graphic of a suitable joint. It's essentially what other contributors have suggested. A router works well in conjunction with a T square guide for the housing, and a router and side fence for the tongue on the end of the shelf or horizontal part as sketched. Slainte.
Richard Jones Furniture
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