Hi Steve, We put a pin on the center unless we need you have a cup without a hole. Then we drill a hole midway between the pin and edge to bleed the air from the part while spinning it. The part comes out nice and parallel with no bell mouth opening. We have made hundreds of spin tools that way in both aluminum and steel depending on how many pieces we run. So far the aluminum tool have produced hundreds of parts but the get soft eventually due to the heat created by spinning. Hope I helped out on this as you and your crew have given me idea also.
Steve K.