When you say "boxes" do you mean jewelry box size
or display stand size? Are you making completely
closed boxes or mitered corners on the front, back
and sides with a bottom in a dado and the lid in a
frame that sits on top?
As others have noted, running ply, even baltic,
russian or finnish ply over HSS jointer knives
is asking for nicked knives. Unless you get the
fence set up just right, exposing the knives
only the EXACT width of the ply you're going to
change the dimensions of the parts if you over
shoot the blade exposure.
There's another way to do mitered corners on
ply that uses a router table, a precision position-
able fence and either a 45 degree chamfer bit
or a "miter lock" router bit. It does require
that all initial parts have
- parallel edges that are square to the face
- straight edges
- square corners
Any errors in the above will show in the finished
joints. The miter lock method also requires some
trial and error set up of the fence AND the height
of the router bit. Here's a link (which has links
to more about this bit and it's set up). All one
line so watch the line wrap.
And here's a web page I just put together on
doing a completely encloded box using a lock
miter bit - just for you. What service!
Again - all one line so watch the line wrap.
hope this gets you further along your journey.