The ribs are bent on a hot bending iron and glued to the shaped blocks. After gluing in the top linings, the ribs are flattened using a glass plate and chalk, which marks the high spots, which are then planed until the ribs resemble the glass (which is to say, flat). This is one of the coolest low tech techniques we use in the shop. The key is a very sharp plane (or you could break the rib) and a bit of patience. I've also shaped the inside of the scroll here.