This print allows you to make your own R/C Heli/Quadcopter training gear, with the similar levels of accuracy and build characteristics as store-bought training gear.
As it's parametric, you can define the number of arm sockets as well as your choice of rod diameter. Additional cutouts are done to minimize the weight of the hub.
Hopefully this will give you optimum flexibility in printing one with the correct size/arm count for your bird.
The default settings in the .SCAD and exported STL are for a 200-size heli, using 3mm carbon fiber rods as the arms.
UPDATE: Download the .STL file for a pre-generated hub set for 45mm diameter and 3mm rods. This .STL has additional fillets to strengthen the hub at its weakest points, and chamfers to ease insertion of the rods.