The taper exists as a pickup or lead-in. Its job is to keep the edges of the sproket from coming into contact with the edges of the chain. Idealy, the only contact you want between a chain and a sprocket is with the chain rollers.
The taper allows us to build robots with misaligned sprockets!
A lot goes into the design of a sprocket. You’re probably best off buying one (unless making one means you’ll get to play with an expensive cutting machine
). The taper should be easy to cut on a lath.
Greg