How to cut dados for a bookcase?

[meackerman]

As for dados, I always test fit the dado to the material.  Also note that it is not guaranteed that consecutive sheets of plywood, MDF or particle board bought at Lowes or Home Depot are of uniform thickness. 

I was building some book shelves from white laminated particle board.  I carefully setup the dado blade for a fairly tight fit.  But the second sheet was thicker than the first and it would not fit in the dadoes.  Both sheets were taken off the stack in sequence. 

So check the thickness of all the sheets before cutting the dadoes.

which is why a router and jig that references directly off the thickness of the plywood shelf really works well.

Course if you then sand the shelf after you cut the dado, the fit won't be as tight.

[mstens]

Maybe you know this, but If you are using “3/4” plywood” it probably isn’t 3/4” thick despite how it might be marked at the store.

If you use a 3/4” router bit or dado blade set up the dado will be too wide.  

Mike

... and this is why I register off of the sheet itself. 
 

measuring just induces error.

[Cooler]

which is why a router and jig that references directly off the thickness of the plywood shelf really works well.

Course if you then sand the shelf after you cut the dado, the fit won't be as tight.

My point was that all the shelves are best cut from one sheet of flat stock.  My concern is not the fit, but that all the shelves are of the same thickness so that the fit is good for all of them.

[meackerman]

I've found the same sheet isn't always a uniform thickness. So I always check the fit for every piece.