[Appears in its entirety here: Cambering a Scrub Plane Iron but what follows is the short version.]
If you have a true scrub plane, like the Stanley 40, then you probably already have an iron with the right camber (curve) on the cutting edge. If you are in need of a scrub plane for flattening a twisted board there are a lot of good reasons to use an old wooden, transitional plane (the ones half wood with a metal carriage on top) or metal bench plane. Personally I like my Stanley #5 Jack plane that I converted to a scrub. I like the longer length and the added mass.
Converting is actually pretty easy, just back the frog back a bit to open the throat and camber the iron appropriately. Frog backed off…check. Iron cambered appropriately????
The question of how big a curve should be put on the iron is tough, and it different for every width iron. but just because it is different for every iron, doesn’t mean you should just fiddle with it and try to get it right. It is too easy to come up with a wrong angle.
Plug in your measurements and pop out an answer. I use 3/32” as the reveal. That lets me take a deep scrub but gives me room to back it off too.
-- Galootish log blog, http://www.timberframe-tools.com