I used splines to construct the core when I built a large (4' x 8')
torsion box table. I used inexpensive plywood I bought at the home
center, and expected a fair variation in thickness, using splines
avoided having to deal with that. But in that case, the load is
distributed across many such joints, and the skins. These shelves will
hold books, and I thought a dado would be a better choice. I didn't
consider biscuits or mortise and tennon for the same reason. Am I wrong
Side rabbet plane will work fine in plywood.
Or, as someone else suggested, rabbet the plywood shelf to fit a
narrower dado. You will still have to adjust it by hand, but it's
easier to trim the thickness of the tongue with a rabbet plane than it
is to trim the dado with a side rabbet.
From other posts it sounds like your router bit fits ok for the thinnest
part of the sheet. If this is the case when you set up your fence for the
router run a layer (or possibly 2 or 3 layers) of tape along the fence. If
dado is too tight, remove 1 layer of tape, run dado again and it will widen
by the thickness of tape. Repeat until you get a nice fit.
Not much more fiddly than a single pass dado and a very cheap micro
adjustable fence at that.
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