I want to make many irregular shaped cutting boards. for making smooth edges , I want to use round over bit. because of the round edges , I don't know how to sand perfectly. I just found
pipe belt sander tool. are there any better way/jig for this problem ?
A well-executed roundover should require very little sanding. I might start at 240 grit. You can back your sandpaper with a sponge or wad of cloth so that it will conform to the edge even better than your hand will. A sharp bit and a light touch are key.
I will note, though, that rounding over plywood edges can be problematic. The layered surface of plywood doesn't always make a smooth surface for the bearing to glide over, and the alternating grain of the layers can leave you with some tearout, even creating voids at times.
Also I would think twice about using plywood for cutting boards if you mean them for food preparation. Plywood's veneer layer may be good quality wood but the layers are joined with adhesives which are not necessarily food safe. By rounding over the edges you are almost certainly exposing more of these adhesives to contact with food. With a countertop or table, you can protect with a polyeurethane finish and come away with a food safe surface, but this won't work well on a cutting board that's going to see a lot of direct knife contact.