[MrForbes]

Without seeing your basic design it's hard to know if this will help, but you migth be able to redesign it so that the sprocket attatches to the arm instead of the shaft (with a bearing between the two)....this might allow you to use a smaller shaft and not have to worry about keying it.

igus parts are really nice for arm pivots, although it takes some effort to figure out how make it all work. And possibly a lathe.