I used a 6" plate shear extensively for my project. I was able to cut 3/16" 4130 using a cheater bar. Makes nice clean cuts. Then take to grinder to finish shape any round corners, etc. I've cut everything with it, steel, aluminum, plastic, composite plate, etc. Very useful.
http://www.grizzly.com/products/Plate-Shear-6-/H0732