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.
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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.
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this gets you further along your journey.
charlie b