Hi Lady,
I’m with Yadda on this one. Using a knife is possible assuming your material (plywood) is very thin. Anything more than 3/16” thick is probably too thick. I tried to replicate what you’re proposing with 3/16” thick birch veneered plywood. Just to make a couple cuts using a utility knife outfitted with a new blade was tedious. For one or two cuts, it will work, but an entire picture will take some significant time and effort to create with a knife. Anyway, here’s what I did:
Using a straight edge (the machinists square) I scored the plywood making 8-10 passes. I flipped the workpiece over and repeated the process. With the plywood scored on both sides, I broke the workpiece over a cylindrical object (the punch). I didn’t get a nice edge so I cleaned them up with a SHARP block plane. Then I made an angled cut (trying to replicate a triangle) going through the same process. Like I said, it’s tedious. Just to make those two cuts and clean them up took about 10 or so minutes. If you’re going to proceed, I’d look for plywood that’s 1/8” thick or less. Is balsa wood a possibility?
Your tools need to be sharp and you’ll need to adequately secure the workpiece once it gets too small to hold and work by hand. That knife is sharp and you’ll be applying force trying to cut through the plywood. If you slip or go out of the score line, the hand holding the workpiece could end up with a nasty cut!! While I went through the process of trying what you’re proposing, after doing so,, I’d recommend looking for an alternative method to cut the plywood, or use a different, easier to cut material like balsa wood.
Jim C.