A hump in the middle. Actually, the whole surface is curved such it is higher at the center than at the edges. You won't need much crown. The crown helps the blade to track. But aligning the wheels is way more important than crowning to get the saw band to track properly. The saw must be made so that at least one of the wheels is adjustable in two directions. Imagine a plane passing through the fixed wheel. Now imagine an in-and-out adjustment on the other wheel that lets you bring that wheel into the plane of the fixed wheel. And the other adjustment tilts the wheel. If a line is drawn through both wheel centers as they lay in the same plane the tilt axis will be parallel to the plane and perpendicular to the line passing through the wheel centers. And I think you will need a lip on the back sides of the wheels. At least all the band saws I've seen that used cast iron or steel wheels have a lip on the back to prevent the blade from coming off. And if the running surface of the wheels is wider than the distance from the bottom of the tooth roots to the back of the blade you will need a rubber belt (called a tire) on the wheels to prevent taking the set off one side of the teeth. ERS