On Sat, 24 May 2008 11:10:52 -0700 (PDT), Fred the Red Shirt
wrote:

On May 23, 7:36 pm, Tom Watson wrote:
...
I usually make a dado that is in width from a
third to a half of the nominal thickness of the ply. Then I cut a
tongue in the other piece to match that width.
...
The offset tongue should be made on the bottom face of the shelf so
that the laminations of the shelf ply are loaded in compression,
rather than being under tension.

I usually make thelength of the tongue very slightly shorter than the
depth of the dado, so that the glue has somewhere to go when you clamp
up. By 'very slighty' I mean no more than the thickness of a sheet of
paper.



That makes good sense, but I would be inclined to make
the tongue more like 2/3 to 3/4 of the thickness. I don't see
any advantage to making it less.


Then you would only have 3/16" - 1/4" of meat left beyond the groove
when using this joint at the top or the bottom of a carcase. Your
theory works fine for a shelf or divider in the middle of the panel
but would require different joinery for the corners, which is not
optimal.



If it is less than half, the some of the veneers that
aren't glued into the dado will be in tension.

The tongue is formed on the bottom part of the thickness of the board.
The shearing load is compressive throughout the thickness of the
board.

Only at the bottom of a carcase, where the tongue would, of necessity,
be formed on the top part of the thickness of the board, would there
be tension on the glue lines of the ply. The reason that you can get
away with that is because you can have a frame below the carcase at
this point.






Regards,

Tom

Thos.J.Watson - Cabinetmaker
tjwatson1ATcomcastDOTnet
www.home.comcast.net/~tjwatson1