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Discussion Starter #1 (Edited)
I've been trying to come up with the best joint detail for dadoing fixed 3/4" plywood shelves into a bookcase on all 3 sides. I need this much support because I'm using the shleves to store heavy reference books and they will be completely loaded down. I will have a 3/4" nosing to add strength and casing trim all around the front face of the bookcases which should hide these dados completely.

I usually try to design my details to allow some tolerance so I don't have to build joints that fit perfectly. I'm using a router and was following an example from a magazine that used zero clearance hardwood templates and the actual plywood shelf as a guide to set the dado template width. This was fine except after cutting I learned a few things.

First thing I learned was not all 3/4" plywood is exactly the same thickness. My first set of shelves were cut from two different pieces of plywood and they vary by as much as 1/64 to 1/32. This didn't seem like much until I installed the shelves. Then a small horizontal gap at the top of the dado shows up like a sore thumb. So I thought I got smarter the second time and cut all the shelves from the same piece of plywood.

However, the second thing I learned was that sanding the shelves prior to assembly will also change their thickness just enough to create more horizontal gaps.

My initial solution was to rabbet the top and bottom of the shelves to create a thinner toungue that would be easily concealed by the thicker shelf. However, since I do not have a router table yet that's alot more setup than I want to do for these. After further thought I came up with the joint on the attached picture. I think by cutting a rabbet just into the top of the plywood shelves I'd create a toungue that would be easily concealed while still allowing tolerance. I'm pretty sure gravity will close up any potential gap at the bottom of the shelves anyway. In order to keep from having to make each shelf exactly the correct width to prevent vertical gaps between the shleves and the sides, I realized I could just make the dado bigger than the shelf toungue in both directions. I think this could solve the problem.

Anyone have any better ideas?
 

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Have you considered gluing strips of hardwood around the edges of the shelves. Align these hardwood 3/4 X 3/4 with the top of the shelves. They will fill the gaps and add strength and stability to your shelves. Of course the top shelves (above eye level ) should have the strips glued on the bottom plane of the shelves to keep that small lip out of sight.

If you're concerned about glue oozing out onto you're shelves, you can finish the shelves prior to the glue up. Just don't get any finish on the edges that will be joined.
 

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Discussion Starter #3
Thanks Bernie,
I actually am planning to add some small scribe moulding to the top joint of the shelves. However, this is not something I want to keep doing. I'd like to find a better joint for the future work. The bottom joints are tight and don't need any help, thanks to gravity.
 

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I like your idea, but ,...

I would have a larger tongue say, 5/8" or 11/16" and a corresponding rabbet just slightly larger. I wouldn't trust a 3/8" tongue with a heavy load of reference books.

You can make exact width dados using a jig:
 

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Discussion Starter #5 (Edited)
Good point Woodnthings...an open joint like I drew would be risking more movement than a closed joint, so a big tongue makes sense. It only has to have enough overlap (maybe 1/8" or so) to conceal the dado, so I would think the tongue could easily be that big.

As they are now, the shelves snug up all the way to the back of 3/4" tall by 3/8" deep dados on each 3/4" plywood side panel and all the way into 3/4" tall by 1/4" deep dados across the 1/2" plywood back and they are glued in solid. Feels sturdy enough for a small family to live in. Hopefully they will hold.

As for the dado jig, that is a similar system to what I used. My dados came out exactly the thickness of the plywood template I used. My brainless issue was not realizing that the template I used from one piece of plywood shelf could be a different thicknesses from another plywood shelf. And that sanding the shleves could thin them down enough to show a gap. If I use that method again I'll be sure to double check all my plywood with a thickness gauge and sand it BEFORE using it as a thickness guide.
 
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