Curving the shield: This applies to curving plywood, not joining shield boards into a curve (more on that later).
After building several shield presses and jigs over the years, I have settled on a method that is simple, inexpensive, and works every time.
First, draw a guide in full scale for the amount of curve you want. Cut it into concave and convex pieces for checking the curve later.
You will need:
4 pipe clamps ($5.00 each at Harbor plus some pipe (at least 3 feet long); I use 3/4 inch)
A dozen or more C or F clamps, at least 6 inch throat
A couple dozen or more plywood blocks cut from scrap, 2x2 inches; mine are an inch thick
A box of 3 inch wood screws
Wood glue (your choice)
Saran wrap
Plywood to make shield (your choice)
A drill, bits and drivers
Small 4 inch painting roller
As before, let's say you're making a shield that's 2x4 feet. You're expecting this to get some real punishment, so instead of 2 layers of 1/4 inch plywood, you are going to call around and get 3/16 inch ply and build this out of 3 layers (for a total of 9/16, which I have found is much tougher).
(Again, this is just an example: there are so many options...)
Cut the plywood to the same size panels.
Cut your 2in. plywood blocks and drill a whole in the center of half of them to freely except the shank of your wood screws, and the other half need pilot holes. Cover in several layers of Saran wrap.
Stack your Panes on each other and drill a hole through them all at top center and bottom center. Use a painting roller to evenly spread glue between each sheet according to manufacturer's directions. Titebond III is great; Gorilla glue is very strong; traditional hide glue works just fine.
Clamp the top center and bottom center together with a C or F clamp.
Stretch your pipe clamps across the width of your panels. Space them along the length evenly. Start tightening the pipe clamp until the panels bow up. From now on the edges will stagger, but you can ignore that for now. One at a time, tighten each pipe clamp a bit more. Get your curve guides and start checking the curve from inside or outside. You may find the center gets to the curve you want earlier than the edges. Use your Cor F clamps to lock these curving edges together, as needed.
You may also find the long edges may splay. Use more C or F clamps to pull them together, but not tight yet.
Continue this process until you get just a tad tighter than the curve you want. One by one you can replace the C or F clamps if you want to use them for the next shield blank. Start with the center top and bottom: Remove the clamp, thread a wood screw through a block, through the hole in the plywood (or drill a new one if needed), and screw into the pilot hole block underneath. Tighten it down. The Saran wrap will keep it from sticking.
Do not remove pipe clamps until the glues cures (24; 48 hours; whatever the directions say).
After that, remove all clamps and screw blocks .
Layout your cutting pattern, and cut into the shape you want. If there are any holes left and you want them filled, simply apply wood glue to a dowel of the same diameter as the hole, push some through leaving it proud. When the glue is cure, trim with a chisel or gouge.
Next, I'll tell you how to figure out where to drill for the shield straps so its right the first time!